Testing H745 Regulators

[Rob Jarratt]

Hmmm this would suggest 200V 20A
https://archive.org/stream/bitsavers_decfieldSeeTechnicalManual1972_19126909
8/DEC_Field_Service_Technical_Manual_1972_djvu.txt rather than 35A. Given
the 20-30VAC input though, I presume a 50V part would be OK still?

> -----Original Message-----
> From: Rob Jarratt <robert.jarratt at ntlworld.com>
> Sent: 17 February 2022 22:38
> To: 'Brent Hilpert' <bhilpert at shaw.ca>; rob at jarratt.me.uk; 'General
> Discussion: On-Topic and Off-Topic Posts' <cctalk at classiccmp.org>
> Subject: RE: Testing H745 Regulators
> 
> 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.