ringing bell in speaker

[radio nut]

I am currently working on a General electric model 810(Sams set 53 folder 12 date 1/49). If I turn the contrast to the middle the speaker has a low pitch rumble that stays. Here is the interesting part. If I tap the tubes or chassis with the plastic handle of a tool the speaker gets a bell type sound as I tap and it goes away. It happens if I tap anything, caps, resistors, coils even the workbench! This set is fully recapped, the resistors were checked in the set. Sound goes through it okay as long as the contrast is turned up past half, below that there is no sound. Please, what do I need to check!!

[radio nut]

I neglected to add that the picture tube lights but what I want to watch is not on the screen

[wa2ise]

A microphonic tube can do that. A tube that has a loose control grid will modulate its electrons when a mechanical disturbance happens. Tapping the suspect tube sideways should make it go worse.

[Old1625]

I have one of those 810s. Cute little set. Some day I may pull the thing out and fiddle with it...

Before doing anything else you might want to try replacing the audio output tube. Can't remember for sure about the 810, but in many of those compact sets the B+ to feed all the Rf and IF stages was taken off the cathode of the audio output tube. If the output tube gets weak then that B+ feed drops below what will make those stages operate well. Poor contrast to nothing but a raster will result. Microphonics can also be another symptom of a tired output tube.

[Don Lindsly]

Check the tuner oscillator as well.