On Sun, 18 Nov 2012 17:54:36 -0800, mike wrote:

On 11/18/2012 4:15 PM, Jeff Liebermann wrote:
On Sun, 18 Nov 2012 16:01:38 -0800, Jeff
wrote:
(...)

Also, you may need a TM502a mainframe schematic if you suspect the
power supply to be at fault.
http://www.ebay.com/itm/390216672603
$7.50

I don't understand why people do stuff the hard way...
http://bama.edebris.com/manuals/tek/tm503
FREE

Ummm.... because I was looking for a TM502a, not a TM503. I didn't
think of searching BAMA (oops) for variations and mutations. Thanks.

Looking at the schematic, I see two overtemp thermostatic switches (S2
and S3) which might be part of the problem. There are two types,
thermal fuses and thermal switches. The parts list description an a
barely readable Sensata catalog page says switch, but I think it's
really a fuse. It's a stretch, but a bordeline thermal switch
activation might fit the symptom.

Also, as you previously mentioned, the power transistors might gone
intermittent where they're soldered to the backplane. The 4500uf 40v
caps might have dried out and/or need reforming, which would show up
as high leakage current. That could be tripping the thermal switches
or blow the fuse until the caps have warmed up somewhat.
http://www.qsl.net/g3oou/reform.html (simple)
http://www.nmr.mgh.harvard.edu/~reese/electrolytics/ (detailed)


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558