Temperatures Nakamichi PA-7

[CptFuture]

Hi all
I've just measured some temperatures (celsius) on my poweramp Nakamichi PA7 (mark 1):
After one hour in idle state, the temperatures on the two large heatsinks are between 40 and 45 degrees. I think that should be ok.
After that, I opened the casing and measured the temperatures of the 4 transistors Q110, Q111, Q210, Q211 (they have their "own" little heatsinks (L-shaped).
These temperatures were about at 80 degrees! I'm afraid that this is too high. Or is this ok? Can I lower these temperatures with another bias-setting?
Thanks a lot and regards from Switzerland

Andy

 

[Nelson Pass]

I don't have a schematic, but I assume you are talking about TO-220 style
devices on little sinks.

Probably they are OK. What is most important is that the chip temperature
doesn't approach the 150 deg C. rating, and it is likely with that setup
that the chip is only a few degrees above the 80.

 

[Old'n'Cranky]

PA7 and PA7E

Resized from much larger tiff images.

 

[CptFuture]

@Nelson Pass: Yes, I'm talking about the TO-220 style devices, see the schematics in the post above from Old'n'Cranky.
Please, can you tell me in some sentences, what is the "task" of these 4 (2 per channel) devices?
@Old'n'Cranky: Thanks you very much for the schematics!

I wonder why Nakamichi didn't put these transistors also on the large heatsinks..

 

[Zen Mod]

if you're asking for Q110 and Q111 , they're emiter followers and part of biasing stage

VR101(201) is for output offset , VR102 (202) is for Iq

random picture from net - I see heatsinks , dunno is that later addendum by owner or factory

in any case , there is a place for some heatsinking

 

[CptFuture]

Thanks, Zen Mod. I understand now why they are getting hot (high bias)
Well, there are tiny heatsinks as in your picture, but inside a casing without any "holes" for airflow, this makes no sense. I'm thinking of modifying the casing for better airflow...

 

[Zen Mod]

if nothing else , put 5-8mm spacers between bottom plate and heatsinks , same for top plate

of course , you'll need longer screws , for same amount

for test , you can use , say , 8mm nuts as spacers ...... later find some nice ones

 

[CptFuture]

Zen Mod, yes, I had exactly the same idea. I will use the following (see picture) for the
top plate. Not sure with the bottom plate, if it is enough stable with these. I will see.

 

[Zen Mod]

well , for air you need inlet , same as outlet

so , you need to do same thing with both plates

I believe bottom one is thick enough , while screws will do fine , even with much greater pressure force

though , greater OD of spacers will serve better than these extending screws