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Model 4P Scrambled 80 Characters Mode - only when floppy cable connected ??

V

VirtualTed

New Member
Joined
Feb 23, 2011
Messages
9
Hey guys,

I'm trying to sleuth out why my 4P has very messy 80-column video (64 is fine) and of course I found plenty of information about C231 here on the site. So, I carefully disassembled and extracted the motherboard, disconnected the floppy and expected to tweak C231 on the motherboard. However, when I turned it on. it looked great. No issues with the video whatsoever.

So, I refrain from playing with C231 and insert the motherboard back into it's spot, plug everything back in and turn it on - and the video is very wavy and messy again. Hmm.

Then I leave it all together and only disconnect the floppy cable from the motherboard - video looks great again.

Therefore, it appears to be related to some interference or something else that is related to the floppies. They are powered, but just not connected to the motherboard. I've done quite a bit of searching and can't find anyone with a similar problem.

Any ideas? I have a FreHD I can use to boot the system, but I would really like to keep using the floppies if possible.

Thanks for any help in advance.

-VT
 
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What happens if you leave the Ribbon cable connected to the floppy's and just unplug the
Power Cables to both floppy's? Is the video correct?

Or what happens if you just unplug the Ribbon cable from the second floppy? Or what happens
if you just unplug the Power cable from the second floppy? Is the video correct?

Larry
 
Hi Larry,

First - thanks for the help. I was actually thinking last night "maybe there's something wrong with the cable?". First thing I did this morning was disconnect each drive cable independently. But what I'm getting now is different than yesterday.

The drives do not seem to make a difference. Whether the drives are connected or not, it looks like I'm getting a wavy screen about 90% of the time when I do a reboot/restart on the machine. I guess the drives were a red herring and I just got "lucky". I think my chances of rebooting and having it look normal improves as the machine warms up, but I can't be certain of that. Once I get it to boot and look normal, subsequent boots are almost always normal unless I turn off my FreHD and the 4P. Then, the next bootup is wavy again and after a handful of resets I can get it looking normal again.

Strange huh? There is some consistency to the screen problems, but it's really odd that anything with the FreHD would have anything to do with it.

-VT
 
I'd suggest checking the Power Supply(s). I've never worked on a 4P, but the Model 3's and 4's that had
the Tandy Power Supply (vs Astec) with a long row of vertical round pins, were bad about having small circles
around the round pins that showed a bad solder joint. The PS just needed to be removed and the
Solder re-flowed with fresh solder for each pin. Also look for any signs of leakage from any Cap(s) that are on
the Power Supply(s) and if any have a bulged end those need to be replaced.

You can also use a LOAD on the Supplies for testing them to make sure everything is working correctly.

You can download the Model 4 Service Manual and check it out.


Larry
 
Can you show a picture of the scrambled text.

There is a sheared view for the C231 issue, but equally it could be power, broken solder joints, etc.
 
Thanks guys! It's starting to appear to be a warm/cold issue from what I have seen. It kinda looks like this...

First two minutes: Rapid screen scroll horizontally. Completely unreadable and unusable.
Next 8-10 minutes: Screen no longer scrolls and is legible, but characters dance back and forth and there is some overlap/folding over of the content on the left side. Like the monitor needs an adjustment.
After 10 minutes: Screen is perfect. Everything is centered, non-jittery, and clear. No folding or overlap of text.

I'll get some video if I can and post it so you can see what I'm talking about.

-T
 
Hey guys,

Here's a picture of the video board: https://drive.google.com/file/d/1WsTIgYulinfsngB4DaXhAAnLJLEv3FpF/view?usp=sharing

Here's the power supply: https://drive.google.com/file/d/1C1pnEeD05WUCk_8sMRyq3CMuSQZCG7_f/view?usp=sharing

And here's a video of the problem happening. About 2 minutes (kinda big). You can see where the screen is completely scrambled, then after it warms up it get "better" and then finally is nearly perfect. The process was very quick this time probably because the system was already somewhat warmed up I think...

https://drive.google.com/file/d/1C1pnEeD05WUCk_8sMRyq3CMuSQZCG7_f/view?usp=sharing

I'll likely pull the power supply off tomorrow and look at the back of the board for any issues. However, the caps on both boards look pretty good. No bulging or leaking of any kind from what I can see.

Thanks!

-VT
 
Well, I'm trying this again to see if the video will post. (I replied a couple days ago but I'm not seeing it.)

Here's a video of what happens, and it's condensed down to show all three phases.

Phase 1 - Totally scrambled, scrolling wildly and unreadable. (up to 10 minutes)
Phase 2 - Unscrambled, but pushed so far left that it's overlapping itself on edge of screen. (5-10 minutes)
Phase 3 - Almost Perfect! No issues except the occasional jiggle, which I'm fine with overlooking. (15-20+ minutes)

I'm also including a photo of my power supply / video driver in case you're interested. All caps look fine to me...

[video]https://drive.google.com/file/d/1ZWVI-Yw7pBhjDe0H9-j1WcuzJ2rNl1hJ/view?usp=sharing[/video]

4p-powersupply.jpg
4pvideoboard.jpg

Thanks again for any ideas.

-T
 
Hi Guys,

Well, I responded to this thread a couple times with videos & pictures but for some reason they never posted or got approved. I wanted to update you on this in case it helps someone else..

To recap, on a cold boot where the machine had been sitting all night, if the floppies are plugged in then the screen is scrambled or very erratic for 10+ minutes. If the floppies are not plugged in, it boots perfect 90% of the time. The power supply and video board look good - nothing burned out, no bad caps visible, etc. Once the machine warms up, the video looks good regardless of whether floppies are plugged in.

I have a spare 4P that is for parts and I pulled the motherboard and swapped it. Video looks great and works as it should no matter whether floppies are plugged in or not. So, from this I can assume it is a motherboard issue and the only thing I had not tried was adjusting C231. So I swapped in the problematic motherboard again, adjusted C231 and after a little back and forth it seems to be working perfectly.

The floppy thing was a symptom of C231 being out of adjustment, and that was surprising (to me). I'm far from an electronics guy, but I wouldn't expect one to affect the other. Let this be a heads-up in case someone else runs into the same thing.

Thanks again to the guys that responded.

-VT
 
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