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Old 3rd January 2008, 01:36 AM   #1
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Default Damaged Adcom 565 Monoamps

I have a pair of these that are damaged, both as a result of the leaking 220uf 25V caps. One has a circuit board that's badly damaged and the other not so bad. Surprisingly enough, the one with the worst damage is the one I am having less problems with.

In any case - having read most of the info in these forums and others, I have replaced all the usual resistors and caps. Cleaned and repaired the PCBs as best I could.

Now my two remaining issues:
1) The acid damage resulted in a problem with a SCR type rectifier. The original part number is 'ADCOM J2 8934M' - it has a TO-92 (3 lead) case. I need a available replacement part number and manufacturer. I usually deal with Mouser.

2) 2nd issue, on the amp with less damage, I have the worst DC offset ever (meaning if you attach it to a speaker - sounds like a 22 pistol going off - after that the voice coil has no more than a minute or two to live. As I said I have replaced all the recommended parts, and using a VTVM have checked about every component and haven't found anything wrong in checking mirrored components and from amp to amp.

I do not have a service manual so I'm a little blind.
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Old 3rd January 2008, 04:05 AM   #2
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Me 'audio_shop' again. One thing to add that might be helpful in diagnosing the problem. On turn-on, the instantaneous distortion light comes on and stays on, and the Q111 transister (main driver left side of board looking from the the front of the amp) becomes hot to the touch in about 30 seconds. Sounds like a short somewhere - any ideas where to start looking?
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Old 10th January 2008, 01:43 AM   #3
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Now that I'm deep into this and still not making much progress (although I did find a source for a service manual), I'm wondering if someone could come up with a design for a simple driver stage that would work with this amp. After reading all the posts on 565 issues (and they are almost all the same), you have to wonder wasn't there a better and more reliable design for this section, and why is DC at the outputs such a big problem with the 565.

I just fixed a HK Citation sixteen, and it was soo much simpler and I really think it sounds better. Of course the HK is not direct coupled, but other wise from a power supply and output stage perspective, the two amps are almost the same piece.

I would be more than happy to make one of these amps into a test bed for an alternative driver stage if the forum members could point me in the right direction.
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Old 10th January 2008, 02:03 AM   #4
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Hi audio_shop,
Sorry I haven't responded until now. Here goes.

Quote:
1) The acid damage resulted in a problem with a SCR type rectifier.
No, those are precision 7 V regulators.

Quote:
2nd issue, on the amp with less damage, I have the worst DC offset ever
You should have watched for DC offset. Your fault harks back to ...
Quote:
Cleaned and repaired the PCBs as best I could.
Not well enough. That's the simple truth - sorry. back out they come. please read carefully the posts of mine you have come across regarding cleaning those boards. I wasn't kidding.

Quote:
you have to wonder wasn't there a better and more reliable design for this section, and why is DC at the outputs such a big problem with the 565.
Bad assumption. The design is sound, just some bad caps that didn't show up until years after the stuff was in the field.
Quote:
I would be more than happy to make one of these amps into a test bed for an alternative driver stage if the forum members could point me in the right direction.
See above. I think you would be making a mistake. These are not the best amps ever, but they are pretty good. At that price point, they are great!

-Chris
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Old 14th January 2008, 12:21 AM   #5
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Thanks for the input Anatech,

I'm purchasing a set of precision 5W Zener diodes to provide the voltage regulation. In the meantime I have completed my repairs on one of the two amps, specifically the one with the severely damaged PCB. Getting good results so far even with the cheap OP07 Opamp while I wait on a better piece.

Regarding the cleaning of the board, I used a mix of baking soda and water with a small brush to do localized cleaning, followed by clear water, and then alcohol. It all worked very well.

The second amp without the PCB damage is the one with the more severe DC offset problem. Because replacement of the typical resistors and Caps, and the IC still haven't resolved the problem, as it did in the other amp, I'm thinking that Q101, 102, 105, or 106 may be damaged. Would it be best practice too simply replace them all, or is there some way to isolate which it might be.
BTW, the DC offset problem is positive.
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Old 14th January 2008, 02:54 AM   #6
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Hi audio_shop,
Those zeners are not correct. You need to get the original types made by Linear Technology (I am guessing here. Memory issues). They are 7V each, two in series.

The op amp is for DC servo use only. No audio passes through it. You need a low current type. If the leads are not rotted off them, they are probably good after a proper cleaning.

Note that the stuff you are trying to get rid of is not visible to the naked eye. I wonder if a UV light would show it up? It's also very difficult to get rid of. I personally don't think any of those you have worked on are completely clean. Sorry, but that electrolyte is devilish stuff to remove.

Replace anything you think is damaged. The diff pair must be matched carefully.

-Chris
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Old 14th January 2008, 03:32 PM   #7
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And how would you go about testing the diff pair to ensure that their characteristics do match or are at least very close.

The original LT regulators are no longer available. they are listed in the Service manual as zener diodes (although voltage is not specified, I am taking you word for it that the circuit requires 7 V). The pieces I am puchasing have a different form factor, but technically replicate the originals. They are rated 6.8V nom, with an operating range of 6.5 to 7.2 Volts
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Old 14th January 2008, 10:38 PM   #8
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Hi audio_shop,
They are two precision 7V regulators in series to give 14 VDC. It's the low noise they are after here, so regular zeners (0.25W) would work, but the noise level might be higher. I bought some last year, so they should be available at Newark or Digikey I would think. The very last thing I'd use is a large zener. The current through there is not high and you'd be operating on a very soft knee.

What are you getting to replace them? Just curious.

-Chris
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Old 14th January 2008, 11:39 PM   #9
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ON Semiconductors Surmetic 40 Zener Diodes. Part no. 1N5342BG. Check out the spec sheet - very stable. Nom. 6.8 volt.

Your saying you were able to get the exact replacement thru digikey or a zener to do the same job. If it was a replacement, you wouldn't happen to have a part number?

BTW, I found a nice relatively inexpensive replacement for the IC101 (ADCOM 2A), its the TI OPA177F. There is also a 177G which I am also buying whose characteristics are most similar to the OPA97ED/LT1012.

The G is about $2, and the F about $6.

The 177F has even lower offset V and is more stable at temperature, although it gain is less at 25 C Nom. I intend to try both. Right now I have one amp working well on a standard OP07 (which I know is just lucky).

- Tom
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Old 15th January 2008, 02:49 AM   #10
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Hi Tom,
I am still looking for those notes I made. The originals were a shunt regulator IC design in a two lead TO-92 case. It's possible you can find the exact part by looking for two leaded shunt regulators. What I bought was definitely not a zener diode, they were shunt regulators.

The exact DC offset is not that important. You will be well below 10 mV even - so who cares beyond that? You aren't going to hear it either. Do check your supply voltage on the op amp. If it draws too much current, the regulators will drop out. That you might hear.

The original op amps might be fine after a really good cleaning.

-Chris
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