Challenger 4P - Let's get started!

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CommodoreZ
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Challenger 4P - Let's get started!

Post by CommodoreZ »

It's about time I got around to posting the status of my Challenger 4P. So far, I've noticed the following irregularities and problems:
  • 2303 Bytes free on startup. Seems low, since all of the 2114's are populated, unless I misunderstand how much RAM I'm supposed to have.
  • Inconsistent keyboard responses. I can't always reset to the "C/W/M ?" prompt, nor does it always enter BASIC after pressing C.
  • The machine faintly vibrates when you turn it on.
  • Visible leakage on the 10000µF capacitors.
  • Rats nest of wires leading to rear panel connectors.
  • I accidentally broke off a pin from the DIP-16 ribbon cable from the keyboard. Oops.
  • Funky rear-panel switches. I don't even know precisely what they do, but they're hooked into the serial ports...

I can handle the last 4 problems. I've got an order of capacitors en-route, along with a fresh set of DIP-16 ribbon connectors. I think I have suitable replacement switches for the back panel, and I have a reasonable stockpile of wire to overhaul things inside the case and eliminate the need for electrical tape.

I'm not so sure where to source 2114 RAM chips, should I need replacements. I'm not even sure if they've outright failed, and I have no easy way to test them individually. I could always pull the lot of them, and test in pairs.

I'm also not sure where to start with cleaning up the keyboard to alleviate the intermittent response (assuming it's the keyboard's fault), beyond throwing a continuity tester at every switch and cleaning the inter-board connectors of any tarnish. That's what I intend to do, but beyond that? I'm not sure what common problems cause these symptoms on OSI equipment.

I seem to be having a hard time attaching images, and it's late so I put them here: http://commodorez.com/OSIC4P/
Superboard Replica (400, 420C, 440) | C4P (502, 540) | Mini OSI-300 | https://www.commodorez.com 🌵
waltermixxx
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Re: Challenger 4P - Let's get started!

Post by waltermixxx »

Regarding the keyboard,

looks like you may need to replace a few switches...
what you can do is, reset the computer then short the C key out manually,
and if you consistantly get into basic doing it that way, yup you will have to
to either clean or replace that switch.

if you need to order new switches,
here is some good info on them. I got lucky and a friend gave me a cash register keyboard that had about 75 keys on it, so I started replacing any key on my OSI that gave me grief.

https://deskthority.net/wiki/Futaba_linear_switch

Regarding the ram, it could be the chip that controls the CS lines for each pair of ram, or it could be the ram itself. Perhaps remove all the ram chips and populate the board 4 chips at a time (2k) see if those work, then test 4 more, if all the ram is working, it's definately the cs (chip select) section (74LS138 most likely) hope this helps a bit?
Turning into quite an OSI Junkie (HTFDTH?) Superboard II REV B-40K, 9600b, HC-05 Bluetooth, Superboard II Rev D, C4P rebuild- 502, 542c, 540B. :)
dave
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Re: Challenger 4P - Let's get started!

Post by dave »

Looks like a nice project.

It seems to be in basic working condition, so you're at a good starting point.

You can order new keys from Electronic Surplus: Futaba MD-4PCS. Only $3 for 20 of them.

Keep us posted.

Dave
waltermixxx
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Re: Challenger 4P - Let's get started!

Post by waltermixxx »

2303 bytes of ram means only the first 3k worth of chips are working.
Turning into quite an OSI Junkie (HTFDTH?) Superboard II REV B-40K, 9600b, HC-05 Bluetooth, Superboard II Rev D, C4P rebuild- 502, 542c, 540B. :)
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CommodoreZ
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Re: Challenger 4P - Let's get started!

Post by CommodoreZ »

Progress report:
I've found a source of 2114s, which are en route but may take some time to arrive.

The DIP-16 ribbon cable connector was replaced with a factory fresh one. Those things are a pain to crimp down just right.

I've swapped the big power supply capacitors, that wasn't as bad as I had anticipated. I still need to figure out where I put my zipties to secure the new caps in place.

I should probably tackle that keyboard next...
Superboard Replica (400, 420C, 440) | C4P (502, 540) | Mini OSI-300 | https://www.commodorez.com 🌵
Jeff
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Re: Challenger 4P - Let's get started!

Post by Jeff »

Excellent! Keep us posted.

/Jeff
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CommodoreZ
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Re: Challenger 4P - Let's get started!

Post by CommodoreZ »

For the first time in forever, I got back to the C4P repairs.

My fresh 2114 chips arrived, and just for the sake of curiosity, I swapped out all of the old ones for now. My new key switches also arrived, and I replaced 23 switches in total that were not giving me continuity when pressed.

I've connected everything back up, but have left the chassis open so I can test things, and use various probes. I noticed that while upside down, if I power on the machine, nothing happens. That's concerning. I've got it sitting on its side now, and it powers up now. This indicates to me that there exists an intermittent connection or perhaps a short somewhere. I don't suppose there is a schematic dedicated to the power supply I can reference...

Anyway, I get the usual pre-reset garbage on screen when I power it up, but it does not respond to the BREAK key. I threw a probe at the reset pin on the 6502 to confirm that when I strike the BREAK key, the processor's reset line is being pulled low. Not entirely sure what to check next on that front.
Superboard Replica (400, 420C, 440) | C4P (502, 540) | Mini OSI-300 | https://www.commodorez.com 🌵
waltermixxx
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Re: Challenger 4P - Let's get started!

Post by waltermixxx »

I'm guessing no power getting to your 502 board, or bad power.
you mentioned power supply issues, so I'm guessing (and this is what happened to Steve's C4P),
maybe too much ripple on the 5 volts? so it's not reseting?... if you have a scope check your 5 volts (this also happened to me, looked like 5 volts on a multi meter, but on a scope it was 5 volts with a ripple of 1 volt or a bit higher.... my computer would not reset, just the screen full of junk.

You mentioned power supply issues, you could try swapping your boards around the back plain.
because the C4P has two power supplies, and it could be that the video card is on a good power supply and the 502 is not?
Turning into quite an OSI Junkie (HTFDTH?) Superboard II REV B-40K, 9600b, HC-05 Bluetooth, Superboard II Rev D, C4P rebuild- 502, 542c, 540B. :)
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CommodoreZ
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Re: Challenger 4P - Let's get started!

Post by CommodoreZ »

I separated the backplane from the 540 and 502, then ran some tests, including checking for proper voltage, and ripple with my scope. Everything checked out normally, I saw 5V as I should and the ripple was negligible. As I started reassembling the machine, I noticed that there was continuity between the 5V rail and ground once I attached the 540 board to the backplane. If I raised it up slightly, the continuity went away. Aha!

So, I've moved the 540 board up one level, and that seems to have alleviated any short circuit issues there. Voltage under load seems to be steady at about 4.8V. Ready, clock, and reset all look how I would expect. 5V reaching the 6502 still looks to be within tolerance.

Image

Still, I have on screen garbage. I probed a data line, and the R/W line on my scope and... those waveforms don't make it look like anything particularly valid is happening. The logic analyzer is not showing consistent results on the data bus directly out of the 6502. Looking at individual data lines shows voltages aren't rising to 5V. The R/W line in particular seems to be stuck in a square waveform hovering right around 2.5V, not reaching 0V nor 5V. Curious. I swapped in another known good 6502, with the same result. So far my only guess is that U25, the 7417 that buffers the R/W line may be to blame.
Superboard Replica (400, 420C, 440) | C4P (502, 540) | Mini OSI-300 | https://www.commodorez.com 🌵
waltermixxx
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Re: Challenger 4P - Let's get started!

Post by waltermixxx »

Hey cool, I didn't know you were the cactus computer guy,
I've seen your presentation at VCF Midwest, and your computer at VCF East.
That's a very cool computer you made.

Ok, Knowing this now, I suspect you know more about your OSI
And how it works, than I could pretend to know. :0)

The pictures help a great deal, I thought your computer was stuck at the random
Garbage screen you might get if the processor board was not even plugged in.

Could it be a bad rom? (It resets then runs partial code than goes to lala land?
Does the screen look similar each time it's turned on?
If you press the m key, can you see the pattern of characters changing anywhere, as you press return?

Not sure any of this helps.

Josh Bensadon and I helped Steve Gray get his up and running, but it was pretty much power supply related.

Please post any updates.
Turning into quite an OSI Junkie (HTFDTH?) Superboard II REV B-40K, 9600b, HC-05 Bluetooth, Superboard II Rev D, C4P rebuild- 502, 542c, 540B. :)
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