Nelson Pass A40 Help

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Robbie, just a kind pointer for next time: when you switch on for the first time, make sure you have both quiescent current settings dialed down to minimum.
Then, make sure you have a multimeter at each channel to measure the quiescent current, then switch on. If too high, immediately switch off and try to find the issue.

That offset you can investigate at leisure, nothing will break from it, but the quiescent is of utmost importance to prevent toasting the amp.

Jan

Thanks Jan,

When you say “dialed down to minimum”, how do you mean?
 
...When you say “dialed down to minimum”, how do you mean?
You need to read, reread and really understand the A40 original article especially the "Final Tests" and "Easy Bias" section. Ask the points you do not understand. Turn on one channel at a time, when both are completely tested then you can turn on both.

I have not seen the exact part number of the transistors used in your actual build. The A40 use PMD 16K100 and 17K100 darlington pairs which are hard to find, what are the actual parts you use in their place? Please do not turn on before this part issue is cleared.
 
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You need to read, reread and really understand the A40 original article especially the "Final Tests" and "Easy Bias" section. Ask the points you do not understand. Turn on one channel at a time, when both are completely tested then you can turn on both.

I have not seen the exact part number of the transistors used in your actual build. The A40 use PMD 16K100 and 17K100 darlington pairs which are hard to find, what are the actual parts you use in their place? Please do not turn on before this part issue is cleared.

I did not build the amp, I bought it as faulty and have been trying to repair it.

The transistors are un-marked, so I cannot confirm a part number but almost all parts used elsewhere were as per the specification, I have no reason to think that the transistors are not.
 
... Poking around with the multimeter, I am now reading 37 volts DC at the speaker posts. ...
Then both Q8 and Q10 are not conducting, either both R18 and R19 are open circuit and/or both PNP are. In any case, 7V at the RCA input indicate C2 may have the inside melted and short circuited. I suggest to disassemble the faulty channel, desolder and test each suspected part.
 
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Afternoon all, I soldered in a set of 4 new 0.68ohm resistors (R16-19) in the faulty channel this afternoon, switched it on and it fried all 4 resistors immediately then blew the fuse.

Can anyone suggest what fault this would point to?

Yes. I think this points to the fault of not reading the advice here and acting on it. Sorry to be so blunt.

Jan
 
Sorry to irk you with my questions, I was just hoping for some pointers. I thought maybe someone would have had a similar experience and could suggest something.

The problem I’m having is that I didn’t build the amp, I bought it faulty, so I am trying to fault find. I am struggling due to the way the amp is constructed, as once it is put together so that I can plug it in there is zero room to access the circuit board and output transistors to test voltages etc.

I may have to start from scratch with this one and maybe even re-house it.
 
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Sorry to irk you with my questions, I was just hoping for some pointers. I thought maybe someone would have had a similar experience and could suggest something.

Yet, when suggestions and pointers are given, you seem to just disregard them.

When those 4 low value resistors blew, someone suggested to check out the output devices, because we know from experience that there almost sure is where the shorts are.
Yet you replaced the resistors and switched on. OF COURSE the resistors blew again, how could it be otherwise?

Jan
 
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