Testing a H744 Regulator on The Bench

Mattis Lind mattislind at gmail.com
Thu Dec 30 15:11:42 CST 2021

Den tors 30 dec. 2021 kl 19:08 skrev Rob Jarratt via cctalk <
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
> <https://www.mouser.co.uk/ProductDetail/?qs=N8%2Bk0S5zvCZ7dMEMH2XU9Q%3D%3D
> >
> 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.


> Thanks
> Rob

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