SMS cards are very easy to reverse engineer, because all the components are discrete and visible. More importantly, the PCB has traces only on one side, with wire jumpers soldered when signals have to move across the component side. The parts are all labeled so that I can tell their value with the exception of three diodes - two Zener and one normal diode (that rectifies the AC bias input). Given the similarity to the previous SMS card, I expect the Zener diode values to be identical in the new card.
I began beeping out the connections from the SMS card fingers to determine where the circuit would diverge from the previous card to this one. During this process I got a beep where I shouldn't. The card has a large 108 PNP transistor on a heatsink that is the final stage which drives current through the output transistors. The emitter and collector had almost zero resistance - shorted out. This is not true with the other card (or any properly functioning 108 transistor).
Therefore this is not functional and can't be inserted into the power supply as a substitute. It reverts to its original status as the parts donor.
DEALING WITH THE 026 TRANSISTORS ON THE ORIGINAL CARD
I desoldered the two 026 transistors that form the comparator on the regulator card, so that I could test them on the curve tracer. I found that there was a 3X difference in beta (gain) between the two transistors. Without a spec sheet or a known industry alternative part number, I couldn't determine which was bad at this point.
I then removed one of the 026 transistors from the ebay card and put it on the curve tracer. I now had two 026 with identical gains and curves, plus a third that was much higher in gain. It seems quite reasonable that the two transistors in a comparator should be as equal as possible, so I put the similar pair back on the card.
INSERTED CARD IN REGULATOR AND TESTED AT LOW LOAD
With the card now possibly repaired, I plugged it into the regulator and set up the power supplies and electronic load. The regulator is fed 8-9 volts DC from one supply, which in the machine comes from the unregulated raw DC supply. The machine also provides 8-9V AC to the regulator which is turned into a DC bias voltage by a half wave rectification on the SMS card. I used another power supply to provide a DC voltage in lieu of the 1130's AC input.
My electronic load was set to 1A, enough to be able to assess whether the machine is holding a fixed voltage. I then adjusted the potentiometer on the card to set the output voltage at just over 3V DC.
This did the trick. The regulator kept the same voltage for long periods of time, unlike its prior behavior where it began to run away after a few dozen seconds. I bumped up the load to 1.5A, 2A and 4A to see how steady the output voltage remained.
I was not completely happy with the outcome. The regulator sat at 3.04V while at 1A, but declined a bit with each bump in current until it was at 2.96V at 4A. This could be one of two things. First, it could be an artifact of how I am powering the supply on the workbench. Second, it could be due to the lower beta of the transistor I used to replace the 086 on the card.
In the real machine, the bias AC supplied to the regulator comes from the winding of the raw DC supply that feeds the regulated supply. This might change under load and somehow compensate while my bench setup feeds a fixed DC voltage to the regulator. If this is the case, then the regulator card is really working properly and I can consider this fully repaired.
On the other hand, the lower beta of the replacement transistor might not produce as much drive current, therefore the regulator might not adjust as much as it should as the load increases. The solution to this would be to find a better replacement or even try the original 086 back in the card.
REGULATOR INSTALLED IN VCF MACHINE
I bolted the regulator back into the machine and installed some of the wires to it. I can do load testing first with my electronic load up to 5A and then with my resistor networks for a much larger load. That will determine whether the regulator is actually working good enough or if I have to keep working on a substitute for the 086.
HIGH LOAD TEST WILL CONFIRM PROPER OPERATION OF REGULATOR
I used my network of power resistors to load up the regulator to 16A, which is higher than the draw from the 1130 it will be used in. The aim is to see if the voltage remains nearly constant when under load, which proves that all four of the power transistors in the final stage are working properly.
The next stage of testing, when all the power supplies are working correctly, is to adjust the voltages to the target levels for the machine by reference to the terminal blocks attached the logic gates where the DC is delivered. I need the 230V outlet in my workshop to power up the machine, but the owner of the unit is waiting on the electrician to arrive to install the outlet.
DESIGNED AND HAD PLEXI COVER BUILT FOR SSM 1053 PRINTER
The 1053 console printer for the System Source Museum 1130 system did not have the plastic cover that fits in the lid. This protects the printer from objects falling in as well as reducing the sound produced as it types. The IBM part is a complex shape which was unrealistic to copy but I had a flat piece laser cut from plexiglass as a substitute.
The plastic IBM part has notches on the underside where springy metal wires hold it in place and against the front of the lid. I cut some small bits of acrylic to provide the same notch and glued them to the bottom of the cover I just had made. It fits well and looks appropriate.
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| Cover without notched pieces added |
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