Help needed to fix channel Conrad Johnson Sonographe amp - diyAudio
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Old 13th April 2008, 09:30 PM   #1
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Location: Brooklyn, NY
Default Help needed to fix channel Conrad Johnson Sonographe amp

Hi, I have no experience in fixing/building amps and your help/advise will be really appreciated.

I have a Sonographe SA-120 amp (Conrad Johnson entry level in the 80s, which I understand is a Fet amp).

I short circuited the left channel speaker cables while trying a new bsc circuit and the channel went off. After replacing one fuss that had blown up I only get a very low level, highly distorted sound on that channel. So it seems that something else got damaged within the channel's output stage. The other channel works fine.

Here is a photo of the open amp:

http://www.flickr.com/photos/1157168...7603119448350/

The question is: Is it realistic to try to fix it myself (ie to get a multimeter and try to locate the damaged component and replace them)?? or I should have it repaired. The problem is that I live in London, England and here these sort of repairs are generally very expensive and I suspect it is not worth paying much to repair this amp. However, the amp has served me for 20 years and I like its sound, so I would like to fix it.

If it is feasible to do a DIY repair I would be grateful if anyone could guide me where to start.

Or maybe someone knows a good repairman in London.

Many thanks in advance.

Antonio
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Old 14th April 2008, 03:35 AM   #2
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Join Date: Feb 2001
Location: Columbia, SC
Inevitable: At least one blown output device
Possible: One or more blown driver devices
Hopefully unlikely: Blown devices in front end
I've got a fairly decent selection of older Conrad Johnson schematics, but they're all for the tube gear--not solid state. Start making a list of the nomenclature on the transistors--particularly the big ones attached to the heat sinks. Those will be the output devices. The next thing to do is to verify that those MOSFETs are still made. That may make up your mind for you without having to get into heavy diagnostics.
It's worth remembering that even if the parts have been discontinued there are people who may have stock of them still, but don't expect them to be cheap in the sense of people trying to clear out old stock. It actually works the other way. There will be other people in pretty much the same situation you find yourself in who are desperate and will pay any price just to get their equipment running again. As a result, the prices tend to go up, not down.
It's possible that you could cross reference the part, but that is not always a graceful transition. Sometimes newer parts work really well in older equipment. Sometimes they oscillate.
If all else fails, you might consider building a project in the case.

Grey
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Old 14th April 2008, 07:43 AM   #3
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Location: Brooklyn, NY
Grey,

Many thanks. That's the sort of answer I needed.

I already asked CJ for the schematics. Lets see if I'm lucky.

The case is top class (big and strong and with elegant real wood panels on the side). So, yest I could use it to build another amp but first I will follow the the steps you advised me to take.

Regards

Antonio
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