VAX 4000-500 PSU Overload?

Brent Hilpert hilpert at
Mon Dec 21 04:11:31 CST 2015

On 2015-Dec-21, at 1:14 AM, Robert Jarratt wrote:
>> -----Original Message-----
>> From: cctalk [mailto:cctalk-bounces at] On Behalf Of Holm
> Tiffe
>> Sent: 21 December 2015 08:51
>> To: General Discussion: On-Topic and Off-Topic Posts
>> Subject: Re: VAX 4000-500 PSU Overload?
>> Robert Jarratt wrote:
>> Just resolder the SMD Caps, they sometimes get a loose pad.
>> You see if that happened while resoldering.
> Shouldn't I be able to detect a loose component by pushing on it with a
> small screwdriver or something? I suspect the caps are the darker coloured
> (orange) ones, but I am not sure, is that right?

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.

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