Philips 14GR1234/75RTV -Periodical surge increases in audio volume

[Technomaniac]

This 14" TV with I believe a GR1 chassis, occasionally bursts up loudly for a second and then back to normal until the next time. It may do it three times in an hour. I tired of babysitting it and connected it to a three channel chart recorder, which shows a high amplitude pulse in the volume control pin of the do-everything-chip 7020. I have not found such a surge on any rails or signal points so assumed it was occuring in the chip itself, until I replaced it, which made no difference. I have replaced any relevant rail bypass electrolytics. I believe that it happens coincident with a scene change. There appears to be a slight vertical jump on scene changes which may not be normal. I couldn't find a vertical hold tab pott to tweak, nor could I find fault with any of the vertical circuitry. I noticed that the vertical oscillator is a division of the horizontal timebase so tried tweaking that oscillator but it didn't make any difference to the vertical jump. Downloading info on the aforementioned chip (which is a TDA8305) revealed that the volume control pin is also used as a start pin for the horizontal oscillator !  So I have tried fiddling the circuitry on that pin to reduce excursions should there be a mains dropout or something happening in the power supply that I missed, but the burst of sound continues to occur. This chassis model was hugely popular - someone must have experienced this problem ! (I hope!)  

16th June 2009 added: Found Philips had a modification for this which didn't fix it in my case. Also found a discrepancy between the schematic and the set w.r. to D6030, 12V Zener. In my set it is on the pos. end of C2050, circuit says the negative end. Moving it to the neg. end didn't help. After much frustrating research and experimentation, I found inserting a 20v zener diode in series with the starting pulse capacitor C2058 solved the problem completely. The rail has to vary 20V now for the fault to occur. The rail variation was only half a volt that I could see and I couldn't find any power supply fault, though there may have been one behind this. Be careful with the PC board foils on these sets, they come adrift very easily.