Thanks everyone for your insight so far! I haven't had a chance to dig much further, but I thought I'd drop in with some more observations and responses to your questions:
As far as diagnostic equipment goes, yes, I do have another scope at my disposal, and a signal generator and multimeter.
The power rail ripple isn't as bad as I first thought - that 20mV figure I quoted earlier was a combination of measurement error and a rather electrically noisy environment. Taking a bit more care about the measurements, I think I'm hitting the noise floor of my cheap modern scope and probes at around 3mV. So it may not be quite within spec, but close. Voltages all seem to be stable and within tolerance.
The connections to the deflection plates appear to be OK. With the the horizontal amp common-moded and X-Y mode selected to un-blank the trace, I see a centered dot, so I think we can sign that section off as OK for now.
I've noticed that the horizontal position in X-Y mode is influenced by the sweep mode, which seems a bit odd to me:
- A Sweep (also A intensified by B): trace is offset far to the right, can be brought partially on-screen with the horizontal position control, but not fully.
- B Sweep: trace is offset by approximately one division to the left with the horizontal position control centered
- Mix: When pressed, the trace starts at B sweep position and then sweeps to the A sweep position at the rate set by the B timebase (and then stays there)
To me this lends credence to the hypothesis that the A sweep generator is 'stuck' - Andreas, thanks for digging that info out of your notes! I'll definitely take a look at that capacitor on the timing board.
However, right now the timing board is out of reach. I'm having some trouble removing the B trigger slope knob; no matter how much persuasion I apply, that grub screw isn't budging. Even with regular dousings of penetrating oil over the past few days, it's resolutely stuck, to the point where I've wrecked a couple of hex keys trying to turn it. Anything else I should try before I do some violence with a left-handed drill bit?
Thanks again,
Richard
