MM11-U G235 bias/strobe jumpers
jnc at mercury.lcs.mit.edu
Tue Mar 1 09:40:20 CST 2016
So, the MM11-U manual (EK-MF11U-MM-003) describes (pg. 3-12) a set of jumpers
(W5-W7) on the G235 card (X and Y selection line current generators - those
for the inhibit lines are on the G114) which adjust the bias current for the
selection line generators. It goes on to say:
"Jumpers W5-W7 are factory cut to adjust the bias current to its
optimal value and they should not be changed."
There's apparently a similar adjustment for the timing of the sense strobe
(although I can't find the description of that circuitry). So I have two
observations, based on this.
The first is that the original procedure for setting those jumpers is likely
lost, it's probably only in some internal DEC documentation. The manual says
(Section 5.4.2, "Sense Strobe Delay and Drive Current Adjustments"):
"Correction of any failure in either the sense strobe delay or drive
current circuits on the G235 module that would require reconfiguration
of the jumpers within these circuits should _not_ be attempted in the
field. Replace the faulty module with a spare G235 module and return the
faulty G235 module to the factor for repair."
These cards are old, component values may have drifted, and so perhaps these
might need to be adjusted - but we'll need to work out a procedures for doing
so, if so.
We _do_ seem to have a test to know _if_ the bias current is properly
adjusted - see Section 5.3.4, "Drive Current Checks", and also for the strobe
delay (Section 5.3.3, "Sense Strobe Delay Checks"). So I guess in theory, if
a G235 card fails one of these tests, we could change the smallest value
jumper, and see if that made things worse or better, and then loop. So
perhaps all is not lost.
The second is that I was worried that these boards were 'tuned' to be part of
a set. E.g. one of the components, in the circuit that the W5-W7 jumpers are
part of, is a thermistor on the core stack board. I couldn't tell if the
jumpers were just for dealing with component variations on the G235 board, or
if they also include variation elsewhere - i.e. that MM11-U's came as tuned
board sets which should not be 'mix and matched'.
However, that second chunk of text I quoted alleviated that concern:
apparently one _can_ replace one G235 with another, without swapping out all
the boards in the set.
Which means that the 'mixing and matching' that has happened to these boards
since they were removed from their machines (I myself am guilty of this - I
pulled a couple of MM11-U sets, and didn't carefully keep the boards in their
original sets) has probably not caused any problems.
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