3b1 Terminal & Scrreching Noise

[Bill Mayhew]

I've had more opportunities to look at sick monitors over the years
than I would like to have had.  Most of the time when the flyback
transformer is bad there are other components bad too.  It seems
that most monitors (90%??) have the flyback capacitor coupled to
the horizontal output transsistor via a cheap electrolytic
capacitor.  The flyback of the horizontal scanning voltage from the
right side of the screen to the left sets up a ringing oscillation
in the flyback transformer, generating the high voltage for the CRT
anode supply.  What usually happens is that the electrolytic
capacitor fails by shorting out.  There is a fairly high AC current
waveform flowing in the capacitor.  A lot of monitors use power
supply filter type capacitors that aren't designed for heavy
currents.  If the coupling capacitor shorts, the flyback is
essentially connected directly to the DC power supply; this in turn
burns out the flyback winding, and maybe the damper diode too.

Some high frequency monitors, such as 31.5 KHz monitors used for
VGA displays don't need electrolytic capacitors; they use
nonpolarized mylar caps for coupling.  At least this is what I
noticed when I disassembled a Samsung VGA monochrome moniotr for
preventative maintenance.  I was impressed.

My recollection is that the monitor on my 3b1 has the typical old
ugly electrolytic cap.  I have seen at least two types of CRTs on
Unix PCs, so yours may be different.  The monitor on my machine has
a small glass fuse where the power supply wires connect.  The
monitor on our Netnews fowarder Unix PC back at the office has no
fuse in the monitor and a different PC board layout.

With monitors' penchant to fail in ugly ways, I opted to install a
a separate power switch on the 12 volt power lead feeding the
monitor on my 3b1 at home.  I soldered a wire to the fuse and
another wire to the fuse holder and put a piece of tape between the
fuse and its holder.  A switch is then attached to the free ends of
the wires.  As such, the switch can be used to shut off power to
the monitor while I'm not at home.  I drilled a 3/8" hole on the
right bottom of the case for the shaft of a small Raido Shack
toggle switch's handle.  On my monitor, there was plenty of room to
install the switch.  Note that it is important that you put tape
under only one end of the fuse when installing the switch so that
the switch is wired IN SERIES with the switch so that you have
continued fire protection.  Of course, if you are going to work on
the monitor, unplug the machine from the wall and leave it off for
30 minutes before opening the case.  This is necessary to assure
that any residual high voltage dissipates; even then, you should
still presume that high voltage charge may remain.  Never work on
anything electronic alone.  Have someone present to call for
assistance if you are subjected to electrical shock.  One more
safety issue is the display tube.  Hitting the tube may cause it to
crack and subsequently impode.  Very serious injury can be caused
by the flying glass of the implosion.  I have seen a CRT implode;
it is VERY ugly.  You don't want to be in the same room if it
happens.

If you have the type of monitor without the fuse holder, you'll
need to splice into the power lead itself if you want to install a
switch.

Summing up, it is best to replace the whole monitor.  The flyback
transformer and associated components are involved in generating
the high voltage for the CRT anode.  Said voltage determines the
amount of X-rags emitted by the CRT (yes, the CRT itself can give
off X-rays).  If you affect repairs, you have to use exact
replacement parts to assure continued safe operation.

One source of Unix PC monitors is Discovery Elecrtonics.  Check an
Issue of Unix World for their address.  Discovery used to be known
as Digital Data Systems, I believe.

One might be able to use something like a NEC multisync GS for a
Unix PC monitor when/if the supply of "real" monitors dries up.  It
would, of course, require consulting the hardware reference manual
to figure out the wiring.

Bill
wtm at neoucom.edu   or    wtm at impulse.UUCP