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Model I Lower Case mod help needed

MikeModifed

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In a first attempt to do anything of this, I'm trying to put lower case into a recently acquired Mod I with a 1700069G motherboard and a 6670 chip in Z29.

In entry #6 here:
http://www.vintage-computer.com/vcf...eHD-NOW-WORKS-ON-YOUR-MODEL-1&highlight=kitsz, Professor Mavric :p says not to use the mod on page 106 of "The Custom TRS-80" but to use Kitsz' mod on page 108 (presumably the same book) if the mb has a 6670.

In my copy and the copy on archive.org, page 108 is a continuation of the reverse video mod.

My confusion, I think, is in the description on pg 106: "Now locate the trace on the circuit board running from Z60 pin 4 to Z30 pin 13. Use an ohm meter if necessary to make sure you have the correct trace (it is and I did). Cut it through. Double check that you have cut the trace that runs from Z60 pin 4 to Z27 pin 13."

I think that I'm to cut the trace shown in the figure in the Electric Pencil mod
http://www.trs-80.org/electric-pencil-lowercase-modification/, right? It does run between Z30 pin 13 and Z27 pin 13, which runs to Z60 pin 4, ASSuming that I'm doing this correctly.

Any advice or suggestions appreciated.

Mike
 

TRS-Ian

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There are two different editions of Custom TRS-80. I'm looking at 1st ed 1st printing (the only one I have at work with me). Page 106 "Lower Case with Upper" is the one to do if you have the 3001 or 6670 chip. 1st ed 1st printing doesn't even mention the Radio shack mod. Maybe my references in the previous thread refer to pages in 2nd ed printings...

In any case, that same trace cut needs to be performed for all lowercase modifications.

Ian.
 

TRS-Ian

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Ok so my copy at home of Custom TRS-80 is 1st ed 3rd printing (Revised ed. June 1983), and indeed page 107 explains the Radio Shack lowercase mod for systems with the 6673 chip, and page 108 explains the mod for systems with the 3001 or 6670 chip.

On the 3rd printing the picture of the female chicken on page 105 is replace with the RS lowercase mod.

Ian.
 

MikeModifed

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Thanks for the elaboration, Ian.

Next problem :) : "Now locate Z25...With a solder wick, suck up the extraneous solder that is present on pins 11, 12 and 13, and with a sharp X-acto knife or razor blade, cut pins 12 and 13 free from each other and from the ground lead going to pin 7."

Done and done, but pin 13 still has continuity to pin 7 (ground) and to pin 2 (which is grounded). It appears that there may be a trace running under Z25 between pin 2 and pin 13. What's the best option for freeing pin 13? Cut it? If desoldered, can it be pulled out?

Mike
 

TRS-Ian

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I just used to piggyback another chip on top of Z25. To me the time to piggyback the chip vs. the time to liberate the 3 pins from the PCB made it well worth piggybacking the 50c chip.

On my own personal machines I went one step further, which was to remove and discard the existing Z25 altogether, install a socket, and a new chip with pins 11,12,13 bent out to the side.

Ian.
 

MikeModifed

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Ok, so it's not rocket science. :p

After flailing around trying to figure out what I had done wrong, I realized that I hadn't done anything (terribly) wrong: the ribbon cable for the keyboard was disintegrating (had disintegrated)!

So, after carefully desoldering the 40 contacts and soldering in a new ribbon, I test zero shorts and get 20 good continuity tests. Firing the unit up as a 16k machine, I get 15572 memory available; all keys work properly (except the left shift which needs its contacts adjusted [I think]), and I can gin up a little ditty to poke the lower case ascii values into video memory: all of the proper lower case characters display correctly.

So, I assume that I've done the conversion correctly.

However, when I plug the unit into a Radio Shack or LWN expansion interface, a known-good floppy won't boot; immediately substituting another keyboard (only) with either ei gets a proper boot.

So, I'm at a loss for what to check next.

I don't think it makes any difference, but the keyboard I've been working with is an original Radio Shack 16k/Level II (26-1006).

Any help, suggestions, critiques welcomed.

Mike
 
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