I looking at https://console5.com/techwiki/images/f/f8/Vectrex-Troubleshooting_Guide.pdf, Symptom "Picutre size changed and unstable when input voltage is normal". There is info that IC501 should have PIN 5 = 5VDC and PIN 8 = 3VDC. I have 5V on pin 5 but 12 V on PIN 8. Something wrong with the system? I looking at LM555 datasheet, its MIN voltage is 4.5V and MAX voltage 16V (it's supply voltage, pin eight). So, how could it be 3V? It simply will not work, if I understand correct.
Also, waveforms of T502 pin 7 and Q502 base (pin 1 according to datasheet) should be the same. It is not. Here is a screenshot (blue is T502 and yellow is Q502). It was made not at Test #3, but as far as I remember it is the same in any mode.
Waveform of IC501 in my board is 45 ms, not 50ms. Wrong? Can change it to 49ms using potentiometer (max position), and no change in wobbling issue.
Also, I have a different input voltage: ~11.5 V instead 8.5-9V. I suppose it's due input voltage 250V instead 220V, and (maybe) not very good chinese AC transformer (I bought 220V 9-0-9 as a replacement to a dead original). However, I can say all these wobbling were with an old transformer too (it was a 220V model in 250V network).
Any ideas what to do?