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Model III CRT Issues

A

ardsleytank

Experienced Member
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Nov 15, 2015
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Tennessee
Okay.

So I have been having a few issues with the CRT on my Model III, maybe you fellas could help?

First off, when the screen is on, every once in a while, the screen twitches and zooms in and out a little. It's been doing this ever since I got it.

Secondly, I just replaced the old B/W tube with a Green one. Now, even with the contrast knob turned all the way down, there screen is mostly green. If I turn up the brightness knob, it goes away, but the image is fuzzy. And if I reset it, the green comes back for a moment, until the TRS-DOS boot screen loads, then it goes away again.

I have fiddled with the focus adjustment that's on the board to no avail.

I am good and truly stumped, any ideas?

Thanks,
Peter
 
Okay, here are the pics!

Just after I press reset:

trs_80_2_by_ardsleytank-dag3fph.jpg



While running BASIC:

trs_80_1_by_ardsleytank-dag3fom.jpg


Weird huh?
 
I'm curious if your computer has a TANDY Power Supply in it versus the Astec? The Tandy's have a bad habit of developing cold solder joints on the Pins at the edge
of the board where the connector plugs in. I've checked and resoldered a bunch of those for intermittent problems. That is where I'd start looking.


Larry
 
What's the story on the original tube? Was it also out of focus or too dim or too bright? Do you have a voltmeter? I can walk you thru some measurements if I'm looking at the right schematic.
http://maben.homeip.net/static/S100/tandy/systems/model III/Tandy Model III Service Manual.pdf board is p76 and schematic is p78. If you look in the upper left of the schematic,
jumpering across R308 should totally mute the video with the screen all black, I think that is what Chuck is suggesting to narrow the problem to this board. Does that happen?

Larry G
 
@ldkreamer he is working on a Model III not a model 4. The 4's had the bad solder joints.
@Tank Mr. Chuck(G) is referring to the ground cable that connects the video board to the main chassis. A thick Green/yellow or Black cable with spade lugs on each end. Creates a common ground. Send pictures of insides. Especially the Top part with the CRT and video board.
 
Perfect. On the TRS80 systems, Sam's is usually much clearer than the Tandy manuals.

In any case, the "wiper" on a potentiometer is the center contact. Grounding the wiper on the 500K brightness controls should make the raster disappear. Grounding the 500 ohm contrast control wiper should make any text go away. Pots can get dirty and create all sorts of havoc.
 
Alright now I'm worried. This morning, I fired her up with the top off, and all was good. I fiddled with the RF shielding on top of the disk drives, and when I flipped the switch, the monitor comes on, but the drives don't spin, there isn't a CASS? and I get no system response. At all.

When I unplug the data cable from the drives, they spin, but no lights come on.

I'm scared that I may have shorted out something...

Please advise,
Peter
 
And, disregard that previous comment.

I accidentally tightened the RF-shielding a little too tight on the right side where it bolts up on top of the keyboard ribbon cable.

I loosened it up, and CASS?. Put the drives back in, Diskette?.

I am so relieved. Now, on to the tests!
 
Picture time!

Okay, so I grounded the brightess pot. This is what I got:

trs_80_3_by_ardsleytank-dag7ab2.jpg


Nothing happened when I grounded the contrast pot, so I didn't take a picture.

Here's a pretty terrible shot of the video board:

trs_80_4_by_ardsleytank-dag7aa5.jpg
 
That doesn't make much sense. I wonder if you broke a wire during your replacement.

Put a voltmeter (set to 200VDC range) on the brightness wiper terminal and ground and measure the voltage when the system is powered up as you rotate from one end to the other. Keep your fingers off the terminals; it's low current, but it could sting a bit.
 
Well, there you go. There's a break somewhere. The Sam's literature should help to find out where it is. Normally, the high end of the pot turned up to maximum brightness should measure at about 100VDC. The symptoms would seem to indicate that the 100V is present at the CRT, but the pot's not shunting this voltage to ground, so the raster is always at its highest intensity.
 
Last edited:
Oops i missed it in the Photo.

I have had dirty edge connectors also on these video boards, Plugging in and out a few times will normally clear the contacts, or wipe (Don't Spray) the contacts with a cleaner on a rag might help. Good luck

Thanks Ray
 
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