RetroChallenge Day 4: MBC020 video and troubleshooting

A while back, I tested out the video output on my new-to-me SYNERTEK MBC020 ExorBus SBC, and noticed problems with the upper half of the video. I wanted to make sure it was not my LCD monitor that was at issue so, I went out and bought a 9" black and white CRT security monitor to use for further testing. Unfortunately, when I recently went to connect it to the monitor, I could not get the MBC020 to boot anymore!

This is a good excuse to share some troubleshooting tips for these vintage SBCs.

With a multimeter or voltage tester:

1. Check the power rail voltage- this may seem obvious, but is an important first step, especially since these boards usually do not have any power indicators.
2. Verify that RTS or DTR goes low on the ACIA (or high on the serial connector)- This is a very simple check and you don't need an oscilloscope. If RTS or DTR goes low at the ACIA on boot, it indicates that the microprocessor is able to access ROM (to read the firmware) and the ACIA (to configure the ACIA), and is probably running OK. If it does go low but there is no activity on the serial connection then you may have a cable configuration problem, a terminal settings misconfiguration, or a problem with the RS232 driver ICs.
3. Verify that RESET goes high-

With an oscilloscope or logic analyzer:

1. Test clock signal to processor.
2. Test chip select signals to RAM, I/O chips
3. Test data bus and buffers

With this procedure I was able to pretty quickly determine that I had no clock signal to the CPU. After I swapped a few ICs in the circuit around the oscillator with no improvement it became apparent that the crystal was likely at fault. This problem seemed consistent with my nearly 2-year-old son knocking the board off of the dining room table a few months ago. I ordered the crystal and that brings us to now, the first week of RetroChallenge.

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Replacing the crystal was pretty straight-forward, despite the large amount of solder on the board in that area. I did have to use a small drill bit to completely clean out one of the vias, but no big deal. I really dreaded turning the board on for the first time though. If the problew was something other than the crystal, I would be very stumped and my RC2023/10 would get stalled right out of the gate.

Fortunately, everything came right back to life! The CRT monitor initially showed the same skewed image as the LCD, but after some heavy adjustment to the vertical hold, I ended up with a nice clear display. Although everything seemed to be working, I did notice some spurious "E" characters popping up after a command is completed. I am not sure if this is normal, or some new fault in the board, but I do not recall it happening before.

With everyting up and running, of course I had to mess with video connector and sudddenly I lost video and the terminal stopped responding. I tracked the problem down to a blown 5V fuse on the backplane. I must have short-circuited something. Fortunately, I still have a few hundred fuses left over from when I first built the backplane, so a new fuse and little soldering got it working again. I think I would definitely make the fuses easier to replace if I make another run of these backplanes.