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Chemistry Electronics Services
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Repair CRT display, Philips TEM CM100 CM200 Electron Microscope
Electron microscope CRT display with crazy distortion. Can't read menu for pushbuttons!
Fortunately this one has schematics for the small PCB that drives the monochrome picture tube.
It's a TEA2037A sweep generator, LM1881 synch seperator, and LM1201 video amp. The ribbon
cable fits a 26pin gold pcb edge-connector, see schematic link below. Orient the connector by finding
+12V supply pins on one edge of the connector, pins 1 and 2 being on opposite sides.
NTSC video input is then pin 11, with grounds on 9,10, on 12,14,16, also 22,24,26.
Normal behavior: it initially draws 2amps at +12V, decreasing rapidly to 0.8Amps.
We had a video source; an old IR camera with NTSC output jack. The crazy distortion in
displayed raster was
both in transients in the vertical sweep, also as large blotches of video-blanking. Accidentally
I found that the CRT monitor worked perfectly at 13VDC supply voltage, but giving the
distorted raster at 12.9V and below. (Note that without a video feed, the vertical
deflection becomes very small.)
The cause was one dead, dried out electrolytic capacitor, C2; the 150uF 16V filter on the +10V supply,
on pin 16 of U1 the TEA2037A chip. (This capacitor is located right against the chip, next to pin16.)
Also a similar capacitor was nearly dead but still functional, C21 100uF 50V on the flyback xfrmr.
Replacing these caps put everything back to normal. The same symptoms still appear, but only
if the supply voltage is decreased down to +11.1V
LINKS
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Department of Chemistry University of Washington Box 351700 Seattle, Washington, 98195-1700 Voice: (206)543-1610 FAX: (206)685-8665 |
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