21 Dec
2015
21 Dec
'15
3:10 a.m.
-----Original Message-----
From: cctalk [mailto:cctalk-bounces at classiccmp.org] 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
example
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
which
is unusual.
Resistors can generally be identified with a multimeter, if you can come
up
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
to
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.
Regards
Rob