Randall short

[DrGonz78]

Oh man!! I just assumed you knew not to attach the speaker until you had the amp working. Honestly do not power this thing up at this point until you have thoroughly removed any faulty components and reinstalled new ones. For example, if you remove Q15 then go measure resistance on Q12's (and the other's) pins to see what has changed. The best way is to pull the part and really test it to see if it has been damaged.

[52 Bill]

If R18 is getting hot, then either Q12 or Q14 has a problem.

[Valvehead]

oK, tested tonight, speaker un -plugged .

resistance measurements in and out of the pcb . " IN PCB" was taken with all parts in the board.

Q12
IN PCB
B -C 2.4 OHM
B - E 2.2 OHM
C - E 0.2 OHM

Q12
OUT OF PCB
B-C 1K
B-E 3 OHM
C-E 1K


Q15
IN PCB

B-C OPEN
B-E 98 OHM
C-E 0 OHM


Q15
OUT OF PCB
B-C 700K
B-E 900K
C-E 1.5MEG


Q13
IN PCB
B-C 700K
B-E 6M
C-E 7M

Q13
OUT OF PCB
B-C 12M
B-E OPEN
C-E OPEN

Q14
IN PCB
B-C 0.3 OHM
B-E 98 OHM
C-E 98 OHM

Q14
OUT OF PCB
B-C 0.3 OHM
B-E OPEN
C-E OPEN


other-- the banded end of D12 and D13 are going right to ground. But test ok out of circuit---resistance and diode test.


looks like Q12 is bad ?? after all , its connected right to r18 which burned...

[The Dude]

Yes, it would appear Q12 is shorted B-E. It also looks like Q14 is shorted B-C. You should check them all via diode check with proper polarity to verify none are open and junctions are good.

Edit: I don't know your abilities or experience, so this is not meant to be condescending, but instead a tip:
If you're not familiar with how to test a transistor using diode check on a DVM, scroll about half way down this page. There is an explanation on how to do it.

http://www.elexp.com/electrical_meas...ts_part_3.aspx

[Valvehead]

thanks Dude !

[Valvehead]

replaced Q12/14 . powered up with limiter, bulb was very dim , so i powered up full ac , POOF ! Q14 instantly went up in smoke.

[Jazz P Bass]

Who are you purchasing the transistors from?

They may be bogus.

When you had the amp on the limiter, you should have made a few voltage test measurements.

It's a Class A/B amp, so the bases of the outputs should barely be on.

Remember, no load on the output until you get it sorted out.

[Valvehead]

some from China , some from usa....

[Jazz P Bass]

And the guy from the USA got them where? (hint: China)

If you have the original failed part, pop it open.
Then pop open a 'new' failed one.
10 to 1 you will see a drastic (ie: inferior) difference.

Maybe not.

http://sound.westhost.com/fake/counterfeit-p2.htm

Start over & take voltage measurements with the limiter.

[Valvehead]

i will eventually....i need to walk away from this one for a while

where do you suggest i buy "real" transistors ? Mouser ?

[Jazz P Bass]

2SA1943 & 2SD5200 are the most popular.

http://www.findchips.com/search