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K200B6 [message #28291] Tue, 10 August 2021 17:08 Go to next message
aeroboy47 is currently offline  aeroboy47
Messages: 15
Registered: September 2019
Junior Member
I posted last year about this amp. It's the one that had a #12 wire soldered across the fuse and had several bad transistors and the shorted bias diodes. After I got the parts replaced and the amp working again, I was concerned about the 120mv DC voltage at the output. I'm back working on it now and I have replaced other parts and the DC voltage at the output is now 48mv DC. I'm guesssing that 0vdc would be ideal but dont see that happening unless someone has ideas about what to do nest. I have replaced Q1-Q4, Q701-Q709 and CR701-CR703. All the voltage readings I have been able to take are very close to the schematic values except Q707 collector is at -2.4 vdc and Q703 collector is at +4.6vdc. Any technical expertise would be appreciated.
Re: K200B6 [message #28292 is a reply to message #28291] Wed, 11 August 2021 09:53 Go to previous messageGo to next message
stevem is currently offline  stevem
Messages: 4728
Registered: June 2004
Location: NY
Senior Member
That's strange that the collector of 703 is positive when it should be negative!

What does the base of 705 test at and the collector of 701 test at?

[Updated on: Wed, 11 August 2021 09:54]

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Re: K200B6 [message #28293 is a reply to message #28292] Wed, 11 August 2021 12:45 Go to previous messageGo to next message
aeroboy47 is currently offline  aeroboy47
Messages: 15
Registered: September 2019
Junior Member
I made a mistake Steve. It's the Q706 collector thats +4.6vdc, and not Q703.
Re: K200B6 [message #28294 is a reply to message #28291] Thu, 12 August 2021 09:24 Go to previous messageGo to next message
stevem is currently offline  stevem
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Registered: June 2004
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What does the base of Q708 test at?
Re: K200B6 [message #28295 is a reply to message #28294] Thu, 12 August 2021 12:14 Go to previous messageGo to next message
aeroboy47 is currently offline  aeroboy47
Messages: 15
Registered: September 2019
Junior Member
Base of Q708 = 1.22vdc
Re: K200B6 [message #28296 is a reply to message #28291] Fri, 13 August 2021 07:33 Go to previous messageGo to next message
stevem is currently offline  stevem
Messages: 4728
Registered: June 2004
Location: NY
Senior Member
What did you use to replace CR703?
Re: K200B6 [message #28297 is a reply to message #28296] Fri, 13 August 2021 12:15 Go to previous messageGo to next message
aeroboy47 is currently offline  aeroboy47
Messages: 15
Registered: September 2019
Junior Member
Can't believe I did this again. I replaced CR700-CR702. CR703 and 704 checked OK and were not replaced.
Re: K200B6 [message #28298 is a reply to message #28291] Fri, 13 August 2021 19:51 Go to previous messageGo to next message
stevem is currently offline  stevem
Messages: 4728
Registered: June 2004
Location: NY
Senior Member
I would first swap 703 and 704 and see if the problem stays as is.
Re: K200B6 [message #28299 is a reply to message #28298] Sun, 15 August 2021 12:03 Go to previous messageGo to next message
aeroboy47 is currently offline  aeroboy47
Messages: 15
Registered: September 2019
Junior Member
I will be away for a couple of days but will do this when I get back.
Re: K200B6 [message #28315 is a reply to message #28299] Tue, 31 August 2021 18:09 Go to previous messageGo to next message
aeroboy47 is currently offline  aeroboy47
Messages: 15
Registered: September 2019
Junior Member
Sorry about the delay here. When I got back from vacation, I found additional problems that required ordering parts. They're here now and I should have answers in a couple of days.
Re: K200B6 [message #28321 is a reply to message #28291] Fri, 03 September 2021 17:53 Go to previous message
aeroboy47 is currently offline  aeroboy47
Messages: 15
Registered: September 2019
Junior Member
The amp is back together. I just went ahead and replaced the FD111's with 1N4148's. It works but still has problems. the voltages at the collectors of Q707 and 706 went up slightly. The cathode of CR704 is -.72vdc (anode is -3.2vdc)and the anode of CR703 is 1.24vdc (cathode is 4.8vdc).The back of the amp seems to be getting hotter than it should but the fuse is not blowing and I have had it on for over an hour.
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