Common items you passed up that turned rare when you wanted
ard at p850ug1.demon.co.uk
Tue Jan 31 15:53:23 CST 2006
> up a keyboard to it straight! I do have one monitor. I haven't tried
> to power it up, but written on the case is the word "DIM", presumably
> identifying a failure in the monitor somewhere. I've never repaired
> monitors, so I'm not sure what a dim image is suppose to indicate.
> Failing HV drive circuitry?
It's not failing EHT (HV) on its own. That will cause a dim picture, but
it'll also cause a large picture (low EHT means the beam is 'less stiff'
and is deflected more easily).
One (embarassing ) cause is notthing more than a dirty screen. Clean it
 Well, it's embaarassing if you're staring at a 'scope and meter, got
the schematics open, etc, and have spent a couple of hours looking for an
Other, more serious causes include :
Low emission CRT. A CRT of that age might take being 'blasted' if you can
find somebody with the right device, or feel like making one (there were
plenty of designs in the magazines between about 15 nd 30 years ago)
Low first anode (g2) voltage. In monitors that produce this from the EHT
by a resistive divifer network it's not at all uncommon for this to fail
and lower the first anode voltage. It's often, alas, part of the flyback
transformer, and those are even harder to find than CRTs.
Incorrect voltage on the cathode or grid of the CRT. Normally the easiest
fault to put right (in that it'll be a common small component in most
monitors), but the hardest to trace.
Power supply problems. A low supply rail to the monitor might cause a dim
picture with no other problems.
Does anyone have a working monitor of the saem type and suitable test
equipment. If so, bribe them (:-)) to measure the supply rails, CRT
electrode voltatges, etc. It'll give you something to compare against. I
actually do this when I get a new monitor, and write the values on the
schematic. Then when it fails I know where to look.
> > The mouse is a 3 key straight quadrature output thingie.
> Do you have any ideas on what kind of commercially available mice
> would be compatible?
If it is a quatrature output mouse, and is TTL levels, then I would think
an ST, Amiga, or Acorn Arc mouse could be used with a connector change.
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