Testing H745 Regulators

Thu Feb 17 17:37:17 CST 2022

I used a Vishay GBPC3506W-E4/51, which is 600V 35A, 400A inrush rated. Looks like they're currently on-order at Mouser, I don't know about other suppliers.

The GBPC3506W-E4/51 is a little thinner than the bridge rectifier DEC used originally, which sometimes matters. For the 5411086 board in my PDP-11/10, height does matter, as it shares a heatsink with other parts. I stacked a few washers under the center of the bridge:

http://www.glitchwrks.com/images/dec/pdp1110_psu_repair/rectifier_spacer.jpg

Thanks,
Jonathan

------- Original Message -------

On Thursday, February 17th, 2022 at 17:57, Rob Jarratt via cctalk <cctalk at classiccmp.org> wrote:

> 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.