Modified SCA-80 with Transcendence 2

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avahifi

Re: Modified SCA-80 with Transcendence 2
« Reply #20 on: 26 Oct 2017, 05:43 pm »
Yes, try the Deoxit D5 contact cleaner first.

A contact dropout in the tape monitor, mono-stereo, or speaker switches will indeed cause the issue you have.

Do the easy things first!

Replacing transistors and opamps come only after careful signal tracing.

If you insist on replacing opamps anyway, use AD845 devices which we know will be stable on those old circuits. Many newer ones may oscillate.

 What did you use for replacement output power mosfets?  The Hitachi 2sk135 and 2sj50 parts are obsolete and essentially unavailable.  Exicon devices can be used as replacements, but the gate resistors MUST be changed to 220ohm parts or the amp will oscillate.

Sorry, we have no schematics or wiring diagrams available.  They would be buried in 35 year old non-computerized paper records filed away somewhere, probably in a land fill by now. 

dmtparker

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Re: Modified SCA-80 with Transcendence 2
« Reply #21 on: 26 Oct 2017, 09:54 pm »
I know it sounds dumb, but getting something like DeOxit here in rural Panama (Bocas del Toro) is not easy. For other electrical things I have occasionally used WD-40 with some success. I have one more place to look for DeOxit.
In the mean time, I have some test results that would suggest it is not contacts, but I simply cannot explain them totally. With the amp assembled as it was from the factory, the R channel plays beautifully and the L is great for the first 10-15 secs and then fades to nothing (actually I can barely hear the music - badly distorted - at full volume). If I switch the wires going from the line level boards to output boards, the R sounds good, but there is a faint scratchy sound in the background that wasn't there before. The L plays very briefly (<5 sec) and then fades out. I have repeated this several times with consistent results.
From this, I would conclude that there is a problem with the L output board.  It really does sound great for the few seconds it plays. What would come on like that and fade out? Maybe a capacitor? That is not the whole story, however, as with the wires switched, the good R channel has some background noise that was not there before, so is there maybe something going on with the L line level that damaged the L output??

dmtparker

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Re: Modified SCA-80 with Transcendence 2
« Reply #22 on: 26 Oct 2017, 09:56 pm »
I forgot to add: I did use the Exicon Mosfets. I don't think they are oscillating at this point, but I'm sure I have some 220 R in my kit so I will swap them out. Thanks

dmtparker

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Re: Modified SCA-80 with Transcendence 2
« Reply #23 on: 27 Oct 2017, 07:26 pm »
Replaced the gate resistors with 220 (were 420?). While apart I took pictures of line and output boards. Not sure if this will ring any bells for anyone.






dB Cooper

Re: Modified SCA-80 with Transcendence 2
« Reply #24 on: 27 Oct 2017, 10:54 pm »
This thing must have been in a fairly dry climate; none of the unsightly blemishes in the plating on the chassis bottom that I have. Very clean. I'm jealous.

Wonder if those output caps are AVA work or somebody's DIY or from updatemydynaco.

What opamps are in yours? Mine has TI TL013 IIRC (retrofitted a few years after I had the rebuild done) but originally came with something else, don't remember what.

There must be some audio packrat out there with a schematic in their basement....

dmtparker

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Re: Modified SCA-80 with Transcendence 2
« Reply #25 on: 27 Oct 2017, 11:05 pm »
The op amps are TI L356N - all 4. I think the output caps are previous owner, not AVA. They look more 'modern' than even the quad set in the power supply. There really aren't that many pieces to the amp. If I only had a schematic, I could probably replace all the bits and pieces from my kit, but I don't know what each piece is.

heiba

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Re: Modified SCA-80 with Transcendence 2
« Reply #26 on: 28 Oct 2017, 08:17 am »
Any resemblance with this 1980 AVA rebuilt Pat-5 beauty? Well, I had to take a look under the lid in 1980, and I remembered those TL L356N opamps.




« Last Edit: 28 Oct 2017, 10:05 am by heiba »

dB Cooper

Re: Modified SCA-80 with Transcendence 2
« Reply #27 on: 28 Oct 2017, 02:57 pm »
That's a beaut. It's AVA, right? IIRC you could get your chassis powder coated as an extra cost option at one point.

heiba

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Re: Modified SCA-80 with Transcendence 2
« Reply #28 on: 28 Oct 2017, 05:03 pm »
Yes, Super-Fet preamp made in the spring of 1980 by Jensens Stereo Shop - later changed to AVA.