Nothing but buzz and hum

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Nothing but buzz and hum

Post by tpaairman on Fri Apr 03, 2009 10:47 am

I'm working on a Silvertone portable(Schematic) and get nothing but a buzz/hum.

I am using my variac as part of a temporary battery eliminator. I have been told this can cause quite a bit of interference, but I think it's more than that here. I turned on my transmitter and found the spot on the radio dial. The transmitter had no input, so only carrier wave going out, and I should have gotten silence on the radio. Instead, it was a loud buzz.

I've replaced every cap in the thing, and half the resistors (some were out of value.)
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Re: Nothing but buzz and hum

Post by Resistance is Futile on Sun Apr 05, 2009 4:29 pm

Your power supply may be shot, you may have a short in the diode section.
You give very little info on the battery eliminator, is it solid state or tube?
What type is your transmitter? How is that powered? Wall wart? DC supply? Batteries?
Normally you should have no hum in a battery tube portable. Introducing a AC power supply can be your culprit.

You have to eliminate by deduction and experimenting. Such as trying a known good radio with your transmitter. Does it still hum? if not then the battery supply is the suspect. If it does hum then its the suppy for the transmitter.

I think your going at this wrong, your variac shouldnt be used as part of a battery eliminator.
You need a constant and measured DC voltage to the A, B and C battery connections. If you are using AC to the filaments that is wrong. However I am assuming a lot without any details of your hookup.
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Re: Nothing but buzz and hum

Post by Bill Cahill on Sun Apr 05, 2009 8:23 pm

Most dc power supplies to replace battery sets:
A Cant handle the current of tubes,
B Have no transformer. Merely resistors off both sides of the line. Also, nearly none of them even have an electrolytic in them.You are likely putting pulsating dc into something that needs pure dc. You need alot of capacity there. Usually 4,000 microfarads.
Bill Cahill

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Re: Nothing but buzz and hum

Post by Tony V on Mon Apr 06, 2009 3:48 pm

If your using one of those power shifter supplies meant to convert a battery/farm radio to work on AC current...you probably need to put new capacitors in it. Also, for elimination purposes, try powering up without the variac and see if you have the same problem. Alot of times, using a variac in conjunction with a power shifter will create some hum. Like others have said, we need more specifics.
-Tony
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