Testing H745 Regulators

[Rob Jarratt]

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
various questions.

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:
https://uk.farnell.com/taiwan-semiconductor/gbpc35005w-t0/bridge-rectifier-1
ph-35a-50v-thd/dp/2677250?st=rectifier which is rated 50V and 35A?

Thanks

Rob

> -----Original Message-----
> 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
max.
> >
> > 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
> confirm this?
> 
> 
> 20V on a 10 ohm load: current = 2A.
> 15V, 1.5A.
> 
> In this regulator design there is no path for more current than that which
the
> 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
regardless.
> 


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
limit?
> Does this bench supply have an adjustable current limit?, so that you
could
> run it up starting at a low current while taking measurements. Or, does
the
> 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
at this
> 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
be nil.
> 
> Are any of the drive transistors socketed, so they could be measured out
of
> circuit? and other R measurements made without them in circuit?
> 
> Pull F1 to isolate circuitry. Still draws current?

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
transistors
> Q2::Q5.
> 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
is the
> supply for the 723 regulator IC. The reference is the internal reference
> provided by the 723, though that internal reference is powered inside the
IC
> from the +15V.