PPI 2150 AM Dead! Need help or good repair person!

[PyroZona]

Wow, that was fast...thanks Perry!

Pics are zipped and on the way to you.

One odd thing I noticed about the amp each time I brought it back from Tran's shop is that the power & ground wires are hot (i.e. - have current in them) and will spark when I wire them back up to my power and ground blocks...or if they're touched together. Am I incorrect in thinking that really shouldn't happen?

Thanks again for your help,

Pyro

[Perry Babin]

The filter capacitors will remain charged until they're discharged by some means. They will eventually discharge internally due to leakage (normal) or possibly through some of the circuitry. If you touch the wires together before something else discharges them, you will get a spark. For amplifiers that have wire leads for power and ground, I generally touch them together to discharge the caps. When I forget, I know it because the owner calls back freaked out/concerned because the amp made a spark while they were carrying it.

It appears that he didn't clean out the old compound. I'd suggest that you remove all of the old compound from the heatsink, the transistors and the insulators and reapply new compound. If you use a solvent like acetone, be careful. Many of the PPI finishes (powdercoating) are damaged by acetone.

When you reapply the compound, you don't need much. You only want to apply enough so that a tiny bit is displaced all along the perimeter of the transistor (see attached) when clamping pressure is applied.

Remove the rubber tape and metal strips he used. If the fingers are weak, pull them out so that they're parallel with the bottom of the cover. That will apply enough pressure when the cover is replaced.


Did you check to see if the screw holes were stripped. This is very common and must be repaired before the new compound is applied.

[Perry Babin]

What are the gate resistor values (the resistors connected to the first leg of the power supply transistors)?

What transistors did he use as replacements?

[PyroZona]

Originally posted by Perry Babin
...I generally touch them together to discharge the caps. When I forget, I know it because the owner calls back freaked out/concerned because the amp made a spark while they were carrying it.

LOL...sounds familiar.

Did you check to see if the screw holes were stripped. This is very common and must be repaired before the new compound is applied.

Yes, that was one of the first things I did. I'm pretty sure I got all of the holes back to a point where they'll provide sufficient grip for the original screws. Luckily I just happened to have a 6-32 tap in my mess of tools. Should be ok.

I'm good on R/R'ing the compound. Just enough to transfer heat, much like one would apply between a computer processor and its heatsink, right?

Originally posted by Perry Babin
What are the gate resistor values (the resistors connected to the first leg of the power supply transistors)?

What transistors did he use as replacements?

Not sure if you're asking what they measure now, or what the color codes on them are.

As for the transistors on the power supply side, there are 4 that say MUR810 on them and then 10 (I think) that are 44N, which I think was mentioned as a possible replacement in another PPI repair thread on this forum. Are these adequate, or is there a superior replacement that I should be looking to order?

Thanks again for your help!

Pyro

[Perry Babin]

The gate resistors are connected to the first leg of the IRFZ44s. From the photos, they look like yellow, purple, gold, gold or yellow, purple, brown, gold. If he left the yellow purple brown gold (470 ohms), they are likely the problem (or at least part of it). They should have been changed to 100 ohms or less.

The MUR810s are the rectifiers.

[PyroZona]

Ahh...I see. Ok, so what resistors should I be looking to pick up tomorrow? What tolerance/wattage and what color bands should be on 'em? And if I replace them, should the Z44's be sufficient?

Thanks,

Peter

[Perry Babin]

100 ohm resistors should work well. The color bands will be brown, black, brown, gold (for 5% components). 1/4 watt is what you'll need.

The Z44s are good enough.

[PyroZona]

Gotcha. Hopefully I can get those at RatShack or Fry's Electronics here in town and won't have to order 'em. I really wanna get this amp rockin' again like it should and make one last trip to Tran's...just to show him that a PPI 2150AM absolutely can push 2 DVC 12's and 4 6 1/2's in stereo at 2 ohms without turning into a small version of a Pink Floyd smoke & laser light show, LOL!

It did it without a problem for at least 12 years without a problem, and his argument of "that was then, this is now" (pretty much) just doesn't fly with me.

OH...one more question about the thermal grease application. Originally, sandwiched between the transistors and the heatsink were strips of clear lexan or plastic. Is there any reason I should or shouldn't be sure to put them back in when I re-apply the thermal grease? Do they serve a functional purpose, or should I not worry about 'em?

Thanks again SO MUCH for all your gracious help!

Pyro

[Perry Babin]

Those strips are mica insulators. They are used to insulate the transistors from the heatsink. If any of the heatsink mounted components is allowed to contact the heatsink, the amp will fail.

[PyroZona]

OOooooo.... Good thing I asked! Thanks for that, I'll be sure to include 'em.

SO glad I found this forum.

And Perry, you rock, sir! Thanks ever so much.

I'll post an update once I get it all up and running again.

Pyro