Friday, June 12, 2015

Possible spare part source, digging into Pertec drive problem


I received a kind offer from another restorer of old computers, who has a spare 1053 printer that is deemed too far gone to restore to operation. He will check it to see of the margin rack latch is there and has offered to send it to me in that eventuality. There is a definite chance the part won't be on the machine, either because it was already removed, it is broken too, or the 1053 does not have the high speed carrier return feature.

 I will be waiting to hear from him, like a child laying in bed Christmas morning eager to unwrap presents and see what delights await. We shall see.


I saw marks on the board at the pins of IC U6 and at all four points of two electrolytic filtering capacitors C75 and C76. The IC is responsible for detecting and smoothing the head retracted switch signal, but the two capacitors are completely unrelated to the IC. Further, I saw signs of solder spatter on the pins of the IC and along one of the leads of C76.

I had assumed these were the points that emitted smoke, but the lack of correlation was suspicious. Even more so, the two capacitors seem to be a late design addition - not on either of the sets of schematics I have access to, yet all the rest matches up well. The capacitors help the positive and negative power rails so they don't sag much when the draw is instantaneously high.

After quite a bit of thinking and inspection, I know think that these two capacitors were replaced (or changed as part of an engineering change). Further, IC U6 is pin identical to the other signal smoothing op amps yet it is a different part number. I believe, on close inspection, that the hold chip was snipped off and this one was soldered to the leads, again probably part of an EC done in the field. Thus, solder resin left on the board causing the marks and bits of solder spatter due to hand soldering.

I am going to ignore those, treating them as false clues for now, and continue looking for the cause of my smoking. Because the initial smoke production was associated with movement of the emergency unload relay, my first suspect is the rather large electrolytic 'emergency unload' capacitor. If that has failed, it well could have caused the symptoms. However, it doesn't appear to be bad.

Next steps are going through each circuit hanging off the board, validating reasonable levels and testing that transistors, etc are working. I will put a bit of time in on this but at some point soon, it isn't worth that much mucking around compared to getting the main parts of the 1130 system and SAC box working.

1 comment:

  1. If all else fails, would it be possible to 3D-print a new part?