30 Dec
2021
30 Dec
'21
1:08 p.m.
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
<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?
Thanks
Rob
30 Dec
30 Dec
4:11 p.m.
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.
/Mattis
Thanks
Rob
4:32 p.m.
Sounds like you're testing things basically the way I was:
http://www.glitchwrks.com/2019/06/05/pdp1110-psu-repair
I used a large Stancor 24 VAC transformer to supply power to the modules, since they
usually see 20-30 VAC from the big transformer in the chassis.
For dummy loads, I like automotive tail lamp bulbs. Series/parallel as necessary to get
the load you want. Do keep in mind they get hot enough at rated voltage to melt soft
plastics.
I replaced them with a LED and a resistor.
Many of the later modules do that anyway! I used incandescent lamps because I had them on
hand. Wouldn't you know, after reassembling and a few hours of bench running in the
PDP-11/10, one of the old ones I hadn't replaced burned out too :P
Thanks,
Jonathan
31 Dec
31 Dec
1:09 p.m.
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?
Regards
Rob
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
<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.
/Mattis
Thanks
Rob
30 Dec
30 Dec
4:55 p.m.
On 12/30/2021 10:08 AM, Rob Jarratt via cctalk wrote:
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?
For the OL-6003BP, the CM7370 will work nicely. The CM7003 is closer to
the original specs (before de-rating for voltage), but doesn't last as long.
See the PDP-8/I section of:
https://so-much-stuff.com/pdp8/repair/bulbs.php
Vince
934
days inactive
935
days old
4 comments
4 participants
participants (4)
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lists＠glitchwrks.com -
mattislind＠gmail.com -
robert.jarratt＠ntlworld.com -
vincent.slyngstad＠gmail.com