Mike's PCjr Page

Greetings.

New PCjr user here. I have a 128k system with a Tecmar Jr Captain and Cadet...or do I? I can't figure out if the sidecars work.

1) Will these do anything without the Captain's external power supply?
2) Can anyone point me to the mod for this to run it off the Jr's power? I can find reference to it on the forum, but no details.
3) Anyone try running the with an original Nintendo NES Power cable? They output 9v ac... which is pretty darn close.

You need the external power supply. A 9.0VAC power supply is tantalizingly close, but still close to 6% over the original power supply - I don't know power electronics so that scares me a little bit. (And the amperage would have to be adequate.)

I don't know if there is a published modification to get it to run from the system power. If I were to do that I'd use a power sidecar before the Tecmar sidecar, and then on the Tecmar sidecar remove/disable the AC to DC circuitry which ultimately has to output what the power sidecar would provide anyway. I don't know if the Tecmars are particularly power hungry so having the extra power sidecar would be safe.

One final note - they are nice, historical oddities. I would never use one on a regular basis. Get a jrIDE. Or find an IBM sidecar that has been upgraded to 512KB.

For the record, I really looked for a mod.

Searched this forum, usenet archives and every instance of the PCjr newsletter. The only relevant result is this:

https://computerarchive.org/files/compu ... 991(s).pdf

If one looks deep, they'll find reference to someone just hooking up a 5V DC power supply to to the 5V leg of the voltage regulator. I'll probably be taking a crack at something similar. I'll attempt to rig it to take a 5V DC power supply in lieu of the original 8.5v AC one and just bypass all of the power conversion circuitry.

https://computerarchive.org/files/compu ... 991(s).pdf

Mike

Spent a precious few minutes on this this evening.

Found a 5v DC power supply. Found a spot where I could easily jump its input to the 5v line...

Powered up...and...190-some kb of RAM reporting? Harrumph, but still better than 128kb. I checked my power supply, only .2 amps. Per the manual, it draws quite a bit more than that, so I'm reasonably confident that it will work once I have something beefier.

In case anyone was waiting in suspense, I can only get 64kb to work in the Captain. A true bummer. The first bank of 64k seems to work fine, but no response from the second one. There is some corrosion on one of the PLAs or PALs...whatever it is, don't have it in front of me right now. Cleaned the legs and reseated...but no go.

The power section on these things certainly is quite wonky and I am glad that I was able to get it powered from a 5v power supply. That was fun and easy, if not successful in getting it fully functional.

I suppose that I have to be on the lookout for a deal on a new memory expansion, now. To that end, I'm tempted by a PCJR on local craigslist that has an expansion chassis and likely some additional ram.

Not a bad idea, since you can do a simple hardware mod to a 128K sidecar to allow it to take 4k256 drams, turning it into a 512K sidecar. Warning: This takes a lot of desoldering/resoldering of chips.

Keep in mind that this might just be a bad DRAM chip. During the boot process the machine scans for memory by writing a pattern and then trying to read it back. A read error will break the search. So what should be a 128KB sidecar might only show up as 64KB because one of the DRAMs in the second bank is bad.

Save it to fix it later. Debugging DRAM is not hard. (Generally just search and replace.)

Brutman wrote:Keep in mind that this might just be a bad DRAM chip. During the boot process the machine scans for memory by writing a pattern and then trying to read it back. A read error will break the search. So what should be a 128KB sidecar might only show up as 64KB because one of the DRAMs in the second bank is bad.

Save it to fix it later. Debugging DRAM is not hard. (Generally just search and replace.)

Thanks for the tip. I haven't 100% given up on it.

Blur2040 wrote:In case anyone was waiting in suspense, I can only get 64kb to work in the Captain. A true bummer. The first bank of 64k seems to work fine, but no response from the second one. There is some corrosion on one of the PLAs or PALs...whatever it is, don't have it in front of me right now. Cleaned the legs and reseated...but no go.

The power section on these things certainly is quite wonky and I am glad that I was able to get it powered from a 5v power supply. That was fun and easy, if not successful in getting it fully functional.

I suppose that I have to be on the lookout for a deal on a new memory expansion, now. To that end, I'm tempted by a PCJR on local craigslist that has an expansion chassis and likely some additional ram.

Techmar Captains require the power supply to report memory in the POST.
There is a driver that comes on the Techmar Treasure Chest disk which must be loaded by the autoexec.bat file.
Do you have the Techmar disk ? My system circa 1986 uses a Captain and a Cadet to get to 640K.