PPI PC2350 Woes/Questions

[briansz]

I picked up a PPI PC2350 awhile back and just got around to testing it. When connected to my 35A Astron bench supply, it fired right up and seemed happy, green power light. I turned it back off and hooked up some RCAs and speakers, and quickly got a little magic smoke. Even more quickly, I turned off the Astron.

What I've found are several visible issues.

Resistors R2 and R3 across the preamp inputs were floating around in the case. They do not appear burned. There's a slight heat signature in the epoxy under where R3 was, very light and hard to see. On the back of the board each of these resistors has one solder joint that almost looks cold (perfect hole, perfect bead) and another that looks melted, but not very thoroughly. I think they both actually pulled out or fell out. There are plenty of leads left on each of them so I don't believe either was cut. Not sure why I didn't notice parts rattling in the amp before disassembly, maybe just luck.





Resistor R78 at the opposite end of the board has one lead detached in the same manner as R2 and R3. Cold joint or very intense, very quick burst of heat that allowed the lead to release.



Some minor arcing took place on the bottom of the board. In two areas the copper trace that runs to the L- and R+ output pins got hot and shorted to the heatsink. Likely the origin of the magic smoke.





When I removed the huge amount of thermal grease and goo, I was surprised that I did not have enough mica insulators for all of the heatsinked devices. Never thought of that, so I did not keep track of what went where <doh!>

I'm far from an electronics repair expert, know enough to be dangerous really. I do have a DMM, scope, and a Weller temperature controlled soldering iron. Some time ago I picked up Perry Babin's DVD. Would like to give a try at getting this thing up and running.

Plan so far is to resolder the three resistors noted above and to excavate the small area of the heatsink extrusion where arcing took place to prevent further shorting to the bottom of the PCB (no devices clamp or mount in this vicinity).

Out of 60 devices, only 8 diodes and 4 transistors appear to be fully encased, and I don't have nearly enough insulators to cover everything else. My mica sheets are not large enough to span the banks of A1303's and C3284's, so I assume each group of six RFP70N-6's gets one. Just need to know what else.

Thanks in advance for any help and advice!

[Perry Babin]

Order the Kapton 200MT from J&R electronics on eBay. If you plan on doing this type of work, order a couple of rolls.
Polyimide Kapton MT Film insulator 0.002" x 5.5?x 72" | eBay

What's the resistance between the B+ terminal and the non-bridging speaker terminals?

Everything that's not fully insulated needs to be on an insulator.

[briansz]

OK, film you recommended is on order. I've read that paste is to be used on both sides of the Kapton, right? Have a tube of GC Type 44 here.

I can't understand how no devices on the power supply side would have had an insulator. Have to assume another ape was in this amp before me. The heatsink was painted a fine chromgenic green/purple, so maybe it happened then.

As the board sits, there is no connection between the B+ terminal and any speaker terminal (or, for that matter, between any speaker terminal and the ground terminal).

I did not mention in the original post that immediately before magic smoke I heard some faint oscillation from the heatsink. This was only present once I connected the low-level source and speakers, amp was dead silent with just ground, B+ and remote connected.

[Perry Babin]

Yes, heatsink compound on both sides of the kapton.

[briansz]

While I am waiting for the kapton film to arrive, I'll test and reattach the resistors and clearance the heatsink where the arcing took place. Guess I could use another piece of the kapton to insulate that trace on the bottom of the board, should hold up to any heat that's generated.

Will also make up some clamps in the meantime so I can test things out with the bottom cover off. Might even strip the hideous paint from the heatsink.



Thanks for the assistance thus far!

[Perry Babin]

If there is paint under the transistors, it needs to be removed.

[briansz]

I've got an unused box of walnut shells up in the garage rafters, will just media blast the whole sink clean and spray it off with dry air. Pretty sure that the overspray visible in the picture was factory original. It's not the same color as the chromogenic paint on the exterior and appears to be darker than any automotive primer I'm familiar with.

I got a PC2100 with this PC2350 and it's the same delightful color. I haven't put power to it yet, judging by the experience with this amp I think I will wait on the kapton to arrive and make sure all devices are properly insulated before the attempt.

Might be a few days before I report back as I now have to wait for the insulating material to arrive.

[briansz]

Success!!

Received the kapton film earlier this week and stripped the heatsink, then soldered the resistors back to the board. Put down two kapton strips on the heatsink with thermal paste and made some transistor clamps. Pasted up all the devices, put a kapton pad under the area where the trace shorted on the bottom of the pcb, then torqued it all down.

Set up the Astron psu with a 7-Amp circuit breaker for safety, got a green power light. Then I connected the low-level inputs, still got a green light. Connected one speaker at a time, and I've been playing/listening to the amp for the last 20 minutes.



This was a pretty easy fix, now to paint the heastsink.

Thanks Perry Babin for your willingness to help a n00b!