Unhappy HP-85

From: Tony Duell <ard_at_p850ug1.demon.co.uk>
Date: Mon Jul 12 16:08:52 2004

>
> Drat... what is wrong with me? :)
>
> The fellow is located on the PSU / Printer board - there are three
> transformers on there; it looks like the two larger ones are
> identical. It's one of the two larger ones - it seems as if the two

OK, I've got out my HP85 service manual. The 3 transformers on the
PSU/printer PCB are :

T1 (Lower of the 2 large ones), converter transformer from +12V to +5V
T2 (small onme), current sense transformer for +12V/-5V PSU
T3 (Upper of the 2 large ones), +12V/-5V SMPSU transformer, +6V PSU

>From your description we can discount T2. Amazingly the schematic gives
the winding details for the transformers, here you are (pin 1 is the top
left pin looking at the componet side of the PCB -- it has a square
solder pad, pins then number conunterclockwise like an IC) (T = number of
turns) :

T1 : 2 windings

pin 1 (start) -- 27T -- pin 2 -- 27T -- pin 3
pin 4 (start) -- 15T -- pin 5 -- 15T -- pin 6

T3 : 2 windings
pin 6 (start) -- 20.5T -- pin 3 -- 5T -- pin 2 -- 17T -- pin 1
pin 5 (start) -- 15T -- pin 4

You would have to get the wire sixe by measuring the wire on the existing
transformer, and you'll have to re-use the original cores.

> giant screw-on caps on that board were shorted... they're going to be

Those are C1 and C8. C1 is the smoothing cap for the incoming supply (26V
AC, rectified on the PSU board), so if that shorts it'll take out the
mains fuse at the back. C8 is the smoothing cap for the 12V line. While
this could damage T3 if it shourted, I am suprised the PSU didn't shut
down. I'd check the rest of those PSU components very carefully (you want
to get the HP85 service manual, although you're not 'supposed' to do
component level repeair it does contain schematics).

> replaced. Can probably rewind the transformer if I can't find
> something suitable, just am feeling a bit lazy. :)

They are HP custom. The only place you'll get them is from another HP85
(I know from experiece that the 9915 (HP85 in a metal case that could fit
into a rack mount kit) and the HP86 have totally different PSU
transformers. I assuem the 87 does too, but I've not worked on that yet
(a project for the near future, actually).

-tony
Received on Mon Jul 12 2004 - 16:08:52 BST