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Elektronika MK-52: The MK-61 big brother?
02-18-2015, 06:22 AM
Post: #27
RE: Elektronika MK-52: The MK-61 big brother?
Thanks again jebem for your detailed repair writeup; it was very helpful in repairing my unit. The power supply needed recapping (all 4 large caps were the flag brand; all tested OK, but had terrible ripple under load). The switch in the power connector and the keyboard foam needed work too.

After I got everything else working, I found that some of the digits and some segments on the display were intermittent. All of the signals to the VFD were OK on the pins, and it changed if I twisted and pushed on the board - it turned out to be the lead-to-glass transition on the VFD itself.
The pins are connected to the traces on the glass with a black blob, then the whole thing is sandwiched with a brown glue and a piece of glass. I'm betting the blob is carbon-based glue and the brown is epoxy; it wouldn't budge after 12 hours in acetone, but some dichloromethane/methanol mixture dissolved it in a few hours.
After cleaning it up I mounted it back on the board with some 90 degree header pins and some carbon conductive adhesive of my own. By connecting the filament first, you can see the individual segments and digits come alive as you drop the glue on to the other pins! The filament pins need metal-to-metal contact, though, because the filament impedance is only 100R. You also have to be careful where the glue goes - a short between any two pins on the order of 50K will cause ghosting, and that's an invisibly small smear of glue.
I also found out the hard way that the front face of the VFD is also coated with a transparent conductive layer and connected to filament potential (-27V + AC). I assume this has to do with directing the electrons towards the rear of the device?
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RE: Elektronika MK-52: The MK-61 big brother? - jhl - 02-18-2015 06:22 AM

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