What To Do When Your PCjr Breaks Down
By Carl Haub

     BeepBeep.  Ever get that when you boot and nothing else?  What do you do?  What if you turn on the machine and nothing happens at all?  These are problems which may well confront you, particularly if your PCjr is getting on in age.  While unsettling at best, many of these problems can be diagnosed or even fixed by you, the user.
     This article is written, not by a technician, just by a PCjr user who can’t tell a resistor from a transistor, but who has futzed about by trial and error and who also came across an unusually detailed list of IBM "error codes."  If I can do it, you can. 
    First, don’t be afraid to have a look inside jr, even if you never have before. Lift the lid by popping it off with a quarter.  What’s inside? Working from left to right as you face it from the front, the first circuit board is the power supply.  You may have either the older "short" card or the newer "long" one which reaches to the front of the machine.  A failure here is often caused only by a blown fuse.  Just to its right, in a metal RF shield is the 64K memory board which brought your original machine to 128K. The first 64K chips are on the motherboard, the large board which completely covers the bottom of the machine.
     Just to its right are two "slots." The first holds the jr’s 300 baud internal modem, if you have one.  The second will have the disk controller board in it with a flat grey cable running to an edge connector on the disk drive.  The cooling fan sits on the back of the disk drive.  Both the fan and the disk drive are connected to the power supply card, which is how they get their power(!). If the fan goes bad, by the way, it can be replaced for $9.95 from PC Enterprises.
     The square silver "box" in the front is the infrared sensor which accepts input from the jr’s cordless keyboard.  You can remove this by gently pulling upwards if you never use batteries in the keyboard.  It saves a little strain on the power supply and keeps the machine a bit cooler.
     If you get the "two beeps" when you boot, it indicates some sort of problem.  The problem may not be serious.  It could indicate that a connector such as the light pen or serial port (which you may not use) has a defect.  Bent pins from careless attachment of plugs are a common problem.  Or it could be something else. 
      You can troubleshoot the machine yourself.  Try removing one board at a time. Turn off the machine. Work from right to left.  Unplug the disk drive from the controller board by removing the flat grey cable. Reboot.  No beeps? It’s something in the drive circuits.  Still two beeps? Remove the controller board. Then try taking out the 64K memory card, then the infrared sensor.  If the problem still exists with all of that stuff removed the problem is in the power supply, motherboard, or monitor.  Often, simply removing these components (and possibly cleaning the slot contacts with a clean gum eraser) will put your machine right back into operation.  Dirty contacts, likely in an older machine, are a common source of failure in electrical components.
     The error codes, if any, shown on the monitor screen provide more clues.  During the Power On Self Test (POST), these codes appear on the screen and normal use can often be continued by hitting the enter key if the failed part is not essential.  The following details the possible error conditions you may encounter.

     1.  Two beeps and a horizontal line across the top of the monitor screen and that’s all.  Errors in CPU registers, ROM chips, 64K memory on motherboard and/or 64K memory board, or something called an 8259 Interrupt Error.  This usually calls for replacement of the motherboard (a $75 exchange item from IBM) or the 64K board.  "Chip-level" diagnosis can, however, often locate the problem part for replacement at less cost.  Few shops, or IBM, are willing to perform that type of detailed repair, but some of our PCjr advertisers will. This could also be a bad power supply, frequently fixed with a 50 cent fuse.

     2. "Error A" shows on the screen.  Memory error on motherboard, 64K card, or memory sidecar.  If the latter, this can be caused by dirty pins on the attachment bus for the sidecar.  Try removing and blowing any dust away or cleaning the pins.  On Tecmar boards, this can be caused by looseness in the plug on the heat sink (the flat plate inside the sidecar). 

     3. Two beeps and "Error B" on screen.  Almost always, this is because you touched a key during while booting.  This is a no-no; don’t know why, but it is.  It can also indicate a bad infrared receiver, however.  If it happens again when you didn’t touch the keyboard, that’s it.  Replace it from IBM or get a keyboard cord.
     Error B can also refer to the monitor, a little known fact.  If none of the above seem appropriate, B may be indicating an error in the CRT/CPU page register logic (gate array, 6845, or associated TTL glue).  In other words, your monitor is has gone over the hill. DO NOT attempt to repair a monitor yourself unless you know what you’re doing.  CRT’s store large, potentially fatal, voltages.  REPEAT, leave the inside of the monitor alone.

     4. Two beeps and "Error C." Unlikely since it indicates an error with a cassette tape storage "wrap back."  Who uses cassettes?

     5. Two Beeps and "Error D."  Serial port ("S plug") on back of machine failed.  Bad news only if you use an external modem or a serial printer (the more usual dot matrix uses the parallel port).

     6. "Error E."  Modem failure. At least it’s (probably) not your jr.

     7. "Error F/G."  Error F means an error in ROM on the motherboard (the chip can be replaced).  G, more specifically, indicates an error in the ROM cartridge space.  The high order byte is shown following the letter.

     8. "Error H." Disk Drive error. It’s the disk drive.

     If nothing at all happens when you turn on the machine, it’s probably a power transformer (the "black brick").  These are available from IBM and, lately, from Lolir Computer in Texas for only $9.95. Call (214)-416-5155 to order a spare brick and for information on current availability.  This is a great buy on a vital spare part and they won’t be in stock at that price forever.
	In our user group, the Metro PCjr Users Group in the Washington Area, we have a "first Saturday" workshop held at the Capital PC User Group offices.  Area jr owners come by with nonworking machines and the group officers try to help.  One owner had a jr which worked fine except that it wouldn’t format disks.  Inspection showed an LED swinging freely above the disk drive. We pushed it back into the hole in the locking arm on the drive and all was well.  Apparently, it senses the indexing hole on the disk to begin the formatting process and won’t work when it’s out of position.  The point here is that none of the assembled group that day knew that problem existed, but it was fixed.  
	On another day, a woman had a Tecmar jrCaptian that did not seem to work.  Turned out that the machine she had bought from a friend had a bent pin on the sidecar bus.  That was that.  Bent pins, by the way are a very common problem.  An Achilles’ heel of the jr is the flimsy pin connectors on the back.  When plugging things on DON’T reach around the back and just shove it in.  Make sure that you have the plug right side up and in the correct port.  Use a good light if necessary and don’t force a recalcitrant plug.  The pins can sometimes be repaired, but not always; that means a new motherboard.
     There, that’s it.  A brief tour through the problems I hope you don’t see.  Hardware problems often cause unneeded anxiety.  You may not be able to repair the problem, but the chances are quite good that you can at least isolate it and get the offending part fixed.