My new TRS-80 Coco 2: First look and immediate upgrade and modding
Entry posted by Gunstar
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I have been keeping an eye out on about half a dozen different brands of 8-bit micros over the past couple of years, looking to finally branch out from my Atari 8-bit roots. I have had C64's, VIC-20's, Apple IIc's & Laser 128's pass through my hands in the last 20 years, especially back in the early 2000's when all could be had for dirt cheap still at thrift stores and flea markets. I use to buy them up, repair and restore them and then eventually resell for a profit after I played with them for a few weeks or months. Those days ended when to many other's started the practice and the vintage sources start drying up due to competition and stores like Goodwill started assessing value by looking to ebay and then selling at ebay prices so there was no room for profit for me anymore.
I wish I'd kept at least the C64, seeing how ridiculous prices have become in recent years, especially for the C64 that sold more than any other 8-bit micro, and there are 50 at any given time on sale on ebay, but still seller ransom them for north of $150 for working units, just like the more rare 8-bit brands are selling for! So I decided I was going to branch out and keep an eye on about a half-dozen 8-bits that I felt I wouldn't be disappointed in compared to my Atari 8-bit, I'm spoiled. The C64 was one of the few that makes the grade!
Anyway, one of the other brands I started watching were always a curiosity to me, but always seemed a "B class computer" compared to the Atari and C64 "A class." Which in my classing, has nothing to do with how expensive they are, the Apple II line is "C class" to me due to poor graphics compared to many other 8-bits, and being a graphically oriented artist this matters to me. The Coco line's graphics are poor compared to Atari and C64, but better than Apple's still. But even though I consider it "B class" to me, I was always curious about it because though it had far less software released for it even than the Atari, one thing it seemed to have tons of were expansions, and first party ones too. So I started watching auctions and buy it now listings mostly to wait until the listing ended without a sale and see what kind of lower price point the seller would relist at.
Anyway, I was watching this one listing of a "buy it now" TRS-80 Coco 2 listed as "powers on, but screen is corrupted and computer is frozen." They were asking $100 for it, and it looked dirty, and slight UV yellowing, but otherwise the case and keyboard were in good shape. I knew I'd be able to repair it.
Then things started to fall in place like it was my destiny to own a TRS-80 Coco 2 or something. First, just a couple of days after finding the listing and adding to my watch list, my business partner, who is well aware of my vintage computer and console and electronic repair hobbies brought a big brown bag full of old cables to me one morning, saying he was going to throw them out but decided to see if I could use them or not. Well hidden in the middle of a huge tangle of cables in this bag were three TRS-80 Coco controllers. A mouse and two analog joysticks. Also I found no less than three TRS-80 Coco tape drive cables in the bag too. So at first I'm thinking that since they are all analog, and my Atari's can do analog controls, That I'd convert the mouse and one joystick to the Atari plug standard and see how they work with some graphic art progarms that work with analog input devices like the Atari Touch Tablet.
Then a week later, already having converted the TRS-80 mouse that works great as a substitute for a touch pad, I get a private offer from the seller on that Coco 2 I was watching. I haven't come across a price that cheap for any 8-bit micro in the last year, working or not. So with that and the advent of now owning TRS-80 Coco controllers, I took it as a sign from the God or the universe that the Coco 2 was to be my first branch-out vintage micro for keeps.
So the Coco 2 arrives, and I open the box and it looks in better condition that in the pictures, in fact, it looks like what I thought was UV yellowing damage to the case is in fact just dirt!
So after a good scrubbing with soap and water and rag and toothbrush, it turns into a perfect looking, mint condition computer case & keyboard!
So I power on the computer, and sure enough, it turns on to a green screen, but with garbage graphics all over it. (sorry no pic)
So I decide to open it up and start trouble shooting. To my surprise (didn't notice while cleaning the exterior) the "Warranty void if removed or tampered with" sticker is still intact! This machine has never been opened since the factory! Sorry again I didn't think about taking a picture before I punctured the sticker to get to the screw underneath.
Upon opening her up, I can see that everything inside still looks brand new and fresh off the factory floor! And the IC's are all socketed too, something I've since learned is actually not common on TRS-80 Coco's at least the model 2 and 3's. So I start pushing on the IC's and several of them were not seated all the way and pushed in. So after doing that, I plug the computer in to see if it will work properly now, and sure enough! It's working perfectly! I got a mint condition, inside and out, TRS-80 for $50! It must have been destiny say's I!
Of course it's a 16K model with standard BASIC, there's an empty socket where the Extended BASIC rom is suppose to go. But luckily I quickly find out that all that is required for a full 64K upgrade are 8 4164 dram IC's, which I have on hand from upgrading 64K Atari's to 256K Rambo's! And connecting a jumper wire between to pads on the mobo. And so 5 minutes later, I have a 64K Coco 2.
I start researching other upgrades and video mods, since the Coco 2 has ONLY RF output and that just won't do! I find an S-video mod that replaces the RF modulator and uses the RCA plug hole for the S-video plug and the channel select switch hole for a headphone style audio output. And start doing the video mod right away, as I have all the needed components on-hand. Picture of board below. Complete and installed.
Though there are several different versions of pre-made drop-in video boards for the Coco's, they are all composite only output. Probably because there are so few S-video capable TV/monitors out there anymore. But I have TV/monitors and video converters that have S-video inputs. So I see no reason to settle for composite out when S-video is quite a bit sharper, and if I want composite video for the Coco's high-res graphics with NTSC artifact colors I have an S-video to composite in-line adapter and combing chroma and luma in-line works just as well for artifacting as straight composite video.
So I found all the schematics I could on the Coco's and with those and some extrapolation from current composite mods I made my own board. Most of, if not all the pre-made composite boards also use the old TV box channel switch spot for a switch to turn color on and off for a sharper monochrome picture for text modes and use a stereo headphone jack with a special DIY cable that has both audio and composite video out. I found this useless with my already sharper S-video board and of course there's no audio out for the S-video jack, so I instead used a headphone jack in the TV switch position that outputs dual-mono audio instead.
Of course there are modern upgrade boards for RGB and VGA output, but those boards all plug into the VDG socket, but I need that spot to add an external character and font board for lower case (which most Coco's don't have) and programmable character fonts (also don't have) which I feel is more important than RGB or VGA sharpness with so low-res a machine anyway.
I've been asked since I did this blog for details on this S-video board I made. The best I can do right now is a electronic schematic of the board. When I did my board, I was studying several different circuit schematics and extrapolating from some and putting them together, leaving out the circuits for the composite mod to make it simpler and only have the luma and chroma and audio circuits left. because I have an inline S-video2Composite and vice-versa adapter that works just as well to turn S-video back into composite for artifacting to work. I'll post pictures of it below too.
Anyway, I'm old school when it comes to designing circuits. I still use a circuit design program on my Atari 8-bit, but they are down for repairs or upgrades at the moment, like the Coco 2. I downloaded TinyCAD and tried to use it, and though it's suppose to be a simpler one, the learning curve of the program was just taking me too long and I'd make errors or accidentally erase or whatever and have to start over.
So in the end, I broke out some paper and my colored pencils and just drew up a complete and concise schematic for the upgrade, and using a different colored pencil for each circuit to help make things clearer to follow. But I also decided to redo my own S-video board because I'm going to test a new circuit of my own I've added to the schematic that I used on my Atari 1200XL when I did a real video upgrade for it and not just creating new video out choices. It's a chroma color saturation boost circuit that makes the colors much more vibrant and rich than the normal chroma without over saturation or color bleed like you get if you turn the color up to much on a TV or Monitor. This is a clean color boost and to my eyes actually approaches the color quality of RGB from a Chroma/luma (s-video) circuit. I love the sharp and colorful video out on my 1200XL now, that I decided to extrapolate that circuit design from the Atari video circuit and add it to this video circuit and board and see if it will work as good on a Coco 2 as it did on my Atari.
But it will need to be tested when I actually get the new board done and installed back in my Coco 2. It may also need a resister before feeding into the rest of the chroma circuit, there was one on the Atari version, but the chroma circuits are different and use different components and values, so I'll see how it works without a resistor and then test several different values to find the one that works best. For this reason I've also added a chroma saturation boost circuit switch to the circuit that will disable it at the flip of a switch and the video will work just like it was never there if I choose. This is an optional circuit that does not have to be included for this S-video board to work. Notes about it all are on the schematic.
This board is meant to replace the RF modulator completely and sit in it's spot, but it does require the MC1372 modulator chip from the RF board. Design of the actual board is up to you, and this is probably best since there are different versions of the RF and where it goes on Coco's so with just the schematics you can design the board to fit your particular Coco. I would imagine this would work on the Coco 1, but it's made for the Coco 2's.
Since I will be rebuilding my own board using this same hand-drawn schematic myself, to add in the new upgrade circuit, I will be posting about here this time step-by-step with lots of pictures. This will give all the "details" that I have been asked about regarding this upgraded. I'll add a complete parts list too, so no one has to scour the schematic and make a list of their own, unless you start on your own board before I get all the details up.
This may happen over the next few days, or it may not happen until after the holidays. I have 3 on going restoration and/or upgrades still not finished yet, representing the first three blogs at the top of my blog page currently.
You'll have to right-click on the images and save to view on your PC to be able to Zoom in and see everything, but it's high-res so you can zoom in as much as needed, once you save it to your PC and use an image viewer, like irfanview 64 that I use. If you want to print out the image, like I always do instead of having to look at a screen that's harder on the eyes, it will require legal sized long paper as you can see, or else it will get printed over to sheets unless you shrink it down to fit standard paper.
I ordered a 24-pin to 28-pin adapter board with a 28-pin prom chip with Extended Basic OS in the original socket that is 24-pin. I also found out that the last model 2's that were made for the Korean market had Hitachi 6309 CPU's in them that are 100% compatible and drop-in replacement with the original 6809, but have new addresses and functions, and also run software up to 30% faster than the 6809. So I ordered one. They have arrived and I installed them. I still need to get the later revision SAM chip though, to take full advantage of the faster 6309, which also has a 1.8Mhz mode doubling the speed but it requires the newer SAM chip to activate it, both are required.
While I have the old girl apart, I thought I'd go ahead and do a couple of aesthetic modifications. One, I prefer the look of the Coco 3 with it's grey keyboard backing and grey cartridge door over the Coco 2's black. So I decided I wanted it to look like a Coco 3, it practically will be anyway by the time I am done with all the upgrades, internal and external*! The grey I used is slightly lighter than the grey used on the Coco 3, but I had some left over from my recent 800CX computer being painted (see 800CX blog), but I think I actually prefer the lighter grey, it makes the grey keyboard keys look darker and makes the white keyboard keys stand out more, than the black on the Coco 2 originally.
A Coco 3 picture for comparison:
Second, Coco's never had power LED's believe it or not, and though I watched some upgrade videos that people added them, generally installing a red one up in the left corner of the Coco's venting, I didn't care for the placement and thought I wanted something unique and with a bit of "BLING."
So I thought it would be cool to put red, green and blue LED's right where those color spots are on the Coco's name badge! Eventually I will connect two of them too other upgrades to show them as active or to an SD card loader to show it's busy or something, but for now all three will be connected to power and all light up when powered.
I took a picture holding the case up to the light so you can see how they will look as they are not connected electrically yet, and if they look crooked and not properly lined up in the photos that is because the LED's aren't properly installed yet and I'm keeping them in place in the holes by wedging them in at an angle. If you look closely at the shot with the case back on the computer, you can tell the holes are aligned and the LED's are in crooked.
These small, round LED's are the only ones I had on hand, but I intend to replace them later with larger rectangular LED's that will be as large as the rectangular color boxes on the name badge now and completely replace them. I decided to go ahead and drill now and use the round LED's since the replacements will be larger anyway and require larger cut-outs so no harm, no foul, and I can enjoy them now and already have them wired up so I'll only need to swap out LED's later.
Interesting to see all these pictures of my Coco 2 through this blog at different times of day and under different lighting and see how the colors look different, isn't it? I noticed while proof reading.
*two external upgrade boards I will be getting, in combination, will actually make my Coco 2 more advanced that the Coco 3
Both upgrades might not be well known amongst Coco users because they were initially created for the Dragon 32/64, a clone of the Coco that has a different OS. But both were made to be compatible with Coco 1 and 2, not sure about 3. They are the MOOH board http://tormod.me/mooh.html and the SuperSprite FM+ 6x09 https://www.dragonplus-electronics.co.uk/product/supersprite-fm-6x09-for-dragon-and-coco-computers/
I will of course need a Multi-Pak Interface to accommodate both of these boards at the same time, and with original MPI units selling north of $150 dollars, and me, late to the Coco party I missed out on the limited runs of the newer home-brew Mega mini MPI and even the 2 slot mini-MPI (originals and Mega have four expansion slots) that wouldn't have been enough for me anyway, I need four. So I stumbled upon a youtube video of a coco owner who found some old Homebrew schematics, re-created them in software and sent them off to a PCB printer to be made. He added all the components and printed a case, and it works! for $50. So I'm going to follow suit, but print my own case as I want a design that matches the curves of the Coco, just like Tandy original MPI's.
https://www.smbaker.com/tandy-color-computer-resources
Another system upgrade that I nearly forgot about and want to do is replacement of all the old 74LSxx(xx) series support IC's with newer equivalent 74HC or 74F series IC's. I do this with all my vintage consoles and computers, especially if they are socketed. In the case of the my Coco 2, though all the main IC's are socketed, the 74LS support IC's are not, except for one. So I will be de-soldering, removing, soldering in sockets and then installing the newer IC's.
I think this should be just as automatic as replacing capacitors on vintage hardware. Why?
- They wear out and often need replacing anyway, so why not avoid replacing them possibly multiple times as they go bad.
- They have faster reaction time (less latency) which is better to help keep clock timing in sync, especially if the machine has lots of upgrades and add-ons, all that extra circuitry combined with the slower 74LS chips often causes timing issues and weak clock signal issues. The newer versions help compensate quite a bit; I have seen extremely upgraded machines that were very unstable and changing one 74LS chip out for a newer 74HC or 74F immediately cured the problem.
- They consume less power, which is also good with heavily upgraded machines that may be close to reaching the maximum amps the PSU can supply.
- They run much cooler which also helps with all of the above and they last longer.
- The 74HC and 74F series are engineered to be running on much faster systems and so are under far less stress and will have greater longevity due to running at speeds well under what they are rated for.
So I will be adding this process into the blog with photos. Also, I have been asked to include details of the S-video upgrade board so those will also be added.
To be continued...thanks for reading!
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