Blown amp (reversed polarity)

[tjmehta]

haha yeh i tested continuity of both the fuses with my multimeter..both were fine

I am completely confused 😕 on what is broken...ive followed the GND and PWR to so many places on the circuit...trying to figure out where the wrong current went... I even got to the point..where i just started popping out parts randomly and testing them..

nothing anywhere seems to be broken.. 😡

I can start taking pictures of more parts..if that would help anyone.

else...

I think i have a working 494 chip from another amp..im going to pop that into the board next and see what happens.

 

[tjmehta]

---
I do have one quick question though..in my rampage of popping out parts..i cant remember what order these two transisters are supposed to be in
http://www.prism.gatech.edu/~gtg158w/gndpic1.jpg
(the two Transistors by my hand)

Its a PNP / NPN pair (its the only pair of its kind on the board)

i looked at other side transistors on the board and all NPNs have a white-filled box under them. while PNPs have a empty white box.

Is this a good assumption or could it just be coincidence? 😕 😕 😕

 

[Perry Babin]

Are you sure that those aren't rectifiers?

The positive rectifier will have the diodes facing inward. The center leg will be connected to the positive terminal of one of the filter caps. The negative rectifier will have the diodes facing outward. The center leg will go to the negative terminal of one of the filter caps.

 

[Perry Babin]

If the amp is still inoperative, post the DC voltages for all of the pins of the TL494. Use the amplifier's ground terminal for the reference (black meter lead on chassis ground terminal). Apply power and ground as you would normally (both B+ and remote powered).

 

[RetroAudio]

Another idea might be to start testing the amp in sections,..such as disconnecting the circuits (if possible) from the power supply and just testing the supply,.and maybe loading it down to draw a few mA anyway. I know some switching supplies need a load to function properly. That's the way I'm doing some amps now and having pretty good luck, still manage to be a spaz half the time though...lol. I dunno, maybe I missed something and you're doing that already.

 

[anatech]

Hi Perry,
I think you may be right from what I'm seeing.

Hi tjmehta,
What are the numbers on those parts? If they are dual rectifier diodes, the common cathode goes to the positive of your filter cap. That you can check with your meter.

Hi RetroAudio,
I normally will run the amp section up off another power supply. If it's fine, I just spark the entire thing up (after checking the major parts first of course). I never run the switching power supply by itself as they often have feedback to regulate the voltage.

-Chris

 

[tjmehta]

...i didnt know what they were..i assumed transistors..they prob are rectifiers.

FMG 22R
-<-->-
38 06

FMG 22S
->--<-
38 20

I just test if these are working prop i just 'diode check' in the directions of the arrows right?

thanks again to everyone thats helping

 

[tjmehta]

anatech:

whats the common cathode.. like what legs?

and which filter cap? is it in the picture i posted above?

thanks
tj

 

[anatech]

Hi tjmehta,
Those would be your rectifiers for sure. FMG 22S appears to be common cathode. The center leg on those are your common terminal, both diode cathodes go there. The center lead will connect directly to the positive terminal of the big, black cylinder nearest your finger in the picture as a guess. The terminals of the cylinders that connect together are your common ground point. The other should be the negative supply filter cap.

The other part is common anode. The center leg will go to the other cylinder. Check this out with your ohmmeter.

-Chris

 

[tjmehta]

i know where the rectifiers go in the board ..the spots are still there

what im questioning which goes where in the board?

both center legs would goto a big cap in either spot.

looking at the circuitry it almost looks like a loop and that the spots of the rectifiers dont matter..

__________ __________
|which here | |which here |
| | | |
| | | |

in ref to the picture...or how would i be able to determine which goes where?


does that make more sense...sorry

tj

 

[anatech]

Hi tjmehta,
Okay. Step one. Identify where each center lead of the part goes with your ohmmeter.

They will go to one terminal on one capacitor (large black cylindrical thingy). The side with the white stripe is the negative terminal. They will also goe to the amplifier from there, but to the capacitor first.

So trace those and post your results.

-Chris

 

[DigitalJunkie]

I've fixed an amp that had the power reversed..
It was frustrating,and took a long time to fix..
There were 2-3 traces on the board that were vapor,I had to jumper them with a piece of tinned wire.
After replacing *every* semiconductor in the amp($80),it works beautifully!


Anyways,your diodes...
Looking at the picture here:
http://www.diyaudio.com/forums/attachment.php?s=&postid=951381&stamp=1151622706

I would guess the FMG22R would go on the "top"
and the FMG22S would go on the "bottom"

That's my guess,based on thier positions,and the positons of the capacitors.

 

[RetroAudio]

so then a way to protect yourself against reversing power is not to do it?

 

[tjmehta]

aright i got the rectifiers back into their correct places.

thanks...i didnt know the white stripe on teh cap meant negative 😀

so im back to testing parts

 

[Perry Babin]

Post the voltages on the tl494. It may save some time troubleshooting.