# Help with US Amps USA-100



## Dawgless (Oct 6, 2014)

I just picked up this amp from a seller on here and it was indicated that it is in non-working condition. The seller indicated the light came on, but it produced no sound. Before firing it up, I opened it up and this is what I found. 

There was no film behind several of the FET's and there's a strange wire that was just hanging out in space. There are several pictures in this group that don't really show much of anything, but I'm including everything that I took. 

Would anyone happen to know what the little wire might be to? I have no experience with what I'm looking at, so maybe there's someone on here that can help me out with this.


----------



## cajunner (Apr 13, 2007)

case ground


----------



## Dawgless (Oct 6, 2014)

Thanks!

Would you suggest just looping it to the screw that helps to bind the rail, that secures the FET's to the heatsink? 

Also, when reassembling the amp, my plans are to use this: Kapton Tape, Kapton in Stock - ULINE in conjunction with this: Super Lube 98003 Silicone Heat Sink Compound 3 oz Tube | eBay

Are there any drawbacks to using either of these products, or anything negative that I should be aware of? Not pictured here, I also picked up a mystery US Amps four channel that is around the same era as this one. It was purported to be working, but I have yet to test it as I'd been on vacation the last couple of weeks. My plan is to pull the board out, clean up the heatsink, apply the Kapton tape, and use the thermal paste on it as well.


----------



## soccerguru607 (Nov 4, 2009)

Those 'film' are transistor insulators. Without the amp is grounded.
Blue=remote, Black=ground, red=power and black/white=speakers


----------



## TrickyRicky (Apr 5, 2009)

Did you check all the semi's that are attached to the heat sink? 

I can't comment on the thermal compound, but the kapton tape will work...also there is mica (clear...the ones on your amp) and silpads (silpads are much cleaner as they don't require thermal compound.).

And yes you can place a round/ring terminal on the wire strand and use the screw from the clamps.


----------



## Dawgless (Oct 6, 2014)

Okay, TrickyRicky, I did some further testing on the amp tonight and while some of the readings I got were strange, majority of all the FET's checked out, with the exception of two. 

In the second picture from the bottom, shown above, the two FET's just to the right of the gain adjustment are registering a short across all the legs. These are TIP35c's. What's strange is that I also tested the opposite side and initially they were reading fine, but later, they weren't... I guess I'm going to assume that all four of these are dead and will need to be replaced. These also appear to be linked to the four blue capacitors showing in the pictures. Should I replace those as well? I'm guessing it'd be a good idea to replace the two brown caps towards the power side of the amp? All the FET's down on that side seem to be testing within normal tolerances, based upon what I read on Perry's website. 
I did have some issues determining whether they were PNP or NPN, of which I'm still a little fuzzy on what they truly are, but the readings were consistent across all the legs, with none reading weird shorts or low voltages.

Okay, so after going back and checking, it appears that the ones to the left of the gain adjustment are testing okay, but the ones on the right aren't. I'm guessing I should just replace those two? Or should I do all four of the 35c's on this end?


----------



## Dawgless (Oct 6, 2014)

Quick update: 

I ordered replacement FET's and enough capacitors to replace every single one in the amp. Just received them today! Mouser rocks!

Not having soldered components on a board like this, I was a bit nervous to start with. As I went, I got way more comfortable with it. Caps are easy, but the FET's are a bit of a pain. Used a lot of solder wick, and eventually they begrudgingly came out. Got the new ones soldered in and I'm fairly happy with my work. A pro would say the joints look like crap, but I'm okay with them, and I'm certain they'll hold. 

However, this story comes with a big hairy butt... I ordered enough caps to replace all of them, but I screwed up on one... Rather than order 25v100uf caps, I ordered 25v10uf's. And I ordered like 20 of them... Went back on and ordered the right ones and they're on their way. DOH! 

I did test the FET's that I installed and they are now giving me correct readings, which leads me to believe the issue with the amp was the FET's. Once I get all the caps installed, I'll work on getting the old girl powered up and hopefully I won't let the smoke out. Even if I do, this has been a great learning experience and has given me confidence to really dig a bit deeper into amps and what makes them tick!


----------



## Dawgless (Oct 6, 2014)

All caps finally replaced! Need some thermal compound for reassembly and then it's time to fire this badboy up! No guarantee's that it'll work, but if I let the smoke out of something, I'm sure I'll be able to replace it. 

going to also have to replace the wires, they're all a little short for my liking...


----------



## Dawgless (Oct 6, 2014)

She's alive after two new TIP35's and all new caps!

I decided that today was a good day to put the amp back together and give it a trial run. Both channels work and it works in bridged mode too! Nothing got hot and no funny smells or sounds.

Now it's time to get the old 425 up and running.


----------



## soccerguru607 (Nov 4, 2009)

Took you awhile but glad to see working


----------



## Dawgless (Oct 6, 2014)

soccerguru607 said:


> Took you awhile but glad to see working


HAHA! Yeah, been kind of tied up with other things lately. 

Just went out and ran it on the sub it's going to be mated to. Ran it at 2 ohms bridged for a good solid 3-5 minutes. Case was slightly warm, but not too bad at all. I was still running my 5amp test fuse and that popped in a fraction of a second. :laugh:

Seems to be running well! Made my sub pound. Gives me motivation to get it completely finished and installed! I still have a long way to go on this whole project...


----------

