Rob Jarratt robert.jarratt at
Sun Mar 29 05:41:43 CDT 2020

> -----Original Message-----
> From: cctalk <cctalk-bounces at> On Behalf Of Matt Burke via
> cctalk
> Sent: 29 March 2020 10:41
> To: cctalk at
> Subject: Re: VAXmate PSU
> On 29/03/2020 08:50, Mattis Lind via cctalk wrote:
> > Hello Rob,
> >
> > söndag 29 mars 2020 skrev Rob Jarratt <robert.jarratt at>:
> >
> >> Thanks Mattis. I do test it with a load, although to be honest I
> >> forgot to do this when I took the measurements yesterday. I use a
> >> modern-ish IDE disk and a load board from a MicroVAX 2000 as the
> >> dummy load. I don’t know if that is sufficient.
> >>
> >>
> > Do you know the nominal output rating for the supply?
> > It might be the case that even with load board the load is uneven. But
> > it sounds less likely.
> The output specifications for the H7270 power supply are on page 9-13 of the
> VAXmate Technical Description (EK-PC500-TD-001).
> +5.1V - Min 6.40A, Max 10.24A
> +12.1 - Min 0.17A, Max 1.37A
> -12.0 - Min 0.12A, Max 0.33A
> +28.0 - Min 0.45A, Max 0.55A
> -9.0 - Min 0.18A, Max 0.20A
> 300 - Min 0.00A, Max 0.46A (for the expansion box)
> I have one of those load boards from a MicroVAX 2000 but the 5V load was not
> connected by default. I had to install some zero ohm resistors. The board I have
> draws 1A at 12V and 3A at 5V so you will need more load on the 5V rail at
> least.

Thanks Matt, I just found that bit in the documentation and realised I am not generating enough load. I will have to add more. I did connect the 5V load because as you say it wasn't connected.

> As Mattis has mentioned there looks to be a crowbar circuit on the secondary
> side (Q2) to provide overvoltage protection. The 5V and 12V output are shorted
> by Q2, which causes the primary side to sense an overcurrent condition and
> shut the supply down.

I can't test it now until I get another fuse, but I am sure I didn't see this happen, although it seems to be quite hard to tell.

> Matt

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