Sorry it has been a while responding. I have been looking further at my H745
regulators this evening. Below is what I have found and my responses to the
In summary, it looks like the rectifier may be faulty. It is marked NSS3514.
I believe it is a 35A part. Can anyone suggest a suitable replacement? Maybe
something like this:
ph-35a-50v-thd/dp/2677250?st=rectifier which is rated 50V and 35A?
From: Brent Hilpert <bhilpert at shaw.ca>
Sent: 27 January 2022 03:33
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 H745 Regulators
On 2022-Jan-26, at 3:41 PM, Rob Jarratt via cctalk wrote:
> I am trying to test a couple of H745 regulators with a DC bench PSU
> and I am having some problems with testing them.
> My bench PSU is a twin unit so I can supply the +15V required as well
> as the "AC" input using 20VDC from the other half of the bench PSU.
> The problem is that I don't think the bench PSU can supply enough
> startup current to allow the regulator to run. It can only supply 5A
I have seen with the H744s that if I put too big a load on them, then
they can't start because of the heavy startup current required. I can
start them with a lower load and then add load once the regulator is
running without breaching the current limit of the PSU.
With the H745s I have tried reducing the load to see if I can get them
to start, but a 10R load appears to be too much and the regulators
draw the full 5A without outputting -15V.
I have two H745s, both exhibit the same behaviour. I suppose they
could both have the same fault, but I am inclined to think that
perhaps they need a higher startup current than I can supply. Can anyone
20V on a 10 ohm load: current = 2A.
In this regulator design there is no path for more current than that which
load draws, aside from temporary peak currents to
charge capacitors. If
you're drawing 5A DC from the bench supply, something beyond 'failure to
start' is wrong. I would expect this supply to operate at small load
That's interesting. On the H744s I have observed that if I have a high load
the bench PSU current limiter operates and the regulator cannot output +5V,
but if I start with a lower load and then add load, it can continue to
operate. Is the H745 different to the point that I shouldn't expect this
kind of behaviour? If it is the same, then why do the H744s do this? I have
tried waiting a few moments to allow the input capacitor to charge up, but
the regulator just does not start.
What is happening to the bench supply voltage? Does it
go into current
Does this bench supply have an adjustable current
limit?, so that you
run it up starting at a low current while taking
measurements. Or, does
> current respond with some linearity to varying the input voltage?
> What happens with no load R?
Yes, it hits the limit, and it does so even if I have no load at all.
> Are you running it for any length of time
at 5A? (Sounds like a bad idea
point) Anything getting warm?
I daren't run it for long. I just tried running it for 15 seconds and
couldn't find anything hot.
> Is the 723 socketed? Pull it and run it up
while watching what happens
> around the drive transistors and elsewhere.
> If the 723 is not socketed, consider pulling Q5 or opening it's emitter
> connection. With no drive to the drive transistors, input current should
> Are any of the drive transistors socketed,
so they could be measured out
> circuit? and other R measurements made without them in circuit?
> Pull F1 to isolate circuitry. Still draws
Well, that is interesting. I pulled the fuse and it does indeed pull in all
the current. According to the schematic, the only thing it could be is C1 or
R1. I tested C1 out of circuit not too long ago and that seemed fine. I took
it out again to be sure and C1 seems fine. R1 also seems to be fine. Which
leads me to suspect the rectifier may be bad, although some in circuit tests
seem to show it is OK. So, to be clear, I tested with the fuse out, with the
rectifier feeding C1, with R1 (3.9K) across C1. The bench PSU hits 5A when
trying to supply 20VDC to the rectifier. As the fuse was removed there
couldn't be anything else that is shorted. The capacitor does not charge up.
This does suggest the rectifier is the problem.
> Have you looked for shorts/leaks?,
especially leaky junctions in
> e.g. R measurements, no F1, no load R, both directions:
> Q2.B-C ?
> Q2.E-GND ?
> Q2.C-GND ?
> -15-GND ?
> Settling time for cap charge/discharge may be needed.
> In answer to your earlier question, no,
the +15V is not the reference, it
supply for the 723 regulator IC. The reference is the
provided by the 723, though that internal reference is powered inside the
from the +15V.