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Keithley 2306 Dual Battery Simulator Repair (maybe)
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Smokey:
Note: As I said above, the main digital board does not consistently power up regardless of whether the display board is plugged in or not.  Also I have captures of the display board's serial communications in both working and not working conditions, so I'm not saying this is causing the issues with the digital board... or is it????.....

The MC34064 undervoltage sensing circuit on the display board is acting strangely..
From this plot in the datasheet...


The output is supposed to snap on around 4.5V input.  It's not exactly doing that. 
At about 4.4V input it starts to turn on the output, but it's output is only 1.5V.
4.5V in = 2.2V out.
4.6V in = 2.75V out.
4.7V in = 3.3V out
4.8V in = 4.0V out
4.9V in = 4.9V out
5.0V in = 5.0V out.

??? That isn't even close to what the plot says it should be doing.  The IC itself doesn't look damaged and it's not getting super hot or anything.

So that may explain why the display only turns on at the full 5V input....
but... at least according to the parts list, there is no voltage monitor IC on the digital board and the main digital board does not appear to rely on the display board for anything since it will power up when it's totally disconnected.....
Kean:
The MC34064 has an open collector output and so depends on a pull up resistor.
Maybe the MC34064 is damaged, or more likely something else is loading down the reset line.
Smokey:
The pullup on the "Model 2306 display board parts list"
R612 = RES, 4.75K, 1%, 100MW, THICK FILM

If the comparator in the MC34064 was acting correctly, I would expect the open collector to pull hard to ground until it hits the threshold and the output snaps on.  But its got this strange ramp that's under spec before the turn on.  Once the input gets to 4.9V it's full on like it's supposed to.  If there was excessive load, I would expect a voltage droop here too, but it's solid above 4.9V.

But even at the top range of the input specifications in the data sheet, the highest it should get before it snaps on is 4.8V.  So even if we ignore the ramp voltage, it is turning full on too high.
Kean:
So two possibilities I see
1. The MC34064 output is not changing state cleanly at 4.5V, which seems unlikely as it has a simple comparator driven output with hysteresis
2. The MC34064 output is changing state at around 4.5V, but between 4.5V and 4.9V other circuitry loads the signal more than the pull-up resistor can handle

The fact you see a change at around 4.4-4.5V makes me think the latter is the case.  You could isolate the output pin of the MC34064 and see what happens to the reset signal voltage with just the 4k75 pull-up as you vary the supply from 4V to 5V.  Or just swap out the MC34064 to eliminate that as the cause.
Smokey:
Success.  (so far.....)

I ended up doing the mod I described above, where I replaced the original LM2940CT generating the +5VD rail with a MIC29302WT adjustable regulator.



I set the MIC29302WT output to 5.10V, and the whole supply works fine now. (The original LM2940CT had an output spec of +4.85 to +5.15, so I don't see this causing any issues with the other parts of the system). 
External VFD display turns on with good brightness (The 5.1V pushes the display MC34064 voltage monitor into the region that's full on, so that problem goes away as well). 
Digital board powers up every time.  I tested the output of channel1 and it's still accurate.  It's been put back together running in it's home on the bench all morning with no issues. 

In my desire to be done with this project, I didn't run some additional experiments that I probably should have:
1) I didn't use the pot to drop the output voltage down to the 4.95V that the original LM2940CT supply was making when the unit wouldn't turn on consistently.
2) I didn't just replace the LM2940CT with another LM2940CT and see if that fixed it as well.
3) I didn't replace the MC34064  on the display board (even though I'm suspecting that the 5V reg was causing this strange issues as well somehow).

If I had to guess, I think the problem was most likely just a faulty LM2940CT (even though the output looked fine on the scope).  If anyone else has a 2302/2306/2308 that isn't powering up, I would start by just replacing the LM2940CT.
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