is there a thermal pad that looks like PCB? HP 6177B

[coppercone2]

**turns out its hidden under a FR4 sleeve**


This HP 6177B I am restoring specifies a BN thermal pad for a TO3.

I see my unit has what looks exactly like PCB thermal pads for the TO3 transistors.

I figure while not as nice as BN, standard Alumina might be a bit better then fiberglass PCB material.

Is there some sort of special PCB look alike thermal insulator pad, or did they just use FR4 instead of boron nitride?

Since I assume it works with what looks like FR4, while not boron nitride, Alumina will be alot better of a conductor at a similar thickness (its a thick PCB). I don't want to deal with BeO.


Am I missing something? It seems kinda big deviation to go from boron nitride to what looks like a circuit board cutout. AFAIK Its not been touched.


I guess its not important, but damn thats some chainsaw killer cost cutting to replace specified boron nitride with FR4. I wonder if someone wrote a death threat on the change notice on this one  the insulator mafia?? it went from 30-50 to 0.3 watts heat conduction. shift the decimal two places over 

its thick too, like 2.5mm. Anyone else think that this is pretty savage??


Could it have something to do with stability and thermal insulation though??? but then why put the heat sink. did this unit have some kinda problem they found late in production?

the change notice says
A boron nitride insulator (HP Part No. 0340-0411) has been installed
under Q25 before mounting it On the Heat Sink.

which agrees with the parts list.


but it does not agree with the PCB insulators I see on my unit.

[SeanB]

Might be a silica reinforced pad, which is there to have high voltage stand off capability.. I do have some BeO TO3 insulators, along with some TO66 ones, which are pretty good, though they are 2mm thick they are as good as not having anything there.

[coppercone2]

a silica reinforced pad looks like a PCB? same color with fuzzy edges? (brown), it also feels exactly like a PCB. I did not dismantle it yet to check, but I bought Alumina spreaders last night

[coppercone2]

Since its in a feedback loop I don't think isolating the transistor thermally would help anything?

I am just worried they did it to quell some kind of low frequency drift or whatever... don't wanna 'improve' it worse.. but I feel like these transistors are usually pretty expenisive to replace and annoying, so I might as well give them better conditions.

[coppercone2]

ah ha I got it

There is a boron nitride base and there is just a FR4 thin sleeve that fits around it. I never saw this technique before.


SO its just undocumented HP quality. I could not wrap my head around WTF was going on.


There is no thermal grease, but that is not a big deal for this IMO


And here is a tip, if you dismantle it, expect it to break after all these years. I wanted to put grease there anyway, and one fell in half as soon as I bearly touched it, and the other was structurally sound except a chipped corner.


if you know the transistor is good and you can't live with broken parts, don't touch it. I am OK with replacing it with fresh alumina that has not been under pressure for 50 years, despite slightly lower performance. I can live with that