Matt Burke matt at
Sun Mar 29 04:40:44 CDT 2020

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.

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.


More information about the cctalk mailing list