Broken H724 power supply from PDP-8/e

Pete Turnbull pete at
Thu Jan 8 09:38:34 CST 2015

On 08/01/2015 13:05, Michael Thompson wrote:

> Thanks Al and Marco.
> I saw Q207 when I inspected the power supply and wondered what that giant
> diode was for. I also saw it in the schematic and was surprised to see that
> the diode installed backwards. Now I see that it is an SCR, not a diode.
> R29 is easy to get to, so I will try raising the crowbar voltage a just
> little. It is also possible that the regulation of the +5V is not working
> correctly and the crowbar should be firing.

I had a similar problem when I converted a 110V version to 230V and 
tried to adjust the 5V regulator according to the PDP-8/E Maintenance 
Manual.  Beware, for the later (ie, most common) version of the PSU the 
manual is wrong where it describes the location of the pots to adjust 
the voltages; it describes an early version of the regulators.  As a 
result I adjusted the crowbar instead of the +5V and it took me a while 
to work out how to set the crowbar correctly as the manual only says 
it's a factory adjustment.


Pete Turnbull

More information about the cctalk mailing list