Heat sink compound or thermal pads for TO-3?

This can be a somewhat generic question, but my specific case is what I'm guessing is a late 70's Peavey "Bass" amp (no other names or identifiers).  Across the back is 8 TO-3 transistors for the power output in what I assume is a generic "power module" for various Peavey amplifiers of the time.

Seeing as how this thing is pretty much 40 years old, I'm guessing that the heatsink compound is pretty much dried up and useless at this point.  Fortunately for me the transistors appear to be socketed.  Perfect...Unscrew the retaining cap, pop out the transistors, clean off the old goop, squirt in some new goop, bolt 'em back in and keep going.

Since I don't have any compound...Does anyone have a part number or something I can get from Mouser?  Should I forget the goop and just use a thermal pad?

Thanks!

If it isn't broken why fix it?

The function of the thermal compound is to fill up air gaps that reduce heat flow. It would have to really dry up to no longer be functional.

The metal cases are electrically hot, connected to the transistor collectors, so typically need to be electrically insulated from the metal heat sink.

First rule of repairs, do no harm.

JR

I've never heard of re-greasing thermal grease.

It isn't to prevent squeaks or wear, like the grease Ford (forgot to) put in my U-joints.

PRR said:

I've never heard of re-greasing thermal grease.

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It's quite a common thing in computer maintenance.
Cleaning  CPU and Graphic card chip old and dried thermal grease and applying new one.

And yes, temperatures really go down after that, I monitored that in quite a few computers.

TheJames said:

Seeing as how this thing is pretty much 40 years old, I'm guessing that the heatsink compound is pretty much dried up and useless at this point. 

Since I don't have any compound...Does anyone have a part number or something I can get from Mouser? 

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Yes you are right, the compound should be quite dry after 40 years.

You don't need to order from Mouser, just go to any local computer store and buy thermal compound they have small syringes.
Any brand will do the job.

Thanks guys!  I appreciate the input.

My understanding of heat transfer compounds is that over time, when they dry out, their ability to effectively transfer heat is compromised causing the temperatures of the silicon to increase beyond the original design.  My background is computer systems and this is (at least was) a big deal with CPUs.

For this particular amp, it passed signal but was weak.  I swapped out the electrolytics, flushed/lubed the pots, and away it goes.  Actually a pretty decent sounding amp.

TheJames said:

My understanding of heat transfer compounds is that over time, when they dry out, their ability to effectively transfer heat is compromised causing the temperatures of the silicon to increase beyond the original design.  My background is computer systems and this is (at least was) a big deal with CPUs.

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Definitely, although CPU's will benefit much more from "precise" heat transfer than heatsinked output transistors on a guitar/bass amplifier.
If I service an old solid state amplifier and the thermal grease is dry I will clean it and apply new grease. Easy to do and the heat transfer can only improve, even if marginally.

that amp wants new grease and new mica insulators,

apply a thin coat, a thick coat will compress and lower screw pressure.

re tighten after one day of new grease,

that PV might have weird screws with a square head, use a flat screw driver to get them loose and then use standard hardware so the next guy does not suffer (might be you)

thermal pads are easy to use but we find for TO-3 hardware that they can get cut easily by burrs and sharp edges of the transistors and heat sink, and when they dry out, transistors will pop,

take a 1/2' drill bit and run it by hand over the mounting holes to remove burrs,

old mica insulators become brittle with heat and crack easy, unless they are super quality like Marantz used to use,

you can buy tubes of grease on evilbay if you on;y plan on fixing one amp,

otherwise MG Chemicals sells larger containers,

a lot of modern transistors and mosfets have plastic cases which need no insulator washers,

this saves headaches as those washers come in a million different sizes so don't lose yours,

run a flat file over the heat sink face to scan for warpage and high spots,

heat the heat sink with a blow dryer of heat gun to make the grease thin out a bit, if you apply grease to a winter time heat sink, it will be hard to spread,

as long as you have the transistors out, you may as well run 5 ma of base current and see how much collector current comes out, you need a dual pwr supply for this,

two of those eight transistors are probably drivers,

if the hfe is all over the place, then call PV for replacements, they might have old stock, matched, and with  lower freq response, you can use ON Semi, but you might have HF OSC problems due to 500 MHZ instead of 5 MHZ response, depends on the design,

While I was working there (85-00) we started tumbling the aluminum extrusion heatsinks to de-burr them (big drum filled with ceramic media to take off sharp edges). Before that there could be burrs a plenty, and it was way too easy to get cut on sharp heat sinks.

As I've already posted the mechanism for heat sink compound, is to be a better conductor than air (tiny air gaps due to irregular metal surface smoothness).  The difference between using heatsink compound or not is incremental, so the difference between fresh and old heat sink grease will be even less than that.

The lowest possible thermal resistance junction to ambient is always a good thing but do no harm and if it's a really old one don't get cut.

JR

I'm guessing this thing is from the late 70's.  No LEDs, Neon power indicator, all discreet, no opamps, no colour knobs, inductor EQ....Ancient.  There really ins't a heat-sink...The output transistors just mount directly to the back panel.  It had these real tall 63V 10,000uf can caps mounted to a chunk of thick sheet metal (along with the transformer) that is screwed to the bottom of the wooden box. 

TheJames said:

I'm guessing this thing is from the late 70's.  No LEDs, Neon power indicator, all discreet, no opamps, no colour knobs, inductor EQ....Ancient.  There really ins't a heat-sink...The output transistors just mount directly to the back panel.  It had these real tall 63V 10,000uf can caps mounted to a chunk of thick sheet metal (along with the transformer) that is screwed to the bottom of the wooden box.

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If it was an aluminum metal chassis, that was the heat sink, if steel a poor heat sink, but still a heat sink.

JR

To add to Whoops post.
I "fixed" a number of slow running PCs with a heatsink compound change and heatsink cleaning. IIRC CPUs after the P3 will lower the clock to lower the temp of the die.
I also "fixed" 100 HP AC drives that overtemped changing the compound for the power devices.

I don't expect a 70's Peavey bass amp to be as sensitive to heat sink grease as an over clocked modern CPU, but YMMV.

JR

JohnRoberts said:

I don't expect a 70's Peavey bass amp to be as sensitive to heat sink grease as an over clocked modern CPU, but YMMV.

JR

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Totally agree with you JR and I also wrote that.
But applying new thermal grease if done properly is always better than 40 years old dry grease, even if marginally better.