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Wang Series 700B Advanced Programming Calculator Refurb.

Aha! Thanks for the pointers to PCBs receiving PIG... My eyes were not able to find those. So, the PIG going to 5921 is actually enabling the main system clock (co-resident on the board with the core memory sense/drive stuff). So, that should prevent the microcode from being executed until reaching PIG. Of course, a flaky/fluctuating PIG would certainly destabilize things. The /PIG on 5928 looks to me to be a suppression signal for read/write (RDWT) and also a de-assert for GPRM (PRIME) which should effectively work the way RESET does on most modern computers - holds things in reset until the power stabilizes, then releases the processor and ensures it starts in a known state.

The whole RDWT thing is a little fuzzy, but I seem to recall that signal being an enable rather than an actual read-write pulse.

Are you able to put a 'scope on PIG and watch it?
 
I did an attempt at getting the signals on the scope (I'm very much a novice when it comes to this scope). The signals are noisy, but I think they are pretty much where they should be. As with all voltages, the high signal goes up as the voltage increases. If I understand the schematic correctly, PIG should be low, so that each time the CLOCK goes low the output on the NAND gate is high clocking the flip-flops on board 5921.

The other one, /PIG... is going to be the inverse of that because there is an inverter between the 2 so should be high, not sure how that relates to the circuit behind it, I have not really looked into that.

Anyway, PIG is indeed low and stays low even when the machine becomes unstable and inoperable. It is also always this noisy.

PIG.jpg

The other one is high, and stays high. But the peek does creep closer and closer to 5 Volts as the supply voltage increases. The waveform stays the same.

PIG-.jpg

Both signals stay roughly the same regardless of the state of the machine.

Could the ripple on either of those signals be enough to throw off the stuff that it relates to?
 
Just to be sure, I put a probe on the crystal. My thought was, maybe the crystal became marginal and it only worked at slightler lower voltage (I guess it would run of the +5).
But it seems to work fine. It runs at 8 MHz which is what the schematic says it should be. This is measured directly from a leg on the cristal.

Crystal-scope.jpg

Getting a scope from the chip that makes a proper square wave from this is a bit more tricky.
 
It might be good to look at the system clock outputs, like P0,P1,...P10, to make certain they are behaving correctly. Of course, if PIG was glitching and only causing the occasional drop-out of clock, that would be hard to detect on a 'scope.

I guess another possibility is that the wire-weave ROM shows unreliable behavior. I know from working with Andrew that those circuits can age and fall out of spec. The schematics hint at the need for some sort of tuning, as there are capacitors that are listed in the notes as being chosen to meet timing or added to noisy bits.
 
durgadas311 said:
It might be good to look at the system clock outputs, like P0,P1,...P10, to make certain they are behaving correctly. Of course, if PIG was glitching and only causing the occasional drop-out of clock, that would be hard to detect on a 'scope.
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To get to those, it would be good to make some riser boards. The crystal was just about reachable with the probe... well, I guess I could use the wires that came with the logic analyzer probe (with the little hooks). But at that point the signal will become noisy just because of the long leads.
If it's only the occasional glitch I can see it being hard to get on the scope indeed...

durgadas311 said:
I guess another possibility is that the wire-weave ROM shows unreliable behavior. I know from working with Andrew that those circuits can age and fall out of spec. The schematics hint at the need for some sort of tuning, as there are capacitors that are listed in the notes as being chosen to meet timing or added to noisy bits.
Click to expand...
Would that include the actual voltage that it gets? So, let's say some component in there drifted far enough that it's no longer happy with +5 or +12 exactly, but when it gets +4,5 and/or +11,2 it is perfectly happy?
This may sound really stupid, but that is what it what "seems" to be happening here. As long as I keep all secondairy voltages sub optimal, I can pretty much do anything (probably apart from using real tapes, as the lower voltage might not run the motor at the right speed). But as soon as those voltages go closer to their optimal values, it starts to break more and more.

You linked to that ROM replacement thing a while back, is it as simple as unhooking the connections from the bottom ROM and just connecting it to such a board with the correct ROMS and code?

EDIT: Why have I not tried loading an actual tape at reduced input voltage... I don't know. But it works! So that means that as far as I have ever used this machine, it seems to be fully operational at 185 V AC mains input.
 
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The thing about the wire-weave ROM board is the circuitry running off the CPR signal, that generates the drive pulse for the ROM wires. The waveform and width of the resulting pulse is critical for proper reading of the ROM, and is controlled by an R-C pair. On the 600 series, the capacitance is adjusted to meet the signal requirements, and on the 700 series the resistor is adjusted/chosen. The 600 schematic actually talks about achieving a specific rise time. The other side is the sense circuits for the "transformers", where the 100pF capacitors have an impact on sensitivity and also the transistors need to drive the output properly for the TTL inputs off-board. The 600 is different, but there it talks about those capacitors being added "as needed to reduce noise". This leads me to think that the wire-weave ROM maintains a delicate balance, so it seems logical that it's behavior might change with voltage. Of course, that board is buried under the chassis and not really accessible on a running system (unless you were able to splay the machine out on a bench somehow).
 
durgadas311 said:
Of course, that board is buried under the chassis and not really accessible on a running system (unless you were able to splay the machine out on a bench somehow).
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Technically... I have a workbench that is big enough to do that (although I might need a larger ESD worksurface then), but half of the bench is used by another system that is a real favorite of mine. It's nothing as special as the Wang 700, it's an upgraded Toshiba T5200 with a 19 inch CRT on top of it. And that CRT is not really easy to just "store" :). It's pretty close to the specs of the first computer we had at my parents home.
And I would really rather not go that deep if it's not strictly needed. Knowing my luck I might break more then I fix.

It also sounds that it would be really hard to diagnose what is out of spec from your explanation, it sounds like they had to "tune" this on a machine by machine basis.

Given that everything does pretty much work on a lower voltage, I would rather leave it be then and not risk other damages. That is, if the lower voltages are not harmfull to anything else in the system. So far, everything seems happy.
But I do understand that IF it's a failing component somewhere down there it will only degrade further over time and at some point it won't work at all.
 
I took a bit of break from this one as I really didn't have time or the workspace to take it apart further. I fired the Wang up this morning and I may have stumbled onto a "clue".

So, I had the machine running at around 185 V AC from the variac and it was doing fine for a while. At some point in going through it's various funtions I loaded a small program via the tape adaptor from the laptop... and it didn't work (had nothing to do with the laptop or adaptor).

When I put the machine into learning mode I could not step through the memory steps normally, it jumped around like crazy through the memory space. This was annoying as this "seemed" to be a worsening of the problem it already has.
While folding away the quickstart thingies underneath the keyboard I lifted the left corner of the machine slightly, this made the screen go dark... just a slight shimmer of 1 or 2 digits remained (the same problem it has when any key is pressed when opperating at above 190 V AC). Put the machine back flat and the screen came back and stepping through memory works again. And I could load and execute the program from the laptop aswell

So it "seems" there is some sort of bad connection somewhere... now to figure out where...
 
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