Olivetti M24 color monitor repair (DSM 2412/C)

[MacFly]

I'm progressing with the restoration of my ebay M24. After fixing the faulty PSU, reviving the harddrive (Tandon TM502: fortunately wiping the metal tab contacting motor "spindle" on the bottom of the drive with contact cleaner was enough to make it spin again), and completely disassembling/cleaning a super filthy keyboard (yuk!) where almost none of the keys had worked reliably, it is almost perfect again. Almost...


Final issue (hopefully) is a defect with the monitor. It's color monitor for the M24, an Olivetti "DSM 2412/C":


Unlike many other Olivetti monitors this one is not made by Hantarex, but by Toshiba: its chassis has a label "Toshiba CDU 1200". The main PCB bears a type label "Toshiba PW 4269-1". This is the monitor also mentioned in this other thread.



Unfortunately I could not find any schematics for this monitor. Neither the Olivetti number, nor the Toshiba numbers result in anything useful.

The actual issue I'm having with this monitor is some defect affecting the monitor's brightness/contrast. Everything is fine when the overall screen is mostly dark. In this case the display is perfectly bright & crisp:


However, as soon as the overall screen is bright (colored background, or graphics filling most of the screen) then the monitor slowly get's brighter and shows terrible contrast (photo doesn't truely reflect the issue, but you can kind of guess):

Switching back to a screen content with mostly black background, slowly restores the picture quality to normal brightness/contrast.

I'm hoping it's going to be a simple issue - maybe a bad capacitor. I'll probably have to completely disassemble the main PCB (yes, after discharging the tube first) and see if I can find any suspicious component. If anyone had any more specific suggestions, let me know...

 

[Chuck(G)]

Check the power supply voltages when the high-brightness screen is displayed. Could be that they sag when a full, bright screen is displayed.

 

[MacFly]

Look! It's working again... It's a beautiful color display (for a monitor from 1984).


I finally took the time to restore this monitor. It was super filthy, everything inside covered in thick black dust. Yuck. Required a good cleaning anyway:


I replaced various degraded caps, none were leaking/completely dead though. I also replaced a power transistor, generating the supply voltage for the CRT neck board. The transistor was weak (hfe way below spec). That was already close - and related to the actual defect. But I missed the main culprit. I took photos of all the PCBs (see PDF below). Luckily the PCBs are quite service friendly with proper silkscreen on both sides.

Reassembled everything - the issue was still there. But with the photos as a replacement for non-existing schematics, I could at least take measurements and narrow down the issue. Indeed the neck board's supply was breaking down as soon as a picture with higher brightness was displayed. What I had missed when I cleaned/checked the PCBs was a blown resistor: a 2.2 Ω resistor protecting a winding of the flyback. It was supplying the suspicious power-transistor which I had already replaced. The resistor was completely open/burnt. But the blister wasn't really visible from above. And it's a dark brown resistor anyway:


Surprising that the CRT was displaying anything at all. The power-transistor couldn't even switch anything without a supply. But it's base current was already enough to charge a capacitor and drive the RGB grid as long as the display was mainly dark/black.

Anyway, I'm really happy to now have a fully working M24 in my collection. I had one of these as a kid - which was lost. Now this ebay find is making up for it: a "hopeless machine" (originally offered as dead/nothing working) is fully restored.

In case anyone else needs to work on an Olivetti DSM 2412-C / CDU1200: I'm attaching a PDF with more PCB photos and a partial component list (things I had been looking at).

 

[1ST1]

Hey, great little documentation, thank you!