TRS-80 Model I upgrade to Level II 16k

[litterbox99]

I've been thinking about buying a TRS-80 Model I.

When I was 13 in 1978 my grandmother bought me
a Model I 4k. We had to wait a few months to get
it upgraded to Level II 16k. I eventually sold it to a
a business a few years later.

I'd like to get just the keyboard/CPU and power supply.
I can use a conventional LCD monitor.

I see plenty of Level I 4k's and was wondering what it would
take to convert it to Level II 16k.

I have the skills, just need some good info
and a parts list.

Todd

 

[Vint]

Ah yes, the Model 1 -
From my recollection (going back to 1978 when I bought my TRS-80 Model 1 Level II machine.)
I walked into a Radio Shack store and told the salesman I wanted a Level II, not a Level I machine. He said it would take a short while (turned out to be an hour), for them to take a Level I machine to their 'other place' where a tech would convert it to a Level II machine. Now, I'm assumming they simply added a daughter board or perhaps just changed out some ROM chips and that was it. I took my Level II machine home and played with it in 4K RAM configuration. Shortly thereafter I ordered the upgrade to 16K in RAM chips from a place called Godbout, out California way, I believe, for a cost of $140. I received a package of RAM chips and installed them. I also had to slit a PCB trace to facilitate the completion to 16K. When I fired the thing up, I now had not only Level II, but also had the gigantic RAM of 16K! That's how I remember it, some 34 years ago.

 

[NeXT]

The Level II upgrade was indeed simply an additional board with three ROM chips and tacked into the system with a ribbon cable and a few wires.

I'm amazed that this is the best photo I can find of the upgrade.

 

[litterbox99]

That was the same pic I found...


I figured someone out there would have
documented this in the past.

 

[Druid6900]

On the version E board, the satellite board was no longer necessary and the upgrade was accomplished by replacing the two Level I ROMs with the Level II ROMs and programming the included dipshunt.

The mod shown in the picture is one I've done, oh, about 100 times and the other, smaller mod is the XRX-3 Mod for squaring up the audio signal from the cassette port so that a wide range of volume levels would be accepted. Probably did about 150 of those.

 

[TRS-Ian]

Todd, may I suggest you just buy a 16K level 2 model 1 and use that. They sold many more of those than the 4K Level I, making the 4K/L1 the rare one. Over time many lesser machines were upgraded to 16K level 2 as well. They appear on evil-bay on average 1 every 2 weeks however lately model 16K level 2s have been showing up weekly.

Don't worry about upgrading it, keep the 4K/L1 original.

Just my 2c worth.

Ian.

 

[Vint]

I totally agree with TRS-Ian’s good call – a Level 1 machine is a desirable piece of equipment in it’s native state. Besides, two Model 1’s are always better than 1, right? I remember the reason I just had to go with the Level II. It was it’s programming ability in BASIC vs. the Level 1 machine. For example –

Level 1, although quaint, and cutesy with it’s What? And How? replies, wasn’t going to let you do much in the way of programming, at least in the BASIC sense. After reading (above pics), in the RS catalog, I just knew I really had to have things like Left$, Right$, Mid$, etc.

But a Level 1 machine, as a collector’s item, I believe, would be the more desirable of the two. Why I’ve seen just the Level 1 manual on eBay for $100. asking. Bit ridiculous, but more power to ‘em if they get it. I’ve also seen the Level 2 manual with the same asking price. Both are available online, but if you’re setting up a display with original equipment, the Level 1 book is desirable too. Just need to shop around.
The system I originally bought back in 1978 for a mere $750. complete - was as the one below, except with the Level 2 mod and Level 2 book. Didn't even have a numeric keypad.

 

[litterbox99]

I just scored a NOS Level II upgrade
kit from Ebay; item # 330727984525

It also came with an unused dip jumper
and a 220 ohm resistor. A very clean kit !

It didn't come with an installation manual.
I'll assume that it wasn't for general public and
the the local store/repair center had their own
bulletin.

There's only six connections that need to be soldered;

A11-A13
Ground
+5vdc
ROM *

The TRS-80 Technical Reference Handbook 2nd edition has all
that you need for schematics.

My question is... What's the 220 ohm resistor for and
where does it go ?

 

[litterbox99]

I agree, I'll keep it a 4k Level 1 until I buy a 16k Level 2 w/10 key pad.

Right now I'm just toying with this unit, it is easily restorable to original condition.

Basic Level 1 sucks, if I'm gonna do any programming, it has to be Level II ;-)

Todd, may I suggest you just buy a 16K level 2 model 1 and use that. They sold many more of those than the 4K Level I, making the 4K/L1 the rare one. Over time many lesser machines were upgraded to 16K level 2 as well. They appear on evil-bay on average 1 every 2 weeks however lately model 16K level 2s have been showing up weekly.

Don't worry about upgrading it, keep the 4K/L1 original.

Just my 2c worth.

Ian.

 

[Druid6900]

I just scored a NOS Level II upgrade
kit from Ebay; item # 330727984525

It also came with an unused dip jumper
and a 220 ohm resistor. A very clean kit !

It didn't come with an installation manual.
I'll assume that it wasn't for general public and
the the local store/repair center had their own
bulletin.

There's only six connections that need to be soldered;

A11-A13
Ground
+5vdc
ROM *

The TRS-80 Technical Reference Handbook 2nd edition has all
that you need for schematics.

My question is... What's the 220 ohm resistor for and
where does it go ?

OK, the first thing is what version is your Model I motherboard?

If it's a Version E, you can just plug the ROMs into the two sockets on the motherboard, replace and program the dipshunt, and throw the rest away.

The resistor is for a mod to the E/I connector on earlier boards and can be dispensed with if it's an E revision as the correction was made on that version board.

If it's not an "E" board, let me know what revision it is and I'll see if I can dig up the installation instructions.

 

[litterbox99]

PCB ver. 1700069D

 

[Druid6900]

PCB ver. 1700069D

OK, well, so much for the easy way.

You're going to have to install the satellite board on a Rev. D board instead of just the chips.

Give me a couple of days and I'll try and dig up the installation instructions.

 

[litterbox99]

Thank you for the effort !

I can figure out where the six wires go, just
not sure of the resistor. I welcome the instructions
as I can add them to my documents.

In the mean time, I built an adapter to read the mask
ROM's. Using the pinout of a 2332B ROM to a standard
2764 Eprom.

They read successfully. I don't know how to tell what
ver. they are, but I did see "R/S L2 BASIC" in the code.

Todd

 

[Druid6900]

That's the last version of the ROMS and were meant as a direct plug-in on the Rev. E board.

If you don't have, or plan to use, an expansion interface, the resistor isn't necessary.

 

[retrobecanes]

I just wanted to pick up this old thread as I am currently considering upgrading a Model 1 Level 1 ("G" model board, already upgraded to 16K) to a Level 2 so that I can use an expansion interface and a drive emulator. What's needed in this case?

 

[cjs]

I'm amazed that this is the best photo I can find of the upgrade.

Yeah; I was just looking for a photo of the North American upgrade board and couldn't find a thing, except for yours. Odd!

Anyway, here's a photo of my upgrade board. You'll notice that these are NEC mask ROMs, and there's actually an extra ROM chip there. That's because, as I've just discovered, the Level II ROMs on the Japanese version are different, and larger.

 

[ldkraemer]

Todd,
Have a look at the Character ROM and see what the number is. You might be lucky and be able to
enable lowercase characters.

Ian's method follows from:
http://www.vcfed.org/forum/showthre...l-1-in-2016/page2&highlight=Model+1+Lowercase

I simplified it this way which has served me well since the 80s.

If your character generator (Z29) last 4 numbers are 3001 or 6670, then you do the mod Lowercase with Upper from Custom TRS-80.

If your character generator last for numbers are 6673 or 6674, then you do the Radio Shack lowercase mod, from the same book.

All other LC mods are complicated or unnecessary in this day and age, and in fact when I buy systems with other mods I undo
them, and set them up as listed, above.

For the record:
3001 : early systems, lowercase but no descenders, and a flying 'a' due to Motorola manufacturing fault (so they sold these
chips cheap to RS who figured they didn't need LC anyway)
6670 : mid-production systems, same as above but with no flying 'a'
6673 : late systems, custom part made especially for the M1 for Radio Shack and included with most late 79 and 1980 machines.
By this time RS was selling the LC modification "kit" for $99 but if the computer already had the 6673 then the 1-min job
was to cut the trace, install the stacked 2102s into the already fitted socket, and solder two wires. It probably took
longer for the soldering iron to warm up than to make the mod.

Ian.

If you want 48K of RAM in the Keyboard, I have an article about how to do that using 64K RAM IC's.
See attached ZIP.

ROM Information:

Model 1 Level 1
Z33    ROMA    MM2716E    4K    Part Number 3108011
Z34     ROMB    MM2716E    4K    Part Number 3108012

Model 1 Level 2
Z1    ROMA    MM2716E    4K    Part Number 3108013
Z2    ROMB    MM2716E    4K    Part Number 3108014 
Z3    ROMC    MM2716E    4K    Part Number 3108015

Model 1 Level 2 Mother Board Revision G - 2 ROM Set
Z33    ROMA&B    8044364    8K    Part Number 7807
Z34    ROMC    8044732    4K    Part Number 7804 

Model 1 Level 2 Mother Board Revision G - Single ROM
Z33    ROM    8044364    12K    Part Number 7809  BASIC I


The model 4 contains 14K of ROM (in Model III Mode) divided into:
U68    ROMA    MCM68A364 8K    Part Number 8048364    0000-1FFF = 8192
U69    ROMB     MCM68A332 4K    Part Number 8040332    2000-2FFF = 4096
U70    ROMC    MCM68A316 2K    Part Number 8042316    3000-37FF = 2048 less 37E8-37E9 = 0xFF

Larry

 

[dittman]

Yeah; I was just looking for a photo of the North American upgrade board and couldn't find a thing, except for yours. Odd!

Anyway, here's a photo of my upgrade board. You'll notice that these are NEC mask ROMs, and there's actually an extra ROM chip there. That's because, as I've just discovered, the Level II ROMs on the Japanese version are different, and larger.

View attachment 1275381

I noticed you are in Tokyo, any chance you've found any of the operating systems that were released for the Japanese Model I with the Kana keyboard and ROMs?