Power supply failure and debugging on the Calcomp 565 plotter

RESTORING LAST BIT OF CIRCUITRY WHEN AN ANOMALY OCCURED

Having previously restored the carriage left/right servo to full operation with the replacement power transistor installed, I turned my attention to the pen up/pen down circuit. This operates a solenoid in the pen that either holds the pen up in the air off the paper or allows a spring to push the pen down for drawing. 

I had discovered a failed power transistor in the circuit and when the spare part arrived, I installed it. As I was probing around investigating the operation of the circuit, I installed a similar solenoid onto the carriage and hooked it up. Apparently Calcomp also made a cutter solenoid that would work on the same plotter mechanism, but place a knife on the paper/plastic/fabric so that patterns could be cut with .01" precision. 

It is almost identical to the pen solenoid, which is why I scooped it up on eBay, planning to convert it into the pen. It has the same inductance and method of attachment to the carriage. It was in place and I began checking voltages, when I saw that the -24V level shot down to -41V. It may have been something I did duriing the probing or it may have been a coincidental failure. 

I turned it off, then when I powered up later the voltages were now all near or at zero. The fuses were good, so it appears that something failed in the power supply. 

FIRST FAILED PART DISCOVERED - A WIRE

I verified that the AC output was coming from the transformer, but there was essentially zero coming out of the bridge rectifier. Checking all the diodes did not find any that were shorted, open or otherwise malfunction. Just zero volts coming out and I had confirmed it was NOT due to a short that the rectifier was feeding. 

After some continuity and other testing, I discovered a defect in the wire which connects one of the transformer secondaries to two diodes in the bridge. The wire was not a wire. It is one of a bundle of wires that are laced together and routed through the power supply, with no obvious sign of damage or overheating. It was just an open circuit. 

ADDED A NEW WIRE BUT THE VOLTAGES WERE NOT GOOD

I soldered in a new section of stranded wire between the terminal board where the transformer secondaries attach and the junction of the two diodes. With continuity, I powered up and began to check the voltages being produced by the supply. The line that should be -24V (or a bit more negative since it was unloaded) was at -41V, the same as what I observed just before the wire committed suicide. 

I checked a couple of the other power rails and they were all off significantly. Something else failed in the supply besides the wire. I ran out of time to find the issue when I had to leave the workshop.

OVERVIEW OF THE POWER SUPPLY

The power supply of the Calcomp plotter is interesting, using just one s et of transformer secondaries and no regulators yet producing -24, -9, -7.5, +1.5 and +3V rails. It depends upon a chain of diodes to generate the proper voltages, a voltage divider taking the approximately 27V produced by the rectifier bridge under load and turning it into the rails above.

There is a small additional part of the circuitry that is connected to the bridge rectifier before the filter in order to have pulsing DC, used for the fast stepper modes that drive the servos at 120 steps per second. It is not salient to the issue we are debugging so I didn't include it in the pseudo-schematic above. 

For me to see 41V instead of the 34V I previously observed, the most likely failure is that the Zener diode has shorted. I tested all the other diodes with a VOM in diode mode and found them all to have a good diode voltage and not pass any current in the reverse direction. However, I couldn't check a Zener with that method.

The acid test for a Zener is a transistor curve tracer, where carefully limited current and controlled voltages can be introduced and the results observed on an oscilloscope. The diode should be high resistance at voltages up to 7.5 then exhibit a nearly vertical slope to the current flow allowed by the tracer setting. 

When I get back to the shop, I will remove the Zener diode and test it on my transistor curve tracer. If I need a replacement I must be sure to get one with a high enough power dissipation capacity. The schematic lists it is a 1N3016A diode so that would be the perfect replacement, or a substitute with the same or higher specification of 1W capacity. Interestingly, the spec sheet does not show the voltage threshold as 7.5V, instead listing it at 6.8V. 

Designed SMS socket and having it manufactured

THE NEED FOR AN SMS SOCKET

The Standard Modular System was the construction methodology used for IBM computers in the generation from the Stretch computer until it was replaced in 1964 by Solid Logic Technology of the System 360 and 1130 systems. The 1401 and the 7094 are two well known systems based on SMS. 

IBM did use SMS components during the later generations such as S/360, both for components such as power supplies and for attaching older peripherals originally designed to connect to SMS machines. In the IBM 1130, many peripherals are attached using SMS paddle cards and SMS power cards. 

The paddle card looks like the end of an SMS logic card, but is attached to cables. SMS cards have 13 pads, labeled A through R (as they skip the letters I and O due to their resemblance to the digits 0 and 1). The power card is similar but has some pads combined into fatter pads to handle more current, e.g. M and N become a double width pad, and it has a notch in the middle. The connector block into which power cards are plugged has a steel bar to block ordinary SMS paddle cards but allow the notch of the power card to slide around it. 

I do not have any reasonable source for SMS paddle cards, SMS sockets, nor the gender changer/adapters that allow two paddle cards to be connected. I had to build my own small PCBs to work as SMS paddle cards and SMS power cards. 

The challenge I face is that there is another option connected to a cable, a socket into which a paddle card will plug. One of the cables coming from the 1053 Console Printer, a selectric typewriter used as the console of the IBM 1130, has a socket on it rather than a paddle card. My 1053 Emulator project was originally built with SMS paddle cards, but one of the cables either needs a gender changer/adapter or a socket.

CONCEPT FOR SMS SOCKET

I realized that I could solder some RF shield fingers onto the pads of one of my SMS paddle PCBs. These are springy gold plated contacts that stand 2.5 mm above the PCB but can compress down to 1.5 mm or less, ensuring a good contact across all thirteen pads of whatever we mate to. I needed a body that would hold the paddle card PCB with its fingers and hold an inserted SMS paddle card against the contacts. 

At first I was working on a design that would put my paddle card PCB inside and have a slot for the insertion of an SMS paddle card. However, I realized that the paddle card PCB itself acts as one side of the socket, thus it does not need to be enclosed. 

I designed a part that will be installed over the top of my paddle card PCB, bolted in place through the four holes I put into that PCB. The opening of that part is 3.8mm high, thus an SMS paddle card at 1.54mm will slide in and compress the fingers on my PCB down to 2.26mm height, giving a good solid contact. The part has a wall to stop the SMS paddle card from sliding in too far. 

It has two additional features, notches for mounting and an opening for the cable wiring to exit the socket. The opening is not particularly important, but the notches are key to use in the 1130.

The IBM 1130 has a U shaped bit of metal, the height of the U much larger than the width of the opening. The opening itself is just over 56 mm. The U shaped bit is rotated 90 degrees and mounted in the 1130. 

The IBM gender changer/adapter parts have notches built in so that they can be slid into the U and thus anchored, with SMS paddle cards plugging into the front and back of the adapter. SMS sockets such as the one attached to the 1053 also have the notches and are slid into the U the same way. 

DESIGNED IN FREECAD AND SENT OUT TO MANUFACTURE IN NYLON

I whipped up a design for the body that converts my SMS paddle card PCB into a socket.

I transmitted the design through Cloudcraft and should have three of them by the end of next week.  

Continued debugging of the 1053 Emulator - part 2

TESTING AFTER THOSE CORRECTIONS

I plugged the emulator into the VCF 1130 system and ran my hand entered code to fire off characters and commands to the emulator. I needed to see it successfully handle a print, triggering the interrupt level so that I knew it had the correct feedback signals from my box. Without the feedback working, I can't do a shift nor test out the characters on the other hemisphere of the type ball. 

The oscilloscope was set up to watch the feedback signals as further confirmation that they are reliably delivered into the 1130 system. I did not see enough swing in the signals to cause the 1130 controller logic to operate as intended. I have to throw in the towel and give up on using the N-channel enhancement MOSFETs - the only type I had in the shop. 

THE PROBLEM WITH N CHANNEL MOSFET SWITCHING

The N MOSFET needs a positive voltage on its gate relative to the source, of a high enough differential to switch on the channel and pass current from the drain through the source (actually electrons flow from the source to the drain, but the conventional notation used in circuits has the current flowing from + to - terminals. That is why an N channel MOSFET has a positive voltage on the drain relative to the source.

The classic circuit for these is to have the source hooked to ground, the drain hooked to the + rail through a current limiting resistor, and the load being switched is hooked between the drain and the limiting resistor. An Arduino delivering 0V to the gate will keep it off since the gate is NOT more positive than the source. The high (+5V) output of the Arduino is positive enough for some N channel MOSFETS, although some require a higher differential, thus it can turn on the flow through the MOSFET and pull the load down to ground. 

The situation with the 1130 and the 1053 is not the classic situation. The load line is pulling current from the + rail down to -3V inside the debouncer in the 1130. When the MOSFET is switched on, the load line jumps to nearly the + rail, when the MOSFET is off the load line is seen to be at -3V. 

The drain of the N MOSFET is thus nearly at the + rail  when the switch is off, but drops down to -3V when it is turned on. The gate is more positive than the source when the MOSFET is turned on, but we have to limit current through the resistor which drops the voltage sent to the debouncer circuit. When the switch is on, the current is flowing through the MOSFET in addition to going into the debouncer. 

This gets more severe with one of the signals, which has a +48V rail, as the current limiting resistor is set for 2.5W of consumption yet that lowers the voltage to the debouncer too much to work reliably. In reality, the classic circuit does not work well for me.

The ideal use of the MOSFET switch is indeed as a switch, with the + rail on the drain and the debouncer circuit on the source with no current limiting resistor at all. That way it works purely as a switch, the perfect analog for the mechanical switches in the 1053 typewriter we are emulating. The challenge comes from trying to control it with the gate.

The source voltage is going to vary now, from -3V when the switch is intended to be open up to the + rail when the switch is closed. The gate has to be more positive that the source to turn it on, but that means it has to be more positive that the + rail. Unless I want to add additional power supplies above the 12 and 48 V rails of the 1130, this is not possible to do. I certainly can't do it with an Arduino's 5V logic high. 

There is even the risk that some current will flow even when the Arduino puts out 0V, logic low, as that is 3V more positive than the source initially. The MOSFET is likely to turn on just enough to raise the source to a point where the gate-source differential is lower than its threshold.

P CHANNEL MOSFET IS THE SOLUTION

If I switch to a P Channel MOSFET and hook it up as a switch, the situation gets better. The rail connects to the source terminal, the drain is connected to the debouncer. The gate has to be more negative than the source terminal, but fortunately in this case the source is steady at the + rail, not varying as in the N channel case. We just need a way to get the gate to sit either at the + rail or about 8-10V below the rail, to have the switch off and on respectively.

That is provided by a simple NPN transistor and a voltage divider between the + rail and the collector. The emitter is hooked to ground and the Arduino will turn this on or off as it drives the base with logic high and logic low respectively. The transistor, when off, allows the MOSFET gate to see the full + rail voltage. When the transistor conducts, the pair of resistors in a voltage divider are sinking current to ground through the emitter. 

Choosing the values of the two resistors lets me set the gate voltage I want the MOSFET to see when we want to turn it on. The MOSFET doesn't need any appreciable current to the gate, thus the resistors can have high values. If the upper resistor is 30K and the lower resistor is 10K, with a +12V rail, the junction of the resistors will drop to 3V when the transistor conducts. putting the gate about 9V more negative than the source. 

Now this is simplified due to things like the diode voltage drop of the transistor and non-zero emitter-collector on resistance, but you get the idea. For the +48V rail, we can choose an upper at 18K and a lower at 47K to produce just under 35V at the junction and thus the gate will be 13V more negative than the source. 

I will order some P channel enhancement, MOSFETs with suitable voltages and thresholds. I have plenty of switching NPN transistors on hand and enough resistors to create the voltage dividers I need. 

Developing SMS socket to connect to paddle cards

WHIPPING UP A FEMALE SMS SOCKET FOR THE BOX

I think I have a good plan for a female socket to plug into an SMS paddle card. I built some SMS signal paddle PCBs that I could adapt.

Using some 1/8" or 3/16" stock, I could cut some pieces and drill holes so they create a channel for the incoming SMS paddle card. Another larger plate sits on top of those side channels and creates the socket. Soldering some spring contacts on the pads of my PCB will allow good contact when the male paddle card is inserted. I could also add a block to stop the paddle card from inserting too deeply. 

I soldered some RF shield fingers as springy contacts - it wasn't a perfect job but they are all there with no shorts. Next I cut up some brass I had as the spacers and a plastic spreader as the top.

Very quick and dirty, but sufficient to test out the concepts and spacing. The brass spacers were .09" high and the tests I did shows that I need the space to be almost1/8" otherwise the contacts have to bend too much. However, by loosening the nuts slightly I could insert another SMS paddle card into this makeshift socket and verify connectivity for all thirteen pads. 

I have one additional spacing detail - I need grooves on the sides that are 2.2" apart. The SMS signal box in the 1130 into which the socket attaches has top and bottom rails separated by just over 2.2", thus the socket is held in place while in the system. 

Based on these tests, I will design a socket structure to connect with my SMS paddle card. It can be 3D printed fairly easily. 

I believe I can design a simple 3D printed part that can be attached with screws and nuts to form a socket, something cleaner than some scrap parts cobbled together. I will work on that but this temporary socket gives me an emulator box that can be long term loaned to the System Source Museum for use whenever the 1053 typewriter is out of service. 

Replaced transistor to restore Calcomp 565 plotter carriage movement

REPLACED - CARRIAGE STEPPER NOW MOVING 

I had found some GM Delco 2N392 transistors on eBay - apparently this was used widely in car radios. The first new-old-stock transistor arrived and I installed it into the plotter. When I powered up for a test, the ring counter was restored to correct operation. The manual control for carriage right or carriage left now shifts the ring counter one step, changing the coil that was activated and moving the stepper motor one position. 

WAITING ON PEN SOLENOID TRANSISTOR TO REPAIR THE REMAINING CIRCUIT

The second new-old-stock transistor is on its way from a different vendor. Once it arrives I can stick it into the pen flip flop circuit and finish restoring that to (hopefully) full operation. However, having done some circuit testing and analysis, I am concerned that another PNP transistor in the circuit is also bad. 

Continued debugging of the 1053 Emulator - part 1

ADJUSTED RESISTOR VALUES FOR MOSFET SWITCHES

The voltage divider network hidden inside the IBM 1130 logic, specifically the switch debouncing circuits, required a much smaller resistor on my MOSFET switches to produce logic low and logic high levels on the gates fed by the signal. I changed over to 470 ohm resistors for the three switches that work with 12V. 

The switch that drives the -TWR CB RESPONSE signal has to deliver 48V when active and that voltage sinks to ground through the MOSFET when it is turned on. While I don't have the exact resistor values inside the debouncing circuit for the 48V signals, I believe that a 1K resistor will allow the gates to see logic low and logic high as it switches. That will consume a bit under 2.5 W when on steadily. 

CHASING DOWN ANOMALY WITH PHANTOM RED SHIFT COMMAND

I can see from the output to the terminal that immediately after the emulator box turns on, it believes it has received a Shift to Red command from the 1130. However, the logic in the 1130 is designed to fire off a Shift to Black when it starts up, so this looks like the wires are crossed somewhere in my box. 

In point of fact, I had misinterpreted the string of characters - it was actually a shift to black exactly as it should be. No issue.

SWITCHED TO DIRECT PORT ACCESS FOR DIGITALREAD AND DIGITALWRITE

I removed the digitalRead and digitalWrite statements, instead using direct access to ports such as PORTL and PINL. Much faster. Running my code in WokWiki, an online Arduino simulator, it appears to not produce output characters at the maximum rate of 15.5 cps. The simulator web page claims that it runs as fast as the Mega 2560 hardware. 

I will need to run some printing software to hit the maximum rate and check to see if any characters get dropped. If there is a speed problem, I will have to get even more clever to meet the timing requirements. 

Attempting to make the 1053 Emulator usable for SSM in time for their pickup of typewriter tomorrow

RESOLVING FLAWS RAPIDLY IS POSSIBLE

The issues I identified with the feedback signals should be resolved with some simple MOSFET switch board rework. The missing line feed is fixed already. The only behavior I still need to investigate is why the ANSI colors aren't working as expected. That seems likely to be easy to resolve too. 

DEALING WITH GENDER MISMATCH IS A CHALLENGE IN THE TIMEFRAME

The physical connection issue is what worries me. The 1053 printer comes with two SMS signal connections - one is a male paddle card and one is a female socket. My emulator however has two male SMS paddle cards. 

I can temporarily fix this by using an IBM SMS gender adapter, a molded device into which two male paddle cards are plugged. In fact, the SMS signal block into which the 1053 plugs has one such adapter already. A male card from the 1130 logic plugs into the adapter, then the male card from the 1053 plugs into the other side. 

The 1130 logic has a second male paddle card, which is plugged into the female socket on the cable that comes from the 1053. When I add a second adapter, I can plug my second male paddle card into the adapter and connect to the second male card from the 1130 logic. 

The only tricky part is that the adapter reverses pads. The leftmost pad on an SMS card is A, the rightmost is R. However, through the adapter, what is on pad A on a card plugged in will be connected to pad R on the other male card plugged in. I have to invert the male paddle card from my emulator to plug it into the adapter in order to get my signal line on pad A to connect to the 1130's paddle card pad A. 

I don't have spare adapter devices. I had to borrow the one from my own 1130 system, the one that normally connects the male 1053 cable to the male 1130 logic cable in the SMS signal block. That leaves my system unable to use its console printer, so I can't loan this to the System Source Museum when I loan them the emulator. 

If I am able to put together a usable emulator by Friday afternoon, I will have to cobble together a female paddle card socket and wire that to my emulator tomorrow. I have a start on this with some experimental work I did, but it is pretty crude and unfinished. We will see if I can pull this off by 4PM tomorrow.