Motorola 7 inch tabletop
 

Ok so I have this vt71 already restored works great but back when I finished it there was a bad hum which sounds like a 60 hz hum. Since then the only thing I did was tried different tubes thinking maybe an HK short but no difference is there any place else I could look for this problem, except caps because thinking at the time maybe I had a bad cap and tried paralleling them and had no change. The more i raise the volume the less you can here the hum but minimum volume the hum is loud. :thumbsdn:

Check grounds/B- on the chassis. If any signal grounds are bad it could add hum. Also look at the heater and line AC lead dress...If that is parallel to any audio lines or if there are adjacent loops of wire, capacitive or inductive hum coupling may result.

I had a bad buzzing in the audio on mine and had to clean and put 'liquid wire" on all the chassis tube socket rivets - the advice from my repairman. It worked!

Is it possible for a component such as a ceramic disc that goes from a given circuit to the chassis ground, which is one side of AC along with DC negative to go bad or short to let AC into the circuit. This set when it's turned off the speed of the hum quickly slows to a stop and the image as well shows it as a kind of ticking. But when it's at full 60hz you don't see it but hear it.

The chassis on that set isn't a true return to B-. The chassis is somewhat isolated from the B- with a resistor and cap network.
Maybe a component is returning to chassis ground instead to B-. It's easily done by even seasoned restorers. :scratch2:

Right. You should measure about 470K between the chassis and B- (one side of the AC line).

It's been awhile but I know there is a 470k resistor there and I may have measured that at the time. If I have 470k between chassis and B- what would you suggest as a possible smoking gun. :smoke:

Maybe one of the filter caps on the filament string is leaky. Ys, ceramic caps can go bad and as I recall some of them look like mica but are really micamold paper caps.

Just FYI, I had a nasty hum after a few minutes in my VT-71. It was in fact the audio output tube. I guess it had grid emission and took time to show up. Not uncommon in tubes with large, hot heaters.

Ok well I cannot find a reason for the hum in the audio so I'm wondering if it's at all possible that the noise I call a hum Could be a buzz , could this maybe coming from the IF and sounding like an ac hum. I had placed an electrolytic cap between the audio amp and output and was able to filter Alittle bit but no matter where a cap is put, audio or power supply the hum or buzz is still there. The ground on the tube sockets are all soldered. Swapping tubes make no difference. All that seems left is the IF or the verticle but I went over the verticle and found nothing.

A hum at minimum volume usually indicates power supply hum or AC ripple getting into the B+. Disconnect the speaker and with an 8 ohm or so dummy load, signal trace the audio circuit to see where the hum originates.

Is it possible one of the 2 silicon diodes I put in place of the seleniums are leaking ? I didn't try subbing them.

I'm not to sure what your saying by disconnect the speaker and trace. I do have 470m b- to chassis but there has to be another way to track this down. Different tubes nothing works and the chassis grounds at the tubes are super clean and soldered . Should there be voltage from chassis to b- , because I'm getting about 239 Vdc at those points.

What I really meant was disconnect the speaker so you can hear the hum where it originates through the signal tracer, if you have one!
Are you reading the 239VDC using a digital meter? You might reading some stray voltages.

Yes I'm using a digital meter and getting 239 Vdc b- to chassis. No I don't have a signal tracer.