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Tandy Color Computer 2 Ram upgrade and strange activity

Is it possible to try your CoCo1 keyboard on the CoCo2? I don't really know if they are pin compatible, but a cursory search seems to indicate they are.
 
This was back in early 90's I had a issue like this on my CoCo2. As another diagnostic test, I jumped the keyboard connector with approate jumpers to simulate the keyboard keys. I found the keyboard was causing issue and replaced it.

Most likely the flexible membrane is breaking contact when those keys are pressed and reconnecting when not pressed. That will drive you nuts!!!
 
Ok I thought I would close out this thread. I Gave up trying to repair the keyboard membrane and bought the COCO 2 keyboard fix PCB from computerconect.com by Rick Ulland for $45.00
It works just fine now. Those keys that were not working before now all work fine. It was easy to install. So that and the 64KB upgrade my coco 2 is up and running.

computerconect.com

KeyFix2 keyboard kit for CoCo2 - replace bad mylar

For those who want to keep their CoCo looking exactly as it did, we present KeyFix2. This is the clicky key version. This circuit board (keyboard NOT included) replaces the metal plate and Mylar sheet with individual switches and a wired connection. Looks and feels like the original. No mods...
computerconect.com
 
Thanks for writing this up. I just brought back a working CoCo2 from VCF East and after trying to run my target game (Sokoban) I found out it only had 16k. I purchased this "new" CoCo2 because my old one wouldn't show a good screen anymore. I followed the instructions and replaced the ram chips with 8 4164 chips I purchased off eBay for this project. I then added the jumper wire to the 64k jumper position on the main board.

When I fired it up it had a screen full of characters that looked like C's. It was green but it would not boot to Basic. When I tried the cartridge it filled the screen with blocks of color but it was a mess.

Hmm. I watched a few video and got the idea that it may be a bad ram chip. But how will I figure out which chip was bad? I purchased 10 chips and used 8 of them. I decided to just try with one of the chips and work my way through one chip at a time, assuming I had a good chip to swap in. I removed the first chip and replaced it with one of the unused chips. Turned on - no change. I put the chip back in place and removed the next chip and replaced it with my assumed good chip. Turned it on - it worked! The second chip I tried was the defective chip.

Now I have a working CoCo2 again.

I put the eight chips from the 16k set into my other non-working CoCo2 and removed the 64k jumper. When I turned it on the screen was still garbled. I pressed on the chips and when I touched one of the PIA chips I could see the screen change. Upon inspection I noticed the PIA chip had some corrosion. I pulled the chip and cleaned the socket. I am going to order a new chip (MC6821) and hopefully I will end up with another working CoCo2, with 16K. Hopefully all it needs is a new PIA chip. Time will tell.

Seaken
 
seaken said:
Thanks for writing this up. I just brought back a working CoCo2 from VCF East and after trying to run my target game (Sokoban) I found out it only had 16k. I purchased this "new" CoCo2 because my old one wouldn't show a good screen anymore. I followed the instructions and replaced the ram chips with 8 4164 chips I purchased off eBay for this project. I then added the jumper wire to the 64k jumper position on the main board.

When I fired it up it had a screen full of characters that looked like C's. It was green but it would not boot to Basic. When I tried the cartridge it filled the screen with blocks of color but it was a mess.

Hmm. I watched a few video and got the idea that it may be a bad ram chip. But how will I figure out which chip was bad? I purchased 10 chips and used 8 of them. I decided to just try with one of the chips and work my way through one chip at a time, assuming I had a good chip to swap in. I removed the first chip and replaced it with one of the unused chips. Turned on - no change. I put the chip back in place and removed the next chip and replaced it with my assumed good chip. Turned it on - it worked! The second chip I tried was the defective chip.

Now I have a working CoCo2 again.

I put the eight chips from the 16k set into my other non-working CoCo2 and removed the 64k jumper. When I turned it on the screen was still garbled. I pressed on the chips and when I touched one of the PIA chips I could see the screen change. Upon inspection I noticed the PIA chip had some corrosion. I pulled the chip and cleaned the socket. I am going to order a new chip (MC6821) and hopefully I will end up with another working CoCo2, with 16K. Hopefully all it needs is a new PIA chip. Time will tell.

Seaken
Click to expand...
Its always good to have a specific machine to test ram chips in. A c64 or apple IIe can test 4164 chips easily. An apple II or II plus can test 4116 chips easily. Etc Etc, you get the idea. Thats what I do instead of hunting and pecking and its cheaper than buying a specific RAM testing device.

Glad you got it working! Congrats.

My advice buy a set of these as they are cheap and hes almost out. His kit helped me fix my coco 1 and my coco2. https://www.ebay.com/itm/113539027035
 
Thanks for the link. I think I will buy that set of chips.

I have both a C64 and an Apple IIe, both are working. Do I need any other hardware to test the chips? Or just some test software?
 
seaken said:
I have both a C64 and an Apple IIe, both are working.
Click to expand...

Fair warning, the 16K chips in a CoCo2 are *not* 4116s. Plugging them into an Apple II might let the magic smoke out. (I'd have to check the pinout. You *might* be okay if the lines that would connect to -5 and +12v are NC. The CoCo chips are a 5-volt only chip, same one as used in IBM CGA cards.)
 
Ok, good to know. I will check here before I do something stupid. Especially since I am still learning about these old 8-bit machines. As of now I have four working 8-bit machines - CoCo2, C64, Apple IIe, Apple IIc. Three non-working - Atari 800, TI-99/4A, Kaypro 2.

Thanks,
Seaken
 
Eudimorphodon said:
Fair warning, the 16K chips in a CoCo2 are *not* 4116s. Plugging them into an Apple II might let the magic smoke out. (I'd have to check the pinout. You *might* be okay if the lines that would connect to -5 and +12v are NC. The CoCo chips are a 5-volt only chip, same one as used in IBM CGA cards.)
Click to expand...
right those are (the 16k stock chips in the coco2) 5v ram chips not 12v correct?
 
I did not even consider that the chips I took from my "new" CoCo2 might have a different voltage than what was in my "old" non-working CoCo2. (Mostly because I don't really know what I am doing, haa!) But I did remove the 64K jumper when I put the 16k chips in the old machine. I don't know if it makes a difference. And I am away from home so can't look at the chips and post their ID.

Seaken
 
seaken said:
I did not even consider that the chips I took from my "new" CoCo2 might have a different voltage than what was in my "old" non-working CoCo2. (Mostly because I don't really know what I am doing, haa!) But I did remove the 64K jumper when I put the 16k chips in the old machine. I don't know if it makes a difference. And I am away from home so can't look at the chips and post their ID.

Seaken
Click to expand...
Well of course. No other system lets you mix two different RAM types like that. So its good he bought it up!

In other words those old 16klb ram chips are good for nothing....... As they wont work in any of the 4116 ram style machines.
 
The original CoCo 1 *did* use 4116s in the 16K spec, which is why upgrading one of those is more involved than a CoCo 2. (You have to switch one of the voltage supply lines to be an address line, which on early boards involved cutting traces; later boards had jumpers, but unlike a CoCo 2 semi-spectacular things could happen if you had the jumper set wrong.)
 
Eudimorphodon said:
The original CoCo 1 *did* use 4116s in the 16K spec, which is why upgrading one of those is more involved than a CoCo 2. (You have to switch one of the voltage supply lines to be an address line, which on early boards involved cutting traces; later boards had jumpers, but unlike a CoCo 2 semi-spectacular things could happen if you had the jumper set wrong.)
Click to expand...
Thats why I am unsure if I want to switch my low SN Rev D coco 1 from 16kb to 64. I found the article in rainbow (I think it was a 1985 issue) which detailed the upgrade and on the rev d it was a rather lengthy process with lots of mods and jumper wires in the end. Should I do it or just keep it original?
 
VERAULT said:
Thats why I am unsure if I want to switch my low SN Rev D coco 1 from 16kb to 64. I found the article in rainbow (I think it was a 1985 issue) which detailed the upgrade and on the rev d it was a rather lengthy process with lots of mods and jumper wires in the end. Should I do it or just keep it original?
Click to expand...

Since you have a working CoCo 2 I'd probably say leave it as it is. Functionally the two machines are pretty much identical; the main complaint people had about the CoCo 2 is because it lacks the +12 and -5v power leads on the cartridge slot (which was a consequence of not needing to support 4116s anymore) there were compatibility issues with some older peripherals, but if you don't have any of those or have a Multi-PAK interface that problem is taken care of.

Later CoCo2s are also somewhat harder than the originals to add composite out too, I guess.
 
Eudimorphodon said:
Since you have a working CoCo 2 I'd probably say leave it as it is. Functionally the two machines are pretty much identical; the main complaint people had about the CoCo 2 is because it lacks the +12 and -5v power leads on the cartridge slot (which was a consequence of not needing to support 4116s anymore) there were compatibility issues with some older peripherals, but if you don't have any of those or have a Multi-PAK interface that problem is taken care of.

Later CoCo2s are also somewhat harder than the originals to add composite out too, I guess.
Click to expand...
I do have a multipak interface... So that means its not a problem?

Speaking of composite I am building a coco av board. This one in particular: https://www.pcbway.com/project/shareproject/Tandy_Coco_Video_and_Audio_out_UVD_Clone_c3102574.html

Just waiting on the last few components to come in.
 
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