Buzzing speakers on CRT TV with TH-11135A ("sync buzz")
Good day folks. Not having enough money or not wanting for whatever reason to get a LCD TV, a friend of mine asked me to take a look at his CRT TV which developed this buzzing sound which reacts to the picture after power went out and then came back during a thunderstorm, supposedly subjecting the set to a power surge. It's audible over the speakers and it changes in tone depending on the picture displayed - it would react to subtitles going on and off for instance. I looked into it a bit and apparently it's called "sync buzz". From what I recall, the buzzing stops when the TV is muted or the volume turned all the way down to 0, but it appears as soon as you turn up the volume even to 1. Not having worked with CRTs before, mostly out of fear of all that HV going on, I was a bit reluctant to open it up and inspect, but nevertheless I did this yesterday, pulled out the chassis, which is a TH-11135A (I've attached the schematic - might not be the best quality, since it's split in half but it was the only one I could find) and located the TDA1112a audio chip which I believe it's the culprit for this - it's located in the top right of the second page of the schematic I provided. I took some pictures of the whole board and the area in question. Trouble is: what's causing this buzzing all of a sudden ? Should I replace the audio chip entirely or some of the caps around, like the VCC one or perhaps the AC coupling ones for the speakers (C606 and C605) ? I couldn't find the datasheet for this audio chip so I don't know what some of those pins do, mainly 3 and 6 which seem to be labeled as CTV or CIV (not too clear) and RR respectively. From what I can tell, I'm either picking up noise from the flyback somewhere, or there's a filtering issue on the 18v bus which also powers the audio amp. The 18v come from a SMPS which also produces a few other voltages. It's not convenient at all to start replacing components blindly, as I'd have to test it out every time, which means putting all the wires back in place which is a chore in itself and then also discharging everything once I see it's not working and I have to go back in there to change something else. Any thoughts ? Cheers.
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