Rockford amp problem

[chipper]

I have a couple punch 150's one i'm trying to get working again... Anyway this one is in good condition but when i turn it on for the first 4 seconds it starts putting out DC on the output the other channel is normal just this one. I can be playing music on it for a good few seconds then it puts dc on the output. It sounds like it would play through the dc but it keeps skipping (maybe because the protection inside?) i tested the transistors 2N6487's and 2N690's all checked good on diode check, What is causing this? Open transistor or resistor somewhere? Also when i lift one fuse out of the socket it will play but all sticky and when i push the fuse back in it will sound normal for a good few secs then back to throwing out dc. Hopefully someone out there can help me.

[Perry Babin]

Have you checked the driver transistors?

[chipper]

No haven't check those yet. Could it be possible a predriver would fail? like a MPSA56? And is a driver fails would it cause that random dc offset to happen??

[Perry Babin]

With an amp this old, it could be any transistor. You'll need to check all of them in the defective channel.

[chipper]

Originally Posted by Perry Babin

With an amp this old, it could be any transistor. You'll need to check all of them in the defective channel.

I have checked every small mpsa56 transistor and others in that channel! But no guess what?! A capacitor blew up! scared to me to death lol. I am suspecting it was that capacitor?? It's small rated 35v 10uf theres a couple of them located near the speaker fuses. That thing just popped like a fire cracker man!

[chipper]

And it caused everything in that channel to fail, shorted the drivers, shorted the outputs. Just what i needed.......damn. I should have checked some caps first.

[1moreamp]

Bad Caps did not cause this, shorted transistors and reversed polarity on the cap caused this, and you not the first person this has happened to lol lol lol...The caps were most likely fine until they got zapped by reversed polarity DC which caused them to explode...Lucky you did not get your eye put out, or any cuts from the metal can debris flying from the big pop...
Gut the channel back to the IC in the center of the board and the amp will play like new again, until some owner bypasses the fuses again...


Oh and remember this happening for this particular name brand and series of amp as its nothing new for the design, and protect yourself next time as this brand of amp can be dangerous to work on if your not used to its idiosyncrasies by design. I suggest eye protection at least, especially until you get past-master at this series of amp.

[chipper]

Originally Posted by 1moreamp

Bad Caps did not cause this, shorted transistors and reversed polarity on the cap caused this, and you not the first person this has happened to lol lol lol...The caps were most likely fine until they got zapped by reversed polarity DC which caused them to explode...Lucky you did not get your eye put out, or any cuts from the metal can debris flying from the big pop...
Gut the channel back to the IC in the center of the board and the amp will play like new again, until some owner bypasses the fuses again...


Oh and remember this happening for this particular name brand and series of amp as its nothing new for the design, and protect yourself next time as this brand of amp can be dangerous to work on if your not used to its idiosyncrasies by design. I suggest eye protection at least, especially until you get past-master at this series of amp.

I'm not sure why this happened? I was checking every transistor possible for any shorts. And then i tried powering it up again and the led didn't light up the second time....hmm then out of no where a big explosion happens lol. I jumped back fast! With fear of the amplifier. The IC your speaking of is NE5532P, right? So far i removed all the outputs both 2N6487's survived and one 2N6490 was shorted. and both drivers where gone too. How do i check this particular IC for any shorts??

[chipper]

Also if someone can post up a scematic for this amp? I heard it was made in 1988? I saw a ad in the rockford forum someone there posted.

[amptech]

hello chipper, your best bet at this stage would be to use a 12vdc 1 to 2 amp power mains pack for repairing your unit.. oh and caps blowing up from the pcb..quick duck and turn the power off..

use this site for more helpful tips how to repair car amps..
the answer to your op-amp question starting from the left side count down to pin 4 you should get a reading of 15-volts and +15 to pin 8..pin 1 is signal out and pins 2/3 are in same for pin 5..6 and pin7 is o/p.

you said you've got a few of these amps does any of them work? if so use one of them to take readings from to aid your repair quicker...