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Old 20th November 2005, 07:11 AM   #1
Yukonsc is offline Yukonsc  United States
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Default Adcom 565 Bias - Issues - Need help

Hi - Have a question I need your guys help with...

I am trying to set the bias on an Adcom 565 and I get 0 -1.5 mv across the terminals. However if I hook a speaker to the outputs the meter goes to 50mv (Also the POT seems to have little to no effect). Still plays music though...go figure

as a side note I set the bias just fine on the other 565 I have - Thoughts? I have the service manual - but am lost at this point. I am just starting in on the repair world.

Thanks in advance.
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Old 20th November 2005, 03:58 PM   #2
anatech is offline anatech  Canada
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Hi Yukonsc,
If you disconnect the speaker from the output terminals, what is the DC voltage across the terminals? The inputs should have shorting plugs installed when you do this, or a know low impedance source with zero signal.

-Chris
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Old 21st November 2005, 05:16 AM   #3
Yukonsc is offline Yukonsc  United States
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Chris -

I'll test it in the morning. Do you mean DC voltage across the speaker terminals? or Bias Test Points?

The Bias Test Points is 0mv (No speaker)
50mv (with Speaker)

I'll measure the DC voltage across the speaker terminals in the morning

Also - What do I short out with a shorting plug - to the point what is a shorting plug?


-Dallas
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Old 21st November 2005, 05:40 AM   #4
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Shorting plug = Device that shorts center conductor of RCA jack to ground. Many receivers/preamps that had more than one phono input provided a pair of shorting plugs for the unused phono input.
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Old 21st November 2005, 03:25 PM   #5
anatech is offline anatech  Canada
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Hi Dallas,
That tells me you either have an offset problem, or the amplifier oscillates with a load.

Either fault requires work on the PCB. We know you have a meter, how about an oscilloscope?

-Chris
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Old 21st November 2005, 10:36 PM   #6
Yukonsc is offline Yukonsc  United States
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Chris - I used a shorting plug and tested again just to make sure. I got 0v across the terminals again.

Are you prepared to be shocked... I do have an oscilloscope. Its a dual trace 100mhz scope. I do need a little (well alot or just specific) guidance in how/what to test.

thanks again. I am learning by doing...

-Dallas
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Old 21st November 2005, 10:36 PM   #7
Yukonsc is offline Yukonsc  United States
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Chris - I meant to mention - I replaced all the electro. caps on the driver board already - The caps were leaking. I followed the repairs suggested on this site

http://www.angelfire.com/art2/stereorepair117/adcom.htm

(I replaced just the caps not the driver board transistors)

However the problem was present Before the repairs. That is what prompted me to to start this journey.


thanks again for your help
-Dallas
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Old 21st November 2005, 11:00 PM   #8
anatech is offline anatech  Canada
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Hi Dallas,
So you are saying that the voltage across the output terminals is close to 0V, less than 50mV? Set the bias current the same as you did.

Set your oscilloscope for 20V/div. Now, connect you oscilloscope lead to the red output terminal. If you get a large 60 Hz component, connect the ground to the chassis, or black terminal. The trace should be sharp (thin, don't turn it up extremely bright). Connect a speaker while watching the waveform. If it gets thick, the amp is oscillating. You may have to turn the vertical sensitivity to a lower setting, say 5 V/div.

If this is the case, check the zobel network (R-C across the output).

What did you clean the PCB with?

-Chris
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Old 21st November 2005, 11:40 PM   #9
Yukonsc is offline Yukonsc  United States
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Chris - Thanks again...

I measured 0mv across the output terminals. Currently the amp and scope are not in the same place. I'll try and get that sorted in the next day or so and follow your directions.

I cleaned the board with a generic pcb cleaner - nothing fancy. The caps didn't look like they were leaking to bad - just smelled really bad when I un soldered them.

I'll do some homework on the Zobel network and see if I can figure out where it is located on the board.

Stay tuned



-Dallas
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Old 21st November 2005, 11:46 PM   #10
anatech is offline anatech  Canada
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Hi Dallas,
To clean a PCB, I normally use "electrowash 2000" and a toothbrush. I may use an aqueous cleaner and toothbrush, removing pots first. Then I let the board dry a few days, typically I shake it and warm blow dry it right after the wash.

-Chris
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