Friday, June 19, 2015

1053 carrier return working well, continued work on Pertec drive


I finished my adjustments and it seems that the carrier return is working perfectly, under motor power as well as with the hand crank. Hurray! The repaired margin rack latch is working fine and the soldered part is not getting much impact since the margin rack movement is buffered by the air cylinder.

I would have been testing with software except that I noticed my tab button is erratic - sometimes it doesn't move the carrier, other times it works fine. Better that I get this sorted out while the mechanism is still out of the covers and accessible. I am agonizingly close to finishing the restoration and putting this together atop the 1131.

This problem could be caused residual gunk on the carrier assembly stopping the tab operation from latching open, it could be a flaw in the mechanism that moves the tab bar to cause that latching, it could be a problem with inadequate spring tension on the mainspring, it could be some kind of drag or interference on the carrier itself, or . . . .? Some of these, such as the residual gunk, would clear out over time with steady use. Others require adjustments.


My 125 replacement capacitors should arrive tomorrow afternoon (Friday), meanwhile I can do some more testing/diagnosis on the boards with the first ten caps changed out. I put it together and fired it up at lunchtime. Alas, the crowbar for the 10V raw supply immediately forced a fuse to blow.

I zoomed in to look at this circuit and adjacent parts. I quickly found the SCR that implemented the crowbar, a 2N3228 device, was defective. It was removed and a replacement ordered, which I should receive on Wednesday while I am away on my business trip. I will be able to put it into place and test everything late next week.

I should be able to run the board with the crowbar SCR missing, as all it does is force a short circuit. I can see that the effective resistance at this point is high enough that it will only draw milliamps, so the only risk I face is if the voltage is considerably above 5V. I did this test quickly and observed that the voltage jumped to over 8.7 volts, also the other crowbar fired due to other voltages also exceeding spec.

Time to dig back through the voltage regulation circuits and diagnose any problems. Starting at transformer, rectification and filtering, where I should be producing +10, +20 and -20V DC, I see I am delivering more than 14V for the 10 supply and almost 23 for the 20V supply. This design requires that the transformer be connected and jumpered for each small granularity of mains voltage. This box is wired for lower voltage, while my outlet is delivering about 122-123V, thus the higher raw output.

It is quite possible this is enough to push the regulators up over their maximum output levels, without any flaw in components. I will rewire the transformer, recheck the raw power levels, and reconnect it to the regulators once I feel the raw supply is working properly. I hate designs that are so finicky they can't deal with the legitimate range of mains voltages that will be found in buildings.

The wiring is now changed. Too, the mail came and I have 125 more tantalum capacitors to swap for the dying ones on these boards. I powered up, found the raw voltages much closer to target, but I am seeing fuses 2 and 3 (+20 and -20 raw supply) blow out when everything is cabled together. Time to back off and test inch by inch through the regulator circuitry.

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