On Tue, Mar 1, 2016 at 10:40 AM, Noel Chiappa <[email protected]> wrote:
> > 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 case. -- @ BillDeg: Web: vintagecomputer.net Twitter: @billdeg <https://twitter.com/billdeg> Youtube: @billdeg <https://www.youtube.com/user/billdeg> Unauthorized Bio <http://www.vintagecomputer.net/readme.cfm>
