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Old 22nd May 2016, 01:00 PM   #1
grazie is offline grazie  United Kingdom
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Default Troubleshooting amp repair

I've got an NEC A230E integrated amp that I'm fixing and I'm now stuck. The repair isn't helped by the schematic diagram being very difficult to read. One channel wasn't working. I isolated at least part of the problem to the output transistors on the left channel. The Sanken transistors are no longer made and were difficult to find NOS. However, after a good search, I found some that I believe are genuine and weren't expensive. In the meantime, the amp started blowing the 1A fuse on the transformer primary (240V). Again, this was due to output transistors failing, this time on the right channel. While waiting for the replacement transistors, I changed the reservoir caps in the PS. The originals were pretty far out of spec., so the power rails are now much better. I'll also replace all the electrolytics, once I've isolated the problem. Visually, the components and PCB look good.

I don't want to replace the output transistors until I'm confident that the circuit is good. Neither do I want to go for the replace everything fix, which often works, but doesn't help with diagnosing the original problem.

All 4 output transistors (TR317-320) have been removed. Then I measured and recorded the bias on the following transistors.

Click the image to open in full size.

However, what's stumped me is that some of the voltages are floating and are all over the place (indicated by ?).

Any suggestions on getting to the root of the problem?

Last edited by grazie; 22nd May 2016 at 03:26 PM.
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Old 22nd May 2016, 01:17 PM   #2
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Have you actually tested the transistors? TR313 and TR315 shows 11Volts B/E. That is not right!
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Old 22nd May 2016, 01:19 PM   #3
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Check your emitter resistors on 317/9 and 314/20
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Old 22nd May 2016, 01:31 PM   #4
grazie is offline grazie  United Kingdom
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Quote:
Originally Posted by JonSnell Electronic View Post
TR313 and TR315 shows 11Volts B/E. That is not right!
Of course, missed that! Looks like they've gone OC. Unfortunately, I don't have any suitable replacements, so I'll need to order some.

Last edited by grazie; 22nd May 2016 at 01:55 PM.
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Old 22nd May 2016, 01:39 PM   #5
grazie is offline grazie  United Kingdom
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Quote:
Originally Posted by JonSnell Electronic View Post
Check your emitter resistors on 317/9 and 314/20
They're good. I think you meant 318/20 (it's not a great drawing)?
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