VAXmate PSU Failure

[Rob Jarratt]

A little while back I posted here because I needed help with analysing the
failure of the PSU from my VAXmate. Since then I have had some comments on
the reverse engineered schematic which I have now improved and which is
here:
https://rjarratt.files.wordpress.com/2020/02/h7270-control-pulse-width-modul
ator.png

 

Following advice from a list member, I have been looking at the 555 and the
PWM that control the switching transistor, using a bench power supply to
power the PWM directly (across pins 5 and 7). When I do this, I find that
Vref from the PWM, which should be 5V to power the 555, does not reach 5V. I
have tried removing the 555 from the circuit, and when I do this Vref goes
to the nominal 5V, once Vcc to the PWM is above 16V. However, if I put the
555 back in, then Vref only goes to 2V. I have socketed the 555 and tried
with two other brand new 555 chips, the result is the same.

 

It seems that just adding a 555 kills the Vref output of the PWM. It can't
be anything else in the circuit because everything else is still in circuit
when the 555 has been removed. Could the PWM be faulty? Perhaps it can't
provide enough current to bring Vref up to 5V once the 555 is in the
circuit?

 

Any suggestions gratefully received.

 

Regards

 

Rob