| Re: A-4 Repair [message #26570 is a reply to message #26568] |
Tue, 13 February 2018 16:40   |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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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.
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| Re: A-4 Repair [message #26834 is a reply to message #26502] |
Fri, 27 July 2018 00:02 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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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.
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| Re: A-4 Repair [message #26878 is a reply to message #26502] |
Thu, 16 August 2018 12:40 |
stevem
Messages: 4775
Registered: June 2004
Location: NY
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Senior Member |
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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] Report message to a moderator |
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| Re: A-4 Repair [message #26918 is a reply to message #26502] |
Sat, 25 August 2018 00:15 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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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?
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| Re: A-4 Repair [message #26924 is a reply to message #26502] |
Sat, 25 August 2018 12:41 |
chicagobill
Messages: 2006
Registered: April 2003
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Senior Member |
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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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