ImageWriter II - 2 Missing Dots

[mars]

The 5th and 8th (from the top) dots on my ImageWriter II (A9M0320) don't print. Things I've already checked/tried:

• Disassembled and cleaned the head. No change.
• Windings on the head all measure ~3 ohm resistance each.
• Each winding pin on the head measure around ~9 to ~11 volts when the printer is on.
• I wrote a small program on my modern PC to test each pin individually over serial (using the ESC G commands documented in the technical reference). I can hear each of the other pins actuating (striking?), but I hear nothing when either the 5th or 8th pins are being tested.
• Using my oscilloscope while the pins are being tested, I see waveforms on the good pins that basically match the drawings in the Sams ComputerFacts document. However, the 5th pin looks like it has no signal (just solid voltage level the whole time) during pin testing, and the 8th pin does have some kind of waveform but it's more square wave looking rather than the curved ramp up and down that the good pins have.
  
  What should I do next? Should I try recapping the power supply? Should I look into replacing the ICs labeled HIC4 and HIC3 that seemingly drive pin 5 and 8 respectively?
  

 

[mars]

I haven't figured anything out yet, but I removed the power supply board and noticed something that may be of interest to someone. In all of the photos of the board that I've seen, the pin driver ICs appear to be coated in epoxy, presumably making it difficult to reverse engineer them. On my board, they are bare. Seems like the components that make it up are clearly visible and they are mounted on some kind of ceramic PCB. I don't know if this would make it easier to reverse engineer these long out of production parts, and make new replacement little PCBs.

 

[hauke]

The 5th and 8th (from the top) dots on my ImageWriter II (A9M0320) don't print.

A long shot, but: Have you checked the needles in the print head?

I have traces of recollection of 1991, when my Imagewriter I was missing a dot. ISTR a needle was broken or missing, and that I broke another needle in half and fixed the print head by substituting the missing needle.

This was 35 years ago, so take with a grain of salt.