I am definitely erring towards a faulty VIA (IC207).
The only other possibility is something else 'dragging' this line. I would suggest looking at the PCB track from IC207 pin 12 and following it to see if there is an accidental short circuit to something. The other possibility is the IC302 input pin that is shorted out internally.
Are either of these ICs in a socket, or are they not being nice?!
Just back from testing. Used another 6522 chip - taken from my perfectly working Commodore Pet. I know this one is 100% working. Same story.
I looked for shorts with GND. But between IC207-12 / GND and IC302-9 / GND I measure 1,9 kOhm (powered off) and 0,4 kOhm (powered on).
No short. Would be too easy, right?
However - I found a signal now at IC302-9. Hardly to see. Just a dot. Took a little video and extracted 2 pictures showing 2 dots. Setting of my scope: 0,5 ms/div and 0,2V/div. This time I connected my external trigger to IC302-10.
Dave, Looking to the schematic and following the path you guided me I guess my issue is on the Analog Board? Or do you see something suspicious on the last pictures?
I can follow up the hunt tomorrow afternoon or in the following days but don’t feel pushed please. You will be probably busy during the working days…
I am not in hurry. I recently bought another, working Vectrex which will probably arrive next week. Maybe a possibility to compare things
Hi Dave. I am reviewing the waveforms I have taken from J301 for X, Y and Z. X and Y are looking pretty much like in the service manual. But Z looks different. Do you think we should focus more on Z? I have attached the screenshot from Z again.
I agree, there is something amiss here somewhere...
Let me describe how I think the Z circuit should work...
That DAC is setup to output a voltage from IC304/1 into the multiplexer IC IC302/13 (IN1). From my understanding, this should be fully positive for 'beam on' and fully negative (?) or 0.0V (?) for 'beam off'.
SEL1 should be set to '1' and SEL0 should be set to '0' with DIS '0' to enable IN1 to OUT1C. In this mode, the input signal from the DAC should be output to the Z channel on IC302 pin 15.
C306 is the sample and hold capacitor. The capacitor is charged up to the analogue voltage that is output on OUT1C.
IC303 (on the Z channel) is a very high input impedance voltage buffer. The output voltage on pin 8 should always be the same as the input voltage on pin 10.
When IC302 pin 6 (DIS) is set HIGH, the output at OUT1C should become open circuit and the voltage that was originally stored on C306 should remain (due to the high input impedance of the following IC303 OPAMP).
Now look at the output of IC303 at pin 8.
If the /BLANK signal goes LOW - this causes the Z signal to go low and blank the beam.
If the /BLANK signal is HIGH - this is the pull-up voltage to turn the beam ON.
However, if the output from IC303 pin 8 goes negative, this drags the Z signal down (via diode D302) and this also blanks the beam.
My expectation is that the Z signal is either ON or OFF (as per the manual) and not as you are observing it.
The question now remains as to why it is like it is. If we can answer this, it may 'open up' the other faults.
Hi Dave. Thanks for your suggestions. Over the last days I reviewed our troubleshooting and my findings. I wondered why the position of Pot0 and Pot1 influences the vectors and why the Z signal looks weird. I thought maybe IC302 is the culprit. And you know what? It is! I just desoldered the chip and installed a new 4052B with a high quality socket and now my Vectrex is up and running again! Just celebrated with my neighbor with a good bottle of german beer
I hope the Vectrex keeps running now. It´s now in an edless Mine Storm Loop for about 1 hour. Fingers crossed...
Thank you very much for your support and help, Dave. You encouraged me not to give up and keep searching