PPI resistor smoked

[markw78]

This resistor smoked a number of years ago and I never sent the amp in for RMA then lost the RMA number!

We replaced the burnt resistor, but when I fired it back up, the same resistor smoked again.

It's just like the one circled in this picture, except its on the opposite side of the board. When we put power to the amp with the new resistor the first thing we noted was that the green power LED was very very faint, then I saw / smelled the smoke.

Whats the next step? Don't know a whole lot about this but replacing parts is pretty easy, may try and find a shop to take it to ... figured I'd ask here first! I did search but the related hits I found were for a different resistor.

Thanks!
Mark

[Perry Babin]

The photo didn't get posted. If it's too large, email it to me. What model amp?

babin_perry@yahoo.com

[markw78]

Forgot to add the picture heh




Hard to see it got a little fuzzy, the circled resistor is the one that smoked, and smoked again when we replaced it.

[Perry Babin]

Measure the resistance from leg 1 to leg2 of all of the heatsink mounted transistors to the right of the black, fully encapsulated rectifiers (top and bottom of photo). Do this with no power applied.

Which ones read near 0 ohms?

[markw78]

I'm not sure what the difference is between the rectifiers and transistors... I measured leg left/middle (I assume thats 1 and 2) on all of the little black tabs attached to the heat sink with thermal paste... I'm not sure I tested the right thing, if I did they were all 0.9 when the DMM set to 2M ohm's. Suspecting I didn't test it right

[markw78]

Sorry, it was at 200M when they all appeared to be 0.9.

I just retested with it set to 2M, and they all read 0, except for 3 of them read 1.

Still think there's a high chance I'm doing something wrong lol

There is a little "i" symbol in one corner, I wonder if that signifies which is leg 1? All the black tabs (transistors or rectifiers?) down the side with the burn say "H0 40" where as teh other side has a mix of "H0 40" and "H0 18".

The burnt resistor is R52 if that is any help at all. The things I was trying to test for you were labeled Q1 - Q12, if that helps?

[markw78]

Ok one more post with info...

I set the DMM to Diode Test / Beep.

When I touch the first 2 legs them, I get a beep and a reading of about .025... on most of the transistors.

Transistors labeled Q1 - Q6 (the first block of them, bottom right corner in the picture) all read the .002 or .025 and beep. Q7 is the first transistor in the next bank over to the left, it beeps and reads .025, but then Q8-Q12 (the rest of the ones in that bank) don't beep, and read ranging from 0.9 to 1.8.

If I count, there are 12 resistors, behind 12 caps, behind 12 transistors. Without looking at the traces, it appears that resistor R46 goes with Cap C23, which goes with Transistor Q1.

Based in this, the smoked resistor lines up with Q7 - but Q8 is where I stop getting beeps. It makes me thing Q7-Q12 are all bad? Or Q7 is bad, making it look like the others behind it are bad? Maybe looking at the traces would help me make more sense of it.

[Perry Babin]

There are 24 you need to check. Set the meter to the lowest resistance range and re-check them.

It's likely that you'll need to replace all 24 and also the 2 HIP4081 driver ICs.

Also check the output transistors (larger transistors on the other end of the board). Measure the resistance between legs 2 and 3.

The 22 ohm resistors go to leg 1 of the power supply FETs.

[markw78]

The Output transitors act weird... There is a cap soaking up when I put on them or something. To get any numbers I have to set the DMM at a high number (200k, 20M, etc) but then it just starts steadily dropping.

The 24 input? transitors tested as such... Not sure what to make of it. Note the "Ohm Setting" column, I had to change it to the highest setting (200M) to get a reading. Leaving it on 200 was not giving a reading. It's weird that on one side, all the transistors say H0 40 and the other side has mostly H0 18, and only a couple H0 40.

http://www.completecareaquatics.com/...stor-tests.htm

Been thinking of just taking it somewhere, but not have a lot of luck finding where to take it for repair.

[Perry Babin]

The link isn't working.