Soldered the diode back, but no lottery wins ever for me

I have shorts on both q13, q14, and i suspect cr38 is also damaged ( will will pull it to be sure.)

 

I wouldn't expect CR38 to have failed, but it's across Q13 and Q14, so you can't test it while the transistors are in circuit.

However, these are the negative side transistors, if they are short then you should have -40V on the output, I though you said 40V, not -40V?.

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I was quick when i did my measurment and must have had my probes backward. I just verified it is -40V. I also removed cr38 and it is fine.

 

You 'might' be able to just change those two then?, have you disconnected them? - as they are in parallel only one might be faulty?, if so remove the faulty one and try the amp with the other one. The amp will work fine with just one pair of transistors, but will only feed 8 ohms (60W) and not 4 ohms (100W) - but it will be fine to try it like that. If you feed 4 ohms at high volume the transistors will overheat and die though!.

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Thanks, ill try that out right now. I dont have them out yet, but working on it. I need more hands and irons.

edit:: sorry about these double posts, i keep pressing back on explorer and it reposts them.

Thank you very much Nigel. I got the first transistor out and it was still shorted. The other was fine, so i left it in and turned it on. 0V at the output now. Dont have my cabinet or guitar here to try it out, but i might as well order the new transistor while i have it all open. Thanks again,
Dave

 

Thanks, ill try that out right now. I dont have them out yet, but working on it. I need more hands and irons.

 

well i picked up a new TIP147 at the local electronics shop. Soldered it in, checked everything over once and powered it on. Measured output and had 0V. Started smelling something so i checked again and i was at -40V.
Powered it off and took the new one out again. The other tranisistor is now shorted.

 

Are the four Vbe diodes all OK?.

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cr25, cr26, cr27, and cr28 all checked out okay.
Any other obvious components to test?

 

Everything around the output stage really, but when you test it, don't bother fitting all four transistors - just one top and one bottom is enough for testing.

__________________
PIC programmer software, and PIC Tutorials at:
http://www.winpicprog.co.uk

 

Should i have replaced these transistors in pairs? I pulled the transistor that just went bad, checked all diodes ect.. around the output stage and turned it back on. I made sure that the lone transistor was not heating up too much and left in on for a min or so, and no damagae was done. Seems to be working now with just one, so i am gonna put it back together and see if it works.

 

Okay, i am really lost now. I used my amp since i last replied to this thread using only the one negative rail power transistor (instead of 2). WEll last week i ordered 2 TIP 147 replacements and put them in. Nothing shorted, but i can hear anything coming out of the amp now. If i turn the volume up all the way i can barely hear the guitar. The wierd thing is when i power it off, it gets really loud (guitar, not noise) as soon as i hit the switch, then obiviously powers off. What do i test now??? what could be wrong. Thanks for any help guys.