21 Dec
2015
21 Dec
'15
2:33 p.m.
On 2015-Dec-21, at 3:10 AM, Robert Jarratt wrote:
SMD markings and packagings are ambiguous, unclear, and all over the spectrum.
Here's a good site for chasing SMD markings:
http://www.s-manuals.com/smd
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.
http://www.s-manuals.com/smd/a4
A good guess for the "2FH" devices are 2F-code PNP transistors:
http://www.s-manuals.com/smd/2f
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.
-----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.