Testing a H744 Regulator on The Bench

Rob Jarratt robert.jarratt at ntlworld.com
Thu Dec 30 12:08:19 CST 2021

Hello All,


Following a discussion on resurrecting a PDP 11/45 I have started looking at
the regulator bricks in the PSU. I have started with a H744. I reformed the
three big capacitors, although they seemed to be fine as they charged up
from my DC bench PSU and then had zero leakage so I didn't reform them for
very long. I then moved on to powering the whole unit from my DC bench PSU
and it seemed to run fine. I am using a panel mount 1R resistor, so drawing
5A of the rated 25A. It drew just under 2A from the PSU which was set at
20V. I didn't run it for very long as I wasn't sure how hot to allow the
load to get.


Is there any sense in measuring ripple when powering this from DC? There
does seem to be some ripple:
https://rjarratt.files.wordpress.com/2021/12/h744-ripple.png hard to say if
that is OK?


Also, the indicator light does not work (as expected). I have seen the
suggestion to replace it with a CM7381. It looks like these can be sourced
with relative ease, but I wondered if there might be a better LED-based
longer term solution? I was also advised that OL-6003BP is good for the 15V
regulators, but these seem to be hard to find, any suggestions there?





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