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Re: A-4 Repair [message #26570 is a reply to message #26568] Tue, 13 February 2018 16:40 Go to previous messageGo to next message
chicagobill
Messages: 1873
Registered: April 2003
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Steve- I'll have to look. I have just been looking at the one posted in the technical section here.

Jonomega3- Well, I hoped you'd found it. Keep us informed of your progress.
Re: A-4 Repair [message #26833 is a reply to message #26570] Thu, 26 July 2018 12:43 Go to previous messageGo to next message
Jonomega3 is currently offline  Jonomega3
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Registered: November 2012
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I,m back on it! Sorry, just had to take a break from the amp for a while. In testing the 6 power transistors found a few that are bad....all are RCA, 4 are the 36892 and 2 have the numbers 3 7 0000 7701 on them, are these the same as the 36892 transistors? replacements would be NTE130?
Thanks!
Re: A-4 Repair [message #26834 is a reply to message #26502] Fri, 27 July 2018 00:02 Go to previous messageGo to next message
chicagobill
Messages: 1873
Registered: April 2003
Senior Member
Those numbers don't mean anything to me other than the date code of 1977, so I don't know if the 2 replacements are correct or not. I would suggest that you replace the 4 outputs with the same number or same types. The generic part number is 2N3055, which you will find a lot cheaper than the NTE parts.

Which of the 6 transistors are bad, the RCA's, the replacements? There are 4 for the power amp and 2 for the low voltage power supply. If the 4 RCA's are good, use them in the power amp and replace the power supply transistors with new ones. If some of the RCA's are bad, replace the 4 power amp outputs with new ones and use any others in the power supplies. The power supplies do not need matching transistors, while it is best to use the same type transistors in the power amp section.
Re: A-4 Repair [message #26835 is a reply to message #26834] Sat, 28 July 2018 14:31 Go to previous messageGo to next message
Jonomega3 is currently offline  Jonomega3
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Sorry bout the numbers. Didn't realize they were all the same type transistor.
Two that are bad are RCA and I suspect one of the no label transistor is bad as well.
They were cheap enough so I ordered 6 today
Re: A-4 Repair [message #26836 is a reply to message #26502] Sun, 29 July 2018 06:06 Go to previous messageGo to next message
stevem is currently offline  stevem
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On circuit traces that have had parts yanked out of them I would also test for good continuity between parts.
Re: A-4 Repair [message #26838 is a reply to message #26836] Sun, 29 July 2018 14:10 Go to previous messageGo to next message
Jonomega3 is currently offline  Jonomega3
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Good point, I will do some checking while waiting for the parts to come, thanks!
Re: A-4 Repair [message #26877 is a reply to message #26838] Thu, 16 August 2018 11:45 Go to previous messageGo to next message
Jonomega3 is currently offline  Jonomega3
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No change...I'm seeing +37 volts dc at the speaker output schematics say +34 , that normal or a mistake in the schematics?
Re: A-4 Repair [message #26878 is a reply to message #26502] Thu, 16 August 2018 12:40 Go to previous messageGo to next message
stevem is currently offline  stevem
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Registered: June 2004
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I believe your reading the schematic wrong as + 34 volts on the red wires is the power supply rail.

If you are seing 34 volts positive on the speaker jack then you have one or more output transistors or driver transistors bad in the positive side of the output stage.

The normal D.C. Voltage seen on the speaker jack 2 seconds after turn on should be less then .045 volts D.C. .

Also if you are seing that level of D.C. Voltage on the speaker jack do not plug in a speaker as 34 volts is = to over 140 watts and you will likely blow the stock speaker (s)!

[Updated on: Fri, 17 August 2018 06:48]

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Re: A-4 Repair [message #26879 is a reply to message #26878] Thu, 16 August 2018 13:14 Go to previous messageGo to next message
Jonomega3 is currently offline  Jonomega3
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Registered: November 2012
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Ah, thanks Stevem testing of the driver transistors, in circuit they don't read as good but not bad either they all read the same actyually so its possible all are bad I guess, ugh! The output Transistors were replaced and all test good now.
Re: A-4 Repair [message #26880 is a reply to message #26879] Thu, 16 August 2018 18:51 Go to previous messageGo to next message
Jonomega3 is currently offline  Jonomega3
Messages: 78
Registered: November 2012
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So, the driver transistors are the 4 inside the heat shields? If so, I have the numbers 38736 and 38737 2 of each, all RCA in there now. I suppose these numbers are not the type of transistor I need to order?
Re: A-4 Repair [message #26882 is a reply to message #26502] Fri, 17 August 2018 06:51 Go to previous messageGo to next message
stevem is currently offline  stevem
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If your fuse is the right value then you likely have no bad output transistors because they would blow the fuse.
Before we get into anything else here it's what you have posted true, that you have a plus 34 volts on the speaker output jack ?
Re: A-4 Repair [message #26883 is a reply to message #26882] Fri, 17 August 2018 06:58 Go to previous messageGo to next message
Jonomega3 is currently offline  Jonomega3
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Steady 37 dc volts, mine is the early circuit breaker model no fuse on this one
Re: A-4 Repair [message #26894 is a reply to message #26502] Sun, 19 August 2018 06:15 Go to previous messageGo to next message
stevem is currently offline  stevem
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Well it's time to yank the driver transistors Q708 and Q704 and test them and also check that the 1 ohm 5 watt resistors have not be blown open.
Re: A-4 Repair [message #26898 is a reply to message #26894] Mon, 20 August 2018 12:18 Go to previous messageGo to next message
Jonomega3 is currently offline  Jonomega3
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Registered: November 2012
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Pulled Q704 and it tested as bad, also replaced one of the questionable 5 watt 1 ohm resistors. Like the output transistors should the driver transistors be matched? Or just get replacements for the one that is bad if that's the case?
Re: A-4 Repair [message #26899 is a reply to message #26502] Mon, 20 August 2018 12:27 Go to previous message
stevem is currently offline  stevem
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Registered: June 2004
Location: NY
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A replacement for that is a NTE128 , this is a TO39 case transistor like the original but without the heat sink so you will need to also get a slip on star type heat sink to make it live.
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