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PCjr "Error C"

S

superspork

Member
Joined
Feb 1, 2009
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20
Hey everyone,

I got a good deal on a PCjr at the local junk shop: $5. Didn't even know what it was at the time; I figured it out after I bought it. All that it came with was the desktop unit itself, no cords, no extras.

Not being able to resist seeing it in action, I pulled out the power supply card and jury-rigged an adapter for a 65w power supply I found. Put the right voltages on the right pins, according to the technical reference, and... BAM! A little smoke and a little glow, and the screen went blank. Strange.

I checked all of my connections, double-checked for shorts on the supply, and tried again. This time it worked. Came up with the blue IBM BIOS screen and detected 128k of RAM... and an "Error C". Doing some looking, this is apparently a "Cassette Interface Error" of some kind.

So, here's the questions: What can cause this? Is this a fatal error? If I got a copy of cartridge BASIC, will I still get this error?

Also, anyone have any spare PCjr parts or know of a good place to find them? eBay dealers tend to charge a lot. (Namely, ~$50 for a keyboard seems a bit excessive.)
 
Most likely you trashed something on the motherboard. Brutman can probably tell you pretty much which thing exactly. By default, the computer will load to BASIC without anything plugged in, so, your motherboard is bad now. You might be able to track down the fried components and replace them.

I would suggest getting the proper power supply next time when you are new to a machine.
 
I figured as much. I was hoping it would be simple like "You need a cassette drive hooked up to use the built-in BASIC", but that's alright.

The part that got (most visibly) fried was an LM358P. I'll replace that first and try again.
 
Nobody should pay $50 for a PCjr power supply, shipping included or not. Contact me on the side if you are interested in a slightly more reasonable price ..


Mike
 
You can use an AT/ATX power supply. Just plug into the molex connectors on the PSU board and floppy (in place of the cable that inter-connects the two).
 
I had a similar issue with mine, and it turned out to be some bent pins on the connectors in the back that where making contact with others. So double check that.
 
Thanks for the offer, Mike. But I ordered the supply from eBay. Should be here in a few days.

In the meantime, I replaced the blown LM358. The power supply I'm using was from an old external drive enclosure; it just has two molex connectors on it. I had made a small adapter board with a scrap PCB with a card-edge connector on it and wired the voltages to the correct locations. Powered the unit up again, but just for a few seconds. Felt the LM358: it's getting really hot. Absurdly hot. Probably why it blew the first time. Used a heatsink to let it boot long enough to see that the error is still there.

Then, out of curiosity, I tried the suggestion of connecting the molex connector to the power PCB. Booted up, part didn't get hot this time. Error's still there. BUT, I also saw the extra power connector I didn't see last time, on the very front of the board, 2-pin connector. PCjr tech ref doesn't mention it.

No bent pins here. Will try again with proper power supply.
 
The more recently pointed out eBay ones were decently priced, so I would have done the same.

A lot of the cards for the Jr have additional grounding pins. The power card and modem card definitely do - not sure about the diskette controller card. You will see the two pins aligned with the card slot closest to the back of the machine. Most people miss the pins when they insert a card, but that usually is not fatal.

Not sure about the LM358 .. make sure there is nothing floating around under the machine that should not be.


Mike
 
Power supply arrived today. After a quick check with the multimeter, I plugged in the power card into the motherboard and powered up. And... error's still there.

BUT, the LM358 isn't heating up anymore. I found the source of all my problems: an inaccuracy in the PCjr tech ref. It cites that there should be two +12V supplies to the motherboard, one on each end of the card-edge connector. Not quite true! There should be one +12V supply and one -6V supply.

Any thoughts where to start looking for problems? I'm going to take a glance through the BIOS listing, but does anyone know off-hand what failed parts could cause an Error C?
 
superspork said:
I found the source of all my problems: an inaccuracy in the PCjr tech ref. It cites that there should be two +12V supplies to the motherboard, one on each end of the card-edge connector. Not quite true! There should be one +12V supply and one -6V supply.
Click to expand...

I don't believe that is true. I'm not at home so I cant look atm, but I powered up my PCjr just fine w/ just 5 and 12. And I'm pretty sure those edge pads are bridged. The -6V is made on the MB

I have been wrong on occasion though.
 
eeguru said:
And I'm pretty sure those edge pads are bridged. The -6V is made on the MB.
Click to expand...

Not the case with mine in particular. I measured -6V at the card-edge connector when the system was running, on the connector farthest towards the back of the motherboard. However, mine has a few reworks on it; not sure if it's 100% original.
 
The PSU schematic lists -12V on pin 1. It isn't regulated though. It comes off the rectifier through that big honk'n coil and through a diode to the pin. So it will probably vary a bit with load.

The mainboard schematic lists the same pin input at -6V. The only things that use it are an amp on the speaker output, amp on the cassette audio line, and the RS-232 line driver is referenced against it and +12V. Never occurred to me that modern charge pump circuits are pretty high tech :)
 
That sounds to me like my PSU is working properly, then.

Ignoring the other things that take -6v, I figure I either fried one or more of the passives around the op amp, or the input that it drives elsewhere. Eeguru, can you check the schematic and see where exactly the cassette input goes? I would very much appreciate it.
 
Checked the passives on the board near the op-amp (the two rows of diodes and resistors). Everything checks out to its marked value.

By the by -- does this look like the correct colors for composite video?
 

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Sorry I'm late getting back. Work sucks. Anyway I quickly scanned in Appendix B of the Jr. It's not in the tech ref PDF floating around in the net. Sorry for the poor quality and large size as it was done in haste. I'd recommend right click and save first (~40MB).

PCjr_Appendix_B_rough.pdf
 
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