Date: Tue, 15 Nov 1994 20:49:40 +0800
Subject: SX monitors, cold water risks
From: B.Robinson@dem.csiro.au (Bruce Robinson)
X-Mailer:

Note for SX-users via John Fournelle

SX green monitors fold-around problem and
Cooling-water low-temperature hazards

Monitor Fold-Over
Terry Harrison, who is the electronics expert in our team, has supplied the
following notes
from our maintenance records. Sorry this is late. Our network dragon
devoured the first
draft yesterday.

We have fixed the X fold-around problem several times on the Thomson M1 and M2
monitors on our 1987 SX-50. We understand that later SX-50s had "Clinton" ?
brand
monitors, which are very similar, I am told.

On our version, we use circuit diagram #29277204 1 of 5. There are three
transistors in the
X-driver chain. They are Q2,Q6 and Q3. These are BD529, BD530 and 2N3792
respectively. We used the equivalents BD131 and BD132 for Q2 and Q6 as they
were
easier to get locally. Q3 is the main driver and is on a vertically mounted
board at the back
of the Monitor box. It is in the centre (of 3) in the bottom row (of two
rows).

Q2 and Q6 are on the Ampli-X board which is mounted horizontally at the
bottom of the
monitor box. The row of transistors nearest the back of the box (starting
from the column
edge of the board) are Q22, Q6 and Q2 respectively. Q22 is nearest the column.

We have twice replaced both Q2 and Q6 at the same time, as recommended by
Claudio
Cermignani (and presumably Edgar Chavez) at Toronto. Terry's reason for
doing this is
that it is very hard to get to the board, so it is simpler to replace both
transistors at once
than to risk replacing the wrong one and have to come back to replace the
other. Claudio
and Edgar may have an additional reason.

We have once replaced Q3 to solve the same problem.
********
If any users have a good third-party source of the monitor CROs, we would
like to know.
We are thinking of getting a spare CRO tube. Ours have a metal mounting
plate fixed to
the glass of the tube.

I would like to thank John Fournelle for keeping the users' list. It
promises to be
increasingly useful. Perhaps we could accumulate a set of maintenance
hints like this which
could be archived somewhere, so when people have a problem they can look at
the index
under Monitors or whatever.
*********
Under-temperature cooling water warning
I have been intending to write a note about the real risks of cooling water
under-
temperature. This has been known to cause condensation of water in the
electronics, inside
the lenses in the column and be generally nasty (water can drip from the
water-cooled green
monitors down the electronics cards). Early SX's had no temperature
sensors. Later ones
have both over and under-temperature monitors. However, even this is not
adequate if the
SX power is off with the cooling water continuing to run. If the water
gets cold, below
dew-point, condensation will form in awkward places. When the power is
turned on again, it is
too late for the sensors to issue a warning. It is possible that the water
has come back to
normal temperature, but the condensation is still wetting things. I
understand that
considerable damage (? "resistors unsoldering themselves and falling on the
floor") has
happened even on a temperature-sensored relatively new SX.

Suggestion. Someone should design a simple condensation-check system which
would
continuously measure conductivity of a test strip attached to the cooling
system piping. If
condensation had occurred, turning the power on would be prohibited (or the
power would be
turned off, if it were on.). The users' group could perhaps collectively
design one, and
distribute notes on how to install it.

We have some plastic drip trays under the monitor cooling-pipe brass joints
to catch any
drips and convey them to the floor, hopefully by-passing the electronics.
So far, our only
problems have been over-temperature in the water (40 degrees C), for which
we have fitted
buzzers and a thermal cut-out. We do not have an "under-temperature" sensor
on the SX
(yet), but we do have one on the new cooling water system.


Regards from Western Australia

Bruce Robinson
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