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Falcon PSU 12V line, what to expect?

Posted: Sun Jan 12, 2020 11:17 am
by pixelpusher
Hi folks,

in the journey to resurrect my Falcon, I thought it would be good to check the PSU before recapping (and replacing the rectifier). Under load (4.7R on 5V, 22R on 12V) the 5V line was ok (4.96V), but the 12V line was just 10.27V ???

I stumbled over this thread viewtopic.php?f=9&t=25, and it seems to indicate that this in "line of expectations" for this PSU? (If so: Is there nothing using the 12 line that needs be within a (+/-) 5-10% window of the nominal 12V, like you would expect it on a modern motherboard?)

Re: Falcon PSU 12V line, what to expect?

Posted: Sun Jan 12, 2020 11:30 am
by czietz
I wonder if you're simply overloading the 12V rail. The Falcon service manual mentions 0.2 A max. current.

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Re: Falcon PSU 12V line, what to expect?

Posted: Sun Jan 12, 2020 11:37 am
by exxos
100R on the 12V would be plenty.

Also (never looked on the falcon psu) but its probably like all the other ST PSU's in that the 12V isn't regulated. The 5V is, and 12V is just whatever turns ratio it ended up on the transformer. Some ST PSU's even had diodes on the output to drop the 12V as it was normally 13V otherwise, then the diode vanished and 12V ended up more like 10V and the diode vanished.

Re: Falcon PSU 12V line, what to expect?

Posted: Sun Jan 12, 2020 12:16 pm
by pixelpusher
czietz wrote:
Sun Jan 12, 2020 11:30 am
I wonder if you're simply overloading the 12V rail. The Falcon service manual mentions 0.2 A max. current.
Blimey, you're right! Not great, but inside of the 10% window now ...