TRS-80 Model 4 Gate Array video problem

[Paul-VT]

Hi All,

New user here. My TRS-80 Model 4 Gate Array has suddenly developed video problems. Video was perfect until I turned it on a few days ago and the screen was way bright with zig-zag lines visible. The machine was able to boot from floppy and display the OS welcome screen but the text was so blurry I could hardly read it. The brightness and contrast control under the keyboard had little affect. I unplugged the 12 volt supply from the main power supply (the Astec model) and ran 12 volts to the video board from my external power supply. Same video problems. Thus I assume it's the video board.

Has anyone dealt with this problem before? Any suggestions?

Thanks.

Paul in Vermont

 

[ldkraemer]

Paul,
If you hold the BREAK Key down, and keep holding it down while you power up, then when you see the display say "CASS??" (or after 8 seconds),
release the BREAK Key and tap the ENTER key twice. What do you see?

A picture is worth 1000 words.

Larry

 

[Tibs]

Might be the video capacitor that needs adjustment on the motherboard, but this is only 80/40 mode. 64/32 mode would not be affected, and what Larry tells you would put it in that mode.

So, if you can read Cass? fine and its clear but not a model 4 OS, you got a little capacitor that needs tweaking

 

[Paul-VT]

Paul,
If you hold the BREAK Key down, and keep holding it down while you power up, then when you see the display say "CASS??" (or after 8 seconds),
release the BREAK Key and tap the ENTER key twice. What do you see?

A picture is worth 1000 words.

Larry

Holding down the BREAK key made no difference. Since there are only a handful of electrolytic caps on the video board, I will change them next.
Thanks for the quick replies.

-Paul

 

[Jay Newirth]

holding down the break key made no difference. Since there are only a handful of electrolytic caps on the video board, i will change them next.
Thanks for the quick replies.

-paul

if you can send me a photo of the board - i may have a spare board (i have quite a few video boards

i will be happy to send you one (if i have it) and you can try that first (no charge)

if it works - we can work out something

if not then just send it back

jay

 

[Paul-VT]

if you can send me a photo of the board - i may have a spare board (i have quite a few video boards

i will be happy to send you one (if i have it) and you can try that first (no charge)

if it works - we can work out something

if not then just send it back

jay

Thanks for the offer, if the re-cap doesn't solve the problem, I'll be in touch.

-Paul

 

[Tibs]

Thanks for the offer, if the re-cap doesn't solve the problem, I'll be in touch.

-Paul

Before you go in there shoot us a picture and lets see what you see. The digital boards have two ICs on them that do the vertical and horizontal, but there is still a choke that can be adjusted for one of the two, h size maybe.

 

[Paul-VT]

I have checked the electrolytic capacitors on the digital video board and all were good.
Here are pictures of the video display problem and the digital video board:


Notice you can just make out "Diskette?" but the text is very blurry and the screen is way too bright. Contrast wheel seems to work as expected but brightness wheel does nothing.
-Paul

 

[Paul-VT]

As it happens, I have another Model 4. It is a very early NGA. This Model 4 was purchased on eBay and was nearly ruined in shipping. The upper case was cracked in several places and the CRT was destroyed.

I decided to swap the working CRT (tube only) and top case from the GA model I am trying to fix into the NGA. After a bit of work adjusting the CRT yoke, I now have a working NGA Model 4. Woo Hoo!

The video is not as sharp as on the newer model but it works. I would still rather fix the GA Model 4.

 

[Tibs]

As it happens, I have another Model 4. It is a very early NGA. This Model 4 was purchased on eBay and was nearly ruined in shipping. The upper case was cracked in several places and the CRT was destroyed.

I decided to swap the working CRT (tube only) and top case from the GA model I am trying to fix into the NGA. After a bit of work adjusting the CRT yoke, I now have a working NGA Model 4. Woo Hoo!

The video is not as sharp as on the newer model but it works. I would still rather fix the GA Model 4.

Yeah, that video issue is in the board as you have figured out. Not many of us have been able to repair those boards, but usually its not that issue but something more serious. I would the pot for internal brightness and see if its shorted out.

 

[Jay Newirth]

I still am making you an offer to get a replacement video board (i have over 10 of the ones in your photo - better video output than the other board you swapped

try and and buy it - or return it

let me know via pm

 

[dittman]

Jay, I'd like to buy two of them myself.

 

[Jay Newirth]

Send me an email to jaynewirth@verizon.net to discuss further

Thanks

Jay

 

[Jay Newirth]

I also have replacement cases (Model 4), screen bezels to mount the CRT onto

Here to help in any way

Jay

 

[mdh]

Sorry to necro an old thread, but I'm having video problems on my model 4. 64 wide video works fine, but not so much 80 wide. See picture below:



Non-gate array model, tried adjusting the pots in the back (but couldn't reach two of them, need to get something non-metalic and long..) I have picturere of video board if needed. I also had to adjust the horiz position (it was smashed up on the right side), but when I went back to 64 wide, it was off the screen...

​​​​​Electrolytic caps look fine, new power supply.