Mag 986FS CRT shimmering picture - schematic?

Jules Richardson wrote in news:khkid4$rqg$ snipped-for-privacy@dont-email.me:

Sounds like a weak powersupply, and/or High voltage supply. Check for bad components around there.

Open it and look for bad caps by the power supply.

That's all I've ever had to replace on a Mag.

Good place to start - assuming its a colour monitor, a faulty degauss system can also cause picture shimmer.

Most monitors use a 2-pin posistor with a relay to take it out of circuit after a delay - a welded contact would cause it to continue drawing residual current through the degauss coil.

Some monitors used the same 3-pin dual posistor as TVs - 1 posistor in series with the degauss coil and the second one direct across the AC mains, that heats the deguass element so it doesn't draw current - a failed heating posistor can cause shimmer.

Well... it is looking like caps. There weren't any that were showing visual signs of failing, but then of course that's often the way. Sadly my ESR meter's 4,500 miles away (so I've put it on the to-do list to buy another).

Anyway, I've replaced the PSU's main smoothing cap and those on the outputs of the 104V and 52V lines using parts from the junk box (which aren't necessarily in the best of shape themselves!), and - so far - that seems to have just about cured things; there are some *very* slight signs of picture instability when scrolling display contents, but far better than before and certainly at a level that I can put up with. I'll give it a week or two and if it's still OK I'll order some new replacement caps.

Thanks for the replies, everyone!

cheers

Jules

Its worth seeing how many caps run hot after replacement as well as when determining which ones need to be replaced.

The ones that get hot obviously won't last long - sometimes you can take the edge off the ripple current with a polycarbonate on the print side (in parallel with the electrolytic).