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Positive power supply rail at a different potential than negative rail?
Positive power supply rail at a different potential than negative rail?
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Old 9th October 2019, 02:10 PM   #51
The Lord Flashheart is offline The Lord Flashheart  United Kingdom
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When the amp is cold the voltages across Q112&Q113 are much lower, around 4v and then climb as it gets hotter.

I'm not sure I understand what might cause that, especially as this amp doesn't have any means that I can see of setting bias?
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Old 9th October 2019, 02:24 PM   #52
Osvaldo de Banfield is online now Osvaldo de Banfield  Argentina
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Bias are fixed by D103 and D104, so no chance to modify it without severe handling of the amps.

Which voltages increases, VBE or VCE?
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Old 9th October 2019, 02:31 PM   #53
The Lord Flashheart is offline The Lord Flashheart  United Kingdom
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VBe Osvaldo
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Old 9th October 2019, 02:56 PM   #54
Osvaldo de Banfield is online now Osvaldo de Banfield  Argentina
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Sorry, I'm a bit dizzy. Four volts across bases? Its largelly excessive. There must be 2VΓ, about 1.2≈1.5V.
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Old 9th October 2019, 03:21 PM   #55
The Lord Flashheart is offline The Lord Flashheart  United Kingdom
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Yes, four volts across the bases whereas in post 49 they were around 10v.

I have swapped the capacitors and it has not corrected the issue.

I think i need to be more systematic about this.
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Old 9th October 2019, 03:27 PM   #56
Osvaldo de Banfield is online now Osvaldo de Banfield  Argentina
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Check Q108, Q109, Q110, D104, D105 and the resistors associated. Something is bad. Very bad :-(.
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Old 9th October 2019, 03:41 PM   #57
The Lord Flashheart is offline The Lord Flashheart  United Kingdom
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Testing them with the diode check seems to indicate that they are ok, will I have to remove them from the circuit and test with one of those transistor testers?
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Old 10th October 2019, 12:17 PM   #58
Osvaldo de Banfield is online now Osvaldo de Banfield  Argentina
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Not necessarily, usually a simple diode and transistor check with the diode drop from any DMM will tell U if there is any damaged part.
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Old 17th October 2019, 05:15 PM   #59
The Lord Flashheart is offline The Lord Flashheart  United Kingdom
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A quick update,

I removed Q901 and tested it as the voltages on Q101 and Q202 were all wrong ( should have been 0.7 volts on the Bases and 0 on the Emitters and Collectors, whereas they were in fact: Q101 - E:7.45v ; C:7.45v ; B:8.16v and Q202 - E:8.65v ; C:8.65v ; B:8.65v).

I obtained one of those transistor boards with the ZIF socket and obtained the attached results.

This transistor is a 2SA733-k and the values given below seem a little off.

the measured Hfe seems a little low for a k variant but is above the minimum given for a general 2SA733.

The Ic seems well out, the data sheet gives it as not more than 100nA whereas the device seems to measure 1.7 mA.

Is this transistor bad?

Datasheet: https://www.mouser.com/datasheet/2/1...733-310191.pdf

Schematic given below, transistors in question towards the top left of the diagram
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Last edited by The Lord Flashheart; 17th October 2019 at 05:18 PM.
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Old 17th October 2019, 05:46 PM   #60
Osvaldo de Banfield is online now Osvaldo de Banfield  Argentina
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No. the transistor gives you the current you program in the circuit, givenm certain base and emitter resistors and other parameters. The data sheet usually gives you a certain point of quisient operation to give you an idea of its capabilities.

Think a transistor like water valve. Given certain aperture, altitude of liquid and pipe diameter, it will give you certain amount of water per second.

The Hfe is the most important parameter in a BJT, but in the manufacturing process is impossible to predict the value of it, so they are characterized into ranges, like resistors. So you will have, say, 100-250 200-500 and 450-800 for example. And apply some code to identify it. Hfe also varies largely with temperature.
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