A log of what's happened, so far. Pictures are clickable to view larger versions. The yellow tag above circuit breakers marks the one that popped out (once). I forgot to jot down which one it is, so I'll have to update that information, later on.
October/November 2011, machine's first observed behaviour: Scrambled picture, with appearance of some picture buried inside noise, but looking like the horizontal scan rate off frequency (interestingly, proc amp controls adjust right parts of picture—sync and picture gain and black levels, correctly and independently). Tracking meter low/no output, tracking control ineffective. RF playback dropouts (used CRO on RF dub output terminal). Characteristic whine of quad heads sounded not quite the same pitch as I expected. Signal doesn't make it through the full E-E path through machine, black lines across picture (some asynchronous addition of sync on top of the picture). Monitor switcher has distorted output (intermittently had async sync drifting through picture, and white crushing). There are two simultaneous outputs from this switcher, one okay (SWR OUT), the other distorted (the picture monitor video output). Separate socket for sync to waveform monitor has a very low output level. Various test points in left hand rack units have no outputs.
All cards in mod/demod and servo racks down the left side pulled and contacts cleaned, no change observed. Pulled out video out/in leads between modules, and inserted colour bars. If replace DEMOD out from left hand rack units with colour bars, get a good clean picture on main output. Suggests AMTEC and COLOURTEC modules are probably okay.
E-E restored: A coax wire had broken in the (module 5) board for the RF-dub/RF-mod input selector.
Black lines over the picture can now be easily seen. Appears to be asynchronous sync over the picture. Careful poking of finger over back of traces on sync processing board around trimpots showed R74 changes the frequency. Playing with R74 can change their frequency to match the picture, but they're still out of phase (locked on frequency, but wrong position, horizontally). Width of pulse looks more like blanking pulses, rather than sync pulses. Temperature/time dependent, needs re-tweaking often. Yes, I know you don't “fix” things by playing with trimpots, but it's probably been fiddled with before, will need fiddling with again, and I know I'll have to look at the circuitry around that trimpot to find the fault.
Control head cleaned, properly. The wrong side was being cleaned before (the shell of the head, not the side that faces the tape). Tracking meter, now, shows a reading (about 1/3 deflection). Haven't checked whether that's a fluke, yet (dunno whether the tracking meter shows control track output, or head RF output level).
Tried changing video head, saw no observable difference to picture. Moments later, smelled smoke and a circuit breaker had popped. Unable to locate actual source of smoke, nor see any apparent damage. Smell most strongly around top left of mod/demod rack, or erase head module behind front panel. Didn't see any change in behaviour when happened, we manually shut off power the moment we smelt smoke, noticed the breaker had popped afterwards. Breaker unrelated to those functions, can't remember what it was, but marked it on the panel. Replaced original head, playback same as ever. Later on, swapped heads, again, playback same as ever. Swapped back to original heads. No further smoke smells, or breakers popping. Not sure if something merely got hot, or actually burnt.
High-band/low-band mode switch very unreliable, but working normally after sprayed with contact cleaner and switch rotated a few times.
Extensive cleaning of tape path, and internals of capstan motor. Got a near-normal monochrome playback picture, from a monochrome recording, when machine warmed up. Before that happened, was playing back with appearance of tv set with horizontal hold slightly off frequency (diagonalised picture, sync locked but off frequency), gradually getting closer to correct rate over time. Interestingly, doesn't appear to have any of the common quad playback errors (16 line segmented or scalloped picture). Tracking meter is reading a steady value, tracking controls have no effect (to picture, nor can we hear the servos do anything). Machine appears to be free-running tape speed-wise and head speed-wise. Minor tweaks of speed controls on servo boards can make the machine drift onto frequency, but it will not lock, behaving like a tape recorded with no control track. With very fine speed adjustment, can move noise bar to bottom of picture, and see minimum of skew error, but can't get a noise-free picture.
Really unreliable audio level pots, for input and output, cue and normal audio.
+135 volt supply is at 150 volts, at the front panel test point. And the +24 volt supply at 25 volts.
More to follow… But really need manuals, and someone experienced with servicing these machines.