New owner of TRS-80 Model 4 in need of help for fixing B/W monitor

[fandenivoldsk]

Hi,
I just got a very nice looking TRS-80 Model 4 computer, and it is quite a cool thing to finally own such a machine, as I am living in Denmark (Europe), so getting one is pretty rare. I imported one from USA, and it finally arrived yesterday - in one piece! The computer is in great shape (it came with a duskcover on), looks like it was well taken cared of and it powers on and disk drive is spinning as it should (I got no software for it yet). However, the monitor is not working as it should... It currently shows some semi-diagonal lines, and that's it. I have taken some pictures of it, in hope that some of you TRS-80 experts can help me out. I hope I am able to fix it myself, but I am no electronic wizard.

 

[Chromedome45]

Good luck fixing it. kind of looks like this doesn't it: http://www.vcfed.org/forum/showthread.php?52712-Model-4-Gate-Array-bad-video
Anyway plenty of help available out there. Try this Hold the "break" key and press the Orange reset button. See if it comes up and says Cass?.
What happens? If you get nothing then try this for now reseat all chips that are in sockets. Be very careful that you do not knock any of those blue wires off any chips
in the upper left corner that is the timing circuit and any other fine white or blue wires. Very critical. And retry power up after reseating. See if it works. If not let us know either way

Just FYI Mine is a different than yours in that its called a "Gate Array" model 4. Yours is a "Non gate array" Model 4 all discrete logic chips and usually easier to fix. Mine is a bunch of programmed chips to act like discreet logic chips.

 

[Avia]

Have you tried adjusting the brightness and contrast pots (knobs) located on the lower left side (underneath) of the case?

 

[fandenivoldsk]

Good luck fixing it. kind of looks like this doesn't it: http://www.vcfed.org/forum/showthread.php?52712-Model-4-Gate-Array-bad-video

Yes, my screen look just like that one.

Anyway plenty of help available out there. Try this Hold the "break" key and press the Orange reset button. See if it comes up and says Cass?.
What happens? If you get nothing then try this for now reseat all chips that are in sockets. Be very careful that you do not knock any of those blue wires off any chips
in the upper left corner that is the timing circuit and any other fine white or blue wires. Very critical. And retry power up after reseating. See if it works. If not let us know either way

I will try this out.

Just FYI Mine is a different than yours in that its called a "Gate Array" model 4. Yours is a "Non gate array" Model 4 all discrete logic chips and usually easier to fix. Mine is a bunch of programmed chips to act like discreet logic chips.

Ahh, good to know - thanks!

 

[fandenivoldsk]

Have you tried adjusting the brightness and contrast pots (knobs) located on the lower left side (underneath) of the case?

Yes I have, but they do not solve the problem.

 

[fandenivoldsk]

Try this Hold the "break" key and press the Orange reset button. See if it comes up and says Cass?.

Yay! It worked!! So the screen is totally black, unless there is a floppy disk in the drive?
Until I get some disks, is there something fun I can do with just the "break" key and press the Orange reset button option, or do I have to get some floppies before I can use it for anything?

Also, now that I have your attention, what else do I need to do, before using the computer? Should I recap something? Can I upgrade the RAM? It says "16k" on the keyboard, but I guess from looking at the chips inside that it got 64k now. Should I go for 128k? And if so, how do I get these chips?
Any other stuff I should check or do?

 

[Chromedome45]

So what the Reset break does is put the computer into Model III cassette mode. Ok in the Model 4 mode there is a problem that can be fixable by adjusting a capacitor near that timing circuit in the upper left corner. I looked for the capacitor but could not see it. So look for something in that area that looks like a small screw head. If you
find it use a small screwdriver and press reset to get that bad video again. Then try to adjust till you get a steady screen. Hope fully it might work.

 

[fandenivoldsk]

When putting my M4 back together, the screen was dead when I turned the power on, and I had to open it again. I found out it was the end part on the back of the tube that was a little loose. Very carefully I pushed it back on, and the screen was working again, but it is still very fragile and the screen is a little shaky and unstable. Is it safe it put the back part back on, or should I be very careful? I know the voltage is very dangerous on a CRT, but I would like to tighten it a little more, so that the screen stays stable. Please take a look at the photo to see what part I am talking about.

 

[Trash-Eighty]

If you like to be on the safe side let the computer be unplugged overnight. Then on the next day it should be safe to work on it. It's generally no mistake to follow the usual 'CRT Safety Procedures' (example) while working on it.

The CRT's neck is the most fragile part of a CRT. Apply only gentle force. Be careful!

There a at least two choices (maybe someone has another) if the socket is still a little loose:

1. - delicate because of the CRT's neck -
Bend all the pins a little (only a few tenth of a millimeter) in outward direction so that they will hold the socket a little tighter. Use needle nose pliers.

2. - much safer -
Try to bend the socket's female connectors internal springs a little inwards with a small watchmaker screwdriver or an other small tool. Do not bend too much otherwise connecting the socket will need too much force.

-Rainer

 

[retrogear]

I would advise against bending the crt pins. The glass tip at the end of the tube is very easy to crack, work on the socket. If the socket seems to be making contact but loose, you could run a line of glue from glass to socket. Some kind of epoxy. Fingernail polish would work too. Of course, let dry before powering on.

 

[retrogear]

A straight pin can be pushed in beside the socket hole to push it tighter. Also can use an emory board to shine up the crt pins and even rough then up a little to hold better

 

[fandenivoldsk]

Thank you for the help guys! I will try it out later today.

 

[ldkraemer]

fandenivoldsk,
I sent you a PM. Did you get it? Please respond!

Larry

 

[fandenivoldsk]

fandenivoldsk,
I sent you a PM. Did you get it? Please respond!

Larry

You got PM!

 

[pski]

You got PM!

***puts on moderator hat***

posts about PMs are redundant guys since they clutter up the threads and since they show up in the user's inbox so we should refrain from those types of posts. Thanks!

***takes off moderator hat***