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Old 9th August 2013, 01:00 AM   #11
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Thanks for the correct drawing.. Looks pretty similar to the one I had, but clearly this one is more accurate.

Unfortunately, it had not helped me solve my problem.

All of the DC voltages seem correct (but, who knows.. I don't do this every day, and may have lost some understanding of transistor bias). Audio seems to flow through the entire circuit, but there is no gain in the final TIP142/147 output. I thought maybe one of the 3067 pairs popped and I was losing the feedback, but pulling it out of the circuit and doing the diode-test on it shows it is OK.

I was going to pull Q4 to test, but when looking for a spec sheet on it I see it has been discontinued. Is there a replacement for the PN4250??

Still considering the shot-gun approach to this, but I have a PPI amplifier in the pile that I never got back to after "throwing parts at it". I'd rather not make that mistake again.

Also broke one of the 1N914's when removing it for testing.. So, I am down to testing with 1 channel anyway (neither work).

Anyone up for providing some help/guidance?

Thanks
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Old 9th August 2013, 01:39 AM   #12
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What do you mean by 'there is no gain in the final...'?
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Old 9th August 2013, 02:54 PM   #13
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The signal is the same amplitude on the input and output.
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Old 9th August 2013, 04:21 PM   #14
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There is no voltage gain in a common collector configuration. This is normal.
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Old 9th August 2013, 04:25 PM   #15
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Ah, ok.. see how dumb I am? But I am back to square one now troubleshooting..
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Old 9th August 2013, 04:29 PM   #16
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What are people using to replace the PN4250?
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Old 9th August 2013, 04:36 PM   #17
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Fairchild says MMBT200 <shrug>
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Old 9th August 2013, 04:42 PM   #18
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I don't have a suggestion for an exact replacement. Is it defective?

What's the complaint if the amplifier has audio all of the way through to the output transistors?
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Old 9th August 2013, 05:01 PM   #19
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Attached to a speaker, there is no output. I wonder if what I was seeing on the scope may have been signal coming back through the feedback (R11).

ON Semiconductor has a MSP4250 that looks to be similar to the PN4250.

I don't know if it is bad, but was just researching it. I know I need to order the diode I broke, and shipping is more than the part- Thought I may order some extra transistors too.

If you have any suggestions on diagnosing the problem, I'd appreciate it.
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Old 9th August 2013, 06:32 PM   #20
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Is the relay engaging?
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