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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
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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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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: 1890
Registered: April 2003
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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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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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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
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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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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 messageGo to next message
stevem is currently offline  stevem
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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.
Re: A-4 Repair [message #26916 is a reply to message #26899] Fri, 24 August 2018 18:22 Go to previous messageGo to next message
Jonomega3 is currently offline  Jonomega3
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found RCA NOS 40409-10 on Ebay and put them in...good news the amp actually works now, bad news the distortion and low volume still there. Checked some voltages and getting voltages everywhere but at the connection at the 40410 base where schematics say should be 15 volts, getting about 1 volt at the 1st diode
Re: A-4 Repair [message #26918 is a reply to message #26502] Sat, 25 August 2018 00:15 Go to previous messageGo to next message
chicagobill
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Registered: April 2003
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The 15 volts is a typo. If it wasn't there would be about 13 volts across the bias diode string.
The usual voltage would be about -0.5 volts.

Does the distortion occur with both preamp channels?
Re: A-4 Repair [message #26922 is a reply to message #26918] Sat, 25 August 2018 07:58 Go to previous messageGo to next message
Jonomega3 is currently offline  Jonomega3
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Yes distortion in both channels,
so 13 volts should at 40410 base, not 15v? I'm not getting any voltage there. I'm getting a little more than 1 volt at the input of the 1st diode
Re: A-4 Repair [message #26923 is a reply to message #26922] Sat, 25 August 2018 11:43 Go to previous messageGo to next message
Jonomega3 is currently offline  Jonomega3
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Shouldn't respond till I've had at least a second cup of coffee, LOL! You are saying there should be -0.5 volts at the base, sorry for the misunderstanding
Re: A-4 Repair [message #26924 is a reply to message #26502] Sat, 25 August 2018 12:41 Go to previous message
chicagobill
Messages: 1890
Registered: April 2003
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There should be a small voltage at the base of the transistor. Because of tolerances of the individual components, the reading will be different for every amp.

My point was that the 15 volts is a typo. In fact one schematic that I have looked at lists .15 volts there. Maybe the decimal got wiped out at some point along the line.
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