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How to trace/fix video RAM problems TRS-80 Model 1

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Chromedome45

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If you ever get a Model 1 screen that is acting almost normal but has odd characters on the screen have a look at the 7 video RAM chips. They are numbered 21L01-04's or something along that line if I remember correctly. With power off and if you have a spare 21L01 piggyback it onto the back of each of the RAM chip making sure that all pins are lined up with the chip below it. Be sure to power down before moving to next chip. Repeat to the next chip etc. Do this till the display clears. When it does clear you have found the bad video ram. So just remove the old chip and replace with the good one.

Hope this helps
 
Frank,
Thanks, for that information on troubleshooting the unusual Video Problems.

I don't know if I previously told you about some other video information on this URL:

http://ts-inc.dyndns.org/Diagnose Repair TRS80 TRS-80 Model 1.html

Also, I sent James some specifications for designing a Z80 Test Board that will assist in testing
those troublesome Z80 Motherboards. He has the Circuit Board designed, and for sale on his site.
It has not been approved for export to the USA just yet..........

As long as the Z80 Control Signals (*RESET, *INT, *WAIT, *NMI, & *BUSREQ) are in the proper state,
and there is a CLK, +5VDC, and GND, the tester should function properly.

http://www.lo-tech.co.uk/product/z80-cpu-test-adapter/
http://www.lo-tech.co.uk/wiki/Lo-tech_Z80_CPU_Test_Adapter

I am waiting until I can get my hands on a couple of these boards for my Model 1's that don't power up.
The Z80 Test Board will allow you to check every Address Line at every IC on the Motherboard.
The only thing it won't do is test or verify the Data Lines, but the Data Lines (and Ports) can be
checked with an In Circuit Emulator (ICE), or another Tester that I am currently in the process
of assembling from other web information.

Z80-1.jpg

Larry
 
Sounds like a really nice piece of test equipment.

In what way will your TRS-80's not power up? I had one acting up that wouldn't power on. Turned out to be the power switch itself! Had to desolder it take it apart and clean the contacts. Soldered it back in and it powered up just fine.

Also check that big transistor. Sometimes they short out.
 
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Frank,
I haven't taken the time to check them out just yet. I may get to it in a couple of months. I've got too much going on right now to
start with the Model 1's.

Thanks for the Suggestions.

Larry
 
Thanks for that reminder about the video RAM on the Mod 1. I've got one out in the garage with just that problem on the screen. Somewhere along the line I picked up some 21L01s thinking that might be the problem. Now I just need to find the chips and the TRS-80 at the same time!
 
Z62! (Sounds like I should shout BINGO!) That was the bad chip on my Model I motherboard.

Of course, once I had the Mod I apart on the bench, I couldn't find the 21L02 chips anywhere in the garage. So, after a quick trip to eBay, the chips arrived today. Using the piggyback method Frank describes above, it took probably all of about 5 minutes to find the defective chip.

Below are some pictures showing what we are discussing. The first is the garbled display, the second shows the display with the piggybacked chip in place, the last one shows the motherboard with the piggybacked chip in place. BTW, the chips I used are AMD's AM91L02APC with 1977 date codes.

Mod1badVidRAM_1.JPGMod1badVidRAM_2.JPGMod1badVidRAM_3.jpg
 
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Hi
Guys, I am trying to bring back my old Model 1 from the dead. I have already repaired the power supply and now it boots up BUT the video is bad.

it looks like the "Mem Size" it there but hidden under individual graphic blocks.
would this be a memory issue?
Rick

20201115_150621-a.jpg
 
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