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Old 10th March 2007, 09:41 AM   #1
leloga is offline leloga  France
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Default citation 12 hum und bias difficulties to adjust

Hello,

I was trying to renew a citation 12.
I've two problems.
In one side there is a hum wich appears loud when we plug the amp and shut down and stabilize.
In the other side, no hum, but it's impossible to adjust the bias correctly ( in the other side the pot is in middle position, in this side the pot is in max position without ability to decrease the bias enough ).
It's a citation 12 220 volts version: Is the bias the same that in 120 volt version?

For the hum, should i replace any condensers? wich one?
Or is it another problem?
Is there somenone who has alreasy have this?

For the bias, i've test the components and the bias trans seems not be breaking, but these characteristics seems to be differents from the other side.

The bias trans is the 2N5232 wich is impossible to find ( in france ), is there an equivalence?

many thanks for all the help you can give me.
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Old 10th March 2007, 11:52 AM   #2
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Location: Kungsbacka on the Swedish westcoast
Hi

The source of the hum is propably a bad electrolytic, replace all of them, they will fail one after another, the amp is more than 30 years and is well worth the cost of a few capacitors. Bias is the same regardless of model. Broken transistors usually results in DC at the output or a silent amp.
If You have more defective parts than caps, take a look at the mosfet rebuild by NP, it makes a realy good amp, look in this thread:Citation12 repair!!

Regards
Ingvar
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Old 10th March 2007, 12:23 PM   #3
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Its been a long time and my memory could be mistaken but doesn't the amplifier have a dual diode that that is part of the bias circuit that is heatsinked? Have you checked it?

I believe a NTE or ECG 199 will cross 100% with your 2N5232. I don't think the 2N5232 is your problem.

What is your DC offset?
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