Testing a H744 Regulator on The Bench

Rob Jarratt robert.jarratt at ntlworld.com
Fri Dec 31 12:09:29 CST 2021

Regarding the ripple, can you see the spikes at the bottom of the troughs? They are a bit faint but they are there. Are they normal?






From: Mattis Lind <mattislind at gmail.com> 
Sent: 30 December 2021 21:12
To: rob at jarratt.me.uk; Rob Jarratt <robert.jarratt at ntlworld.com>; General Discussion: On-Topic and Off-Topic Posts <cctalk at classiccmp.org>
Subject: Re: Testing a H744 Regulator on The Bench




Den tors 30 dec. 2021 kl 19:08 skrev Rob Jarratt via cctalk <cctalk at classiccmp.org <mailto:cctalk at classiccmp.org> >:

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.


I think I have been through at least 10 H744 or similar bricks. Helped Update Computer club to repair a bunch of them. Most common fault was the rectifier bridge. One had blown in the most spectacular way. The metalhousing was gone completely. One connector element in the AC in had fused off.


I used 30VAC input and a 0.4 ohm load resistor when testing. I have a 0.2 ohm but my variac is not able to supply the current needed for full load. 

I checked if anything became hot apart from the load resistor.


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?

That ripple is normal for H744. It is actually designed to have this type of ripple since the LM 723 regulator is connected with positive feedback creating a sort of schmitt trigger. It will turn on the switch transistor when the voltage after the LC network is under a certain limit and then turn off when it is over another limit, creating this charge/ discharge ripple seen in the trace.


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?


I replaced them with a LED and a resistor. 





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