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I'll check Q1, Q6, Q7 during the course of the next days out of circuit.
...
With GREY (33R) and WHITE (60R) loaded down:
C4: 9.8 DCV (always), mystery device right leg pair: 8.6 DCV, or virtually no voltage?
minimal loads:
C4: 10.0 DCV, mystery device right leg pair: OL, 3.98 DCV, flashing bar graph?
Not sure if something is intermittent and I'm getting different results on the mystery device, but it's probably wise to exercise the pot and reflow some joints. If you think it's a good idea I could also try to scope the voltages around that area, maybe the multimeter doesn't like the output.
Btw, R1 = 1k (if it's the "dog bone" style resistor) and measures as such, R3 = 2k and is at 2k3
Q:You said that R1 measures 1K but is it marked 2K?
If true, then that changed the current from 11mA to 22mA (based on 22V unreg. rail) but this should not be enough to kill the LED (if it exists) in the mystery device because many old optocouplers have robust LEDs that can take 40+mA continously.
I guess you can try to test Q1 out-of-circuit.
Q: Are you using one of those component testers that shows transistor type, EBC pinout, hFE, etc?
When you do remove Q1, confirm from the underside (based on the tab) exactly which pin is going to GND, zener, and mystery device upper-left leg. Datasheet: https://www.web-bcs.com/transistor/tc/2sc/2SC857.php
If Q1 is good, then you can remove the mystery device and test it separately by connecting:
i. upper left to 0V, lower-left to 1K to Vcc,
ii. upper right to 0V, lower-right to 10K to Vcc
iii. with your multimeter on VDC on the right pair.
iv. use 22VDC for Vcc from a lab power supply or lacking that a wall-wart/laptop power supply that can do any fixed value between 12VDC and 22VDC.
If it's a working opto coupler then connecting and disconnecting the 1K should cause the MM to go from 0V to Vcc.
Also, report back the Vdrop on the left pair when the 1K is connected.
EDIT: Also, while you have the mystery device out, confirm with your MM set to ohms, that there is very high resistance between any pin on the left side to any on the right side.
I found a 4n35 (Vce=70mV, still ok?)... quick check before I power it up:
Is it wired in correctly? Or do I need to swap the C / E around?
The 4n35 should be fine.
And you have it the right way assuming the LED is on the left, phototransistor on the right.
But before powering up with the logic and display boards installed, let see what the output voltages are under load.
Connect to GND: 10K*, 33R*, 33R, 22R, for RED(1), WHITE(5), GREY(3), ORANGE(6), respectively. (*=more realistic)
Measure them WRT GND (BROWN(4)) and WRT chassis (GREY(3)).
No 10K available (of that wattage) for the RED supply, but with 2x22k at about 10.8K I get 207 DCV WRT GND, 193.5 DCV WRT GREY respectively.
With the 33R load, WHITE is: 8.8 DCV WRT GND, -4.9 DCV WRT GREY, but I noticed that it creeps up with temperature. Is this something to worry about? Is the 2SC830 failing or is this normal behaviour? It went from about 8.6ish to 8.8205-ish (sometimes creeping up, sometimes going back down, more or less remaining there now). If I blow on it changes a little...?
The optocoupler is slightly warm..
I think it's repaired? Time for the logic boards? Or is it better to implement the protection circuit against shorting transistors first?
Good and bad news:
It powers up and the nixies light up too. There was even a 0 on the rightmost nixie, but after 15 seconds it showed random numbers and went comatose.
Here are the voltages, with the RED pot all the way down (one dot on the nixies still lights up, I guess that's good enough voltages)
WRT GND, chassis in VDC:
RED: 190, 176.8
WHITE: 5.6, -7.7
BROWN: 0, -13.4
GREY: 13.4, 0
ORANGE(2): 2.9, -10.5
ORANGE(6): -9.8, 23.3
WHITE and ORANGE(2) deviate from the prediction.. close, but no cigar
EDIT: Attached are some scope pictures.. not sure what to make of it, WHITE still looks weird compared to the others
WRT GND | WRT chassis | My Comments | |
RED(1): | +190V | +176.8V | Good. Within acceptable limits. Nixies light-up (typ. >165V) nor excessively bright. |
WHITE(5): | +5.6V | -7.7V | Bad. Likely a consequence of ORANGE(2) droop. |
BROWN(4): a.k.a GND | 0V | -13.4V | Good. No drooping. |
GREY(3): a.k.a chassis | +13.4V | 0V | Good. Confirms reverse of BROWN(4). |
ORANGE(2): | +2.9V | -10.5V | Bad. See below. |
ORANGE(6): | -9.8V | +23.3V | Good. No drooping. |
ORANGE(2) should be close to +8.1V WRT GND or -5.4V WRT chassis. Because it is derived from a resistor divider, that implies that whatever it feeds must be high impedence otherwise it will cause it to droop. Which is what is happening. Unfortunately, you are going to have to trace where the ORANGE (2) wire goes; which board(s), which component(s) it immediately touches. It is probably used as a reference (my guess) so it shouldn't be used in lots of places; probably leads to a [bad] buffer amp that's loading it down.
Q1: Immediately upon power-up, is ORANGE (2), WHITE(5) normally at +8.1V, +8.6V, respectively, and then they droop?
Q2: Immediately upon power-up, can you type on the keyboard and does the machine respond normally before it goes comatose?
Q1: Immediately upon power-up, is ORANGE (2), WHITE(5) normally at +8.1V, +8.6V, respectively, and then they droop?
It looks like it stabilises at 5.6 V immediately.
I traced it to yet another alien looking thing (see attachment). The second legs seems to go to a ground plane as the trace then stretches across the board connecting many other components...I haven't found out if it's one of the established grounds (BROWN or GREY). I guess to know what the voltage should be we need to figure out this component too.
Quote
Q2: Immediately upon power-up, can you type on the keyboard and does the machine respond normally before it goes comatose?
See my second post after the scope pictures. It seems to power up correctly without keyboard and there is possibly a fault there.
So, without the keyboard (all other boards installed), the voltages are still those below?
WHITE(5): +5.6V -7.7V WRT GND/chassis
ORANGE(2): +2.9, -10.5 WRT GND/chassis
And, also without keyboard, does it still go comatose after 15 seconds?
If it DOES NOT go comatose without the keyboard then I'd give the keyboard a good cleaning and test each of the switches for continuity (high-resistance when not pressed, low-resistance when pressed). Then reboot with keyboard installed and see if it still goes comatose. A key may be loading down a power rail.
The comatose issue or low ORANGE(2) may not be related.
The voltages remain the same with the keyboard disconnected, the only difference is the 0 doesn't show up (it only appears comatose with the keyboard connected).
Working assumption: the boards regulate ORANGE(2) and WHITE somehow and there are no faulty components that influence it.
Any idea how to disassemble those type of keyboards? I've never handled one like that... the wires are soldered to phenolic boards sandwiched between metal plates. Is it possible to bend those metal pieces back..? I guess that's how you un-sandwich it...
I would like to throw this in an ultrasonic cleaner but I don't own one and the keyboard can't be disconnected from the backplane. Alternatively, I'll try to map out the keyboard. My hope is that maybe just one or two keys are stuck close and hinder the calculator from operating correctly.
PS: There's a really bizarre connector on the back.. what could be the purpose? Remote control... factory testing?
PS: There's a really bizarre connector on the back.. what could be the purpose? Remote control... factory testing?
Maybe you can first separate all key groups from the top of the enclosure. It looks like it can come off as one unit. I think that row of brass phillips screws above & below the blue wires (from your first message) secures them. That should give you access side-on to spray contact cleaner and exercise the keys. I don't think you should separate those plates as they are punched along the left and right edges (4 times) of each key group. It'll probably cause you grief to put back together with multiple springs and alignment points. And never be flat again.