Shorted Power Amp when in heat sink; No short when no heat sink

electronhick said:
Maybe the description wasn't clear.....

There are 6 transistors per each channel on the heatsink:

One NPN and one PNP driver

Four NPN outputs
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ok, that sounds better. I was reading that as one channel was all NPN and the other had a PNP part in the mix and that just didn't make any sense.
 
Since one channel is working you could compare resistance/continuity between the two looking for inconsistencies.
 
You might also try measuring across the emitter resistors when you test it with the dim bulb . . . it will tell you which output devices are conducting.
 
Another Observation: When the suspect channel is powered up with TO-3 screws attached to outputs and no speaker connected, the DBT dims as it should. I tried a 4 ohm and 8 ohm speaker and the DBT stayed bright and didn't dim.
 
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electronhick said:
Another Observation: When the suspect channel is powered up with TO-3 screws attached to outputs and no speaker connected, the DBT dims as it should. I tried a 4 ohm and 8 ohm speaker and the DBT stayed bright and didn't dim.
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So fully assembled with no speaker comes up clean, if I hear you right? And no input signal, correct? In that state, do you see any voltage on the speaker terminals?

(And generally, for the sake of the speaker, it's a really, really bad idea to bring up a repair with a speaker on it. You risk destroying the speaker.)

I'll have to do some real noodling to figure out how what *should* be an idle channel can react to a load being attached.
 
Voltage (DC Offset) on working channel is 0.5V.
Voltage (DC Offset) on suspect channel settled at 0.5V, but was close to -15 initially.

Not too concerned about the speaker. But, yes good point. Don't want to troubleshoot with good speakers.

I realize 0.5 V is too high. The other amps I have worked on were like 0 - 0.010 V
 
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In post # 49 you said "These amp modules are interchangeable." If you were to make a good wiring hook up diagram first ( photos are the best) and were to swap the modules. you could find out if the short follows the module or the heat sink.
 
electronhick said:
Another Observation: When the suspect channel is powered up with TO-3 screws attached to outputs and no speaker connected, the DBT dims as it should. I tried a 4 ohm and 8 ohm speaker and the DBT stayed bright and didn't dim.
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Sounds like its trying to dump rail voltage to the speaker. With no load its not flowing any current. See if the voltage is positive or negative at the speaker terminals, that may give a clue where the problem is.

I strongly suspect this is all entirely unrelated to the heat sink, and completely related to the transistors actually being connected to the circuit. Pull the screws and the collectors are open so it can't flow any current.
 
Yes I agree the problem is in the module, The transistor is open when not connected via screws.
I do have it narrowed down to + Rail side, but not sure yet what is causing it
 
ok now we're getting somewhere. See what the voltages look like at the driver transistor.

also I think we confirmed this already, but all of the transistors have been tested good right?
 
I would have it both modules set up so that I could measure DC resistance or you could use the diode test function of your meter. And do a comparison between the two.. I am not sure if you have mention if the short occurs if one transistor mount screw is mounted in any of power transistor screw mounts positions or it did not matter which screw mount it was put in? Using only one transistor mount screw and place it in any 12 transistor mount positions , does the short show up?
 
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The short occurs when the last two MJ15003 are connected via screws on the + Rail side. of the amp.
I haven't had a chance to dig into it any more yet.
 
electronhick said:
The short occurs when the last two MJ15003 are connected via screws on the + Rail side. of the amp.
I haven't had a chance to dig into it any more yet.
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I've been looking around for something that could help and found this. It looks like all of the image you posted part of earlier.
Don't know how much it actually compares to your unit but it may help.


1768253965987.png
 
I am not sure if I am clear here. What happens when one screw is one used? All the output transistors should be mount except for the mounting screws, in which you use only one screw to test each mounting holes to see if the short shows up. You say "The short occurs when the last two MJ15003 are connected via screws on the + Rail side. of the amp." that statement is not clear. Due you mean all the transistors are mounted with mounting screws and when the last pair of MJ15003 transistors is mounted with the last 4 screws the short appears? The idea here is to test each mounting hole.
 
Stereo2 said:
I am not sure if I am clear here. What happens when one screw is one used? All the output transistors should be mount except for the mounting screws, in which you use only one screw to test each mounting holes to see if the short shows up. You say "The short occurs when the last two MJ15003 are connected via screws on the + Rail side. of the amp." that statement is not clear. Due you mean all the transistors are mounted with mounting screws and when the last pair of MJ15003 transistors is mounted with the last 4 screws the short appears? The idea here is to test each mounting hole.
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In post #69 he stated the short follows the module, it's shorted using both heatsinks so that eliminates the heatsinks since one channel works good.
 
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