View Full Version : Motorola tabletop


timmy
01-01-2017, 10:41 AM
Ok, I have 3 Motorola tabletop 7 inch sets all recapped but one since I got it done has what sounds like a 60hz hum from the speaker. Is there any place else this hum can come from or be generated from. The set works perfect it's just for the annoying hum that can be drowned out by turning up the volume but it's there always. Any hints would be great.

EdKozk2
01-01-2017, 10:28 PM
Timmy,
I have a VT 71 that works great except for the same hum you described. When I first finished and tested the set outside of its' cabinet, I couldn't hear any hum. When I put the chassis back in the cabinet there was a great deal of hum. The first problem I found is that the 6SQ7GT I was using had to be changed to a metal 6SQ7 . This first audio tube was picking up hum from the voltage dropping filament capacitors I used to replace the ballast. The tube change helped a great deal but the TV still has some hum. I'm curious as to what chassis type your set with this hum is? Mine is a TS-4C. The only caps I didn't replace were the filament bypass caps that look like micas. I think Banderson mentioned in a post that bad filament bypass caps were a source of hum in the audio and picture. I won't be opening up my set for a few months yet, working on another 14 inch Admiral -run6 or 7 currently.
Ed

timmy
01-02-2017, 06:23 AM
Well mine is a ts4d and I used just resistors no caps for the ballast. With the chassis out of the cabinet it still has the hum. I'm not sure which tube I have in there for the audio , metal or glass. But the hum has got to go.

EdKozk2
01-02-2017, 10:05 AM
Timmy,
Please let us know what the cause was if you find it.
Ed

timmy
01-02-2017, 10:23 AM
Timmy,
Please let us know what the cause was if you find it.
Ed

Oh I'll find it one way or the other since this set survived so many years and now it works great except the hum, I'll let you guys know what I find, for sure.

decojoe67
01-03-2017, 08:00 PM
I had buzzing in the audio of my VT-71 and my repairman told me that it could be a ground problem due to the riveted tube sockets. He told me to connect the center lug of one audio tube (I forget which) to the chassis. I ended-up doing it to two tubes and it seemed to solve the problem. Occasionally I get times where a buzz will come in intermittently, but for the most part it plays well now. In my experience the VT-71's are decent performers, but finicky.

timmy
01-04-2017, 07:38 PM
I had buzzing in the audio of my VT-71 and my repairman told me that it could be a ground problem due to the riveted tube sockets. He told me to connect the center lug of one audio tube (I forget which) to the chassis. I ended-up doing it to two tubes and it seemed to solve the problem. Occasionally I get times where a buzz will come in intermittently, but for the most part it plays well now. In my experience the VT-71's are decent performers, but finicky.

Well it would be nice if it were a buzz but I'll look into those rivets . But I have what sounds just like a bad power supply cap.

timmy
01-12-2017, 12:12 PM
Well I guess I could call this an update but I found nothing wrong with any of the caps, electrolytics. I do know that the chassis is hot and in the IF section there are connections to the chassis, ground and I do remember when I finished the set I had to tune a few slugs to get a clear pic as well as sound and I remember while doing that I started to get the hum so I'm thinking that either I tuned the hum in or a possible mica or dogbone cap to ground my be leaking or bad , in the IF section. So for now I still have the hum. I also tried to filter the hum at the audio amp and output tubes which I was able to rid the hum completely but had to turn the volume all the way up to get loud enough sound. So I left it as it was since I would rather find out what is really causing this rather then put a bandaid on it.