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In Reply to: RE: I think i found the problem posted by DAK on April 18, 2016 at 19:23:51
The bias supply feeds both the 2A3 and the 845, correct? Pull those tubes and see if it's stable. If not, you'll probably need to sketch the bias circuit to make troubleshooting more logical. It should be pretty simple, unless it's regulated.
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Follow Ups:
The bias supply is on a circuit board. On the board are a bunch of rectifiers maybe zeners and 2 SS devices. Plus an IRF840 and a Linear LT1083CP and 2 bridge rectifiers 2W10M. Is there any way to test the transistors? I think I know which caps are used in the bias voltage. I was going to change them out.
Edits: 04/19/16
Sounds like that PCB might have more than just the bias components on it. In any event, I think you'll either have to sketch a schematic for the board so you can troubleshoot, or design your own bias supply from scratch. Sorry I can't be more help.
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Buy Chinese. Bury freedom.
Do you think I should change out the caps in the bias supply? I have found them based on the residual voltage on the metal body. They are some fotoflash type by Elna? You know the ones that they use in the cheap cameras. I know those are not so reliable. regards, Dak
Dak
Take out the signal tubes and measure the negative bias again.I mean take out the 12AT7 and the 2A3 and measure the negative bias.I almost sure it's one of the Jensen caps leaking and like TK says,you have to measure DC leakage with the cap lifted on the side going into the grid of the 2A3 and or 845.Photoflash caps can be questionable because they can have a short lifespan and higher leakage if run near their rated voltage.
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Before I took new readings. The bias voltage is still unstable. I will try without the at7 and 2a3 and see what happens. If no difference I think I will change out the 3 caps in the bias supply. thank you for the tips. Dak
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