TRS80 Model 3 - Alternative monitor

[demonlg]

Hello,

now I have a question for a TRS80 Model III, this computer was sent to me from the USA but during the trip it suffered serious damage, the CRT tube cracked at the neck and also two fixing brackets breaking sent to slam the CRT, with all its weight, against the flyback which in turn lifted off the PCB, and cutting the 6 wires of the primary coil.
I tried to rebuild the whole base part of the flyback and to solder the cut wires, according to how I found them in position, I only found 6 as there are 6 pins of the base, but on one of the pins I actually found 2 coil wires welded on it, I have not seen this seventh wire, perhaps broken inside the coil.

Replaced the CRT with one taken from a monochromatic AMSTRAD CPC464 monitor and mounted the original deflection on it, but in fact the monitor does not turn on, or rather only the heater in the neck of the CRT turns on but I don't see any image, even by changing the contrast and the brightness, checking the voltages I see missing voltages on the deflection yoke and the 80 volts that the transformer should provide for other functions, and probably the 12Kv is not present on the tube!

For this reason I have two solutions, either some kind soul up here has a video card for the TRS80 Model III to sell me (or even just the flyback) or I have to replace the monitor with some other. (With a logical pricing!!).

For the second case I was thinking of adapting a 12" PHILIPS monitor (monitor80) used on a C64, it has an RCA input, I think composite. If it possible i will try to place entire PHILIPS monitor tube and electronics into the TRS CASE.
To do this, however, I would have to convert the video signals of the TRS80 Model III but I still don't know where to start.
Does anyone have any advice or adaptable solutions for this? (Please not LCD adapting :D) .

Thanks

Emanuel

 

[Eudimorphodon]

For the second case I was thinking of adapting a 12" PHILIPS monitor (monitor80) used on a C64, it has an RCA input, I think composite. If it possible i will try to place entire PHILIPS monitor tube and electronics into the TRS CASE.
To do this, however, I would have to convert the video signals of the TRS80 Model III but I still don't know where to start.

It is possible to convert the Model III’s video output to composite with just a chip or two and a few resistors to massage the sync signals; schematics for this are out there. There is one problem, though; the Model III does a 512 pixel wide display with only a 10mhz pixel clock, so the active screen area goes well out into the overscan of most composite monitors. If your monitor doesn’t have horizontal size/centering controls you might have to change some components to make it fit.

 

[demonlg]

Hi, thanks for response and comfirmation of this mods. And where i find this schematics?

 

[Eudimorphodon]

Here’s a pretty elaborate circuit that includes some adjustment circuitry and output amplifier transistors:

TRS-80 Model X Composite Video - EasyEDA open source hardware lab

In EasyEDA open source hardware lab, open source square pools all kinds of electrical design engineerings together to realize resource sharing easily.

oshwlab.com

I think you could get away with a bit less, depending on how forgiving your monitor is.

 

[demonlg]

Ok thanks for link, is not a compless circuit to realize, only attention is for the 74c04 but is not a big problem to find this, probably i have this.

 

[pjhacnau]

Ok thanks for link, is not a compless circuit to realize, only attention is for the 74c04 but is not a big problem to find this, probably i have this.

One question - where are you in the world, and would you be working with NTSC or PAL? I raise this because I had a chat with the guy who made this board and he stopped producing its himself because it had issues when built for NTSC more often than not.

(I'm demonstrating my lack of knowledge here as I thought there was no difference in monochrome monitors and it only became an issue when you went to color . . .)

 

[Eudimorphodon]

I'm demonstrating my lack of knowledge here as I thought there was no difference in monochrome monitors and it only became an issue when you went to color . . .)

PAL and NTSC monochrome monitors run at similar (but not identical) line rates, but their vertical refresh rates, and hence the number of lines per frame, are different. (NTSC draws 30 interlaced 525 line frames per second, PAL is 25 interlaced 625 line frames. Or, if you’re talking about the non-interlaced hack version of of the standards that most 70’s and 80’s computers used, it’s 60 262-ish or 50 312-ish line frames per second, respectively.) Some mono monitors are able to lock onto either and, if they have enough slop in the vertical hold/size controls, display both acceptably, but I feel like that may be more the exception than the rule.

Without trying to analyze in detail what the circuit is doing specifically I would guess it is spitting out sync signals that are pretty marginal for either standard but they’re at least closer to PAL. (Again, without checking I think the standards differ somewhat in details in how long things like the horizontal/vertical sync pulses are, and like I mentioned, the Model III cuts its hsync front/back porch margins down to the barest minimum already for NTSC.)

 

[demonlg]

I'm in italy and i have buyed TRS80 model 3 from USA it's an original 110volt only power supply not convertible to 220v, probably it is NTSC:

In fact, i have rebuilded the primary coils of flyback, now have all voltage on the pin but only one its wrong, in the 17,X volt line i have 32 volts! I have tested for 1-3 seconds with a 12v battery for not damage other component and without tube connected.

I Don't know the number of round of two coils at the bottom of flyback, its necessary to have correct voltage output 80v, 17,7v 17,8v 19v.....I have to rebuilding again coils because i think got in a error, but without any specification it' impossible to have all correct output.

On the schematics diagram of sans manual i have any value of coil but i don't have to interpreting this value!

Emanuel

 

[pjhacnau]

I'm in italy and i have buyed TRS80 model 3 from USA it's an original 110volt only power supply not convertible to 220v, probably it is NTSC:

The question is about the monitor you would connect to. The board we were discussing provides composite output so you would be plugging into the composite input of an external monitor.

In fact, i have rebuilded the primary coils of flyback, now have all voltage on the pin but only one its wrong, in the 17,X volt line i have 32 volts! I have tested for 1-3 seconds with a 12v battery for not damage other component and without tube connected.

I Don't know the number of round of two coils at the bottom of flyback, its necessary to have correct voltage output 80v, 17,7v 17,8v 19v.....I have to rebuilding again coils because i think got in a error, but without any specification it' impossible to have all correct output.

On the schematics diagram of sans manual i have any value of coil but i don't have to interpreting this value!

Emanuel

It might be worth looking at the service manual for the Model 2 - If I understand right the M2 video board is "similar" to the M3 board. Using the voltages specified there may be a good start. YMMV though.

PJH

 

[pjhacnau]

For reference, my board(s) arrived today, should have it built in 2-3 weeks (spare time, what's that?) I'll let you know how it goes when set up. It's primarily for my M2 (fighting rolling problems on the driver board there) but I'll try it on my M4 as well.

 

[demonlg]

I have resolved with my TRS-80 model 3. I have writed to Ian Mavrick from Australia and it have sended me a working flyback for my original TRS video board. Then i have replaced the cracked tube with an other tube taked from Amstrad GT65 monochrome monitor, it's green fosphor not white but have compatible characteristics from the original tube. Then i have placed the original yoke to this tube.

First boot up gived me a vertical line compressed to the center but with blinking cursor, then i have found 4 wires cutted in a yoke coils, the glass cracked have truncated this 4 wires... Very hard to find with lens, but i have resoldered all 4 wires and now the computer lives again!

I have to rotate little the yoke to centering the image but for other it's ok.

Emanuel