Commodore (AOC) CM-312 EGA/CGA CRT Repair

[sean0118]

Ok this might be a challenging one.

I've got an old CGA CRT monitor I'm trying to repair. It's definitely not worth it, but I've gone too far to give up now. I've replaced every electrolytic capacitor and trimpot in the thing (I trimmed them to try and match the ones they replaced) 

Schematic is attached, but it has some mistakes, wrong designaters and connections which don't exist. It also doesn't show the actual IC and diode part numbers, so I'll copy those in later, I'm just going to give the CRT some time to discharge before I disassemble it again.

I'm getting a raster and vertical, brightness and contrast adjustments all work. The voltage rails out of the PSU are correct (except for white wire P, that's 0V but maybe it's meant to be?) and +5V and +15V on the main board are correct. The vertical and horizontal widths also change if I plug a PC in. There's a switch on the front which lets you change the background between black (blue if contrast is turned up) and amber and green, that all works.

But, I'm not getting any text displaying at all...   I'm not sure what fault causes that? maybe it's really out of focus? Or a sync issue?

The last component that I know is faulty is ZD401, the corrosion had got into it and was measuring ~0.2V in each direction, some of the regular diodes had failed the same way. Except, I'm not sure what the function of ZD401 is or what its value should be? Is it just to protect the input to IC400? I've placed in a 5.1V one for the moment, I think the horizontal width changed after I did that. Unfortunately I don't understand the function of the circuit its in.


No PC plugged in:


PC plugged in (horizontal and vertical width adjusts and colour changes when on blue / black setting):


This pattern isn't a camera artifact, might be a sync issue?


Switched to the green background mode:


Faulty diodes, you can see they have faded from their normal orange colour, the leads were super corroded too:



Bonus question, is this a standard 1uF, 160V electrolytic? It's end cap is epoxied and it's massive, they used other 1uF/160V caps which are much smaller. Maybe it's unpolarised? But the case marking looks like it is polarised:






Any ideas? I'll update with photos of the mainboard and IC part numbers soon.

[sean0118]

Ok, I got the main board out again.

I should have mentioned, I replaced Q500 and Q07 (originally C945 Q29XC) with KSC945YBU and Q501 (originally A733 Q183C) with KSA733CGBU. I think these are okay as replacements, but their current gain is lower and the pinout is EBC not ECB. I did have the base and collector of the Q500 and Q07 replacements switched for awhile before I realised, so hopefully that didn't damage anything.

IC list as promised:

IC1:   LM341T-5.0 (5V regulator)
IC02: SN82S147N (this is some Signetics chip I can't find info on, I think its programmable logic?)
IC03: HD74LS05P (hex Inverters)
IC04: HD74LS86P (Quadruple 2-input Exclusive-OR Gates)
IC05: HA11235 (does both vertical and horizontal deflection?)
IC06: HCF4053BE (triple 2-channel analog multiplexer/demultiplexer)
IC07: HCF4053BE
IC08: HA17324 (quad opamps)
IC400: HA17324
IC500: HA17324



















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Here's some before pics:     

[vk6zgo]

Ok this might be a challenging one.

I've got an old CGA CRT monitor I'm trying to repair. It's definitely not worth it, but I've gone too far to give up now. I've replaced every electrolytic capacitor and trimpot in the thing (I trimmed them to try and match the ones they replaced) 

Schematic is attached, but it has some mistakes, wrong designaters and connections which don't exist. It also doesn't show the actual IC and diode part numbers, so I'll copy those in later, I'm just going to give the CRT some time to discharge before I disassemble it again.

I'm getting a raster and vertical, brightness and contrast adjustments all work. The voltage rails out of the PSU are correct (except for white wire P, that's 0V but maybe it's meant to be?) and +5V and +15V on the main board are correct. The vertical and horizontal widths also change if I plug a PC in. There's a switch on the front which lets you change the background between black (blue if contrast is turned up) and amber and green, that all works.

But, I'm not getting any text displaying at all...   I'm not sure what fault causes that? maybe it's really out of focus? Or a sync issue?

The last component that I know is faulty is ZD401, the corrosion had got into it and was measuring ~0.2V in each direction, some of the regular diodes had failed the same way. Except, I'm not sure what the function of ZD401 is or what its value should be? Is it just to protect the input to IC400? I've placed in a 5.1V one for the moment, I think the horizontal width changed after I did that. Unfortunately I don't understand the function of the circuit its in.


No PC plugged in:


PC plugged in (horizontal and vertical width adjusts and colour changes when on blue / black setting):


This pattern isn't a camera artifact, might be a sync issue?


Switched to the green background mode:


Faulty diodes, you can see they have faded from their normal orange colour, the leads were super corroded too:



Bonus question, is this a standard 1uF, 160V electrolytic? It's end cap is epoxied and it's massive, they used other 1uF/160V caps which are much smaller. Maybe it's unpolarised? But the case marking looks like it is polarised:






Any ideas? I'll update with photos of the mainboard and IC part numbers soon.

If you aren't seeing text I suggest you look at the picture tube cathodes with an Oscilloscope.
You should see different drive signals on each.
If you then follow the schematic back, you may find where the error is situated.

[sean0118]

If you aren't seeing text I suggest you look at the picture tube cathodes with an Oscilloscope.
You should see different drive signals on each.
If you then follow the schematic back, you may find where the error is situated.

Good idea. 

I haven't done that yet. It might be hard to access the mainboard while it's powered up though, I'll have to see if I can move the power supply far enough out of the way.

I did trace through the "blue" signal path just using a multimeter while it was powered off. I didn't find any issues, but that also doesn't really test the ICs in the path.

[sean0118]

Hmmm it might not be the monitor, I should have checked this earlier.

Probing the neck of the CRT I get regular pulses at 56.4Hz on all three RGB signals. If I unplug the PC they go to 40.8Hz.

I probed the vertical sync pin of the PC and also got 56.4Hz, so the CRT is locking to it.

BUT I'm not getting anything on the RGB colour pins of the PC output, so maybe it's not configured correctly. I had tested the CRT on a different PC earlier, and it only displayed a raster then too, but I have done more work on it since then.