Mike's PCjr Page

A friend of mine offered up his family's old IBM PCJR (complete with second floppy, printer port / rtc sidecar, monitor, BASIC cartridge and Keyboard) for free on Facebook and I told him I'd take it.

So far, it doesn't seem to work. I stripped it down to just the motherboard, power supply and monitor and here's what happens when powered on.

The monitor lights up the CRT slightly grey and a roughly 200HZ audio tone comes out of the monitor and nothing more. I scoped the tone directly from the RCA audio port and it is what I call a shark wave. Sort of like a saw tooth wave but the sides are curved, so it looks like shark fins.

The audio is still there even if the monitor is not connected. There seems to be an onboard speaker next to the RCA jack but no sound comes out of it.

Unfortunately this one has no socketed chips.

So it doesn't matter if I connect everything to it, or just run bare motherboard with power, the results are the same.

I've checked the external power brick and get the required 18V AC. I have a second power brick which came with the second floppy and get about the same reading.

The DC power supply board seems to work fine as well. I get all the required voltages on all the outputs and there doesn't seem to be any problems with load, burning parts, bad caps or anything like that. However I am somewhat confused from all the reading whether or not the board is supposed to put out -6V or -12V. Unloaded it puts out -12V on that line.

Some random pokes around hitting the opposite corners of the chips usually gets me 5V readings.

The only thing I see that looks remotely wrong on the board is on my board, this little light blue part part near the power board with several resistors and a diode. The part is marked 407. One lead on that part is green, the other side looks dark, but the part itself appears to have no damage. There is no evidence of any liquid damage anywhere on the board.

One more thing. At one point I had left the board on for about 15 minutes continuously and suddenly every so often, the cassette relay on the board would click a few times randomly it seemed.

I'd really like to get this guy going again.

Where would you proceed next? I've downloaded the references from here and have also obtained full schematics which I have here:
https://drive.google.com/folderview?id= ... sp=sharing

Specs of this model as far as I can tell.
My friend told me it was a 128K model and it has the memory board installed, but there is a second memory board installed in the disk drive expansion box. (Made by Rapport / Racore, dated 1987) So I suspect it might be a 512 or 640K model.
The sidecar has a parallel port with clock chip and a PC / PCJr switch on the back.

All chips on the motherboard are soldered.
I have the newer keyboard with cable.

Just a long shot but look bent or shorted pins on the sidecar and back. I have had a bent pin create a no POST situation

James wrote:Just a long shot but look bent or shorted pins on the sidecar and back. I have had a bent pin create a no POST situation

I remember you bringing that up before, I've also seen some other people mention the same problem.



I've actually seen these results happen with the CPU not being seated properly, but that's not relevant, considering that this one's cpu is soldered down.

I will check again, but I'm pretty sure I didn't see any before.

Right now it just the motherboard and power supply board sitting on a table, with no other accessories installed.

No shorted pins. Can someone tell me, the jumper pins next to the relay that connect to the power supply board. Do they go into the first 2 pins of the 3 pin socket, or last 2 pins?

Stonent wrote:No shorted pins. Can someone tell me, the jumper pins next to the relay that connect to the power supply board. Do they go into the first 2 pins of the 3 pin socket, or last 2 pins?

They are not jumper pins - they are grounding pins for the card. Do not jumper them as there are no jumpers anywhere on the motherboard.

Stonent wrote:No shorted pins. Can someone tell me, the jumper pins next to the relay that connect to the power supply board. Do they go into the first 2 pins of the 3 pin socket, or last 2 pins?

They go in the last two sockets (closest to the rear of the card).

Brutman wrote:

Stonent wrote:No shorted pins. Can someone tell me, the jumper pins next to the relay that connect to the power supply board. Do they go into the first 2 pins of the 3 pin socket, or last 2 pins?

They are not jumper pins - they are grounding pins for the card. Do not jumper them as there are no jumpers anywhere on the motherboard.

Yeah I know, I just couldn't come up with a better name for them.

So I'm sort of stuck on what to test next. I found my old Micronta multimeter has a frequency counter so I probed the CLK pin on the CPU and got 4.77MHz.

I guess what I'm wanting to test is where can I probe to get some evidence of brain function? Such as probing some lines on the ROMs to see evidence they are being read, or probing some lines on the 4164 chips to see they are doing something etc.

I did probe the audio line with the the frequency counter and it was actually about 1.2KHz, not 200 like I had previously thought.

Again, symptoms are display lights up but just grey (as in the screen is blank but the brightness makes it look grey) and a continuous tone from the audio. After about 10 or 15 minutes I'll hear the relay clicking somewhat randomly. All voltages seem to be in spec. The motherboard has been completely removed from the case and has nothing connected to it except the power supply and keyboard IR device (makes the same tone whether or not that is plugged in). All chips on the board are soldered so I can't play switch-a-roo with them unfortunately.

I'm not sure how applicable this will be to your situation, but when I was fixing my PCjr, I was able to connect to the serial port, and watch for post codes there while holding down some combination of buttons on one or the other joysticks (barring a joystick you can of course short out the appropriate pins). I can't recall which buttons you had to press (I think it was both, have to look through the BIOS listing again to see). The codes that are spit out are non-printable characters so you need to make sure that your terminal program has a hex display mode....

From my original foray
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My PCjr is having some trouble and I'm trying to figure it out. The system beeps twice, and doesn't display anything. After looking through the BIOS listing, I discovered the POST test can be setup to spit out some error codes to the built-in serial port (at 9600 8-O-2). So, I set this up tonight to try that out, and sure enough, it spits out three bytes:

<FF><04><FF>

If I read the BIOS listing correctly, this is memory failure in the built-in 64k RAM. I poked around on the RAM with my oscilloscope and everything looks about like you'd expect... except Dout from each RAM chip. I see that they go to the logic 1 state just fine, but when it looks like it should be low, it looks like something else is driving onto the bus... the state is somewhere between 0 and 1. Looking at the schematic, I can't figure out how this could happen, as there are several latches that use this bus (labeled ED{0-7} on the schematic), but it doesn't look like anything else should be able to output onto that bus.

Has anyone encountered this before?


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In my case, it ended up being a bad PE-21212 chip, which is responsible for delaying CAS for memory access. However, the above diagnostics can help even if the RAM is bad, it should work for a number of other reasons during POST before any display is possible.