Fisher 400-CX preamp tranny runs a bit hot?

gkargreen

Well-Known Member
Hi all, working on this Fisher that had been overhauled by ? and the Fisher doctor, what a rat's nest of parts all over the place, over-built power supply, super-sized Mills (?) resistors (big green body resistors) and dripping with Deoxit from every tube socket! While it does seem to work OK and sounds great, the power tranny seems to run hot on this bad boy, not sure if that is normal or not. I can touch it and have run it for hours, but I have never had any preamp where the tranny ran hot, usually the opposite. Is it SOP for the power trannies in the 400-CX preamps to be hot to th touch? Not burning hot, but definitely beyond "warm". Thanks!
 
The Human body is a poor thermomter for the taking of temperatures. Anything over 110*F is considered "HOT" to Human skin and the brain. @ 140*F you can get a blister from the contact. I woud suggest if your meter has a thermocouple for temps, tape it to the transformer and run it for an hour. Anything up to 150*F or so is good, as Max Temp for the transformers is about 190*F at which point most start breaking down or at worst short out. Or borrow an infrared laser thermometer.

If you can hold your palm on the top of the transformer for a count of 5 comfortably it's probably 110*F or a little less. Getting uncomfortable? 120*F or Less. Can't Hold it for a countof 3? about 130*F.
 
They do run hot. Even with appropriate adjustments for today's higher line voltage, they will reach 140˚F at the hottest points. The earlier CX transformers were apparently a weak point, so the transformer was subsequently improved with the later CX2 units and is generally quite reliable -- but they do run hot none the less.

Dave
 
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