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TRS-80 Model 1 - Display problem (Hoizontal Sync)

M

MLMorris

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Feb 5, 2024
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Hello
After reading a little on this forum I decided to dig out my old TRS-80 Model 1 to see if it stll works. As you'll see in the photo, it boots and runs BASIC code. However the display is distorted. The vertical is very stable and I can adjust it as you would expect using the various pots. The horizontal is wobbly and characters which should line above each other vertically are not vertical. The knob on the back of the display is very sensitive, moving it just a tiny bit messes up the display badly. The horizontal positioning pot inside the keyboard seems to have no effect at all. I have checked the Sync Generator circuits within the keyboard unit with an oscilloscope and they seem to be working fine. Therefore I'm suspecting the display circuitry. I know very little about how CRTs work. Any advice on where to start?
 

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It looks like it's on top of an expansion interface. If you have floppy drives, they should be on the right side of the computer as you face it. The transformers can interfere if they are on the left.
 
Good question. I once had FDD but they're long gone. It is sitting on an Expansion unit which is currently disconnected and not energized. No obvious sources of mag field in present location. The display has been in storage for a long time. I wonder what it was sitting near. The distortion does sort of look like a magnetic field effect.
 
If you just watch the screen display (i.e. don't touch the TRS80) does the display artefact change with time?

I am just wondering whether there is some ripple on the power rails (possibly due to a failing smoothing electrolytic capacitor) that is 'bleeding' through to the display.

Dave
 
Vertically the image is VERY stable. Horizontally It does wobble a bit which looks to me like a 60hz related ripple. I plan to replace the power supply caps in the keyboard unit this evening. I'm pretty sure they are one of the problems.
I'm not so sure about the way the text swoops across the screen (the first character in each line is not lined up with the one above it). This swoop pattern is pretty stable and very sensitive if I touch the horizontal position knob on the back of the display.
 
The 'swooping' could be the PSU capacitors in the monitor is what I am thinking.

But there are also some other capacitors in the monitor that could be responsible..

What is the part number of the monitor and I will look up the schematics for it.

Dave
 
Dave,
The Radio Shack TRS-80 Model 1 Monitor is actually a RCA Television Chassis with
the Channel Selector Removed. Chassis is an RCA KCS203A/D 1713-2

I also sent MLMorris this information.

Larry
 
Dave:
Thanks for the help and I agree with your thiniking. I replaced the power Caps in the keyboard unit which improved the 60Hz ripple but didn't fix the "swoop". Into the display next, ... although I think I killed something in the keyboard unit video out circuit while poking around. I'm guessing the output transistor may now be dead :) By the way, I've figured out that there were actually two (or more?) different designs for the video display, all based on RCA TVs. Mine actually has three PCBs inside. The other (which I think is a second generation) has only two PCBs (it is the newer design which the Sam's reference describes). Larry was able to share the service manual for the version I own.

Larry:
Thanks again for the info you provided! I'm going deep into CRT territory now.... the dark arts!

All:
I'm VERY aware of all the safety concerns and cautions when dealing with a CRT display. I'd welcome any specific suggestions around troubleshooting the horizontal control circuits. I'm pretty comfortable with dgital electronics, much less so with analog. For those familiar with the specific device, I'm concerned about the variable inductor which is the "horizontal hold" control. It is behaving poorly. In my understanding there is little to go wrong with inductors (unlike electrolytic caps), but perhaps there are known/typical problems with variable inductors?

-Mike-
 
Is there an adjustable core in there or do you need to use a tool?

If a tool, the last person could have cracked the ferrite core using a screwdriver (which is not meant for the job in hand).

I suspect it should have a knob that adjusts the core though if it is designed to be adjusted from outside the monitor by hand.

However, if someone has used too much force, the ferrite can become cracked...

It has been known for the inductor winding to fail... Or a poor solder joint...

Dave
 
FWIW, it might be worth trying to definitively narrow down the issue to either the monitor or the keyboard by building a composite cable and connecting the keyboard unit to another monitor. (The composite input on a TV or one of those dirt-cheap composite to HDMI adapters should do.)

I repaired a Model I last year that had wildly horizontal "shimmering" that was caused by rapid oscillation of the +5v regulator; the culprit was one of the ceramic disk capacitors, not the big electrolytic that filters out 60hz hum. The "swoop" in your display is by comparison a lot more gentle. It could still be an issue in the keyboard, either a much slower oscillation than I was seeing or an issue with the horizontal positioning pot, but I kind of lean towards this being a monitor thing.
 
An update for those interested: I fixed the "swoop" depicted in the photo above. I did so a little bit by accident. I did something dumb which fried the outputs in two ICs on the TRS-80 Model I motherboard. After replacing them, the display problem was corrected. The ICs replaced were the two devices which feed data and sync pulses to the video output transistors (Z41 and Z5 respectively).
 
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