Saturday, August 26, 2017

Display repaired for Digibarn Alto, first test of disk emulator tool


I hooked the emulator up to the Alto but it did not boot from the virtual disk. We did this very late in the test session, when there was not enough time to start scoping signals. Therefore I have no idea what may be going wrong - whether it is electrical, a chip malfunction on my daughter card, a logic flaw in the FPGA or something else.

I have to wait for the next session, which Marc may not be able to schedule for a few weeks as he attends to other projects. Since our Alto is in his basement, no work can proceed except when he can host us.


At our last session, we found that the Display for the Alto owned by Bruce Damer/Digibarn was displaying black but seemed to have scan and an image in the instant when it is powered down. My suspicion was that one or more of the three grids had a potential that was blocking the electron stream, but as various capacitors discharged during power-down the stream could temporarily make it through.

The three grids have potentials of up to 1000V so we proceeded carefully in our testing! I have an old vacuum tube voltmeter with a probe good to 30,000V (implements a large voltage divider and has an impressive plastic shield to keep hands away from high tension). It showed zero volts on all three grids.

Studying the schematic gave us a few places to probe between the flyback transformer winding that delivers 600V as short pulses during each horizontal retrace and the other circuitry, some that doubles up to about 1000V and others that generate -100. We found that a small incandescent bulb on the PCB was the fault!

The design routes the short retrace pulses of 600V through this bulb, a 28V 40ma mini lamp, depending on the shift in resistance of the filament from cold to warm to provide a kind of current regulation. The bulb had a lead cracked off where it entered the glass envelope.

Marc was able to reattach a lead to it, using his binocular microscope, a soldering iron and lots of patience. With it installed the monitor worked beautifully.

We then installed the four power supplies into the Alto and began inserting cards into the backplane. Unfortunately, the machine lacked one card, the memory address terminator board, and needs several cables that run between various boards. We didn't have time to proceed any further on this machine.

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