MM11-U G235 bias/strobe jumpers

william degnan billdegnan at
Tue Mar 1 10:27:50 CST 2016

On Tue, Mar 1, 2016 at 10:40 AM, Noel Chiappa <jnc at>

> 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.
>         Noel

I have found (oops) that I could swap the G235's without issue, although I
had been keeping them with the rest of the core set they came with just in
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