Punch 45 - Weird Problem with Start-up

[bigphil]

I purchased an older RF Punch 45 from ebay. It's supposed to be working fine, and it does except it won't turn on normally. I've checked all of my connections, including the remote wire. The power light turns on, but the amp won't start producing sound unless I physically give it a bump, or drop. I can hear a high pitched winding noise when it's in its pre-start stagel; it goes away once I hit the amp and it turns on properly.

I've openned it up and the only thing I can find amiss is a scarred power capacitor which I intend to change, and what I'm guessing is a thermocouple that has come loose from its thermal paste against the heatsink. Could the loose thermocouple put the amp into protection and then out when I bump the amp? I was imagining a broken relay or something or a loose solder, but I couldn't find any. I'm more familiar with vintage 2ch amps than car audio so any help would be appreciated. Thanks in advance.

 

[Perry Babin]

Which revision is this?

What are the numbers on the power supply transistors (the ones with their center legs connected to the power transformer)?

If it doesn't have a TLx94 driver IC, you should change the B+ filter capacitor and all of the small electrolytics near the transformer. That should solve any start-up problems unless it has a shorted rectifier or output transistor.

If it has a TLx94 driver IC, post the DC voltage for each pin.

IC#
Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:
Pin 9:
Pin 10:
Pin 11:
Pin 12:
Pin 13:
Pin 14:
Pin 15:
Pin 16:

 

[bigphil]

Thanks for the help. It's not a chip amp version. The power supply transistors are D44VH10. I think i'm just going to change all of the electrolytics while I'm at it. I really don't think anything's shorted because once the amp is on it works consistently and sounds perfectly fine on both channels. Any other tips?

 

[bigphil]

I changed/checked all of the electrolytics a few were out of spec but the problem is still there. There's a buzz, seems like it could be some sort of short becasue the power LED is buzzing in tune. As soon as I drop it hard the amp trips on and starts working fine. If anyone has a schematic or knows where I could get one that would be great.

 

[I Am An Idiot]

With the power supply towards you and the LED away from you, heat up the solder joint on the BYV-27 or 28 on the left hand side of the board nearest you, the joint towards the middle of the amp. If that fixes it, it would be a good idea to take the board out of the amp and do it from the bottom of the board.

Also check the solder joints on the 120 ohm 2 watt resistor.

 

[bigphil]

I found the problem. After getting fustrated at spending the whole day on this amp, including retouching all the joints on the bottom I gave up on it. Just now, I took out my transistor testor to try to compare values from side to side. The first power transistor I put a clip onto, the solder pops right off... couldn't believe it. I went through and retouched all of the transistor joints, including the cold one I found and now the amp is working perfectly fine. Thanks for your help guys.