17 Feb
2022
17 Feb
'22
2:57 p.m.
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?
the
That's interesting. On the H744s I have observed that if I have a high load
the
-----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 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.
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.
couldn't find
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
anything hot.
all the
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 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.