VAX 4000-500 PSU Overload?

Brent Hilpert hilpert at
Mon Dec 21 16:33:26 CST 2015

On 2015-Dec-21, at 3:10 AM, Robert Jarratt wrote:
>> -----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
> 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.

SMD markings and packagings are ambiguous, unclear, and all over the spectrum.
Here's a good site for chasing SMD markings:

REing boards like this is an iterative process with consideration of the device markings, pin use, measurements, what makes sense electronically, and design period.

In the two cases you mention it looks like the H's are a slightly different font or size and spaced slightly differently, I suspect the id codes there are A4 and 2F.

An initial guess for the "A4H" device that's right above the LM339 is an A4-code dual diode with only one diode used, as it looks (from the photo) like one pin is unconnected.

A good guess for the "2FH" devices are 2F-code PNP transistors:

Similarly, the "1PG" or  "1PC" device may be a 1P-code NPN transistor.

Diodes and transistors are best double-checked by first characterising your multimeter with PN junctions in known transistors or diodes so you know what ohms-range and what reading to expect on your MM when you have a PN junction in forward-conduction. Look for the lowest ohms range that will give you a reading for forward conduction. Then use that knowledge to check the SMD transistors and diodes for sensibility.

Some strategy:

	- Sort out all the network connections for the board, by visual observation and continuity measurement.

	- Identify the power supply pins to the board by chasing the power supply pins of the IC packages.
	  This will include or begin with identifying the ground connection(s) to the board in part by chasing the
	  ground from the main unit to the pins which this board plugs into.
	  Is an internal layer on the board a ground plane?

	-  From what I can discern it may be that the two ICs are supplied by +/- supplies, that is, the V- pin for the
	  ICs may go to a negative supply rather than ground, so there may be ground(s), V- and V+ connections
	  to the board, rather than just ground and V+.

	- I'd suggest drawing the two op amps one above the other with a positive bus horizontally at the top of the page.
	  If there is a negative supply draw a negative bus horizontally at the bottom of the page.
	  Draw in the networks around the op amps, it looks like they are similar and feed into the 3 terminal device located to the upper-right of the LM339,
	  and thence into one of the comparators. That should take care of a lot of the board, then do the remaining 3 comparators.

	- within sensibility, try to draw everything so electron flow is 'up' the page, this means for example that PNP transistors
	 will be drawn upside down, with the emitter at top-right, while NPNs will be conventional with emitter at lower-right.
	 Everything should sit vertically in between the negative bus below and positive bus above or between grounds towards the bottom and the positive bus above.

	- within sensibility, try to keep signal flow left-to-right with only feedback paths going 'backwards' right-to-left.

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