Monochrome MDA/CGA monitor - no HV, cut video cable, no pinout

[Vein]

Hello, I stumbled onto old monochrome MDA/CGA monitor. It's basically "noname", production date was 1987, model 14HP22.
Issues that i have:
1) Someone cut the video signal cable, I'm not experienced at all, to find out what is what (besides ground )
2) After turning on, there is no HV, is it somehow connected to vertical/horizontal sync? I have tested all transistors and some diodes, all are OK
info: IC that most signals are connected to (directly, or indirectly) is 7407PC
For identification, I'm posting photos of PCB

 

[Chuck(G)]

Looks like a typical Asian (Taiwan, most likely) import MDA. You're not going to get HV without a horizontal sync signal. Try applying an 18 KHz sync signal to each of the pins, one at a time. If everything's working, on one of them, you'll get action.

Likewise, proceed with vertical sync., then video and intensity. Just be sure to exclude monitor outputs in your probing!

Also, be aware the brightness and contrast controls may be on other connectors.

 

[Vein]

Thank you, I'll have to figure out how to generate those signals using some of my stuff (arduinos etc.), or I'll just simply use one of my computers

 

[Vein]

btw. are you aware of some project, that uses Pi or some variant of arduino that can generate mda/cga test signal?

 

[Hugo Holden]

Looks like a typical Asian (Taiwan, most likely) import MDA. You're not going to get HV without a horizontal sync signal. .

I am inclined to agree.

A quick inspection of the pcb components and tracks suggests that there is no independent horizontal scan oscillator in this set, just three transistors leading to the transformer that drives the H output transistor's base, very reminiscent of the IBM5151 VDU, so to get an H scan (and any EHT) it relies on being fed with the correct H scan frequency and duty cycle from the MDA card in a computer.

Be aware that if you cobble an H drive or H sync source from something like an Arduino and get the frequency and or duty cycle incorrect, it will damage the HOT and possibly the EHT rectifier, integral to the lopt.

 

[Vein]

Hey, Thanks for your replies. I have hotwired this display and:
1) I need to have connected Horizontal Vertical and video signal to have any effect from HV side
2) no matter how i connect, i have mumbo-jumbo on screen and "ticking" from HV transformer (or its vascinity) at 50hz(?)
Will have to work on it more, it's becoming no fun at all, but I'm glad to see that tube itself is working,
will post tomorrow video of effects of my work.

 

[Vein]

Hey, I have managed to figure out pinout. I have another issue, very poor h-sync, I have to trim it with potentiometer, but if ambient temperature changes by 0.001F or someone thinks about this, picture slowly creeps up or down. I have even change h-sync potentiometer from 100k to 10k+14k resistor to have much more precise control, but helped to extent of having picture creep slower.

 

[Chuck(G)]

Sounds like v-sync to me. Bad h-sync usually looks like the image diagonally "tearing". This unit does have a vertical oscillator circuit.

 

[Vein]

Yup, my bad, brain fog I have disconnected vsync from connector and nothing has changed. Also, today, i have experienced "waves" in video signal, but figureing v-sync is more important

 

[Vein]

Ok, i have fixed synchronisation issue, messed up pinout. Still does not work CGA mode but i does not care, i have different projects to chase. Wilk post my collection on apropriate forum.

 

[Chuck(G)]

Why would a monochrome monitor that works with an MDA work with CGA? Radically different hsync frequencies, for one.

 

[Vein]

Then why there are all wires in connector (9 pins) with RGB included, and they are somehow wired to U201? I have monochrome monitors that support CGA signal (different shades of amber for every color). In attachement, it's part of service manual for simmilar display. Maybe mine is "cheaper" version (not every IC place is populated) and because of that it does not support CGA.

 

[kdr]

Then why there are all wires in connector (9 pins) with RGB included, and they are somehow wired to U201? I have monochrome monitors that support CGA signal (different shades of amber for every color). In attachement, it's part of service manual for simmilar display. Maybe mine is "cheaper" version (not every IC place is populated) and because of that it does not support CGA.

Yep, I've started to make a habit of checking all monochrome monitors for resistance on the RGB pins, and a surprising number have these pins connected internally. Sure enough, they'll sync to CGA and display it in 16 glorious shades of mono. (And what's more, I find the mono CGA experience to be a far more pleasant than the cyan-magenta garbage on a colour CGA.)

I think that for a while, it was just kind of expected that a mono monitor would support both MDA and CGA, because old advertisements that I have tracked down don't seem to even bother to mention it as a feature. And quite a few turbo XT clones had combo MDA/CGA cards with a single output and automatic software mode switching, which obviously only makes sense if you have a multisync 15.7khz/18.4khz monitor attached.

 

[Chuck(G)]

I guarantee that won't work on an IBM 5151 or a very wide range of contemporary monochrome monitors.

 

[Vein]

I agree, not EVERY monitor suports CGA, but many of them do. I have about 6 or 7 monochrome monitors and at least two of them, supports CGA.

 

[Chuck(G)]

Looking at the board, I'm of the opinion that it's not dual-purpose.

 

[Carles20VT]

Hello, I stumbled onto old monochrome MDA/CGA monitor. It's basically "noname", production date was 1987, model 14HP22.
Issues that i have:
1) Someone cut the video signal cable, I'm not experienced at all, to find out what is what (besides ground )
2) After turning on, there is no HV, is it somehow connected to vertical/horizontal sync? I have tested all transistors and some diodes, all are OK
info: IC that most signals are connected to (directly, or indirectly) is 7407PC
For identification, I'm posting photos of PCB

Hello Vein,

I have the same monitor with the same problem (found with all wires cut). I'm trying to rebuild it but I had no luck trying to find some schematic or anything.



I would be happy if you can share the pinouts that you find to try with my monitor.

Mine has some differences with the ICs, maybe it accepts also CGA? You can try to insert the IC that is missing with the same part number that the following picture shows:



If you need more detail I can check with my monitor to copy the installation. I hope we can discover the entire pinout and rebuild them!

 

[jjplano]

Hey, I have managed to figure out pinout. I have another issue, very poor h-sync, I have to trim it with potentiometer, but if ambient temperature changes by 0.001F or someone thinks about this, picture slowly creeps up or down. I have even change h-sync potentiometer from 100k to 10k+14k resistor to have much more precise control, but helped to extent of having picture creep slower.

hi Vein! have you wrote an arduino sketch to test MDA monitors? I would love to have it! I've come across a couple of monochrome monitors, but I have not computers to go with them, and I would really like to test if they work first... thanks!

 

[x86_guy]

Hey, I have managed to figure out pinout. I have another issue, very poor h-sync, I have to trim it with potentiometer, but if ambient temperature changes by 0.001F or someone thinks about this, picture slowly creeps up or down. I have even change h-sync potentiometer from 100k to 10k+14k resistor to have much more precise control, but helped to extent of having picture creep slower.

Can anyone please share the pinout?
I stuck in the same condition - similar monitor with cut video cable.

Thanks!

 

[x86_guy]

Any guess what is the description of each wire judging by the header pin arrangement/wire color?
Is it safe to try different wiring combinations between the monitor and video card? Can anything go wrong during this kind of test?

Thanks.