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Monitor fun

Gary C

Gary C

Veteran Member
Joined
May 26, 2018
Messages
2,743
Location
Lancashire, UK
After putting a cheeky bid on a Commodore monitor with a free Amiga 500 thrown in (seriously the price was so cheap, the Amiga could be called a free gift) I then realised it wasn't in a place nearby, but near Bridlington on the east coast and I live on the west coast. Ah well, even with the 300 mile round trip it was still cheap.

Put on one of the museums machines to replace a wonky monitor and all looks good

So the Acorn version of the phillips CM8833 now on the bench and everything looks ok, but no blue or green.

Now the issue. The Acorn and the Commodore have male 9 pin D plugs on them and they are now obviously wired different.

With a male plug, pin 5 of the plug connects to what is labelled pin 1 on the PCB, pin 4 to 2 and pin 3 to 3. How very odd

1752363927449.png This is from above and matches the silkscreen of my monitor, and if you present a male 90 degree D, you can plainly see it doesn't match !
Obviously Acorn used a different pinout to Commodore even though they both used a cable with a female into a male socket on the monitor.

What a pain, as they should be a universal monitor !
 
I think these were "universal" as in anyone could buy them as OEM with whichever input signals they wanted (RGB, composite, S-video (with standard or Commodore 8-bit timing, various types of sync signals and whatnot).

if the connector is just mirrored and the pinout otherwise is the same, you could just solder two DE9 connectors to each other to swap gender and mirror the signals again in one go.
 
Sorting is simple, it just really confused me.

The cm8833 is a nice monitor in multiple cases (we have quite a few now) but swapping the female for a male was a sneaky trick.

It must have been to make the monitor more specific
 
Next up is an Acorn AKF18 multisync. Been looking for one of these as we need a VGA CRT monitor thats capable of a syncing down to 15khz for a A7000+ and not many do. Its will also be nice to have an Acorn monitor on an Acorn machine

It doesn't work though (which is why its cheap), I 'think' the commodore 1960 has the same PCB so fingers crossed.
 
While I am waiting for the Acorn monitor, decided (now that I have a working Atari ST) to plug in the SM124 monitor and it works ! Geo a bit out and height a bit reduced, but nice crisp screen.

Quick look inside and yuck, One capacitor is definitely knackered with dried gunk all over the place, the big 400V has a dome that St Paul's would envy and another has gunk but no visible sign of escape.

New units ordered, but damn, didn't notice one is bipolar, so back to RS.

Surprised it worked at all but a quick clean up and calibration and it should be ready to go.
 
AKF 18 next

No response to power at all.

The circuit is almost exactly the same as a commodore 1960 monitor (slight differences it seems)

Good thing, it uses a SG3842 to drive the PSU flyback which is much easier to diagnose than a analogue circuit (well, it is for me)

Time to try out my new battery scope. Bought so I can connect it to the hot ground and do proper measurements (just need to remember to keep fingers off the ground which is now sitting at 130V)

No voltage at pin 7 which is picked up from the +ve from the rectifier via R804 & R805. R805 is open circuit !

1752921641598.png
 
Some of it is, but the lumps and bumps on the case aint.

And it had virtually no measurable capacitance in the uF range
 
Ouch ! Just caught the ground ring on my probe.

Anyone know of a probe with no ground ? I use channel 2 for the ground so I dont need it on the measuring probe

But on a positive note, the power supply has fired up and the HV & heater are now running. Need to get it all into a testable configuration to see the screen.
 
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Oscilloscope probes tend to have a "hook" add-on that can also be used to poke things like with a regular probe, albeit a bit more clumsy, and it covers the ground ring.
 
Monitor fired up and its working

However the colours are blotchy. Hopefully it just needs degaussing (which would mean there is something wrong with the power up degauss circuit/coil)

Hopefully its not something to do with all those fiddly rings on the neck.
 
Ah ha (again)

Was testing the coils continuity and getting some flakey readings. Gave the connector a good clean, readings good. Plugged it back in and now the colours are fine :)

Need to check some more to make sure its working ok as I am getting 240V across the coil at all times (its meant to tail off I believe). Might need a new PTC resistor pack.

But at least it isn't the tube geometry.
 
Fired up the RISC PC (well after reseating things) and tried it. Most of the modes display well. Individual height adjustments for CGA, 768, 400, 350 etc so that when switched to Auto it sets the height correctly which is nice.

Interestingly, the monitor is specified to work upto 1024x786 however it seems to work perfectly fine at 1600x1200 though thats a bit too high for such a small screen.

Had problems with the 800x600 mode folding over to the left which could not be adjusted but then tried a different monitor spec from the list of monitors and now 800x600 works fine.

Just found out that the RISC PC uses monitor definition files that specify in detail the full video specification and timings. For some reason though it doesn't include the AKF18. Acorn did produce a program called !Makemode which can be used to define a monitor so I can make one :)

Also bought a new scope lead that has a cap over the ground ring and fully covers the ground lead connection too. Its also a 100:1 probe which is more useful on a monitor with my battery scope as it puts the range up to 2kV. That and the 40KV probe means I am now set for tools.
 
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