Nixie DMM repair update
Trouble shooting
...
...
So... making progress I think. Now have a plan of attack. Work on that dodgy oscillating comparator, and fix the non-working trigger circuitry.
Live update.
OK so I am following my plan above. After only a few minutes playing and scoping around, I noticed interesting things...
When the counter is free running, (ie one of the comparator is misbehaving, wrong state + pulsing), and I briefly ground the comparator output to get it back on track and stop the free running.... I notice that
1) No, it does not always display valid data.. it does stop free running (I check the clock output to make sure of that), however some, not all, of the Nixie tubes display garage, they display several, not all, digits at the same time. For instance right now I have tube #2 that displays '7' and ' 9' at the same time, and tube #3 that displays '4' and '8' at the same time, and the other two tubes display a solid digits, only one. We know that the Nixie boards work just fine because when swapping the comparator board for the good one, all Nixie boards work just fine. This means the bad comparator board can somehow cause some Nixie boards to display garbage, even though I am 100% sure the counter is not being clocked. The most likely suspect now, is the Reset signal, because of course if it's invalid/crap, the flip-flops / counter on all the Nixie boards, will not reset properly and therefore the counter outputs can be in an invalid state, I would think. The reset signal is generated by the comparator board, so it adds up.
2) If I wiggle the Auto / Manual trigger switch, it can make the comparator output misbehave again, therefore causing the counter to free run again !
It is very repeatable / predictable, there is no red herring here I think. So, there is interaction between the comparator and the trigger circuitry, and it's the trigger that makes the comparator misbehave, not the comparator on its own. Well, looking at the schematic, one can see that on both comparator, one of their two flip-flop output is connected, via a diode, to the Reset signal generated by the Trigger circuitry ! Hmmm....
So, al in all, it looks like :
A) There is only one problem, the trigger circuitry. The "faulty" comparator is only misbehaving because the trigger is sending it a crap reset signal.
B) Looks like the display problems, be it free running, or displaying random garbage, is all due to a crap Reset signal.
Since the reset signal is generated by the Trigger circuitry, and since the latter doesn't even work at all at large.... the plan is now to leave the "faulty" comparator alone for now, and instead concentrate 100% on getting the Trigger circuitry working, and pay special attention to the "Quality" of the reset signal it generates.
Schematic below to help you out. trigger circuitry is circled in green, bottom right. In red, the output of the trigger section that goes to the ramp generator, where it splits in two paths : upwards, it goes to trigger the ramp generator. Downward branch, it generates (by RC differentiation), a reset pulse. Marked ' Z '. Stands for 'Zero', aka the Reset signal. Signal is marked ni yellow... also in yellow where it gets to the comparator flip-flops.
This update was brought to you by Nixie Frog Inc, patent pending.