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Old 11th November 2010, 02:47 AM   #11
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Hello, I ran across this post while searching for something else.

It's possible that a feedback optocoupler's CTR has drifted out of spec, pushing the PWM into a high duty cycle as the input voltage is turned up. Rare, but it happens. I think the Adcom has two.
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Old 11th November 2010, 11:40 AM   #12
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thanks.
I'll take the amp off the shelves, dust it and check the rails. this has been sitting here for quite a while now.
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Old 17th August 2011, 02:10 PM   #13
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upping the thread. still looking for the schematics
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Old 7th May 2013, 03:59 PM   #14
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Sorry for bumping this. Does anyone know the rail voltages for this and the 4404 are ? I'm curious what power you can expect these to produce real world.
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Old 7th May 2013, 04:00 PM   #15
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Also, i have the service manual for both.
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Old 10th May 2013, 02:03 PM   #16
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Quote:
Originally Posted by kappa546 View Post
Also, i have the service manual for both.
Hi,

can you please send me a copy of the service manual? I still have this in my possession so I could work on it once I find the time.

Thanks in advance!
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Old 11th May 2013, 10:24 PM   #17
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Here you go.
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File Type: pdf adcom_gfa-4402_sm.pdf (241.7 KB, 17 views)
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Old 12th May 2013, 03:42 PM   #18
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Thanks! Had a look in the schematic and figured out what to check for the OVP problem.

It turns out the problem was a leaky 15V 1W zener D305 which should trigger an SCR during an overvoltage condition. I have no idea why they had to use a 1W part where a 0.5W part should work just fine. Going to replace it tomorrow and see how it goes.

I feel stupid now lol this amp has been with me for 4 years hahaha Lack of time and motivation (as this is just a favor for a collector friend) could have been the cause of taking this long.

Quote:
Originally Posted by kappa546 View Post
Sorry for bumping this. Does anyone know the rail voltages for this and the 4404 are ? I'm curious what power you can expect these to produce real world.
The rail voltages for the main power stage are listed as +/-24V on the SM but I can check the actual voltages tomorrow if you like.

Last edited by djQUAN; 12th May 2013 at 03:50 PM.
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Old 12th May 2013, 04:35 PM   #19
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That would be great! I'm wondering about the output from 2-8ohms stereo. Thanks!
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Old 13th May 2013, 04:59 AM   #20
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I measured the unregulated rails and saw about +/-22V with 13.2V input at the battery terminals.

I also realized there was an error with the PCB. The 9V regulator and bypass caps (red circle in attached pic) are not connected to the main ground which caused the regulated +/-34V rails to go as high as 45V with high battery voltage. I couldn't find any burnt trace and there aren't any signs that this has been repaired except for a few bodges on the fan circuit which seems to be from factory. A simple wire jumper fixed the problem.

I also replaced the leaky zener with the same 1W part (1N4744) because that's all I had in stock. Now the OVP trips at about 16V whereas it tripped at ~13V with the leaky diode.
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