Symptom: High, uncontrollable volume: channel functions lock up.
Set ID: MGA model CD1945R Photofact: 2435-1
This set would work properly when first turned on, but after a while (usually several minutes, but sometimes just a few seconds) the volume would rise to a very high level and the volume could not be turned down from either the re mote control or the TV itself. In addition, the channel up and down functions would usually lock up.
Initially we thought it was a thermal problem, but liberal use of cooling spray and a heat gun brought no changes. A scope check of the microprocessor, IC701, revealed some noise on at least one of the lines feeding the volume control circuit. We replaced the new microprocessor with one we had in stock, but the symptoms remained.
As a shot in the dark, we wondered if the volume-up control was somehow intermittently on, so we exchanged it with the volume-down switch. Again no change. We also tried replacing diodes D725, D774 and D785; if one of those was leaky it might have been the source of the noise we saw on the scope. Again, there was no change in the symptoms.
As you can see in the accompanying portion of Photofact 2435-1 (see Figure 1), the four resistors feeding the two volume functions and the two channel changing functions are either 47 k-o or 4.7k-o. Because resistors R815, R816, R817 and R818 in this set were 47 k-ohm, we decided to parallel them with other resistors to get the value down to near 4.7k-o. When we did this, the set appeared to function normally. Apparently this change in resistance was enough to somehow reject the noise signals.
Knowing that this wasn't an entirely valid fix, we pursued the matter with MGA. After we described the symptoms and the troubleshooting steps we took to the MGA representative, he suggested that the spurious noise might be coming from the channel LED display. That circuit unplugs easily from the main board, so it was easy to check. With the LED display out of the circuit, the volume remained where it was set. When we in stalled a new LED display we obtained from MGA, everything operated normally with none of the original problems. After bench testing for several days with no recurrence of the symptoms, we returned the set to service.
Michael B. Danish; Steven D. Ashcraft; Aberdeen Proving Ground, MD.
Figure 1. Although paralleling resistors R815, R816, R817 and R818 with other
resistors to reduce the resistance treated the uncontrollable volume symptoms
in this TV, a new channel LED display treated the underlying problem.