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Dolch VPAC Plasma Display blues (in orange)

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Jun 26, 2019
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Portland, OR
I bought a Dolch VPAC 386 but I can't get anything to display on the plasma screen.

The plasma display (a Panasonic MD480L640PG3) runs on high voltage DC and there's a boost converter inside of the PC, that is powered from a 12v line from the power supply. The boost converter has three output lines: HV+, HV- and Vcc-. It is currently outputting 178-180v DC when tested across HV+ to HV- and HV+ to Vcc-, and nothing across HV- to Vcc-.

The video card, a Paradise Systems PVGA1A-JK, will display to an external monitor. The edge connector on the card appears to be a regular old feature connector, and I'm about to test a different video card tonight (I had to order a 26 pin ribbon cable).

There's one more component, a small board that converts the feature connector output to the plasma display's input, it's got a couple of tants on it and a small IC (I need to look up it's function).

Does anybody have experience with plasma displays that can point me in the right direction? This is my first foray into the world of plasma display technology, and cannot locate a datasheet for that display, so I don't even know if the boost converter is working right. Or the PDP, or the mystery video converter.

Thanks!
 
Update: bought another MD480L640PG3 panel, still no activity on the display. DAC board has a flip flop on it so I'm pretty sure it's a DAC now. I think the display will take VGA input so I'm going to try to figure that out from the DAC board. I'll post a pic of the DAC, display and DC DC boost converter soon. Dtech, I hope you're still active, I saw a post you made on vogons from 2013 about your work on Dolch computers and I could sure use some insight! 😅
 
I've played around with that panel quite a bit. It's also used in the Dolch 486 PAC, and the Toshiba 5200 386 based clamshell. I believe it's picky about it's timing, only works with a 70hz refresh if I remember correctly. On my 486 Dolch, it's connected to an Orchid Farenheit Video card- really looks like a vesa feature connector, but the video card does have a rework wire running to the connector- maybe a 'turn on' signal? Also the Farenheit card had a dip switch that lets you specify the refresh rate. I did try to run the display off of a Matrox card, but the display didn't seem to light up at all. Also tried to run one off of a Raspberry Pi-- https://github.com/raspberrypi/firmware/issues/734 -- but the Pi won't do the correct pixel clock.
Somewhere I've got the pinouts for the panel, and the toshiba's connector. It's digital, I've had it hooked up to a logic analyzer to figure out the timings. 4-bit grayscale, dot clock, enable, h-sync, v-sync I believe.
 
That's great info! My Paradise card has an extra wire running to the feature connector as well!

Is 180v the correct operating voltage?

Here are the front and back of the video converter board in between the VGA card and the display.
IMG_20190912_201319.jpg
IMG_20190912_201303.jpg
 
my 486 Dolch has that board as well, if the toshiba has it it's integrated on the board (very proprietary video card, PVGA based). Looks like the plasma display should be getting 205v according to the Toshiba service manuals- http://www.minuszerodegrees.net/manuals/Toshiba/Toshiba.htm . Look for the T5200 manuals, they've got the most published info on this panel I've been able to find.

Power Requirement - MD480T640PG3
E1 5v +- .5v 400ma
E2 205v +- .5v 200ma
E3 5v +- .5v 60ma
 
I've been tracing out the video converter board today, and it appears that Dolch re-purposed the unused pin 25 on the vesa feature connector to power the converter board. I suspect that's what the bodge wire is on VGA card, I'll try to confirm this later.
 
Okay, so the the bodge wire on the VGA card is supplying 5v to the converter board. It makes me hopeful that I'll be able to use any video card with a feature connector. The Dolch will fit a regular old baby AT motherboard, so my plan is to make a sleeper with a plasma display (I have dreams of running Half Life 1 on the orange plasma display in time for Halloween!).

I've re-capped the original power supply (3 boards and with 22 electrolytic caps!), and now I can confirm that the DC-DC boost converter is running on 12v from the PSU, but it is still only outputting 180v. I'd like to crack open the DC converter, but its a sealed black box, so I've ordered an adjustable converter. I'll update once that comes in. If I get the original display working, I'll have an extra PDP, maybe I could make an external plasma display :p
 
While I'm waiting for a DC-DC adapter to arrive, I swapped the CRT in my Compaq Portable with one from a Macintosh Classic. Here's a picture from the first smoke test.
IMG_20190922_223754.jpg
 
Hi,
I'm looking at a similarly dead display on a Dolch. How do you disassemble the screen please? Did you say the HV converter is a sealed unit, what replacement did you find?

Regards,
John
 
1980s_john said:
Hi,
I'm looking at a similarly dead display on a Dolch. How do you disassemble the screen please? Did you say the HV converter is a sealed unit, what replacement did you find?

Regards,
John
Click to expand...

On my Dolch there was 4 screws under the motherboard that held the front of the case to the internal frame. Once those are removed the whole frame should slide out of the case.

I don't know how similar Dolch computers are, but my plasma screen was powered by a separate boost converter. It should be outputting 200v and 5v. My dolch had a small 4"x4"x1" black box that was only supplying 180v. Very sadly, after attempting to pull apart the box, I found it had an adjustment pot under a sticker that I could have used to correct the voltage.

Unfortunately, I've so far not been able to come up with a replacement boost converter. I have a chinese 12v to 200v boost converter, but when I supply the additional 5v to the plasma screen it blows out a resistor on the screen's controller board.
 
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