Too bright a signal makes monitor shut down

[James Olson]

I have an Apple Multiple Scan 17 monitor M2494 with a Sony chasis. I have
failed to locate schematics (Apple is notorious for withholding tech data).
I bought this unit together with a PowerMac 7600 at a flea market for a
measly $20.00 US.

After taking it home, the PowerMac won't boot. I find out it's a missing
ROM SIMM. Like any PC missing its BIOS, no startup program, no boot. The
good news is that I know of sources to get a replacement.

The story with the monitor is not so good. Turning it on with no signal input
results in green power light for about 5 seconds then it goes out. I read on
another post that these units are designed to do that. Ok, so that's not a
bug. Hook up the monitor to my trusty known-good Mac II. Turn the monitor on,
same sequence. Ok, now what? I walked away. When I got back, there was a dim
raster on this unit!! The "dimmer" screen saver on classic Macs was running.
This thing is so simple, it doesn't run any graphics, just drops the
brightness.

The raster was perfect in size, color, and shape. When I bumped the mouse,
the screen saver exited, the signal output by the Mac immediately went back
to its normal brightness, and the raster on the monitor followed suit for
perhaps one second. Then I saw a horizontal white streak flash across the
middle of the tube as it shut down. The green power light went out as usual.

When I pushed the mouse to the lower left corner of the Mac "screen" (Mac
screen savers let you define which of the four corners will immediately
activate it if you park the mouse cursor there - wish PC and Linux did this).
By causing the signal brightness to drop, lo and behold the monitor turns
itself back on! I got back the dim raster. I found that this monitor keeps
a memory, and it knew it was still set to ON. Pressing the power key once
sets it to OFF, and a 2nd press puts it back into my dim state.

I examined the mainboard for burned components, broken/bad solder joints.
None found. The back of the CRT neck board also looked OK, but I didn't have
time yet to remove the board to examine the front/component side. I wrote down
the sizes of the electrolytics and plan to order replacements.

I've poked around the inside of CRT TVs and monitors and am familiar with the
major components, good soldering practices, read the sci.repair FAQ, etc.
I recently built a 100W light bulb device, have a borrowed variac and scope.
I plan to begin looking for the fault right away. Any help pointing me in the
right direction (which parts to probe) would be appreciated.

Jim Olson
Software Engineer
PHLX - The Philadelphia Stock Exchange
www.phlx.com
Email: e1-0j2z-sn0w-jcbd@emailias.com
Temporary address - spam it and I'll expire it, rendering it obsolete!)

 

[Zeljko]

One test:
Try to decrease G2 voltage (lower pot on LOT, do not attempt to measure G2
voltage). Brightness will be lower - it's OK, and see if it works OK then.
Put some bright picture.

I have this problem very often on IBM 14x monitors.

If it works OK with lower Ug2, and switches off if You increase Ug2....
.... then clean everything around CRT socket with acetone (CRT neck only with
alcohol or dump cloth). Remove any glueing which can turn conductive. Check
for leaking elko - spilled electrolite around HV tracks on CRT PCB. Change
few nF/few kV capacitor which goes from G2 to ground. Remove that damn foam
block which manufacturers puts for some pressure on the back of CRT PCB - it
can turn conductive.
I also clean main PCB around HV stabilizer circuit, and check-change small
elko's there. Clean CRT around HV connection.

 

[James Olson]

"Zeljko" <zeljko.hrvoj@zg.htnet.hr> wrote in message news:<c351eq$57d$1@ls219.htnet.hr>...

One test:
Try to decrease G2 voltage (lower pot on LOT, do not attempt to measure G2
voltage). Brightness will be lower - it's OK, and see if it works OK then.
Put some bright picture.

I have this problem very often on IBM 14x monitors.

If it works OK with lower Ug2, and switches off if You increase Ug2....
... then clean everything around CRT socket with acetone (CRT neck only with
alcohol or dump cloth). Remove any glueing which can turn conductive. Check
for leaking elko - spilled electrolite around HV tracks on CRT PCB. Change
few nF/few kV capacitor which goes from G2 to ground. Remove that damn foam
block which manufacturers puts for some pressure on the back of CRT PCB - it
can turn conductive.
I also clean main PCB around HV stabilizer circuit, and check-change small
elko's there. Clean CRT around HV connection.

You're not going to believe this. I opened this monitor, extracted the
mainboard so I could examine both sides for bad solder joints and bad
components (found
none), then I set up for the G2 brightness test you mentioned by
putting the
board back in, hooking the unit up to the variac and connecting to the
Mac
(which had been left alone long enough for Dimmer to kick in). With
everything back together and the back still off, I powered up.
Wouldn't you know that I got a good full-brightness raster?? After
dabbing the pot with whiteout to mark its
original position, I did move it. The raster got fuzzy quickly in both
directions. The original position gave me a sharp picture.

When I got the monitor completely back together, a 2nd test confirmed
that it
is now working. I chalk this up to dirty connections.

Notes for anyone reading this:

1. The flyback/LOPT has no controls. Instead, this design has two
wires coming
off: one is the HV anode wire going to the CRT, and the 2nd is the
focus
wire and it runs to the CRT neck board. The focus wire dead-ends
into a
black module with three pots labeled: FOCUS1, FOCUS2, and H-STAT.
Cutouts
in the metal shield provide access. It was FOCUS2 that I adjusted
above.
2. There was no foam brick on the neck board shield
3. All cables running to/from the mainboard have either inline
connectors or
terminate at a connector. This made removal of the mainboard quick
and
painless. I have to thank Sony doing this!


James Olson
Lansdale, PA

 

[Mike]

that wasnt the G2 pot you turned, that was the focus pot you turned.



"James Olson" <e1-0j2z-sn0w-jcbd@emailias.com> wrote in message
news:4a15e416.0403201423.3dbcea9a@posting.google.com...

"Zeljko" <zeljko.hrvoj@zg.htnet.hr> wrote in message
news:<c351eq$57d$1@ls219.htnet.hr>...
One test:
Try to decrease G2 voltage (lower pot on LOT, do not attempt to measure
G2
voltage). Brightness will be lower - it's OK, and see if it works OK
then.
Put some bright picture.

I have this problem very often on IBM 14x monitors.

If it works OK with lower Ug2, and switches off if You increase Ug2....
... then clean everything around CRT socket with acetone (CRT neck only
with
alcohol or dump cloth). Remove any glueing which can turn conductive.
Check
for leaking elko - spilled electrolite around HV tracks on CRT PCB.
Change
few nF/few kV capacitor which goes from G2 to ground. Remove that damn
foam
block which manufacturers puts for some pressure on the back of CRT
PCB - it
can turn conductive.
I also clean main PCB around HV stabilizer circuit, and check-change
small
elko's there. Clean CRT around HV connection.

You're not going to believe this. I opened this monitor, extracted the
mainboard so I could examine both sides for bad solder joints and bad
components (found
none), then I set up for the G2 brightness test you mentioned by
putting the
board back in, hooking the unit up to the variac and connecting to the
Mac
(which had been left alone long enough for Dimmer to kick in). With
everything back together and the back still off, I powered up.
Wouldn't you know that I got a good full-brightness raster?? After
dabbing the pot with whiteout to mark its
original position, I did move it. The raster got fuzzy quickly in both
directions. The original position gave me a sharp picture.

When I got the monitor completely back together, a 2nd test confirmed
that it
is now working. I chalk this up to dirty connections.

Notes for anyone reading this:

1. The flyback/LOPT has no controls. Instead, this design has two
wires coming
off: one is the HV anode wire going to the CRT, and the 2nd is the
focus
wire and it runs to the CRT neck board. The focus wire dead-ends
into a
black module with three pots labeled: FOCUS1, FOCUS2, and H-STAT.
Cutouts
in the metal shield provide access. It was FOCUS2 that I adjusted
above.
2. There was no foam brick on the neck board shield
3. All cables running to/from the mainboard have either inline
connectors or
terminate at a connector. This made removal of the mainboard quick
and
painless. I have to thank Sony doing this!


James Olson
Lansdale, PA