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PCjr issue...

Stone

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Joined
Nov 3, 2009
Messages
12,814
Location
South Jersey, USA
My PCjr doesn't do much of anything when powered up. The fan starts and runs and the monitor goes from gray/white to black and that's about it. This is the same with and without the floppy card and the ram card. Does that point to motherboard ram and if so what's a good way to go about diagnosing those soldered chips? Are there other possibilities to check besides the main ram?
 
Do you have equipment to check your clock signal? I had issues very similar to that when I first installed my speed-up mod and I knocked one of the wires loose off it. You might want to check your 8284 chip.
 
Hard to tell, but I repaired a PCjr that behaved like yours, black screen when powered. It turned out being a fried 4164 RAM IC.
If you at least have a way to measure voltages, you can check the +5V is present...
 
Also, feel for heat deltas on any of the chips. If something feels really hot, or everything else feels hot but one chip, it might give you a clue where to look.
 
This might sound like an odd thing to check, but see if the stupid IR sensor on the front is seated right; or even better, remove it.

When it's not mounted or improperly mounted the system will boot up and give the same "Error B" as it does when things are wrong elsewhere involving the keyboard...

...but if the sensor unit is burned out/shorted or not in all the way, it can often make the system do exactly what you are describing. It powers on, video output starts by blanking the display, but that's as far as it gets. The Junior I picked up had this problem as when I bought it the owner said it was delivering error B, by the time it was shipped to me it was going to a blank screen and no further until I opened it up and saw the IR unit half-in half-out. Removing it and reseating it properly was the only repair needed. (well, other than replacing the banjaxed floppy drive).

It's silly, minor, but something to check; and just another of those corners cut on the Jr's design that reminds me a lot of the "wishful thinking" in designs like the Apple II or worse, Apple III -- where things like expansion cards are held in not by latches or screws, but by the edge connector and wishful thinking.

A LOT of times all you need to do with systems of said age that are having problems like this is to just take it all apart and put it all back together. One wrong voltage, one unseated connector, one chip only half plugged in, and kiss off the whole setup.

See a XT clone I picked up recently that was shipped to me "working" and by the time it arrived half the DRAM had fallen out and were floating around loose inside the case.
 
+5V is OK.

Nothing feels even a little hot.

IR sensor is seated properly but I can't test it since I don't have the possibility of using the keyboard yet.
 
I think you've already done this, but remove all accessory cards from the main board, leaving only the power supply and then the monitor plugged in.

Double check that the power supply ground connection is made: the power supply board plugs into a card edge connector, but there's also two small pins that slide into a connector at the rear of the card, near the power switch.

Also, inspect your CPU chip. Some on the PCjr were socketed. While you're at it, check for any other socketed chips. Press them firmly to make sure they're seated properly.

If that doesn't do it, in theory you can attempt to repair a PCjr main board, but I'd just shop for a replacement. PCjr's should be plentiful.

-Matt
 
I don't have any left, however if you know someone in your area that has a Jr-IDE card, it does have a POST display. Will at least tell you if the CPU is running or not.
 
Everything seems normal. The CPU is good. The monitor gets some signal because when I turn the system on the monitor goes from light gray to black. But that's all it does. :)
 
But as with all POST solutions, it should tell you where in the BIOS executions it gets stuck. I assume you don't see any drive access. So if the CPU is OK and starts executing, POST should narrow down the problem significantly.
 
Everything seems normal. The CPU is good. The monitor gets some signal because when I turn the system on the monitor goes from light gray to black. But that's all it does. :)

Did you try the composite video out, the digital RGBI out, or both? Do you hear any beeps from the machine?
 
No composite video or audio.

No beeps whatsoever.

How is the digital RGBI labeled? I'm using a PCjr monitor.

The PCjr. monitor uses the digital RGBI port. Because you have no audio something on the board may be gone.

Did you visually inspect the pins on all the ports on the back and the sidecar to make sure none are touching each other?

Did you try re-seating the three socketed ROM chips?

Its probably not a short, otherwise the fan attached to the floppy drive would probably not run. I think you may have to replace the mainboard.
 
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