• Please review our updated Terms and Rules here

Newly acquired TRS-80 4D

M

morpheus256

Member
Joined
Jul 18, 2009
Messages
24
Location
Kelowna, BC
Well tonight after 2hr drive each way, I've brought home my first Tandy find.

The good, the case keyboard and unit overall is in great shape, and I didn't pay a dime for it, except for the $30 in gas.

The bad, it's having a display issue with the vertical deflection. at first i could see a bit on the screen, but it was very unstable. turned it off, on a few hours later, no vertical deflection what so ever. one solid line running running from left to right. My experience in the past with problems like this, is that it could be either the deflection yoke has a short/open. which would be a worst case scenario or its a deflection circuit.

are there any known issues with the TRS-80's displays that could cause this? is the video monitor circuit the same in the model III's and model 4's?

I'd love to get this up and running, thanks everyone!
 
Well i found more information today, model 4 video is different, ive found the wiring diagrams and component lists for both the RCA, and TCE versions. I'm going to reflow the entire board tonight, and see if there is any ovious damaged components. I'm not sure if its RCA or TCE but we'll find out soon.
 
so tonight i reflowed the entire board, it is a TCE version, but its a newer part number then whats in the model 4 technical reference manual, TCE 8790615 is the part number. re-flowing did not work, so I'm now at a bit of a loss.

does anyone have a schematic, or manual for the 4D?

at this point, i feel that I'm just talking to myself
 
I'll see what manuals I have scanned in for the Model 4. The 4d is one machine that I don't have yet. I don't know much about video troubleshooting at all.
 
morpheus256 said:
The bad, it's having a display issue with the vertical deflection. at first i could see a bit on the screen, but it was very unstable. turned it off, on a few hours later, no vertical deflection what so ever. one solid line running running from left to right.
Click to expand...

If the vertical deflection fails on any CRT, you'll get a line across the middle of the screen. Never leave it running like that for more than a few moments or you'll damage the picture tube.
 
well i made huge steps forward today, had a friend help me who's a tv tech, and we were able to get vertical deflection back, and discovered that my sub brightness pot was also toast, which explains why i couldnt dial down the brightness when i had no deflection to troubleshoot.

3 caps were replaced coming off the vert-def IC, once those were swapped, deflection came back, but brightness was way over done, could see the shaddow mask and everything, now that the sub bright pot was replaced, i can see a decent image on the screen. i say decent because a 3rd and possibly related 4th problem was uncovered, shipwrights disease anyone? :)


everything can be read on the screen, but the letters are unstable, looks like worms wriggling and moving around, this is with no floppy drive activity. you can read it fine, but every letter on the screen is moving.
the 4th problem is whenever the floppydrive does a seek, or access.the screen size grows, as soon as it stops seeking it returns to the normal size. while the disk is spinning the letters seem to be moving more.

also the DEC CP/M banner along the top that should be solid green, has scan lines in it breaking it up.

I think the next part is all related to a key component, but i'm unsure.

Thoughts?
 
We did use a inline ESR meter, and measured the capacitance of them, most were good, except those in the vertical deflection circuit. however I do agree, after 25 years, capacitors can look good, smell good, but can still have dried up inside.

I'll make a list and get them ordered. never used Mouser, either local or digikey but if you've had good luck with them I'll give them a try.

would you replace the electrolytes with Tantalum ? or just do a stock for stock replacement?
 
I'm not sure about the 4D but with my Non-Gate-Array model 4 (Original version) on the motherboard there is a trimmer capacitor that you could use to stabilize the images. Mainly the jaggies in the letters. All's you need to do is remove the metal shield that is covering the Motherboard (the board along the back edge) by removing 6 or 8 screws i can't remember the exact amount but after you get the shield off look in the upper left hand corner (as viewed from rear) and you might see the trimmer I am talking about. That's if it has it, if it does turn the trimmer with the screen laying to the side of the computer and tweak it till you get a good picture. If no trimmer present than replace caps as suggested use the same type (stock) electrolitic capacitors. Good luck!
 
You must log in or register to reply here.
Back
Top