KME MB11 Monitor module (from Gould K 50 Logic Analyzer) dea

[Robert Obermayer]

Hi,

i just collected an Gould K50 Logic Analyzer from my Uni's trash.
Which, well, doesnt work, no picture displayed.After discovering a
"Comp. Video Out" BNC on the back side and connecting a regular PAL TV
to it, i got a picture, and could confirm the analyzer part works fine.
Just the monitor doesnt.Its a small (6"ish) monochrome (probably green
or amber) raster scan monitor accepting a BAS composite signal (probably
PAL, or at least similar).It rund off 12V from the large main power
supply, which is okay.

Now for what i found, after looking at the monitor PCV for some time i
identified a burned 1/4W resistor (R9).Its completely burned, no color
code to see anymore.Being mounted a few mm off the board (the only
resistor to be mounted that way) it could well have been a fuseable
resistor.
Now for its environment, its connected in series with D7, a BA159 Diode
being connected to one of the many line output transformer pins.The
other side goes to 2 unsuspicious 630V film capacitors and a red wire
going to the CRT socket, but also to one of the many ceramic thick-film
circiuts in there.

Now, having a shematic would be great, but KME denys any knowledge of
ever having made a MB11 Monitor, and if they would, they wont give out
any service info.Great.

Does anyone have a remote clue of the resistors probable value?
Id think of something in the 10Ohm-ish range...

Also, im fairly sure the problem is somewhere else and it will burn up
right after replacing, but after testing all testable semiconductors
(thats all, except the vertical output IC and the 6 thick film hybrids)
i coundlt yet locate any other fault.

Does anyone have any service information on this or any similar monitor
module?

If all fails, i guess i will somehow fit one of these small LCD monitors
designed for car use in there, which certainly isnt a very good thing
and will look ugly.

Regards, Robert

 

[Robert Obermayer]

Hi,

now after recieving a much more useful answer from another KME employee
i now have a somewhat blurry and hard to read schematic and parts list,
which identifies R9 as 470R 0.5W.
Soldering in one brought back a perfect picture after fiddling with the
brightness control. However, the picture just lasted a few more seconds,
until R9 emitted smoke and flames :/
I dont remember if there still was any picture, i was too busy with
pulling the plug at once...

So, second attempt.
Figuring out the mentioned hybrid is responsible for dynamic focus, i
disconnected it and bridged over the connections to the focus pot.
Another new R9 and put everything back together again.
Power on, useful picture even without DF, then smoke again.
So the Hybrid probably isnt responsible.

Which leaves for failing short after a few seconds:
a BA159 (1A 1kV UF) diode that tests well and wont fail given the
limited current and if it fails, itd be shorted right away
some 630V film capacitors, which test okay, and would rather loose
capacitance but not fail that way

the Focus pot (which looks like its okay but could fail at high voltage,
perhaps.I couldnt hear/see any arcing to occur, but further testing is
rather limited cause burned resistors smell real bad and my roommate
complains about the smell by now.(he doesnt complain about the stack of
various, open electronics however).

The CRT.Is it common for a CRT to short out between screen and/or focus
and heater/cathode?It tests all right when cold, i dont have the
equipment right here for doing more sophisticated testing (well, at home
id have most thats useful, including a variable HV power supply up to
2.5kV).


//some further testing:

With CRT neck disconnected, it still burns.
So the CRT likely isnt faulty.
I guess im gonna unsolder the focus pot now

//Even more testins after my roommate left for his girlfriend
//this is turning into a live report

Focus dusconnected, stil burns.
Surprising how often and how bad a single reistor can smoke without
changing value too much

//Even further testing
This time i gave up on the diode and substituted some SMD diode from the
PFC boost converter from an electronic ballast i decided to be good
(itll have at least 600V and it has to be fast).
Now with CRT and focus connected back, but without DF hybrid (i dont
want to burn it).
Guess what: it works!It doesnt burn up and produces a good picture
(rather sharp actually, i wouldnt see much need for DF).

Whatever happened to the diode...
It tests perfectly on a multimeter, im going to bring it to the uni
tomorrow to see if they still have that neat old TEK curve tracer and
could test my diode.

For the monitor, i guess im gonna have a look for some MUR1100 or
similar and a flameproof 470R resistor.I cant really leave the SMD diode
in there, mostly because it is just hanging from 2 wires on the back
side of the PCB.

Lesson learnedont be too sure that some component that tests okay and
normally would fail in an obvious way is okay...

Regards, Robert