VAXmate PSU Failure

[Jon Elson]

On 02/15/2020 12:17 PM, Rob Jarratt via cctalk wrote:
> 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
>
>   
>
>


The UC3842 datasheet does show you can draw
up to 20 mA from the Vref pin, which should certainly power 
up a 555.  But, this is a 5 V regulator,
and may not be working properly.  it may be that driving 
several mA to the 555, E2 and other
components has cooked it over time.  With the 555 removed, 
apply resistors between pins 1 and 8 of
the 555, and see how much load the UC3842 can supply.  I'm 
guessing that at 1 - 2 mA the
Vref output is going to collapse, indicating the UC3842 is 
"weak". If the Vref does droop, make sure
the Vcc is still well above 5 V.  If so, it has to be the 
UC3842. If the Vcc is also drooping, then
follow the chain back to the power input to find the 
defective component.  It could be Q1, R27 R28,
R32 or for some reason the crowbar D19 is being fired.

Jon