repair Peavey Firebass 700

[jmgoins]

Repairing a Peavey Firebass 700 and there are no mica insulators on the TO-3 outputs. I'm a noobie here, and a little nervous about +/- 75V on these heatsink fins. I know I'm not the first invader to look at this chassis so thought I would ask around while waiting for some parts to come in.

 

[Khron]

1) Might have been worth mentioning the device make and model in the thread title; "repair" doesn't really tell much, i'm sure you'll agree.

2) There seems to be no actual question in your question..? (unless i'm horribly mis-reading / misunderstanding something)

 

[Tubetec]

plus/minus 75 volts on external metal surfaces , no ,
but enclosed within a chassis its not a problem .
Never probe dangerous voltages with two hands .
make ground with with a crock clip and probe single handed ,
tucking the other hand in a back pocket is good .
it means their no path across your heart in the event of mishap .

 

[jmgoins]

Repairing a Peavey Firebass 700 and there are no mica insulators on the TO-3 outputs. I'm a noobie here, and a little nervous about +/- 75V on these heatsink fins. I know I'm not the first invader to look at this chassis so thought I would ask around while waiting for some parts to come in.

1. As I said, I'm new to this forum and every time I tried to be specific in entering a title I got it kicked back as unacceptable. I'll learn(I hope).
2. The question is whether or not to put mica insulators under the output devices when I re-install them. Sorry if that wasn't clear.

 

[jmgoins]

plus/minus 75 volts on external metal surfaces , no ,
but enclosed within a chassis its not a problem .
Never probe dangerous voltages with two hands .
make ground with with a crock clip and probe single handed ,
tucking the other hand in a back pocket is good .
it means their no path across your heart in the event of mishap .

Figured that out after getting the board out. Thanks for the reply and the safety lesson. You mean you don't discharge your PS filter caps by attaching them to your nipples? Thanks for pitching in.

 

[industrialarts]

Quick look at the schematic shows complementary pairs with bipolar supplies. I would have to guess the chassis is ground, so yeah, insulators would be required.

I personally don't use mica for the most part, just because of their potential to crack but they are usually fine.

 

[JohnRoberts]

Quick look at the schematic shows complementary pairs with bipolar supplies. I would have to guess the chassis is ground, so yeah, insulators would be required.

mica insulators increase the thermal resistance from junction to heat sink. The lower that thermal resistance, the hotter the heat sink, the more heat it dissipates.

I won't second guess the amp design from a distance.

JR

I personally don't use mica for the most part, just because of their potential to crack but they are usually fine.

 

[jmgoins]

Issues resolved, and my thanks to everyone for chipping in.

 

[Philbo King]

A silpad is better than mica.
https://media.digikey.com/pdf/Data Sheets/Bergquist PDFs/Sil-Pad_Guide.pdf
Also, sometimes TO3 cases are *supposed* to be shorted to the chassis. Job #1: Get a schematic

Edit - ninja-d by the OP... Nevermind

 

[jmgoins]

The problem was an open 1k thermistor. Caused thermal shutdown and fans to run at high speed. Not easy to get to, but the tip off was the odd voltage at the base of the transistor controlling the fan speed. Got it up and running after replacing both thermistors. Now I get my workout trying to get this 50lb monster back in it's case. Thanks for the suggestions.