Saturday, June 20, 2015

1053 console printer assembled in covers, fully tested, not quite ready. Pertec power supply travails filled my remaining hours


The tab operation is working well now - minor adjustment. Carrier return works almost flawlessly - occasionally it won't trip as it strikes the left margin, instead reaching further left before resetting. If I could tab/space past the left margin position, it would be a pretty benign failure, only impacting left registration of certain print lines. However, the carrier has great difficult spacing past the left margin stop.

I suspect there should be a way to move past it, or to slide the margin over past the carrier, but I haven't figure it out yet. The parts diagrams for each section don't show parts that might be different with rare options - there is one page covering the high speed carrier return feature but it doesn't snow the parts on the carrier itself related to the changed margin tops. The motor power is reconnected to the 1130 itself, so that the typewriter motor comes on when I power up the 1130 computer.

I have the front panel and covers ready for assembly, once I do a software driven test to be sure my selector magnets can correctly trigger the five operational actions - space, tab, carrier return, index and backspace. I will also validate that it shifts upper/lower, shifts between red and black ribbon, and of course that it still types characters correctly.

As I prepared for the test, I decided to reverse the order of my plan, fitting the front panel and then the cover on the typewriter before I did the testing. It just felt better to see it coming together as a complete unit, even if I run the risk of having to open it up for future adjustments.

I found several ways to assemble the two parts - front and cover - in the wrong sequence. I also learned to make sure the column pointer from the carrier is free to move in the front panel before tightening it up. Since the screws go in from the bottom, a very inconvenient angle, it took a few tries. Then, the cover is a bear to get seated properly without hanging up in a position neither on nor off, but unwilling to budge further. A few tries of this and I finally had it together.

I powered up the 1130, hand entered code to make the typewriter type characters or execute various functions, then began putting it through its paces. It did type characters and do spacing operations, as commanded. However, I didn't get a good tab, backspace or carrier return from program command. The front panel buttons work fine for CR, tab and space.

I saw the nylon tape change tension when I issued a "shift ribbon color to red" function, but I didn't see any sign of the counterpart, "shift ribbon color to black" at least in visible tape movement. One of the characters I requested typed as something different - might have been my error setting up the program, or something different. In fact, it could be errors due to my SAC Interface box, which is cabled on the system but was not powered on.

Given the chance that the behavior will change when the SAC testing resumes, I am going to put this down into the recess on the 1131 and install some paper. There is one part missing right now, a spring loaded bar that holds the incoming paper forms, but I know where to look for it.


I have the power supply out of the drive and set up on the bench alongside the servo board which includes the regulators. I will check out as many components as I can in this way, but I will definitely need the switching transistors hooked in. They are installed on big heat sinks on the drive chassis itself, so connecting to them will be an added hassle.

After moving everything close to the chassis and setting various boards on insulating layers, I validated that my +10V and -10V supplies are spot on. The +5V and -5V were essentially off, which I suspect is because they have a 5VSense line back from the using boards that is used to maintain the right voltage level for the logic board. This normally is looped through the plug P210 that delivers power to the logic board, delivering the +5 supply back as +5sense, -5V back to its sense and even a ground sense line looped through the cable.

I temporarily connected those three to their supply equivalents, to permit me to verify the power regulation circuitry on the servo board without actually hooking up the logic and downstream boards yet. I still see fuses blowing and thus can't get reliable +5 and -5V on the board.

The resistance I see on the +5 and -5 outputs is about 2.8K ohms, which would only draw milliamps, so the only way I can get this dramatic suicide of the raw supply is for another of the crowbar circuits to be firing, shorting the input rails. To finish the debugging, I tagged and removed the other two crowbar SCRs, after which I was able to get the power regulators to stay up long enough to figure out what is wrong.

With no crowbars firing, I checked the voltages on TP4, which should be +5V, but was +11V. That certainly would fire the crowbar. The voltage on TP12, which should have been -5V, was hovering at less than one volt. There is definitely something wrong but at least I can now check levels against what I expect and hopefully zero in on the failed component(s). There are less than fifty to check out, since I know that +10V and -10V are correct at TP21 and TP25.

Further, there should definitely be a load on each supply output line, since the servo board uses all four voltages with other parts. Before I dive any deeper, I am going to verify that my sense lines are feeding back the voltages as they should, since these are essential to the output level of the regulators. My current method, having jumper wires pushed into the sockets, doesn't guarantee a solid connection.

The regulation itself is driven by an op amp, fed the +5V sense level on one input and a voltage divider, in parallel with a zener diode and two ordinary diodes, as the other input. The op amp drives a cascade of transistors to drive power at the selected voltage. The -5V regulator has a similar mechanism. The ground sense line is part of the voltage divider for +5V, making it as critical as the other two sense lines.

With the logic board plugged in, ensuring good sense voltage values, I again found the +5V running high at over 8V, but the -5V was near zero still. Further, I am getting fuses blowing in the main supply. How a 10A fuse can blow with a 2K to 3K resistance for a load is beyond me, unless there is another crowbar somewhere causing mischief.

The board has only one other SCR and it is part of the brush motor control circuit, not used for a crowbar. My assumption that the load is almost 3K must be wrong - it may be at the low voltage of the meter I am using, but there are tons of zener diodes in the regulators. Any that fire above the VOM voltage could produce a short in conjunction with another failed component, something I am missing at low voltages.

The main power transistors are mounted on heatsinks on the chassis, connected by cables to the servo board. This allows me to easily test their condition with a parts tester hooked to the cable. All of them appear to be functioning properly.

I will do some continuity testing of all the paths where a short could be in series with a zener diode.
Nothing found by the onset of evening. With Father's Day tomorrow and a business trip beginning early Monday, I will not be getting back to the workshop for quite a few days.

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