PDP-8/M H740 PSU woes

[David Comley]

--- 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