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Model 4 Screen 'wobble'

tempest

tempest

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Sep 16, 2014
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SE Michigan
I just noticed that on my TRS Model 4 (with a FreHD) that the image on the screen seems to shrink or 'wobble' for a split second now and then. Is this 'dirty power' issue, one of the components going, or is there possibly something loose inside?

Is there anyone going to VCFMW that might be able to take a look at this? Monitor and power issues are a bit beyond my ability to fix.
 
In many cases the power supply is the culprit of such unpleasant appearances (on the Model III it's similar by the way). The reason for an unsteady screen picture could be cold solder joints on the output connector (and other places) of the power supply. These cold solder joints do often occur on this specific output connector and could lead beside screen issues also to random reboots and much worse defects inside the computer. So I would check the power supply first (including a check the often bad RIFA capacitors).

An example of the cold solder joints and repair on a Model 4 power supply is documented on YT (TJBChris): TRS-80 Model 4 Restoration: Disk Drives, Testing & Reassembly

My Model 4 had those cold solder joints on connector TB2 also:
Netzteil ASTEC AA12090 01.JPG Netzteil ASTEC AA12090 06.JPG
 
Is there recap kit I can buy? I see one for the Model 1 and 4P but not the 4. Do the RIFA caps have to be replaced? I always thought you could just forget about those these days.

I can check the connectors when I open it up, but since it seems to happen more when the screen has a lot to display I'm guessing it's got to be a cap on the power supply or video board.
 
>>> Do the RIFA caps have to be replaced? I always thought you could just forget about those these days.

Unfortunately not (unless they have been recently replaced of course). Your particular problem is unlikely to be a RIFA capacitor - but when they go - they go!

Yes, go for the obvious faulty solder joint, dirty connectors and damaged wiring first.

Also, clean all the dust and dirt out.

Just out of interest, do you hear a 'crack' at the same time as the picture issue occurs? If so, this could indicate a 'flash over'.

Dave
 
daver2 said:
Just out of interest, do you hear a 'crack' at the same time as the picture issue occurs? If so, this could indicate a 'flash over'.
Click to expand...
No, no sound at all. The wobble isn't constant either. It will do it for a second, go back to normal for a little bit, then do it again. Sometimes it does it more frequently, but there are periods of normal picture.
 
My 4P had two screen issues - the first was that the screen dimmed and contracted slightly when the floppy drives were active, and the second was a slight wobble which was more noticeable when it was warm, but sometimes present and sometimes not.

In my case I took the easy option and replaced the power supply with a new-build unit, which solved both issues completely. I had planned to recap the existing unit, but some of the supplies Radio Shack/Tandy used back then were fairly poor, and a new one seemed a better long term bet.
 
I'd consider one of the prebuilt ones if I was 100% sure it was my power supply. It probably is though.

Where can you buy one?
 
If I recall, Jay Newirth/newSOFT, Pikesville, Maryland. That was a couple of years ago. I have seen the PSUs on eBay too, but Jay was very helpful, and had lots of parts and expertise.

(@Jay Newirth ? Are you still here?!)
 
 
I had my friend look over the power supply board yesterday and he spotted two iiify looking solder joints. One was one a resistor and one was on the power connector. He reflowed both and now the picture seems solid. I'll have to put it through a thorough stress test tonight but all seems good.
 
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