Sound Problem

[VA561]

I have a '55 Philco Model 4113. The tv has a great picture but has developed a sound problem. I watch the tv on average 2 hrs per week. Sometimes when I turn the tv on the sound is great at first with no problems at all. Then for no apparent reason the sound will will just fade away. I will watch the tv with no sound and all of a sudden it will come back again maybe 10-30 minutes later.
At other times I will turn the tv on and no sound from the beginning.
If I turn the volume control all the way up I just can hear the show but nothing if I turn the volume control to where a normal listening level would be. All advice would be greatly appreciated.

[jr_tech]

When the sound fades out, are the heaters of the audio output and other audio tubes lit up? Perhaps cleaning the pins would help?
jr

[Zenith26kc20]

Check the audio tubes for grid leakage. One of my TS-18 motorolas did that and it was one of the audio tubes.

[kramden66]

when a set is restored always check or replace a tube or tubes before you start doing anything else , replacing the tube / tubes is probably better then testing them , spent some time on a couple different sets once only to find it was a tube and i thought geez why didn't i check that first , now i know better ... i think.

mike

[VA561]

Based on the schematic which tunes do you think would be the most likely suspects?

[Don Lindsly]

6AU6, 6T8 and 6W6 are the tubes most likely to affect sound.

Also try slightly rocking or tapping the sound discriminator transformer, 32-4450 or 32-4631, if I recall.

[Phil Nelson]

Originally Posted by jr_tech

Perhaps cleaning the pins would help

Cleaning tube pins and sockets is a good thing to do at the beginning of every restoration:

http://www.antiqueradio.org/FirstStepsInRestoration.htm

Phil Nelson

[Mal Fuller]

"Fading away" suggests a tube filament cooling off. I'd check the ground connection to the audio output tube's filament and, if applicable, the ground on the pc board upon which the audio output tube is located.

[old_coot88]

Originally Posted by Mal Fuller

"Fading away" suggests a tube filament cooling off.

..Or a rare possibly is a floating grid condition, ie., an open in the DC return path to ground. That would allow the tube to self-bias by space charge, eventually cutting it off.

[VA561]

I am crossing my fingers but it appears as though the culprit was a bad 6T8 tube (FM DET/ 1ST AUD) on the schematic. I replaced it with a good RCA tube and so far it is working fine. As usual, there is always a gold mine of great advice from everyone who responds on this forum. To everyone who took the time to try and help, thank you.