Things have been going in fits and starts. I’ve had some very helpful direction from a chap on the Vintage Computer Forum, which has led to a bit more troubleshooting.
Replaced some more capacitors that were bad.
That got pretty much everything stable on the power supply. I cleaned up the main power connector pins and plug, that got the voltages to the boards stable also. Got the voltages set up to within 2% so that’s working properly.
Started troubleshooting to see if things were alive. Pulled the keyboard off and vacuumed out underneath it.
Took a look at the keys, put a meter to a few of them and got funny results.
It’s a Micro Switch keyboard, manufactured by Honeywell. The reason for the odd readings is that the switches are operated by the Hall effect, magnetic sensors within the switches, with no moving parts or contacts other than the switch plunger itself.
Had a poke about on the board, and the big controller chip appears to be alive- the keyboard seems to be a discrete device, and thoughts are it’s an RS232 style ASCII-encoded device. Makes sense to be.
Pressing keys makes various codes on the pins of the controller chip appear:
Had a better look at the RAM and found the correct pinout for the chips. Looked at the Q (output) line. There’s definitely something happening (the camera on my phone appears to be dying):
After this, attention turned to the monitor and video card, as it’s not really doing a whole lot.
The suggestion was to try and drive one of the video RAM lines high or low to see if it caused the generation of junk on the screen- this would cause the request for a lot of empty “space” characters to be drawn normally.. driving the line high or low would force different character codes and hopefully fill the screen with trash.
Drove the output high from Vcc (+5v).
Okay, so we can try driving the line low instead:
Result? The same neat(ish) raster.
So, with raster, at least the V-sync and H-sync are operational. Poked about, got a nice happy V-sync:
The input to the monitor, looking at the board from the back, left to right has the following inputs:
Ground | V-sync , Intensity , n/c , H-sync , n/c , Brightness COM , Brightness , Brightness inverted
Took a wire and put it from 5V to the brightness input that makes the dot on the screen bright. Up until now I did not know if the monitor was actually working- that is, if it was fed a signal, would it display said signal on the screen?
Yes, it does.. and because it’s connected to the circuit board still, there’s a ghostly image on the screen also.
Traced back from the pins to the chip (7406, buffer inverter) and poked with the scope to see if there was anything on the input. Nothing much, just a high signal (expected), so grounded the input pin.
Got this on the screen:
Lots of squares! Very poor contrast (to be expected, due to the grounding of the pin), but I need to see if it’s actually coming from further back or if the chip, loaded down is making the image poor.
Junk on the screen is better than a blank screen.. it’s signs of life, and just means there’s troubleshooting ahead. It’s something to start from, at least!