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.jarr...@ntlworld.com> > Sent: 17 February 2022 22:38 > To: 'Brent Hilpert' <bhilp...@shaw.ca>; r...@jarratt.me.uk; 'General > Discussion: On-Topic and Off-Topic Posts' <cctalk@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 <bhilp...@shaw.ca> > > Sent: 27 January 2022 03:33 > > To: r...@jarratt.me.uk; Rob Jarratt <robert.jarr...@ntlworld.com>; > > General > > Discussion: On-Topic and Off-Topic Posts <cctalk@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.
