Sunday, June 14, 2015

Failed component found in Pertec drive, plus spent some time with 1053 and SAC Interface box issues


My thanks once again to Peter and Johannes who offered spare parts from 1053 printers. I am no less appreciative even though my model is different and their machines didn't have the same part. Unless I am extraordinarily lucky and find yet another owner of 1053 devices or someone with stock of this rare part, I have to abandon the hope that I can find a ready-made part as a replacement.

I feel that finding a way to 'fix' or modify the current broken part is still the easiest path. I might be able to drill a hole and fasten something to the bracket, although space is limited. The smaller pivoting piece doesn't impinge on the wee space where I might be able to insert a teeny screw to hold on a bit of metal that will curve down in place of the snapped off piece.

Potential area for a drilled hole to affix a new part
The piece that snapped off was at the right side of the part inside the red oval, and it extended downward out of the picture plane for about 3/16". I may be able to create an L shaped piece that is bolted through a hole I drill in the middle of the red oval, the piece underneath what we can see and the 90 degree bend taking it downward where the original end projected.


Jack Rubin posted a suggestion to use a Genrad Bug Hound, a tool that helps find tiny variations in current flow to pinpoint where a short exists in a situation such as mine. While I don't have that device, I do have an HP Current Probe and Logic Pulser, which together accomplish the same thing. The pulser injects a series of voltage pulses to the shorted circuit, while the current probe lights with intensity related to the amount of current flowing.

There are over sixty components that sit between the +10V rail and ground, any of which could have been the cause of the short. Using the probe, I followed the current and was able to rule out sets of parts when no current flowed in those branches. Further, I found a spot where there was heavy current flow in the ground side, as well as heavy current flow on the +10V side. They triangulated to a 2.7uf filter capacitor (C36), which I then desoldered from the circuit.

The capacitor was a dead short when tested out of circuit. Further, the board itself now had reasonable resistance, over 2K, between +10 and ground. I will order the 2.7uf capacitor and a replacement for the one LM741 op amp that I cut off the board earlier in my debugging.

The capacitor was a tantalum polarized capacitor - these are notorious for failing with a short circuit and often with dramatic results. in this case, the case of the capacitor was barely affected, too subtle to notice on visual inspection. Tektronix oscilloscopes and plug-ins are notoriously prone to tantalum capacitor failures. Unfortunately, IBM uses these on almost all of their SLT cards and Pertec used them for the PCBs in the disk drive.

Once I have the new parts installed, I will put the drive together and go back to testing its startup. Should be just a few days for parts to arrive and I have other things to do, which is why I don't just drive down to Anchor Electronics and pick up the replacements right away. I am still waiting on replacement fuses anyway, which come early next week.


I am trying to load the fpga configuration bitstream in the flash chip onboard, which appears to complete normally. However, when I drop power and reapply it, the fpga fails to load that bitstream. At this point I am going to look carefully at the bitstream I produce, which loads just fine as a direct operation from the USB but fails from flash. This has to be some subtlety which impacts self-load by the fpga or a configuration issue I don't understand.

I read through the entire Ztex site and its associated wiki, but didn't find any help. Several similar problems occured when the USB firmware didn't have support for loading fgpa from flash, but both firmware images do. Other problems occured if the fpga chip wasn't correctly described, but my parameters are correct. Time for more digging and testing, I guess.


  1. Two thoughts about the broken part: it is quite small and I can well imagine why a welding shop would not want to work on it. And the broken end has such a small area that it would be dubious that a butt-weld would hold. However, a curved piece that starts under the horizontal part and follows the inside curve and extends out to make a tang -- that would have enough mating area to be strong, even using perhaps solder.

    Second, there are people who are accustomed to welding, or at least brazing or silver-soldering, small parts, namely jewelers! You might be able to find an amateur or professional with jewelry-making experience who would be able to affix an extension on that curved piece.

  2. Great idea, David. I found a jeweler who believes they can use a laser welder to reattach the tang, although there will be a built up area around the join. I don't care, as that spot doesn't need to be smooth and any exact size.