Monday, August 22, 2016

Blocked on a couple of fronts with Diablo tool wiring and 3178 monitor repair


I have worked out all the wiring assignments and FPGA signals involved and was ready to begin soldering the cable to one extension board to implement the disk driver role - able to run a real Diablo 31 disk drive. To do so, I had to unambiguously identify all the wires from the plug and put them at the correct location.

I quickly discovered that my cable is missing the signals for sector count that the drive would report back, nor the unit select line, but it does have index marker and write check llines which I didn't need. I have found a site that claims to offer the pins which would let me add the missing lines. I hope they will respond and allow me to buy a small quantity.

I can use the Index Marker and Sector Marker pulses to manage my own Sector Counter in the FPGA, but without a way of selecting the drive it would stay inert. Not as simple as I had hoped to leverage my existing cable exactly as the Alto is wired to the drive.

I need at least one more socket pin, or have to remove and transfer the pin that currently delivers the Write Check status since the Alto ignores that signal. Another option might be to find a way to bypass the select line and 'hot wire' the drive to be selected at all times, but this requires me to do something inside the Diablo drive which is suboptimal.


I opened the third monitor I received, the one with a broken plastic top cover, which was suspected to not work properly. Indeed, when I had tested it, there was no vertical deflection but I could see that a whole screen of content was making the one horizontal line very bright.

I could see a burned spot on the PCB in the section marked Vertical, where an electrolytic capacitor was connected. I will remove the part and check to see if it had shorted, causing the trace to heat and melt around it. There may be collateral damage, such as driver transistors, but first step is to remove and test the capacitor.

Unfortunately, the capacitor is just fine. I am guessing this was a bad solder joint which led to overheating and failure. The problem is to figure out where the pad is supposed to be connected, then ensure good traces exist. The surface is pretty damaged looking and I may have a trace failure.

Burnt land and traces(?) on 3178 circuit board
I think I will have to open a working monitor and photograph the same area to learn what the undamaged traces look like, then go back and repair my errant board.


I had some discussions with others who might want a replica solenoid and pen for their plotters, because it may make sense to build a small batch at one time given how many people have 'penless' plotters out there.

There are a few levels of fidelity I could seek, which have increasing difficulty and cost but also increasing value to others. I am going to sketch out some ideas over the coming weeks and see what makes sense.

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