PCJr Motherboard Troubleshooting

[FlaminJoe]

Hi all,

I recently received a PCJr Motherboard for a PCJr I'm in the process of gathering all the various parts for to one day hopefully having a fully working PCJr system. The motherboard was purchased under the provision it was untested so I assumed it was most likely faulty and it turns out it was.

The sympton: Board powers up but I get no display using Composite (the only output I can use at the moment) and there are no other signs of life. No beeps, nothing.

I'm testing it with just the ATX to PCJr power adapter of which I have confirmed my ATX power supply is outputting the correct voltages. I can't remember off the top of my head what the 12V rail is, but with the all important 5V rail I'm getting exactly that, 5.00V. Nothing else is connected.

In going around the circuit board with my multimeter I'm finding the IC's are getting around 4.80V which I assume is with tolerances, that's until I found the DS1488N/MC14488P as shown in the picture below was reading 0V. Then upon further investigation I found that the two capacitors (note they have since been replaced hence the newer caps) and the voltage regulator sandwiched between were also giving me 0V. As I mentioned I replaced the two caps since it was a quick and easy job for me to do to rule out them being a cause but it is still the same.

Now I'm no electrical engineer so I'm having troubles working out what to check next. I've been looking at the schematics but I really can't make head or tails of where to look next and the lack of any markings on the board really just adds to the confusion. I can get hold of a replacement IC and LM7805 no problem and try replacing them but could there be anything else might explain the lack of voltage across all these components? I'm not seeing any damaged traces or anything else which stands out.

Thanks in advance.

[FlaminJoe]

Ok it seems I managed to sort out that little issue. Turns out the 12V pin on the PCjr motherboard was slightly out of alignment meaning it wasn't getting a proper connection to the board hence the lack of voltage. I'm now measuring 12V going in the voltage regulator and 5V coming out and the DS1488N/MC14488 is also measuring 12V which is correct.

However I'm still not getting any POST not even a beep out of the speaker.

Do PCjr's suffer from the same issues as XT/AT's where if you have a faulty chip in Bank 0 (the onboard 64K in this case) that it would result in a dead motherboard? This was my initial line of thought so I was thinking I would go to the trouble of desoldering the onboard RAM, soldering in sockets in their place and then trying another batch of chips. Unfortunately I don't have any suitable chips to use at the moment but I have found a source online.

[alanh]

Do you have a JR-IDE? Do you get any POST writes?

[FlaminJoe]

Do you have a JR-IDE? Do you get any POST writes?

Unfortunately no I don't have a JR-IDE. Would be very handy to have it's POST code feature but I'm currently limited to just a multimeter and a logic probe so I feel like I'm flying blind at the moment.

[FlaminJoe]

It looks like I may have stumbled across the problem. Just when I was about to give up and resign myself to needing to find another motherboard, I thought I would play around with the trimmer pot on the off chance it would do something and it did. Unfortunately my multimeter is a cheapy which doesn't have a megahertz reading so I was basically moving it blindy to see what happens when I adjusted it, but I heard what's best to be described as a muffled half beep coming from the speaker. It was the first time I've heard any sign of life from this motherboard so I knew straight away I was onto something.

Playing around with it a bit more I then managed to get it to do the two beeps no post error but it wasn't consistent. Other times I would again get nothing although one time I managed to get it to do the two beep no post about 3 or 4 times in a row switching it off and on before it stopped doing it again. So it looks like I may have narrowed down where the issue is (I suspect it's the trimmer pot) which is finally some progress. I'm also hoping that the two beep no post is also a symptom of this and not a secondary problem. From what I understand that's usually due to a display issue which I assume the timmer pot is an adjustment for like it is on the 5150 motherboard so hopefully sorting it out takes care of it as well. But I won't be unhappy if it doesn't as at least I'm getting somewhere!

[FlaminJoe]

Finally making some progress. Turns out it was the 14.3mhz crystal that was at fault which I've now replaced with a spare salvaged from a dead Atari board I have.

I'm now getting two beeps consistently and my composite display is picking up a signal whereas before it was getting nothing. Still no display just a black screen.

Is it safe to assume the video circuitry is good considering there is now some sort of signal coming out?

[Gremlin5]

Have you been making anymore progress?

I got 2 motherboards here that are both giving me the two beep signal.

I have tried piggybacking known good 4164-20 chips on the main ram but that didnt seem to help either one of them. I'm still trying to research before I go blindly probing around the board not knowing what I'm doing.
I have checked the powercards in a known good machine, one of the boards is just being stubborn but the other one is a bit interesting.

The 2nd board I powered it on, got the 2 beeps and left it running for about 15 mins to see if any of the any of the ram chips were getting overly hot. I didnt notice any difference so I rebooted the machine and then it posted and seemed fine.
I rebooted the machine again and this time the graphics were a small bit corrupted so I decided to shut it down for a few mins. After that when trying to power it back on I was greeted by the 2 beeps and no video once again, so I tried the same trick
of leaving it running for a bit then rebooting and yep it posted again but with even a bit more corrupted graphics.

Not yet sure what to make of that one yet but at least it shows a bit of life where the other one does not yet. One of the main probs of trying to work on both of the boards is only one of them came with socketable chips so if I wish to try to work on the other
one I need to pull the cpu, both bios chips and the Motorola XE1503723 and move them over to the other board. Guess I need to focus on the board with the corrupted video that at leasts posts before I end up breaking the legs off
those socketed chips by moving them back and forth then end up being completely SOL.

I know 8088 processors are faily cheap and I think I saw somewhere that was selling the Motorola XE1503723 but I have yet to find the 2 bios chips but no reason to get them if I cant fix the board.

[FlaminJoe]

Have you been making anymore progress?

Unfortunately no, I've put it on the back burner for the time being as I don't seem to be making any further progress.

Also I ended up buying another (this time known working PCjr) which should arrive tomorrow. It was bundled with quite a few extras (2 x keyboards and a couple of sidecars I just happened to be after) so it was too hard to pass up.

I may come back at a later date and continue troubleshooting it however.

[Gremlin5]

Well the board I have that I got with no socketable chips and was giving me 2 beep error seems to be doing just fine.

I got no idea why but it decided to start working perfectly (knock on wood) for the last day or so....

Hope you have some luck with that board and congrats on what sounds like a cool new score of Jr & extras.