27 Nov
2015
27 Nov
'15
9 a.m.
On Nov 27, 2015 8:04 AM, "tony duell" <ard at p850ug1.demon.co.uk> wrote:
>
> Hi folks,
>
> I've begun repairing my PDP 11/40 and have tracked down the first major
issue to the H745 (-15v) regulator.
Before you dive into the brick (sounds painful!) check that the +15V DC
input is
present and correct. Unlike
the +5V units (H744, etc) the H745 needs both the 20V
AC from the
transformer and a 15V supply which
comes from either the PCB on top of the transformer
(in an H742 supply
used with the BA11-F) or the PCB
under the transformer in the supply used with the
BA11-K
If that's OK, then it's the usual suspects in the brick. The capacitors
can
dry up. There is a fuse, but it
never fails for no good reason, so if it is blown you
need to find out
why. Power semiconductors on
the heatsink bracket.
Whatever you do do not disable the overcurrent trip (e.g. by desoldering
the
collector of the sense
transistor). If you do, and the crowbar fires then you
will be cleaning
bits of transistor off the
ceiling. Do not ask how I found that out!
-tony
There is a pdp 11 troubleshooting guide on Bitsavers that covers the h742
powersupply and regulators 744. 745, 754. Assuming that is what you have.
They're all tough to work on especially the regulators but I agree with
Tony that you need to lock down the main suppl first.
That troubleshooting guide very useful just be prepared to completely
disassemble the parts.
Bill Degnan
twitter: billdeg
vintagecomputer.net