PDP-8/M H740 PSU woes - test-drb@ccmp.vtda.org

2 Jul 2009

--- On Thu, 7/2/09, Tobias Russell <toby at coreware.co.uk> wrote:
...
  I've cleaned the PSU up and reformed its
capacitors,
 checked the
 transformer is working fine. On replacing the switch mode
 circuit board 
This looks like a linear PSU to me.
...
  and powering up, I'm getting a blown fuse (F1 on
circuit
 diagram) which
 supplies the 5V and 15V rails. The -15V rail is working
 fine. 
...
  Does anyway have any suggestions as to the best
approach to
 locating the
 problem? I've been pondering whether it would be possible
 to isolate
 either the 5V or 15V circuit so I can see which of them is
 causing the
 problem, but I can't see any simple way to achieve this. 
Based on my limited experience:
Start by checking the bridge rectifier D14.
The +5V line has overvoltage protection via Q11 and D12. It may be crowbarring the 5V line
causing F1 to blow.
Did you verify the condition of the big cap C7 with an ohmmeter ? Make sure it's
discharged and then measure the resistance across it. It should start at zero and climb
slowly and steadily upwards.
I'd also look around Q6 for a collector-emitter short, and Q7 for a short.
...

 Also, is my practice of powering the PSU up without any
 load a foolish
 one? I've not had any problems in the past using this
 approach but I
 have seen people previously mention that running switch
 modes without a
 load is a bad idea. 
A dummy load may be useful as you get further into testing; you'll want to be sure
that the outputs are functioning correctly under load.
Regards,
-Dave