Early AM and FM radios were made up of transistors, variable tuning capacitors, inter mediate frequency (IF) transformers, and AM and FM oscillator coils, all mounted on a printed circuit (PC) board. The coils were tuned by variable tuning capacitors, with a dial string moving the dial pointer. The AM/FM front-end circuits were switched in and out with a slide or rotary switch. Miniature IF coils or transformers were found in such early AM/FM stages (Fig. 1).
By removing the back of the case or cover, you can easily locate each section. The ferrite antenna rod serves as the antenna coil, whereas the FM coils are made up of larger wire and soldered directly to the PC board. Usually these radiofrequency (RF) and antenna coils are located near the variable tuning capacitor. Both AM and FM IF coils or transformers are shielded and found in a row in front of the variable tuning capacitor. The AM coils are wound with many turns of fine wire, whereas the FM coils form a bare solid wire.
FIG. 1 Miniature IF coils are found in portable AM/FM radios.
FIG. 2 A block diagram can help to locate the different receiver stages when
no schematic is available.
The AM and FM bands may switch in supply voltage to separate AM and FM circuits, whereas in deluxe radios a band-switching rotary switch may be found. You can locate each transistor stage to the coils and shielded IF transformers. The audio output stages are found near the volume control leads and can be traced back from the speaker terminals to locate the output transistors.
The front-end circuits
Use either a comparable schematic or a block diagram when the actual schematic is not available (Fig. 2). Early front-end circuits consist of an AM RF coil, a converter, an IF detector, and audio stages. The FM front-end circuits may consist of transistors in the FM RF coil, an FM oscillator, an FM mixer, first and second IF transformers, a discriminator coil, and audio circuits. A separate tuning capacitor with fewer plates is used to tune the higher FM frequencies, whereas the AM stages are tuned with many aluminum plates. Both AM and FM variable capacitor sections are rotated with a dial cord and dial indicator (Fig. 3).
FIG. 3 A tuning knob rotates the dial cord that turns a variable tuning capacitor
in an AM/FM/ MPX radio.
Locate the separate AM and FM front-end sections. The FM coils are made of solid wire and are mounted on the PC board. Locate the AM ferrite antenna rod with coil, and trace the wires to the tuning section and RF AM transistor. The FM/MPX stereo section should be mounted close to the FM stages. Early AM/FM radios had IF transformers sticking up from PC board. The audio section may have power transistors or integrated circuits (ICs) on heat sinks, whereas the low-voltage power supply can be located by the power transformer and large filter capacitors. A quick voltage test across the filter capacitor can indicate if the power supply is working.
Note the receiver symptoms, and apply them to the components located on the chassis. If the AM reception is weak, intermittent, or has no reception, go directly to the front-end circuits. Locate the antenna ferrite coil and tuning mechanism. In early chassis, two or three ganged tuning capacitors tuned the RF and oscillator stages. Today, varactor diodes tune the RF and oscillator sections. In digital tuned receivers, a tuning controller IC provides control and tuning data that are applied to the RF, mixer, and oscillator circuits (Fig. 4).
FIG. 4 A leaky capacitor C31 (0.02 uF) caused a no-station symptom in the
AM converter stage of a Radio Shack clock radio.
Common problems
Look for circuits common to both the AM and FM sections when stations are not received. Check the low-voltage power supply and regulator source feeding the AM and FM circuits. A leaky regulator transistor or zener diode can produce a dead AM section and a normal FM section. Critical voltage measurements on the AM or FM transistors and IC components can help to locate a defective low-voltage source.
Check the IF stages or combination FM IF and AM converter circuits. Sometimes the first FM IF transistor serves as the AM converter. Notice if both the AM and FM IF stages are common to one another (Fig. 5). For instance, an open second IF trans former connection can destroy both AM and FM reception. A dirty AM/FM slide or function switch can cause dead AM or FM reception.
A big change
Today, AM/FM/MPX receivers have many new changes since tube and transistor radios. Instead of a variable tuning capacitor, varactor diodes tune the AM and FM coils. A central phase-locked-loop (PLL) processor provides the varactor diode tuning voltage. The IF transformers have been replaced with ceramic filter devices that are cut for the desired IF frequency of both AM and FM bands.
Instead of transistors, the AM and IF front-end section may consist of IC components throughout the receiver circuits. Silent switching of the various receiver sections is done with silicon diodes. Dual IC power audio outputs are found in both stereo channels. In fact, a front-end IC may contain all the RF, oscillator IF, and pre-audio amp circuits, whereas one large power output IC may contain all the audio circuits (Fig. 6).
FIG. 5 IF transformers were common to both AM and FM circuits in early AM/FM
radios.
FIG. 6 Block diagram of an AM/FM receiver with separate AM and FM stages.
FIG. 7 A leaky or open Q103 results in poor or no AM reception.
No or poor AM reception
The AM section may be dead as a result of any faulty transistor or IC component found in the front end, and FM reception may be normal. Dead AM reception can be caused by a leaky or open AM transistor or IC component. Check each transistor with in-circuit tests. If in doubt, remove the transistor from the circuit. You will find that the suspected transistor is open or leaky with in-circuit tests but normal when removed.
Replace the transistor if in doubt. Sometimes the AM/FM front-end IC is defective in AM and normal in FM circuits (Fig. 7).
Clean the function switch assembly with cleaning spray when the AM or FM is erratic or intermittent. Spray cleaner into the switch contacts, and work the switch back and forth to clean the contacts. Replace a broken function switch with an original part number, if it is available.
Suspect a defective AM RF transistor when a local station can be heard with a volt age probe on the collector terminal. Test the transistor in-circuit for open or leaky conditions. Critical voltage tests will indicate if the transistor is open or leaky. Check the antenna coil for open or broken leads when there is no AM.
Actual AM problems
Radios that use a ferrite antenna coil can produce a weak radio symptom when the ferrite core is cracked or broken. Some of these ferrite cores are round, and others are flat.
Sometimes the core will break inside the coil winding, resulting in weak AM reception.
Suspect a defective front-end IC for no AM reception (Fig. 8). Replace the front-end IC component if there is no AM reception. Check the RF transistor for weak or no AM reception. Replace the AM IF transistor or transformer if there is no AM reception. Resolder the RF and converter transistor terminal connections for no AM signal. Replace the RF transistor even if it tests normal because it can break down under load. Check for improperly soldered contacts on the AM oscillator coil when there is no AM reception. Suspect a leaky RF AM transistor when you can get only one local station. Inspect all leads off the ferrite antenna coil when there is no AM reception. A poor board connection of the detector diode can result in no AM signal. Suspect a broken wire or trace to the AM tuner assembly when there is no AM reception. Do not overlook bad board traces around the AM section when there is poor or no AM reception.
A dead AM section can result from no or improper supply voltage to the AM circuits. Check the supply voltage to the AM RF coil, converter, and IF transistors or IC components. Suspect a leaky or shorted decoupling capacitor or transistor voltage regulator when there is no or improper supply voltage. Replace the leaky IC regulator when only a hum is heard in the AM or FM band.
FIG. 8 Check the AM front-end circuits when there is no AM reception.
FIG. 9 A leaky transistor can be identified by improper 0.2-V bias voltage
when it should be 0.6 V on an NPN transistor.
FIG. 10 For no AM reception, check for a broken coil wire or connection on
the antenna coil.
Intermittent AM reception
Intermittent or weak AM reception also can be caused by a defective zener diode in the power supply. Weak AM reception also can result from a leaky transistor regulator in the AM supply voltage source. A weak AM symptom can be caused by a bad AM converter transistor with only 0.2 V when it should be 0.6 V between the emitter and base terminals (Fig. 9).
An open primary winding of the AM IF transformer can cause a rushing noise on the AM band. A dirty AM/FM switch can cause intermittent or no AM reception. Suspect a bad oscillator padder capacitor for intermittent AM reception. Intermittent AM reception can be caused by a bad or open IF transformer. Simply move the IF shielded transformer and notice if the AM signal goes in and out. When only one station can be tuned in at the lower end of the dial, suspect a broken base capacitor within the AM IF transformer. Check for a broken metal slug in the oscillator coil when some stations are weak or drift off channel.
Determine if both the AM and FM stages are intermittent or just the AM circuits. A cracked or broken antenna core can weaken AM reception. The crack or break may be inside the coil winding, so take a close look (Fig. 10). Some cores can be replaced.
The exact replacement may be available from the manufacturer.
The small Litz wire used in antenna coils is easily torn loose or broken off, and poorly soldered connections can produce intermittent AM reception. Check the windings for continuity. Use the lowest scale on the ohmmeter, and check each winding. You should get a direct short or very low measurement.
Push up and down on the PC board within the AM section. Probe around on small coils, capacitors, and resistors with an insulated tool. Sometimes by moving a small part you can turn up a poorly soldered connection or terminal. An intermittent terminal lead or junction within the transistor can be located in this manner.
The trouble may be between the AM converter stage and the antenna coil. Grasp the body of the antenna coil to see if the stations become louder. If they do, there is continuity between the converter and at least part of the antenna. Look for a break or poorly soldered connection under a layer of coil dope or wax.
Weak or intermittent AM reception can be caused by a defective RF transistor.
You will find a separate RF transistor in the better receivers, but many receivers use a single transistor AM converter that functions as both the RF and oscillator stages.
When a unit has only two variable tuning capacitor sections or varactor diodes, you can be fairly sure that the unit uses a converter stage with no separate RF. If several stations can be tuned across the dial, you can assume that the converter transistor is normal.
No AM reception
No AM could be tuned in on a Radio Shack 13-1267 receiver. Critical voltages were taken on IC101. Zero voltage was found on terminal 23, and the rest of the voltages were way off. The tuning voltage on D102 at test point TP201 was normal. Tracing the secondary winding voltage back to TP203 indicated very little voltage. Resistor R125 (22 kilohm) showed signs of overheating and was cracked on the bottom side.
Replacing leaky LA1266 and R125 solved the problem (Fig. 11).
FIG. 11 Replace leaky IC101 and resistor R125 (22 kilohms) for no oscillator voltage in the AM oscillator circuit of a Radio Shack 13-1267 stereo receiver.
No AM, normal FM
Suspect the AM front-end section when there is normal FM but no AM reception be cause both bands join together in the IF section. Poor contacts or broken antenna coil leads can cause a loss of AM reception. Check for a leaky or open RF AM or converter transistor when there is intermittent or no AM reception. The RF transistor can break down under load and still test good. Clean the AM/FM band switch for no or intermittent AM reception. Check the first AM IF stage for open or broken internal windings. Resolder all PC board contacts when there is intermittent AM and normal FM. Replace IC3 on the tuner PC board when there is no AM reception in an Akai ATM77 receiver.
No FM Suspect a defective FM stage when the AM reception is normal. A leaky or open FM RF FET transistor or IC can cause no FM reception. Sometimes, by placing the meter probe on the base terminal for a voltage measurement, a local station may be tuned in, indicating a leaky or open RF FM transistor. Take critical voltage tests on the RF transistor. An open FM RF transistor will have zero voltage on the emitter terminal and a higher dc voltage on the collector terminal than normal. A leaky RF FM transistor may have a low voltage on the collector terminal and no bias volt age between the base and emitter terminals. Check all transistors within the FM front-end circuits with the diode tester of the digital multimeter (DMM). Some times spraying the transistor with coolant can make it act up or begin to operate (Fig. 12).
FIG. 12 Spray coolant on an intermittent transistor to make it act up or
return to normal.
A loud rushing noise on FM can indicate a defective mixer or oscillator transistor or IC. No FM reception can be caused by improper voltage from the low-voltage circuits. Check for leaky or open regulator transistors and zener diodes. Sometimes a dirty switch contact supplying voltage to the FM circuit can prevent FM reception.
A defective IC voltage regulator can cause no FM reception and only a hissing noise.
Check for a bad zener diode on the power supply board when there is no FM reception. A red hot low-ohm resistor within the voltage supply source may be caused by a defective 47- uF electrolytic. Replace a leaky voltage regulator transistor when there is no FM reception. Replace defective bypass capacitors in a tuner section with low supply voltage.
Poor soldering on the emitter terminal of the FM RF transistor can cause no FM reception. A bad padder capacitor in the oscillator circuits can cause no FM reception.
Realignment of the padder capacitor on the FM antenna coil can result in no FM stations on the lower end of the dial. No FM signal can result from zero voltage found on a leaky or shorted FM FET RF transistor that might lower the supply voltage source.
Replace the first FM IF transistor when only a local FM station can be heard. Check for a defective IC ceramic filter network when there is no FM reception. Check for an open FM coil within the tuner when there is no FM reception (Fig. 13). No FM reception was found in a Marantz SX5 receiver with normal AM, and it was caused by a defective C124 off of Q107-2.
FIG. 13 No AM reception with a leaky RF FET transistor (Q101).
FIG. 14 Clean the AM/FM switch (SW 701-A) when there is intermit tent or
no FM or AM reception.
No FM, AM normal
A Soundesign 5154 receiver came in with normal AM reception and no FM reception.
When both front-end signals were not found at the first or second IF transformer, the FM RF coil, FM oscillator, and FM mixer transistors were checked. Try to locate the bare-wire FM RF coil and FM oscillator coil on the PC board. The RF FM transistor is located between the FM antenna terminals with a bare FM RF coil in the base circuit of the FET transistor. When the positive probe of the voltmeter touched the base terminal, a local FM station could be heard. The open SK41 FM transistor was replaced with an RCA SK9164 universal replacement.
Dirty AM/FM switch
Suspect a dirty or worn AM/FM switch when either section is dead or intermittent.
The dirty contacts can cause no AM or FM reception. A dirty AM/FM switch will not apply a dc supply voltage to one or both bands. Replace the worn switch with a toggle or slide switch found just about anywhere. Spray cleaning fluid down inside the switch area, and move the switch back and forth to clean up the contacts (Fig. 14).
Replace resistor R154 (1.5 kilohms) for slow switching from AM to FM in a Mitsubishi DAR11 receiver.
Weak FM reception
When only one FM station can be heard with two AM stations, replace the FM IF and AM RF transistors. Distorted and weak FM reception can result from a leaky FM/MPX IC. No voltage on the collector terminal of the FM IF transistor can be caused by a badly soldered IF connection and will result in very weak FM reception. A defective or dirty local-distance switch can result in a weak FM signal. Check for an increase in resistance of the resistors in the FM circuits when there is weak FM reception.
Replace a leaky voltage regulator transistor with low supply voltage to the RF section when it causes a weak FM signal. Weak or no FM reception with a hissing noise can be caused by a defective IC voltage regulator.
Check for a bad FM padder capacitor when the FM starts fading out with a lot of noise. Realignment of the padder capacitor may be needed on the antenna coil when there are no FM stations on the lower end of the dial. Replace the FM oscillator transistor when the stations drift off channel. Adjust the FM discriminator coil when the FM reception drifts off. Readjust the alignment of the FM detector coil when the receiver will not lock on FM reception. Resolder a bad connection and tighten the mounting screws on the PC board when an FM station drifts off with only a hum in the audio in a Randix KR4140 receiver.
If the AM section is normal but the FM is weak, prime suspects are the FM RF section and the FM antenna. The FM RF transistor is typically mounted on the PC board next to self-supporting coils. If a chassis layout is handy, use it. If not, signal trace the wiring from the FM RF variable capacitor section, varicap, or varactor diodes to the RF transistor.
The FM RF transistor can be checked with in-circuit transistor or diode tests with a DMM, which will indicate if the transistor is open or has a beta reading. Don't adjust any FM screw trimmers, variable capacitors, or FM RF transformers until proper repairs are made.
FIG. 16 Suspect the front-end section when the cassette or CD player operates
in the audio section.
Really weak FM reception
Suspect the AM/FM tuner front-end section when the FM reception is weak. Measure the supply voltage at the supply pin terminal (Vcc). Extremely low voltage in a Realistic receiver was caused by a very low power supply source (Fig. 15). Remove pin 20 from the supply voltage and notice if the voltage increases. If not, repair the low voltage supply source.
FIG. 15 Weak FM reception was caused by improper supply voltage at pin 20
of an AM/FM tuner assembly.
No AM or FM reception
Look for defective components that are common to both the AM and FM bands. The power supply, IF transistors, IC components, and RF transformers and ceramic filters are common to the AM and FM sections, so if both sections are dead, chances are that the cause is one of the four. Rotate the function switch to the tape or phono position to check the audio stages. If they work, the problem is located in the IF stages (Fig. 16).
Take voltage measurements on each IF transistor and IC. Compare the voltage with the supply source. Improper readings can indicate a defective transistor or IC. Test all transistors in-circuit, or remove suspected ones for testing. Normal voltage readings indicate that the IF transistors and power supply are functioning but will not always identify a defective IF transformer.
A defective IF transformer can be located with signal injection or continuity tests.
Use the ohmmeter to check each winding. The AM IF transformer will measure a few ohms, and the FM IF transformer will measure less than 1 ohm. Erratic or intermittent AM and FM reception can be caused by a poorly soldered connection to the transformers or a broken IF coil winding. Ceramic IF filter networks have no resistance measurements and seldom cause problems.
FIG. 17 Resolder poor connections on Q105 for both intermittent AM and FM
reception.
You can locate a defective IF transformer by signal tracing. Turn the function switch to the AM or FM position. Inject a 455-kHz signal to the base of each consecutive IF stage, starting at the last IF stage and working toward the converter stage. Likewise, inject the FM IF stages in the same manner with a 10.7-MHz signal. When no tone is heard, you have located the defective stage. Signal from a white noise generator can signal trace the RF and IF stages.
Replace a bad regulator IC in the power supply when there is no AM or FM reception. A shorted zener diode in the power supply can cause no AM or FM reception.
Check for a defective decoupling electrolytic in the voltage source when there is no AM or FM signal. A leaky IC regulator can cause a loud hum on the AM and FM band when the volume of the receiver is turned up.
Clean the AM/FM selector switch when both bands are intermittent or dead. A bad front-end AM/FM IC can cause no AM or FM reception. Replace both the AM converter transistor and the RF FM transistor when there is no AM or FM reception. Resolder the antenna input connection when there is no or weak AM and FM reception. Replace the common AM/FM second IF transformer when there is no AM or FM reception. Resolder the AM/FM first IF transistor emitter terminal when there is no AM/FM reception (Fig. FIG. 17). Replace Q202 and Q203 in the low-pass-filter circuit near IC201 in a Pioneer SX6 receiver when there is no AM or FM reception.
FIG. 18 Shorted D104 and burned resistor R125 (180 ohms) caused no AM or
FM in a large Xtal XA-800 car radio.
NO AM OR FM, XTAL XA-800
No AM or FM reception was noted in an Xtal XA-800 car radio. A quick voltage test on the AM/FM front-end circuits showed no dc supply voltage. On checking the power supply, several components were found burned. D104 had become shorted and caused resistor R125 (180 ohms) to become charred. Replacing R125 and D104 restored the 7.75 V to the AM and FM circuits (Fig. 18).
Intermittent AM and FM reception
Determine if the audio section is working in both stereo channels by clicking the center terminal of the volume control with a screwdriver blade. If hum is heard in both stereo channels, you can assume that the audio section is normal. Try out the cassette or CD player if there is one in a combined AM/FM receiver. When both AM and FM sections are intermittent, you may think that the trouble lies in the front end circuits that are common to both bands, such as the IF or ceramic filter stages.
Locate the FM IF transformers or ceramic filters, which are usually wired in the series circuit (Fig. 19).
First, try to move the IF coils or transformers around with your fingers. You might have a badly soldered connection. Move the IF transistor or IC with an insulated tool or pencil. Sometimes a transistor lead will have a poor internal connection or poorly soldered terminal connection. Since IF transistors and ICs frequently cause intermit tent conditions, spray each one with three coats of coolant. Let each coat disappear before applying the next one. Take your time with each transistor or IC before going to the next.
Monitor the low-voltage source feeding the AM and FM IF stages. Intermittent volt age can produce intermittent reception. Clean the AM/FM slide and function switches.
If the intermittent trouble still exists and it seems to be around the IF transformer, remove it from the board. You may be able to repair the IF transformer by removing the insides. Pull back the tabs or indented area, and pull out the coil from the outside shield.
Check each winding for continuity. Place the coil under a magnifying light, and solder the suspected terminal. These transformers should be replaced with factory originals.
After repair, tap and twist the transformer slightly to check the success of the repair.
FIG. 19 Look for common components, such as faulty IF coils, a bad ceramic
filter, or improper or intermittent supply voltage, for an intermittent
AM/FM symptom.
FIG. 20 Check for bad traces and electrolytic capacitors in the front-end
circuits with an ESR meter.
AM up, FM down
Suspect an FM front-end circuit when the AM stations are normal and the FM is dead, weak, or intermittent. The defective component must be in the FM RF coil, FM oscillator, or FM mixer circuits, ahead of the common IF circuits for both AM and FM reception. Check for a leaky or open FM RF transistor or IC when no local stations can be tuned in. A badly soldered connection on the FM RF transistor or IC can cause a dead, weak, or intermittent condition. Check for open coupling capacitors within the FM front-end section with an equivalent series resistance (ESR) meter (Fig. 20).
A badly soldered connection on the first IF transistor can cause a very weak FM signal. A weak FM station can be caused with a dirty local distance switch. Intermittent FM reception can be caused by a badly soldered IF transformer connection. Replace the FM oscillator transistor when there is only a loud rushing sound. Resolder all connections on the FM band for intermittent FM reception. Excessive static can result from a noisy FM IF IC.
No FM reception may result from a low or no voltage applied to the FM RF coil or FM oscillator transistors. Check for a broken or open voltage source resistor with low voltage applied to the FM circuits. A bad voltage regulator transistor or IC can cause a no FM symptom. Only a hissing noise on the FM band can be caused by a leaky IC regulator. Check for a leaky IC regulator for hum on the FM band.
FIG. 21 Take critical voltage and resistance tests on IC1 when there is no
AM or FM reception.
Components that cause intermittent
AM/FM Double-check all combination AM/FM components such as the front-end IC, which might include all circuits and feeds directly to the FM multiplex and detector AM circuits (Fig. 21). Check for a defective ceramic filter with the crystal checker if one is handy. Test the continuity of each primary and secondary side of the filter for poor connections. Check for intermittent AM/FM IF transistors. Replace the IF transistors when there is noisy reception.
Intermittent AM and FM reception may be caused by a leaky or open voltage regulator transistor and zener diode in the voltage supply circuits. An intermittent transistor or IC voltage regulator can affect both the AM and FM bands. Replace open or dried-up filter capacitors within the same voltage supply source when there is no AM/FM reception, when the AM is okay and the FM is distorted, and when there is no FM with weak and intermittent AM reception. Check for a defective electrolytic in the power source when both AM and FM reception are weak and distorted.
FIG. 22 Warped plates in the variable tuning capacitor can short out the
various stations within the AM and FM bands.
No tuner action and intermittent and excessive static with intermittent shutdown were caused by a bad ground with loose screws on the main PC board of a Sony STRAV201 AM/FM/MPX receiver. Intermittent reception in early radio receivers can be caused by the metal plates meshing in both sections of a variable capacitor tuner when the tuning control is turned (Fig. 22).
FIG. 23 A bad first IF connection within an AM/FM IF circuit can result in
no AM or FM reception.
FIG. 24 A bad ceramic filter (CF101) in a Realistic 12-2103 in-dash AM/FM
car radio resulted in no FM reception.
Defective IF circuits
A badly soldered connection inside the IF transformer can cause intermittent AM and FM reception when both IF coils are in a series circuit (Fig. 23). Dead AM reception can result from an open IF transformer winding. Small bypass capacitors inside the IF transformer can cause a dead or intermittent symptom.
The IF circuit can be dead or intermittent as a result of a leaky or intermittent IF transistor or IC component. Poorly soldered terminal connections on the IF transistors or ICs can produce a dead or intermittent symptom. Resolder all connections on the IF board when there is intermittent AM and FM reception.
Intermittent supply voltage feeding the AM or FM IF circuits can cause an intermittent AM/FM symptom.
Although ceramic filter networks found in AM and FM IF circuits cause very few service problems, sometimes you will find a defective ceramic filter component. Take a continuity measurement with the ohmmeter on both sides of the ceramic filter. No resistance should be measured. If a resistance below 1 kilohm is found, remove one terminal on that side of the ceramic filter and take another test. Sometimes a low-ohm component such as a transistor, resistor, or diode may be wired in parallel with the filter and can cause a low resistance measurement. Remove the ceramic filter from the PC board, and take another measurement. Both sides of the ceramic filter can be checked with a crystal checker (Fig. 24).
DEAD GE IF SYMPTOM
No AM or FM reception was noticed in a GE 7-4665B clock radio. Most of the volt ages checked out on the RF and converter circuits. Very little voltage was found in the second IF stage and was traced through T2 and T3 IF circuits. A continuity check was made on all IF transformers, and a badly soldered connection was found at the top of T3 of the 455-kHz IF transformer. Resoldering T3 solved the problem.
Weak AM and FM reception
Sometimes you may find a defective front-end IC that causes weak reception in both AM and FM bands. Check the IF and power supply circuits when there is weak AM and FM reception. An open or leaky IF transistor or IC can cause a weak symptom in both bands. Suspect poor IF transformer connections causing weak and intermittent reception. Take critical voltage tests on each IF transistor or IC. Check all IF transistors with in-circuit tests with the diode tester of a DMM.
Low supply voltage from the power source can cause a weak symptom in the AM/FM bands. Take a critical voltage measurement on the suspected transistor or IC.
Trace the low voltage source back to the power supply with critical voltage tests.
Check for a leaky or open transistor or IC voltage regulator in the voltage supply source. Sometimes there is a leaky voltage transistor or zener diode within the base circuit of the regulator transistor. Check for burned resistors or a change in low-ohm resistors within the voltage regulator circuits. Make sure that the resistor is defective by removing one end and taking another reading. A change in a decoupling electrolytic can cause a low supply voltage (Fig. 25).
FIG. 25 A leaky regulator transistor (Q110) provided weak FM reception in
a Pioneer VSX-452 stereo receiver.
FIG. 26 Replacing a leaky voltage regulator (IC202) in a Sharp SC210 receiver
solved the weak AM/FM reception.
WEAK AM/FM RECEPTION, SHARP
The voltage supply source to both AM and FM circuits of a Sharp SC210 receiver was quite low at 0.58 V. Since the supply voltage on the voltage regulator IC (IC202) was around +19.5 V, the output voltage on pin 14 was only 0.58 V dc. The voltage regulator (ZD201) was suspected of leakage because the collector terminal was between R314 (100 ohms) and R257 (47 ohms). ZD201 tested normal on the diode tester of a DMM. Replacing leaky IC202 with an ECG1243 universal IC solved the weak AM/FM reception (Fig. 26).
Rushing noise in FM, AM OK
Check the FM RF transistor or IC when there is a rushing noise on the FM band with normal AM. The open FM oscillator transistor or IC can cause a rushing noise on the FM band with the volume turned up and no stations tuned in. A noisy RF or oscillator transistor can cause a hissing noise in the FM band. Replace the FM oscillator transistor when there is a low rushing noise. Microphonic reception can result from a defective FM RF transistor. Excessive static can result from a defective FM IC component. Realignment of an FM IF and MPX IC transformer can cause a noisy FM channel. Check C412 (6.68 uF, 35 V) for a leaky condition when noise and a loud roar are heard on the high-end stations on the FM band in a Sony STRVX22 receiver.
FIG. 27 Checking suspected transistors with the diode tester of a DMM in
an AM/FM receiver.
Transistor in-circuit tests
You can quickly check all transistors in the circuit with the diode tester of a DMM or transistor tester. The quickest method is to use the diode tester of the DMM because you can complete the job in minutes. Check for open, leaky, and high junction resistance tests. A high-resistance test will be a lot higher in resistance than with a reverse test between two transistor terminals. Sometimes the transistor will test normal within the circuit and break down under load (Fig. 27).
At other times a transistor may test open with an in-circuit test and when removed test normal. Make sure that no low-ohm components, such as coils, diodes, and low-ohm resistors, are tied across any two transistor terminals. FM transistors should be tested with in-circuit tests because they are difficult to re move and replace.
Another method is to take a bias voltage test between the emitter and base terminals to make sure that the transistor is good. Measure the low voltage between the emitter and base terminals. An NPN transistor should measure 0.6 V, whereas a PNP germanium transistor should measure 0.3V. Usually the transistor is normal if it has a correct bias voltage measurement. Measure the emitter voltage to ground and the base voltage to common ground, and the difference in voltage should equal the correct bias voltage.
After locating a defective FM transistor, check the location and lead length and how the transistor is mounted. Obtain a new universal replacement, and cut the leads the same length as the old ones. Mount the transistor back in the same spot. Redress leads around the transistor. Test the new transistor in the circuit. Sometimes when replacing the FM mixer and oscillator transistors in older receivers, FM alignment must be per formed. With the latest receivers, especially with ceramic filter IF stages, alignment is not needed.
FIG. 28 Check all IC trace terminals after the IC has been removed.
Replacing AM/FM RF-IF IC components
Make sure that the suspected front-end component is leaky or open. Check the sup ply voltage (Vcc) that is applied to the IC part. Take critical resistance measurements from each IC pin terminal to common ground. Make sure that the low or shorted measurement does not tie directly to common ground with a wire trace. Take signal in and out tests on all audio IC components to locate the defective IC.
Remove the defective IC by applying solder wick material on each side of a row of IC terminals. Place the hot soldering iron on the mesh material, and suck up the excess solder. Move each IC terminal with a pen or screw driver blade so that the terminal is free from the wiring trace. Be very careful not to pull up the PC wiring with too much heat from the soldering iron. Double-check each terminal after the IC has been removed (Fig. 28).
Replace the defective IC with the original part number if available. Universal IC re placements also work well in AM/FM front-end circuits. Look up the part number stamped on top of the IC in a semiconductor manual. Make sure that pin terminal 1 is marked on the PC board. Likewise, mount terminal 1 of the new replacement in the correct position. Solder all IC terminals with a 30-W soldering iron. A battery-operated soldering iron is ideal when soldering up IC terminals.
FIG. 29 Improper adjustment of the FM discriminator coil in an early transistor
AM/FM/MPX radio caused distorted FM reception.
FM discriminator and MPX circuits
Early FM discriminator circuits consisted of an FM IF transistor, discriminator coil, and two diodes in a full-wave circuit (Fig. 29). A left and right audio channel were found after each detector diode. Later, a multiplex IC component was found at the beginning of the right and left stereo audio channels. Sometimes, just touching up the alignment of the discriminator or multiplex can cure a loud hissing noise.
A shorted or leaky diode can cause distortion and hum within the audio channels.
A red hot MPX IC can cause only a local FM station to be tuned in. Weak and distorted FM can result from a bad FM/MPX IC. Extremely distorted FM music can be caused by a leaky MPX IC. Replace the MPX IC when there is no FM reception. Check the FM IC in the tuner, and readjust the MPX for normal FM stereo. When the FM quits after 10 to 20 minutes, solder around the adjustment control in the matrix circuits. A weak FM/MPX audio can be caused by a defective IF transformer, RF-IF tracking, or an FM/MPX IC or a poor FM antenna circuit.
Replace the FM detector coil when there is no or intermittent FM reception. Suspect that the FM discriminator coil is out of adjustment when the FM station begins to drift off channel. Check D26 for a shutdown of the MPX IC when the FM drifts off in a Randix KR950B receiver.
Distorted FM tuning
When each FM station was tuned in, distortion was found in a JC Penney 853-3218 four-channel receiver. T3 and T4 on the discriminator coils were adjusted to no avail.
D5 and D6 checked normal on the diode tester of a DMM. When the capacitors were tested in the output, C46 (4.7 uF) showed less capacity, and ESR problems were found on the ESR meter. The tuning meter responded and distortion was eliminated after replacing C46 in the discriminator circuits.
FIG. 30 Check the low-value bypass capacitor, resistors, and IC301 MPX when
D301 stays on all the time.
No stereo light
Try to adjust the SVR control in the MPX IC circuits when the stereo light is not lit or will not go off. In early FM circuits, a panel light was used, and in today's FM/MPX circuits, a light-emitting diode (LED) component is found. Check all low-capacity electrolytic (10 to 47 uF) in the MPX PC board when the stereo light is out (Fig. 10 30). Replace the MPX IC when the stereo light cannot be adjusted and the light stays on all the time. Suspect low-capacity electrolytics when there is normal FM or AM audio and the stereo light will not light up. Check for an open regulator transistor (Q803) when there is no signal strength meter or no FM tuning meter movement in a Marantz 2253 receiver.
The signal and recording meters may have analog meter movement or a bank of LEDs. The audio signal can be traced right up to the meter or LED circuits with an external amp. These indicator LEDs can be tested with the diode tester of a DMM. If one becomes dead or weak, all have to be replaced in one large component. Determine if signal is applied to the indicator circuits before checking the LED indicator circuits. No LED display in a Sony STR53555 receiver was caused by an open lamp on the LED panel.
Drifting FM The FM stations would drift off channel after a Sanyo FTC26 auto radio operated for 1 hour. The stereo light would go out at the same time. All voltages were checked on the MPX IC (IC301). D301 and resistor R308 (560 ohms) were replaced on pin 10 with no voltage indicated. The FM coil (T201) also was touched up to solve the drifting FM signal.
FIG. 31 Take critical voltage measurements on the LCD driver IC for no display.
Liquid crystal display (LCD)
The latest AM/FM receivers have a liquid crystal display (LCD) mounted right on the front panel. The LCD indicates the station that is tuned in, numbers, and the various receiver operations. Usually the LCD is driven by an IC from a data-clock crystal control microprocessor. Suspect a driver IC when the display appears dead (Fig. 31). Check all voltages on the display and driver IC. Scope the data crystal for a waveform.
A very dim display can be caused by a leaky IC. Check for poorly soldered joints on the panel display board for intermittent display functions. For no functions and no display, check for badly soldered connections to resistors and silicon diodes in the power supply. Replace C908 for no digital display in a Technics SA410 receiver.
FIG. 32 The tuner controller varies the voltage on the varactor diodes to
tune the AM/FM bands.
The varactor-diode tuner
The varactor diode changes capacitance when a dc voltage is applied to it. By varying the voltage on the varactor diode, the diode changes capacity and tunes both the AM and FM coils in the receiver. The varactor diode is found in both AM and FM front-end circuits. In early radios, a variable capacitor tuned the various bands, and now the varactor diode takes its place (Fig. 32).
Besides the FM varactor tuner, you may find varactor diodes in the RF coil, mixer, and AM oscillator circuits. The same signal voltage is supplied by the controller. In some chassis there is a test point to measure the controlled voltage. The voltage applied to the varactor diode is a different voltage for each station.
Determine if the varactor diode is operating by taking a voltage measurement fed to the diode. Rotate the AM dial and notice if the voltage applied to the diode changes as the knob is rotated. If so, the tuning controller IC is functioning. Suspect a leaky varactor diode when the voltage does not change across it. Check the different voltages found on the receiver's AM RF coil and converter varactor diodes. If one of the diodes' voltage does not change, check the diode for shorted or leaky conditions. The varactor diode can be checked with the diode tester of a DMM. Remove the varactor diode's ground lead for a correct reading in only one direction. Compare the diode resistance reading with the other varactor diodes in the radio.
Suspect a bad digital tuning controller IC when there is no change of voltage across each varactor diode. Double-check all voltages measured on each diode. If the voltage does not change at all, make sure that the controller IC or corresponding circuits are defective. Measure the supply voltage to the tuning controller IC. A low or improper supply voltage (Vcc) may indicate a defective controller IC or power voltage source.
FIG. 33 Check TP1 and TP2 for a variable voltage applied to the varactor
diodes in the FM RF coil, FM oscillator, and FM mixer stages.
Varactor tuner controller
The varactor tuner controller IC provides the correct voltage to each varactor diode within the AM/FM RF coils, AM converter, FM oscillator, and mixer circuits. The dig ital tuning system provides control over the supply voltage fed to the front end (AM/FM tuning), fluorescent display, signal indicator, and switching in some digitally controlled receivers. A microcomputer chip provides digital tuning control and remote data to several stages of a digitally controlled system. The synthesizer processor (IC) provides controlled voltages to the AM/FM RF and IF circuits (Fig. 33).
Check the supply voltage at the FM tuner. This voltage will vary as the tuning knob is rotated. Suspect a faulty transistor regulator or improper voltage at the processor chip when there is no voltage at the FM tuner. Take critical voltage tests on IC101, TR103, and TR102. If voltage is found at the front-end tuner, suspect a defective tuning system. Check each stage in the same manner as those found in conventional front end tuning circuits. Take a scope test of the crystal-controlled phase-locked-loop (PLL) circuit of IC101.
Very little tuning voltage was found on the AM RF coil, AM oscillator, and FM circuits in an integrated Radio Shack 13-1267 component receiver. The variable tuning (VT) voltage source was traced back to AM diodes D101 and D102. This voltage is fed through a 100-kilohm resistor to test point TP201 (VT). The VT voltage is derived from PLL IC103, which provides tuning voltage to the AM and FM tuning circuits. A B+ voltage check at driver transistor terminals on Q105 was normal, except very little volt age was derived for the VT voltage source. Replacing transistor Q105 (2SK583) solved the tuning voltage problem.
FIG. 34 A positor component is found on the power transformer and output
heat sink to shut down the receiver with overheated parts.
Overload protection
Some deluxe receivers have a shutdown circuit when a power transformer or power supply becomes overheated. A new positor component is inserted in series with a circuit that shuts down the receiver circuits when these parts are running too hot.
The positor part is mounted directly on the transformer and power output transistor heat sink to protect the power supply and amplifier components. When the temperature of the positor changes, the other circuits are shut down (Fig. 34).
When the temperature of positor TH601 (the one installed with a heat sink) and power transformer positor TH602 rises abnormally, the resistance of the positor becomes larger, and the pin 1 potential of IC602 will increase and cause transistor TR606 to turn off. The result is that pin 2 of IC611 is now cut off from the power supply voltage (Vcc) line, and the power transistors are protected.
Check D603 and D604 with the diode tester of a DMM when output transistors have overheated and have been damaged. Make sure that positors are mounted against the heat sink. Take a resistance measurement of each positor and compare with a new component. Replace IC602 if all other components are normal. Measure correct +14.5 V feeding positors and diodes.
Power output transistor protection
A transistor within the output circuits of a high-powered amplifier protects the output transistors when abnormally high current flows through them. Transistor TR604 protects TR602 and TR603 from overheating. TR604 shuts down if there is excess input drive or when the overload impedance connected across the output is too low (Fig. 35). If the current increase is excessive, the voltage across R610a, b will turn on TR604. Then the pin 1 potential of IC602 will increase and cause transistor TR604 to turn off. Now the result at pin 2 is that IC601 is cut off from the power supply voltage line.
To suppress power turn-on noise, a time delay is provided by time-constant R622 and C313. This time delay is set to activate transistor TR604 to connect the +Vcc line and pin 2 of IC601 after enough time has elapsed for the tone-control amplifier and the preamplifier to reach a stable operating condition (Fig. 36 on p. 340).
Take critical voltage tests on each transistor and IC. Test each transistor in-circuit.
Check the supply voltage on IC601 and IC602. Measure each bias resistor for a change in resistance after other tests are made to service the special circuits in the latest receivers.
FIG. 35 TR604 and TR605 protect the power output transistors from overheating
as a result of excessive current or leaky conditions.
Remote control circuits
The remote that comes with a large deluxe receiver may control the on and off buttons and volume up and down. The remote also may control the tuner bands to tune the different stations up and down and have a separate button for muting the sound circuits.
Besides controlling the receiver circuits, the same remote can control the TV and VCR functions. The remote may be powered by two AAA batteries (Fig. 37 on p. 340).
Remote volume control
In many of the latest surround sound receivers, the remote control tunes in the various radio stations on the AM and FM bands. The volume of the receiver can be rotated up or down by the remote. The up and down volume control is rotated by a dc motor and motor driver IC. The volume control driver IC provides up and down voltage to the variable dc motor. The up and down signal is fed into pins 1 and 9, whereas the motor out is fed to pins 2 and 7 of IC381 (Fig. 38 on p. 341).
FIG. 36 Positors TH601 and TH602 are tied to pin 1 of IC602 to shut down
the receiver when components are overheated.
FIG. 37 A simple infrared remote control can operate the tuning, up and down
volume of the volume control, and off and on operations in deluxe and surround
receivers.
FIG. 38 The system control IC controls the up and down volume of the receiver
with a driver IC381 and dc motor tied to the volume control.
For further troubleshooting, see Table 1.
Table 1. AM/FM/MPX troubleshooting.
[Symptom Receiver inoperative (no LED or light)
Fuse blown with power on Speaker inoperative One channel does not work Power indicator light but no sound from both channels One channel dead when signal test applied to volume control No headphone reception No cassette operation No phono operation No FM Poor or no multiplex Stereo indicator does not light FM mute inoperative No AM
Bass no effect
Treble control no effect
No meter or display
Remote control inoperative
]
[ Cause and remedy
Replace faulty cord
Defective power switch
Blown main fuse
Bad power transformer
Defective power transformer Shorted silicon diode Shorted main filter capacitor Shorted output transistors Defective dual power output IC Defective power output transistor Defective speaker relay Defective ac relay Bad speaker fuse
Speaker relay or dirty contacts Bad or open speaker
Blown speaker fuse Defective output transistor Defective output IC Bad copper traces in the output Shorted or open speaker Replace burned bias resistors Defective coupling capacitors
Defective driver or AF transistor Defective driver IC Replace open or burned bias resistor Replace open or dried-up electrolytics Defective balance control
Bad headphone jack Open headphone resistor Dirty headphone jack contacts
Check and clean tape heads Check for torn headphone connection Defective transistor or IC preamp
Bad phono pickup Torn crystal connection Broken wires on crystal pickup Bad phono plug
Defective front end Defective IC front end Dirty AM/FM switch Defective voltage regulator transistor Defective MPX IC and circuits Defective IF FM circuits Defective ceramic filters
Improper adjustment of FM discriminator Improper adjustment of VR control Replace discriminator transistors Replace MPX IC
Defective lamp or LED indicator Improper VR adjustment Improper MPX adjustment Defective resistors in MPX circuits Improper supply voltage
Defective FM mono/FM switch Check for leaky mute transistor Check for no mute control IC
Check AM front end Defective AM IC Defective AM/FM switch Defective AM varactor diodes Defective controller circuits Damaged AM RF broken coil
Defective bass control Bad connections on bass control Shorted internal bass control Open or dried-up coupling capacitors
Defective treble control
Torn-off connection on treble control
Internal shorted control
Open or dried-up coupling capacitors
Defective LCD Improper supply voltage Bad display IC driver Open voltage regulator transistor Defective crystal
Replace battery Dropped or broken PC wiring Test with portable radio
]
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