621TS Sound Problem
 

621 Experts! Having quite a problem on a 621 which I'm working on for a friend. The set has always had intermittent volume problems. Sometimes the volume would be fine, and sometimes it would be low to imperceptible.

I'm not very familiar with the sound circuit this TV uses, which is inhibiting my ability to try and fix it. Recently the sound has started to break into whistles and tweets when fed with a fixed tone through the test pattern generator. Tube substitution does not help, and the chassis has been found to be microphonic. Voltage checks in teh tuner and sound I.F. are all on the mark.

Here's a youtube I've made demonstrating the problem, I really appreciate any input.

http://youtu.be/cNx9pm-7ck8

It's a dirty tuner or audio IF issue for sure, sounds as if it's on a blank channel.

On two different 621's that I've fixed, there have been faulty (broken) tube socket pins on the 6AV6 audio amplifier tube. I've been able to cut away the bad pin and insert a good one from another socket in both cases. Very strange that I saw this apparently obscure problem on two different sets for exactly the same tube. Maybe there was a manufacturing defect?

I've also noticed that on the 621, if the shield is missing from the 6AL5 FM detector, the sound is very unstable and has all kinds of problems.

Also if the set uses the center connector (if present) on the 6AL5 FM Det tube socket as a tie point, verify that they trimmed the leads off and a wire does not protrude through the topside of the socket and keep the tube from properly seating. The older 6AL5's had a larger recess in the base than the modern ones.

For a general audio or visual microphonic problem, simply tap on each and every tube gently with an insulated stick from the top of the chassis.

If that narrows down the search area, try gently tapping each resistor and capacitor in the same general area underneath the chassis.

If tapping about the volume control changes the problem, consider cleaning the control.

Coating the pins of a suspect tube with contact cleaner and inserting and removing a few times will clean them. Wait long enough for the cleaner to dry before turning the set back on.

James

Thanks for the input! Here's what I ended up finding... there was a white ceramic "Hi-Q" 1500pf capacitor shunting the input of the 1st sound I.F. from the tuner to ground. Looked a bit suspicious to me since it had wax in either end of it. Clipped it and measured ~1200pf. Replaced it with a silver mica BAM sound problem fixed.

Unfortunately, a couple of days later the tuner started bellowing smoke and I'm only left with raster. Very frustrating.