Advice for Tek 4051 vector-drawing issue?

Josh Dersch wrote: Good call with the X/Y connector. They might have pulled them off the board layouts and not updated the schematics... Not a very Tek thing to do, but that sort of thing can happen. Divide-and-conquer's great, isn't it? :) Which leaves you -- as you quite rightly said -- the path between the DACs and the deflection system, which is basically the filters. It'd be worth looking into, and the first thing I'd check. The schematic (Vol2, p.2-12) shows no electrolytics in that path other than two pairs of 1uf decouplers on the opamps -- C853, C854, C871, and C872. I'd ignore those for now, unless the power supply is abnormally noisy. That part of the circuit is basically all 'dry' capacitors, i.e. no electrolyte. You've got two 0u01 (10nf) Paktron "plastic" capacitors per filter, and a 470pf ceramic. I suspect those are probably good, they don't usually fail unless overvoltaged or otherwise abused in some way. The parts manifest suggests that R965 (one of the 39meg resistors) is a carbon-composition type. Just about anything will make these things change value, but the main killers are age and (again) voltage stress. They're a known cause of failure in old kit, and generally the value will increase over time. That screws the time-constant of the filter, lowering the cutoff frequency. My plan-of-action would be something along these lines: 1) If you've got a DMM that goes as high as ~40 megohms, desolder one leg of each of the 39M resistors and check them. If not, desolder the existing resistors and replace them, but keep the old ones for now in case their replacement causes problems. 2) Also check R894 and R961 (100R 5% carbon-comp) and their mates in the X filter. These supply power to the opamps. If these have gone high in value, the opamp may have trouble sourcing current into the output circuit. This will reduce the filter's cutoff frequency as well... 3) If that didn't fix it, check the other resistors in the filters. For the Y filter, that's R962, R963 and R964. 4) If the opamps are socketed, swap them both out. You'll want a Signetics N5558, which Google tells me crosses to the MC1458. Which is still fairly common after all these years :) This wouldn't be my first choice, though. Again, unless overstressed these don't generally fail. 5) Those plastic and ceramic capacitors are IMO the least likely to fail (but tied with the opamp in the "chance of failure" stakes). Let me know how you get on, this is starting to get interesting :) Cheers, -- Phil. classiccmp at philpem.me.uk http://www.philpem.me.uk/