Video Corner [Jan. 1990--Electronic Servicing & Technology]

When the cures become the symptoms

By Wayne Smith, CET [Smith is an electronics servicer in Arden. NC.]

A VCR is a complex device - actually a complex system - consisting of drive mechanisms to move the tape and the video head cylinder; electronics to convert the magnetic signals on the tape to electronic signals that produce a picture on the TV; and control circuitry to make sure everything works together properly.

Because of this complex nature, troubleshooting a VCR can become a complex procedure. You must keep an open mind about symptoms and their possible causes. Perhaps as important, you must make sure you understand what the manufacturer meant by all of the terms in the servicing literature.

One other important item: If the symptoms point to a problem in an area that has already been repaired, don't be blind to the possibility that the repair may not have been made correctly. This article describes the problems that can occur when you lose sight of these suggestions.

Don't ignore the obvious I was repairing a top-loading ma chine, Panasonic model PV1270/ PV1275, for another shop. The original trouble reported was that the tape would load, then unload about 10 seconds later.

On my bench, the unit was almost dead.

The power-on indicator was illuminated and the clock was working. However, none of the front-panel switches had any effect.

A visual check revealed that there were no obvious troubles and that the tape sensor lamp was illuminated. In most machines, if the sensor lamp or LED is defective, the symptom is shut down, which is what this unit was exhibiting. System control microprocessor checks revealed that the V DD (B+) Power supply, the oscillator and the reset circuit were operating normally. The four scan pulses – C0, C1, C2 and C3 - were present at the microprocessor output pins. (See Figure 1.) When the Play button was pressed, scan pulse C1 appeared at input port B3. The scan pulse proved that the microprocessor was being given the command to load the tape, but something was overriding this command. One of the safety sensors obviously was activated.

The Mode Select switch is coupled to the mechanical portion of the machine.

It should have been and was in the stop position as verified by a logic low at P6002, pin 4. The Cassette Compartment Up/Down switch was in the correct position (down) as verified by a logic low at P6003, pin 2. The unload and pause logic levels were verified at their respective connectors.

By this time, I was thinking that the clock could have a problem, although it seemed to be working. The proper logic level was verified at IC6005, pin 24.

Figure 1. If the scan pulses are present at the microprocessor output pins, the microprocessor is receiving the command to load the tape. Before a servicer delves into the microprocessor, he should first check the end-of-tape sensor lamp.

Figure 2. The high symbol at JA2 does not indicate a TTL logic high (2V or above).

Know the schematic symbols

The only sensor input that had not been checked was the auto stop at the JA connector, pin 2. I didn't suspect this input because it connects to the sensor lamp circuitry and I knew that the sensor lamp was lit. Just to be on the safe side, I checked the voltage at JA2. It measured 0.6Vdc. The block diagram plainly stated that the condition at this point would be a high if the auto stop was activated.

I knew the microprocessor almost never failed, but what else was left? IC6003 and IC6004 are tri-state buffers.

They couple the four scan outputs into the eight data inputs. The program in side the microprocessor knows which switch or sensor has been activated by the scan output pulse (or pulses) that is (are) coupled into the data inputs (A0 through A3 and BO through B3). Could a buffer be bad, causing the wrong pulse to be applied to the input ports? One way to check this is with a dual-trace scope. Channel A is connected to TP6004, the C0 pulse. This pulse is used to sync the scope and act as a reference to check the timing of the other scan pulses. Channel B is then used to troubleshoot.

Pins 14, 16 and 17 appeared to have the proper scan pulses present. Pin 15 had scan pulses C1 and C2 present.

From the block diagram perspective, the only thing that could turn on both of these buffers would be a logic low at the junction of D6049 and D6030. If the VCR is not in auto stop, JA2 should be low (and was measured at 0.6Vdc earlier). This low is inverted by Q6010 into a high, which would not al low the C1 and C2 scan pulses to be coupled into pin 15. A voltage measurement at the D6049/D6030 junction con firmed that it was logic low. Could Q6010 be defective? The collector voltage measured 0.01Vdc, meaning this transistor was either shorted or turned on. The base measured 0.65Vdc, which was just enough to turn it on. I had mistakenly taken the high symbol at JA2 to mean a TTL logic high (2V or above). Before fighting this dog anymore, I wanted to verify that it would work, so I ran a clip lead from JA2 to ground. The machine operated normally.

Check the replacement history

The sensor circuitry connected to JA2 obviously should cause the voltage to be less than what is needed to turn on the transistor switch. In addition to the sensor lamp circuitry, the dew sensor circuitry is connected to JA2. Subsequent checks proved that the dew sensor and its components were good. The resistors and zener diode (D6504) in the sensor lamp circuit checked out OK. The +12Vdc was normal. The only thing left that could be causing the problem was the sensor lamp.

Time for a call to the technician who referred this problem to me. During preliminary checks of the unit, he recalled, he had found that the sensor lamp was broken and had replaced it with a close substitute.

The resistance of the substitute lamp was 31-ohm. The original lamp for this machine measures 5 ohm. Even though the re placement lamp looked physically the same and had about the same brightness, the higher resistance allowed the voltage at the anode of D6504 to rise to 0.65Vdc. With the exact replacement lamp installed, the voltage was close to zero.

Ambiguous schematics

For a final check, I went back to pin 15 of IC6002 and was surprised to find the C1 scan pulse present. The machine was operating satisfactorily, so it was obvious that this pulse was supposed to be present. Nevertheless, how did it get there with diode D6049 biased off? It's not apparent from the block dia gram, but the schematic will reveal that pin 4 of IC6003 is connected to pin 2 of IC6002 by diodes D6014 and D6034.

The purpose of this port is not discussed in the service or training manual, but it seems to stay at logic low after an initial high at power-on.

This repair seems to have two morals: Don't assume that something called a high or low is necessarily a logic high or low, and always use the correct re placement part.