It Can Happen To You

mlucitt

Veteran and General Yakker
Joined
Jun 24, 2011
Messages
2,501
Location
Jacksonville, FL
#1
During a routine customer WOPL upgrade on a PL400, I noticed two of the Nylon shouldered washers would not fit into the backplane of the amplifier, after I had chamfered all the holes and flattened the back and heatsinks with my calibrated flat bastard file. This is a well-documented problem and can be easily remedied with my cobalt, Letter "A" (.234"), drill bit (from www.CMLsupply.com).
09 PL400 Two Holes Too Small.jpg

After resizing the holes, the Nylon shouldered washers dropped right into place. I proceeded to continue with my assembly. When it came time to power up the amplifier on the DBT / Variac combination, things started to go downhill. First the DBT went very bright. So I backed off and started troubleshooting. Finding nothing wrong, I powered up again and blew the B- rail fuse. I pulled the Backplane Board fuses for the Left Channel, and promptly blew the B- rail fuse again. So I pulled the Backplane Board fuses for the Right Channel and the B- fuse held.

The problem was on the Right Channel Backplane but the Control Board test points were out of spec as well. What to do? The wiring checked fine and resistance checking for shorts with the power off showed no shorts. So I powered up the amp and applied a test signal to the Right Channel. The output showed distortion, so I increased the input signal to take a better look at the distortion (THD? IM? Crossover?). And then I heard a loud pop! The B- rail fuse blew and my distortion disappeared. I did not see a flash, so I wondered where an arc like that could have happened? Are you with me so far?
I pulled the Driver Transistors (bottom row) because they were the only outputs that were populated, and this is what I found:
Cut Sil Pad Shorted to Case.jpg

I had forgotten to chamfer the holes I had drilled out / resized and there was a barely perceptible burr on the edge of both holes.
I had suffered the dreaded Sil-Pad cut-through. I studied the burnt Sil-Pad and the other one that was cut but had not burned through, and I filed the burnt ridge from the transistor case, and reinstalled it with a fresh Sil-Pad.

When I powered up the amplifier everything was fine. I installed the remaining outputs and the amp sounded great. I will never forget to de-burr the holes I re-size from now on.
 
Last edited:

Vintage 700b

New Around These Parts
Joined
Mar 13, 2019
Messages
31
Location
Wisconsin
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Fabricator
#2
Hello Mark,
Was there another picture that you were going to post? I had an arc from a transistor to the heat sink, because one of the Sil-Pads tore on a sharp (burred) hole on the transistor base. I found that I had to check each of those holes as several were never deburred. Even with 5 inch pounds the Sil-Pad tore, and allowed a direct short (arcing). I found only one that did arc. and have a picture of it before I cleaned it up. No damage was done other than the short, but it took a bit to find it.
 

mlucitt

Veteran and General Yakker
Joined
Jun 24, 2011
Messages
2,501
Location
Jacksonville, FL
#3
No other pictures to post, I'm not much of a photographer. I checked all the driver locations on the bottom row and the only Sil-Pads that tore were on the driver holes that I had resized. I know better, I should have turned the amp over and quickly deburred the two resized holes. I didn't.
My transistors were new and the bases were flat, so no issues there.
I have a calibrated torque screwdriver set to 6 inch-pounds, and this is the first cut-through that I've had.
I was surprised at how hard the metal had become where the arc occurred, a case of instant heat treatment. But it cleaned up with a hard file.
 

Vintage 700b

New Around These Parts
Joined
Mar 13, 2019
Messages
31
Location
Wisconsin
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Fabricator
#4
I had the twin to that one. When I first pulled up your post, I did not see the transistor picture or any text past "here's what I found": That's why I asked. It's a bugger, and a tiny cut in the Sil-Pad will do it.
Thank You for posting that info.
 

George S.

Chief Journeyman
Joined
Feb 18, 2020
Messages
694
#6
I tried deburring with a drill and had minimal luck, the drill itself raised a ridge. So I finished mine off with very small swiss files, just too labor intensive. Next one I do I will lightly hit with a ball or cone fine grinding stone.
 

MarkWComer

Veteran and General Yakker
Joined
Nov 1, 2014
Messages
1,996
Location
Gaston, SC
Tagline
Victim of the record bug since age five
#9
Another task to add to the "troubleshooting" list! Thanks for the heads-up!!
I wonder if that's what blew up the left channel of my 400, I remember that I had to ream a few of those holes for those shoulder washers.
 

FredR

Chief Journeyman
Joined
Jan 26, 2014
Messages
838
Location
DFW
Tagline
WTF?
#10
Has happened to me. Will not again though. Also, be sure to apply proper torque to the TO-3 screws.
 

Mohawk

Journeyman
Joined
Oct 29, 2019
Messages
173
#14
Snugging up sil pads always freaks me out a bit... I like the old mica pads !
No argument that the sil pads are way more convenient and no mess from the thermal compound .... But.....
Exactly how do they compare as far as heat transfer?
 
Joined
Apr 26, 2011
Messages
497
Location
Central Ohio
#16
When using mica, which I don't, a drop of grease the diameter of a number 2 pencil, between the pins is all that is needed. Have to use it on both sides. Less is more. I've never had a silpad fail. Of course I never reuse one either.
 

Mohawk

Journeyman
Joined
Oct 29, 2019
Messages
173
#17
Agreed,
Thermal compound spread all over like you're making a peanut butter sandwich is just a mess and nothing more !
Sil pads definitely not to be trusted
If reused !
 
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