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Post by Deleted on Feb 20, 2012 22:21:45 GMT
My SCHA, wi' all da mods maan, has been giving stirling service for years and has been untinkered with for a long time too. In the middle of being lost in some fine tunes I slowly became aware of that distinctive hot electrical odour. Bow locks, I thought, my PK/I2S combo is up the creak! Nope, smell quickly determined as coming from the SCHA, should take the headphones off really (music was still fine). On doing so the transformer was buzzing like a goodun and the power was cut that instant. With my SCHA it means a total dismantle to get to everything, which I did AQAP to catch any hot components. The transformer was the only thing that struck me as too hot, everything else, by this time, was cool. Anyway, closer inspection revealed charring around the first Tosh A1930 on the JLH board (a Canjunkie jobby) which was worse on the under side with tracks having peeled away from the board. I'd never noticed before, but one 1930 (the frazzled one) and one 5171 were of a bronze colour rather than black, though sporting the correct T emblem. So I am assuming dodgy copies despite being bought from a proper vendor, not ebay. So maybe something worth keeping an eye on The amp is working fine without the JLH board, albeit not as good, until I get around to knocking up a new dual JLH. I intend to rescue most of the components of the outgoing board except all the Toshes. Are there any other components I should bin? There is no sign of charring/bulging elsewhere on the board or on other components. Attachments:
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Post by Deleted on Feb 20, 2012 23:38:57 GMT
Hi Chris Can you be more specific about which devices have overheated ? Are you using parallel 2,200uF or parallel 4,700uFs ? It looks like 4,700uF ? It's best to stick with 2 x 2,200uF when using the CL section. I only use the 4,700uF without the CL section fitted, in which case the voltage regulators do the initial current limiting instead.If this is the original SC design with the LM4562 then I presume that the CL isn't fitted , and that the 4,700UF should be O.K. to use.Perhaps a faulty low ESR electro has caused this problem ? Regards Alex
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Post by Deleted on Feb 21, 2012 0:09:45 GMT
Hi Alex, In my SCHA my dual JLH has the CL section bypassed with twin 4700uf on board. The component that failed, looking at the board with input to the left, is the left one of the S1930 pair. The transformer is a 15-0-15. Cheers, Chris Attachments:
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Post by Deleted on Feb 21, 2012 1:03:13 GMT
Hi Chris Check that it's 33 ohm resistor looks and measures O.K. If it does, it seems to point to the 4,700uFs as causing the problem. You could always go back to 2 x 2,200uF if you are concerned.You should only lose a minute bit at the low end ? I didn't go to 4,700uFs with mine as I couldn't prise it back off my son who uses it all the time. Regards Alex
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Post by Deleted on Feb 21, 2012 23:48:07 GMT
Hi Chris
I've just had a quick look at the first picture that you posted and that cap that's just visible behind the Tosh looks like it's got a little warm.
it's sleeve looks like it's lifted a little around the top.
just a thought
take care
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Post by Deleted on Feb 22, 2012 17:47:08 GMT
Alex and Shaun, Thanks for the replies and suggestions. I depopuleted the old canjunkie board, during the process I found many tracks that had lifted and others that had only tenuous connections. Possibly my own heavy handed soldering back then All componenets that I intended to re-use were tested for holding value. The cap that Shaun pointed out is normal, its a starget type that has sloped shoulders, I compared it to others whose foils looked the same BUT replaced it anyway! I took Alex's just-in-case advice and reverted to the 2200uf caps that were removed before. Everything was stuffed onto a virginal GE dual JLH board that I had put aside for a rainy day. On test now, behaving and test points are correct. Cheers guys. Chris
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Post by Deleted on Feb 22, 2012 23:49:38 GMT
Alex and Shaun, Thanks for the replies and suggestions. I depopuleted the old canjunkie board, during the process I found many tracks that had lifted and others that had only tenuous connections. Possibly my own heavy handed soldering back then All componenets that I intended to re-use were tested for holding value. The cap that Shaun pointed out is normal, its a starget type that has sloped shoulders, I compared it to others whose foils looked the same BUT replaced it anyway! I took Alex's just-in-case advice and reverted to the 2200uf caps that were removed before. Everything was stuffed onto a virginal GE dual JLH board that I had put aside for a rainy day. On test now, behaving and test points are correct. Cheers guys. Chris Hi Chris good work and a job well done. take care
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Post by Deleted on Feb 23, 2012 0:09:03 GMT
Alex and Shaun, Thanks for the replies and suggestions. I depopuleted the old canjunkie board, during the process I found many tracks that had lifted and others that had only tenuous connections. Possibly my own heavy handed soldering back then All componenets that I intended to re-use were tested for holding value. The cap that Shaun pointed out is normal, its a starget type that has sloped shoulders, I compared it to others whose foils looked the same BUT replaced it anyway! I took Alex's just-in-case advice and reverted to the 2200uf caps that were removed before. Everything was stuffed onto a virginal GE dual JLH board that I had put aside for a rainy day. On test now, behaving and test points are correct. Cheers guys. Chris Hi Chris good work and a job well done. take care Chris If you feel the low end has lost a bit ,you could always fit the 4,700uFs again a little later on. Try to "acclimatise" them on the bench first if possible. If it was the Toshiba device with the 33 ohm emitter resitor that had badly overheated, you could also try fitting small heatsinks to them as with the PSU PCB. Regards Alex
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Post by Deleted on Apr 17, 2013 21:28:03 GMT
Quick update; I was missing that little extra low end of the 4700uf caps so did as Alex suggested above. Four, well run in, 4700uf caps (taken off from the JLHs on the A Class Pre/HA) went back in, I didn't have a heatsink small enough to fit the limited space so left the Toshes as is. I have a feeling that my old case for the SCHA may have been too small with not enough air space for cooling. The new case has much more room and far superior convection cooling. Hopefully all will be fine, certainly sounds better
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Post by Deleted on Apr 18, 2013 14:47:34 GMT
Hi Chris
good work
my SCHA is still in regular use BTW and sounding great.
take care
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Post by Deleted on Apr 18, 2013 15:03:11 GMT
Hi Chris good work my SCHA is still in regular use BTW and sounding great. take care Thanks and ditto.
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