Realistic sta 2100 repair issues.

Audi0

Audi0

Super Member
My other thread is going off topic I thought I would start this one.

I followed a few rebuild threads using the recommended parts here on audio karma.
The only unknowns were the ksc3505 replacing the 2sd688 and the the itt-73n diodes were replaced with 1n4148 diodes.
All resistors were pulled checked and if bad replaced. Same with caps and transistors

One channel wants to short out and the other I don't have bias control with dc offset. Amp will not pass dim bulb test. And ideas how o can track this down.



20180817_172709.jpg
 
Check the driver transistors and how they are mounted. The picture of the heatsink on the board seems to show that the drivers are installed wrong (180 degrees out of phase). I would check the other transistor orientations as well because many time replacements can be different (ECB vs BCE, vs EBC)
 
Tom B said:
Check the driver transistors and how they are mounted. The picture of the heatsink on the board seems to show that the drivers are installed wrong (180 degrees out of phase). I would check the other transistor orientations as well because many time replacements can be different (ECB vs BCE, vs EBC)
Click to expand...

I can double check pinouts. However I know those ones that appear backwards are installed right. I questioned that to. I believe the board has a microfacture somewhere in one of the traces. Not near a soldier joint.

The other board wants to short and I don't know why. Outputs are isolated and where they attach to the board is isolated. The shorts coming from the board somewhere.
 
Some progress both ksa1013 were in backwards. Dc is 2.1mv. No control over offset still. Pot tracks. Output cycles when speakers connected. Other channel still detects short and I can't find that still.

Whoever repaired the amp before me did a hack job. Trying to save it.
 
I installed the 1013 backwards. No progress to issue. Pulled all transistors to make sure I didn't blow anything and put it all back together. Pulled a trimmer out of another amp to see if it was the issue. Not the issue. The watts meter when trying to drive the channel that will click over goes up to like 10 watts. No audio comes out of it. Checked resistor values. Not the issue. I really dont know what I did wrong. There is 2 transistors and 4 diodes that are un-recommended as there is no info on replacement. Had enough parts to to put together the 1 channel to factory original. Amp card is acting not right. I'm lost to why.

I'm at the point of pulling every component out of the amp cards and bending them to find a crack in the traces. The other board keeps trying to short and I tried and tried 16 hours in to find the short. So I got to up my game a bit.

20180823_073727.jpg
 
It's probably a trace. I lost my mind trying to fix a STA-2080 amp board that would jump drastically in bias even after replacing every single component. Ended up just buying a parts unit and switching boards.
 
Every single cap was replaced and every single resistor was pulled from circuit checked and if I had a replacement replaced it. I know I ain't supposed to bend the boards as I run a high chance of breaking them but I'm out of ideas. The original repair job was a hack job. Chances are within the last 5 years.
 
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What the best way to find broken traces that are kind of still touching? I dont want to bend the boards. Put the cards in the freezer to make things move around?
 
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You could do continuity checks with your DMM on the traces = point to point. The board components as you know are far from original.

I have rebuilt the STA-2100 a few times. A few members have listed good replacements for most of the transistors - google can help you there.

http://audiokarma.org/forums/index....habbing-an-sta-2100-what-would-you-do.613155/

Bias trimmers work in opposite directions on the left / right amp boards. Am a hobbyist and struggle through when things go off the rails.

Bias trimmers should not be trusted at this age... one of the most relevant replacement on any rebuild in my mind.

I spent hours on my amp boards and staring at the schematic when problems popped up... stay with it my friend.
 
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Ok I eliminated the short. It was just was on the bias comp. It was when the board was all together it managed to touch the housing. It was the the tiniest hole in the sleeve which touched when it was all together. Also who ever messed with this originaly wired the output stage with the thermal comp stage. Same wire colours but they mixed them around. Wires I know I never desoldered. Still both channels dont work. I keep thinking I have a transistor back wards and checked the pinout several times. Got 1v solid going to speaker output and to protection relay. Referenced from pin 13. Other channel which was shorting at one point is getting a solid 6v dc to protection from that pin 13. Relay won't click over of course.

Any ideas? This amp is a basket case and the output stage is probably not the only part with issues.
 
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Ok so here is the list. Anyone spot any parts that dont make sense?

2sa872 - ksa922
2sd668 - ksc2690ays
2sb649 - mje15033
2sd675a - mj21194g
2sb655 - mj21193g
2sa659 - ksa1013
2sd669 - mje15032
2sc1175 - ksc2383
2sd400 - ksc2383
Vd1221 - 1n4148 in series
Rd13e - bzx79b13
Sr1k8 - 1n4007
Itt73n - 1n4148


Bad resistors were replaced as needed.
 
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Anybody see anything off? I think one of the ksc2383 is a ksc2380 but I'm going to have to double check. Potential typo.
 
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