VAX 4000-500 PSU Overload?

Robert Jarratt robert.jarratt at
Mon Dec 21 05:10:50 CST 2015

> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at] On Behalf Of Brent
> Hilpert
> Sent: 21 December 2015 10:12
> To: General Discussion: On-Topic and Off-Topic Posts
> Subject: Re: VAX 4000-500 PSU Overload?
> The lighter-beige ones, some of which are wider, are likely caps, for
> the one beside the 34072 dual op amp looks to be a simple power decoupling
> cap (across  the IC power pins).
> The reddish-brown units with 4-digit numbers are resistors: 3 sig figs &
> zeroes multiplier, e.g. 1002 = 10,000 ohms, 1330 = 133 ohms, 1181 = 1180
> ohms.
> The unmarked(?) 'redder' ones I expect are resistors, for example the one
> above the "di" in "digital" looks like the feedback gain control resistor
for the
> op amp, although in other places that would make 4 resistors in parallel
> is unusual.
> Resistors can generally be identified with a multimeter, if you can come
> with the same value when swapping the multimeter leads (polarity) and
> stepping through the multimeter ohm ranges you've probably got the
> resistance - making the multiple measurements to identify non-linear
> responses to eliminate conductivity readings from semiconductor junctions.
> There looks to be two very similar networks around the two op amps in the
> 34072. It wouldn't really be that difficult a board to RE, although it
does look
> like some poking around with a multimeter for continuity will be necessary
> track internal traces, but they look to be relatively limited in number.

Many thanks. My first measurements with a multimeter had suggested to me
that the darker ones were the caps. Thanks for putting me right before
getting too far with the reverse engineering. The multi-layers mean I won't
necessarily know where there is a connection, I can deduce that there is one
where a through hole appears to go nowhere, but who knows what other
connections I might miss.

Any idea what an A4H and a 2FH are? They must be some kind of transistor,
but I am not sure what kind.



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