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My P68 sub amp just went poof - help me fix/troubleshoot  it!
My P68 sub amp just went poof - help me fix/troubleshoot  it!
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Old 23rd December 2002, 03:56 PM   #21
R. McAnally is offline R. McAnally  United States
diyAudio Member
Join Date: Jan 2002
Location: California
Default Re: Re: OK - Amp is now at least powering up, but R14 is getting HOT!!

Originally posted by moamps

Q8 is blown, V across R14 is must be 0,65V
If you short D2 and D3 , Iidle will be around zero.
Are collectors of Q4, Q5, Q6 isolated?

sorry to hear about your troubles. p68 is an excellent amp and has served me well many-a-party blasting into a 10" sub all night long. =) BTW, it has pretty smooth clipping performance.. does not produce the typical barking sound most amps do. just seems to stop getting louder.

I suggest taking a step back and checking EVERYTHING. start with the power supply. is the ground (center tap) making it to the board? is the signal ground isolation resistor on the board (around 10 ohms i believe) the proper value? are all the Q's correctly inserted and properly chosen for their spot? any cold solders or short circuits?

you should invest in a diode checker (cheap transistor tester in my book =). these are included on almost any multi-meter. remove every single Q from the board and test them individually. if one bad Q is left in there you risk blowing up several more each time.

I am willing to bet there is a stupid mistake somewhere. just start all over from scratch with known good components.
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Old 23rd December 2002, 07:08 PM   #22
Jonathan M is offline Jonathan M  New Zealand
diyAudio Member
Join Date: Nov 2002
Location: New Zealand
Default Thanks for the advice - yes Q8 is shot.

Hi all,

I've checked and rechecked every connection. I found one slightly dodgy connection, but I believe it was fine - has been fixed. Q8 is open (Thus R14 is now close to toast).

I've also rechecked all the transistors and diodes + remeasured all the resistors - all appear fine. None of the caps are shorted, either.

I've ordered some more transistors as replacements. Hopefully they'll arrive in the next few days.

Hope you all have a great Christmas and a good New Year. Hopefully there won't be too much more wrong with this thing!!

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