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TRS-80 model 4 - garbage on screen, now every boot

I

Iainjh

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Joined
Feb 4, 2011
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37
Hi

I've recently got myself a model 4. Seems overall good condition. I have already received a frehd kit and a U72 Pal from ian mavric.

HOWEVER!

before I've even started to build or fit those an intermittent boot problem has got much worse. It worked when i first got it, then a week later it suddenly crashed to a garbage screen, and would only boot 1 time out of 10. It no longer boots at all, just sits there with a corrupted display. The drive spins for 30 seconds or so and.. thats it. Same if I hold down break while powering on.

Once or twice today it has spun the disk for 10 seconds or so, stopped, then respun the disk, then again. But thats rare.

I have reseated every IC I can - no change

Ive temporarily swapped out the 4164 rams - (though i dont know if my replacements are good or bad) - no change.

One time I swapped out the U70 and U72, put in the extra 64k ram and it worked. But one time only. So i have reverted back to stock and its exactly as it was.

I note the corrupted display is almost exactly the same each time (see pic) except the time it 'almost' booted when only half the screen was garbage, the rest blank.

Ive also swapped the z80 for a new (but also untested) Z80 - no change, exactly the same.

Can anyone offer any advice or help please? steps to eliminate?


TIA, Iain, UK.
 

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lain,
I'd suggest VERIFYING the Power Supply Voltages. If you have two Power Supplies VERIFY Both. You should have +5.05, +12, and -12. The +5.00 should be within +-3%,
while the +12 and -12 may vary a bit more.

Next I'd try booting with the BREAK KEY held depressed. Then depress the ENTER key at least twice. See if there are any changes in the display.

Then try the F1 (1), F2 (2), and F3 (3) keys held depressed. You are looking for error messages during boot, or any changes in the display.
Reference the Model 4 Service Manual about page 30.

Larry
 
Thanks, will do. It has 1 x psu, an Astec AA12090.

I've bought some caps to replace the usual suspects in the psu, (the rifa 0.01 and 0.1uf's that blow) - I'll get the meter out too. I hope there are trims/ ways to adjust the psu voltages... service manual here I come :)

thanks I'l keep you advised!
 
The voltages in that PS are not adjustable. Also is it Gate array or a non Gate array see here: http://www.trs-80.com/wordpress/trs-80-computer-line/model-4/ Also what revision level is the mainboard. I had one do the same thing. Ultimately it died completely. Hopefully yours won't I could get mine
to boot if I heated certain areas of the board. Like near the Video controller the big 40 pin 68xx chip near the cpu if it's a Non Gate array. On mine I Think it was an intermittent trace or something along that line. Good luck.
 
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Non gate array

Motherboard I shall check, along with the psu and I'll reply in a few days

Thanks!

Ps thanks also for the manuals, Larry

:)
 
On those non-gate array boards, carefully check all the blue extra wires and make sure they are all connected - I had one of those blue wires come loose and got the exact same problem until I found the culprit - re-solder and its back up and running.

Because Tandy used wire-wrap wire, if one comes loose, it won't move far from where it has disconnected from.

Ian.
 
Chromedome45,
The ASTEC AA12090 Power Supply doesn't have an easily adjustable pot to fine tune voltages, but it is somewhat adjustable, via resistors, according
to the Documentation on Section B -- p.113
The Power Supply regulates off of the +5V and +12 V (V3) outputs. If these outputs are low, it could cause the others to be low. If so, adjust
+5V and +12 V (V3) outputs by removing or adding R27 and R28.
Click to expand...

Also, it might not hurt to inspect Capactors: C13, C14, C15, C16, C17, C18, C19, C20, C21, C22, C23, C24, & C25 for any Leaking, Bulging Tops,
or suspect/bad/cold solder joints with a magnifying glass.

Check this posting:
http://badcaps.net/forum/showthread.php?t=31322&highlight=Astec


Larry
 
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Hi all
Sorry for delay, I guess I have notifications turned off, just come back and there's been a few replies - sorry.

Anyway, sit rep. The psu outputs +12.05v, 5.10v and ( I forget exactly) but very similar -12.0x v also. At this point past swapping rams and reseating ic's I need more help. So I've bought some nos and tested rams ( of the right timings etc) and Friday I'm taking it to a friend who'll get the scope on it and get it fixed.

I'll report back in a week or three.. The bargain is that he'll fix it but wants to play with it himself for a few weeks.

Ok, more soon inc details and thanks again, Iain
 
quick update - I dropped it off this afternoon, he's just emailed me to say hes already fixed it - it had 2 dead buffer IC's, 2 74LS245's.

Said he was a star:) OK So I'm without for a few months but hey I'm v happy!

THANK YOU for your help guys, and +1 also for the service manual larry sent. thanks!:)
 
I am pretty sure I have a 4p board that behaves like yours. May or may not be a buffer IC. What makes it more fun is that it is a pre-production board I got from Tandy a long time ago that used to work. The IC numbers don't jive with the service manuals. Curious if your friend might be able to fix it if I can't figure out what is wrong with it. :)
 
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