IBM Engineers

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Fri Apr 9 15:42:05 2004

> > Have yuou checked _all_ the electrolytics, even the low-value ones? On=20
> > both sides of the isolation barrier.
> I've replaced all the electrolytics on the LV side that I could see. I've
> just noticed a 1uF 50V that I missed - I don't think I've got a 1uF 50V 1=
> 05
> deg C in stock though.

I think any 1uF 50V (or higher) would do to prove the point :-).

>
> > Have you looked for a current sense=20
> > resistor that's gone high?
> There is some visible heat damage around some fusible resistors, but they

Sense resistors are not normally fusible (you don't want them going
open-circuit, all hell breaks loose when they do and the output current
tries to flow through the current sense transistor's BE junction).

> checked out fine. There are two diodes in the vicinity of the FRs that I
> haven't checked yet. I really need to get this thing back on the bench fo=
> r
> testing.
>
> > This may mean they didn't actually design it, but bought it in from=20
> > somebody like Astec who didn't supply schematics.
> Looks like that's what happened - the PSU bears the logo of a company cal=
> led
> Phihong.

Right. Never heard of them... Of course some companies then went and
reverse-engineereed the unit to put the info in their service manuals.
The Tandy M3 and M4 technical manuals contain what appear to be
reverse-engineered schematics of the Astec PSU boards. Well, they claim
to be PSU schematics, but actually could never have worked as drawn. I
speak from experience, having produced a more accurate (I hope)
reverse-engineered schematic.

> What really annoys me is that they went to the trouble of picking a plast=
> ic
> that would shatter when you removed the case screws. Needless to say, the

Actually, I object to self-stripping [1] screws going into plastic anyway.

[1] A much more accurate description than self-tapping :-)

-tony
Received on Fri Apr 09 2004 - 15:42:05 BST