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Four-Phase Systems IV/90



That took way longer than I wanted it to but the cables are done and the ends are properly marked so we won't go through that confusing mess again.
Now I have to figure out how the weird vinyl jacket it was in goes back on. It's got like a zipper on it but it's almost impossible to handle when it's at room temperature.
I have been unable to zip the sleeve back together. It's too stiff. I might try this weekend to try powering up the IV/70 attached to the IV/90 and see if we get ANY signs of life out of the front panel.
Well it didn't blow up...



...but it didn't wake up either. Other than most of the indicator lights going out when the IV/90 was switched on the controls remained completely unresposive. No the group of lamps at the end are not stuck from a bad console ASIC as I can move them around and the problem remains on the last few bits.
I mean it was kinda expected since we are still missing parts on some of the boards but no arrangement of boards that I found could get me ANY other signs of life and in fact, too few boards and I would lose the only sign of life out of the console. The NP80 at this point was more responsive.
That damn resistor pack on the DT board also started getting hot again, so something's shorting somewhere and I can't see it.
I'm going to leave this thread open-ended but I'm out of documentation and I'm out of ideas beyond (and I know this is something I should of done already but this is going to be a ball-buster) find an open space, pull, clean and reseat every chip in the system which I would not be looking forward to since right now I can't tell what the pinout of the bus is. Trying to even get the CPU alone to show life is requiring schematics I don't have and technical knowledge I don't have available. I'm basically looking at indicators and LED's that I have nearly no information about.
MITA sent me another set of diagrams for the DT board (would of been nice if that included schematics or a parts listing but this was still extremely helpful) and I learned that all of our cooking components are connecting on the bus on pins 9 through 16.


These are part of whatever the MMA signalling is. I was suspecting that these lines are being shorted to ground, but it's weird that nothing else using those pins was not complaining just as hard and I didn't see anything else causing the failure. I pulled the logic probe out and began to poke around with and without the IV/90 cabinet powered on and found everything was dead in state. I brought out a scope and again found the majority of the bus was doing absolutely nothing other than going low once the expansion cabinet was turned on. I checked for the four clock phases which are accessible from four headers at the top of the memory boards (and on the backplane) and phase 1 and 3 were missing, phase 4 would pull itself down to a negative voltage and do nothing and phase 2 was generating something that looked more akin to a shark fin. So yeah, missing clock phases would totally explain why everything is completely unresponsive.
Checked to see if it was the front panel in MANUAL and it was expecting single-stepping an no.. Checked to see if the clock was being generated by the IV/90 and no the waveforms were the same even with the expansion turned off.
I started pulling boards until the phase I DID have went away. Finally I was able to conclude that if the DT board was pulled I'd lose it, but if I instead pulled the IV 70 Driver Receiver Board that too would take the clock phase away. Furthermore the resistor pack and transistors stopped overheating and the last six bits of the front panel would get stuck all-on, so this was a clue. I didn't know what it meant, but this was progress.


It is the least complicated board in the system but I'm not sure what it's doing. We can clearly see pins 9 through 16 here as they run into a 7414, a 72365 and an N8242. I still do not know what this board is doing and it doesn't actually use phases 1 or 3 but it does use the other two phases we saw doing *something*. Nothing that I can tell on this board is hard shorted and being a 5v-only board I can power it on the bench. I do however see a few warmer-than-normal IC's, such as that one 7414. I wish my Logic Comparator didn't suck.
I thought about this overnight and decided the best action was to look again. The thermal camera was telling me this 7414N was getting warmer than the other one on the board so I might as well poke that with a logic probe as it was simple enough logic.


Sure enough I found output 3 (Phase 4 with a pulldown resistor on the input) was dead and 6 (Phase 2 with a pull-up resistor on the input) was stuck high. (with the resistor removed) So yes chuck, while I didn't need another reason to drink we indeed have bad IC's sprinkled around.
this board is easy. This one IC was straightforward to even test in circuit. The rest, not so much. This won't be fun.
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Erm, yes, sorry.
I'm just pulling it to test it now. Many of these IC parts are old enough top the point datasheets are being fun to find.
It's laundry day though, so I'm not gonna spend all day poking at this board. :U
It used to be that the 74365 was encountered mostly as the Signetics 8T95. I still have some old stock labeled with both numbers. That 8242 is a real bugger, though--same pinout as a 7486, but XNOR instead of XOR. The 74266 is a quad XNOR package, but the pinout is wrong.
Yeah and unless I'm looking at the wrong datasheet it's stone dead and the neighboring 7438 isn't responding to the logic table as I'd expect. I've pulled out the Heathkit ET-3200 to start testing these out of circuit and even then it's not looking promising. I think it's time to invest in an IC tester before I pull my hair out.
Bear in mind that the N8242 outputs are open-collector. The idea was that you could tie any number of outputs together with a single pullup to get an n-bit comparator. Without the pullup, you might not see the logic function.
According to the truth table, if I short A0 and B0 low to ground, f0 should be high.


  • SIG_N8242_DTS.pdf
    99.1 KB · Views: 0
It might just be a one-off. Looking at the seller's store there's only six items (randomly catagorized enough to say someone's going through Goodwill) and two other items that seem to of sold in recent history. There isn't anything to indicate they specifically have broken a system up and have been piecing off the boards online, or at least they sure being picky about what they list.. I can ask though, but you damn right I ain't paying $1000usd for $75 in gold.

Project-wise btw I'm running out of steam. I'm working to rack it before it takes up more space on my video table and I am still accumulating a Digi-Key list. I don't this thread is going to be as active as it was the last five months but is people got things to add I have notifications set.
I feel sorry for the guy who took Symbolics from the grave and managed to reanimate it. That must of taken years of work and so, so much alcohol.
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