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Old 14th Oct 2021, 10:21 pm   #293
SiriusHardware
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Join Date: Aug 2011
Location: Newcastle, Tyne and Wear, UK.
Posts: 7,527
Default Re: Sharp MZ-80K help needed on repair

This forum has probably the highest density of CRT circuit experts anywhere in the world but most of them probably don't read this (computers) section and I don't count myself as anything more than an interested amateur when it comes to CRT stuff.

You could try a separate thread in 'Vintage TV and Video' with the heading 'Sharp MZ-80K CRT monitor fault', post a couple of images of the fault condition and and point them to PDF page 42 / Manual page 40 of the service manual (Monitor circuit diagram) and PDF page 41 / Manual page 39 (PCB layout of monitor). Also the monitor block diagram, PDF page 19 / Manual page 17. For any CRT gurus who do happen to read this, the service manual which includes the monitor circuit details was posted in #3 of this thread.

Given such good service information and good images of the fault condition I'm sure someone in the TV section will be able to point you in the right direction.

All I can tell you is that the upper left and top of the circuit is the video section, upper right is the audio circuit, the middle-left section is the sync separator, the centre middle to centre right section is the vertical deflection / scan circuit and the bottom section going across horizontally is the horizontal oscillator, drive and line output stage. If the fault is that the scanned picture is full height but the top line is bent sideways I think you are looking at a problem in the lower third of the diagram, if the top of the picture is compressed (all the scan lines at the top squeezed into a smaller space) then more likely the middle (vertical scan) section.

From your point of view it's good that the monitor is powered from 12V so there aren't any mains voltages backed up by unlimited current lurking in there and waiting to kill you, but the monitor circuit itself does generate some high voltages especially the voltage (Kilovolts) fed to the tube via the thick rubber lead and cap going to the tube, and there are also voltages of well over 100V in the area around the line output transformer / tube neck.

Don't forget the simple stuff as well, try scoping the +12V supply to see if it is dipping or sagging every time the display sets off on a new scan down the screen. It should be steady 12V, maybe with a slight sawtooth shape to it but without any big dips or downward spikes.

Don't just plonk the scope probe on any random point in a monitor like this, if you accidentally hit one of the high voltage points you could potentially damage the scope.

With care you could also use freezer spray to try to isolate this fault as long as you kept it away from the high voltage areas.
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