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Commodore Plus/4 - Diagnosing problem

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rlauzon

Experienced Member
Joined
Feb 18, 2018
Messages
236
Location
Grand Rapids, MI
I recently picked up a nice (looking) condition Commodore Plus/4 and I've been trying to get it working.

I've replaced the TED and PLA. I'm waiting for a CPU replacement.

It powers on and I get something on the screen (Commodore 1702 display using the Luma/Chroma inputs), but it sort of looks like the horizontal hold is off.

A question popped into my head today: What would a PAL Commodore Plus/4 look like when hooked to an NTSC display? Or does that make any difference since I'm not using the RF modulator?
 
Depends. You would definitely get a black and white image, and you might get some rolling, though I would think that would be more vertical than horizontal. You should be able to see something recognizable on the screen, though, if that's the only problem. The CPU doesn't fail much on these, so if you've replaced the TED and PLA you're mostly there. TED is not intrinsically NTSC or PAL, but the Kernal ROM and crystal are specific to the TV standard (I have an "NTSC 116" which was a PAL 116, but with a USA 16 Kernal ROM and USA crystal).
 
Well, I've validated that the device is certainly NTSC. So my original idea of it being a PAL system is invalidated.

I don't see anything recognizable on the screen. There's something displayed, but it's just "noise".

Everything that I've read said that one of the main failures of the Plus/4 was the CPU. The manufacturing process wasn't that great and most of them fail over time.
 
I located 2 more Plus/4s on eBay - all with a similar issue. So I'm hopeful that it's the CPU.

Someone had suggested to Deoxyit the sockets. So I tried that with no change.
 
Did it work after you replaced the CPU? Throwing a TED, PLA, and CPU at it; just need the kitchen sink next!

I am curious what you get if you burn a Diag264 to an EPROM and plug it into the board. When I had a Plus/4 with what I suspected were stuck bits in RAM, the Diag264 Kernal version helped me diagnose it.
 
I haven't received the replacement CPU yet :( International shipping. *sigh*

Do you know of anyone who sells the Diag264 EPROM. I don't have the ability to burn an EPROM. That would help to keep me from "easter egging" the different parts. I'll soon have enough parts for 2 units.
 
Sorry, but I don't. I saw someone on ebay selling one in a cartridge, which you can do. But its in Spain, so international shipping for you again. At that point I'd just wait until the CPU arrives and see.

I got a MiniPro TL866 II Plus a while ago. Its an easy to use programmer, so I burned Diag264 onto a 2732 (or maybe a 2764, I don't remember), while fixing mine.
 
The CPU replacement arrived today (https://www.sellmyretro.com/offer/details/30475). I installed it and, lo!, it works!

Keyboard is good - all keys worked.
I tried out the 3-in-1 software for a bit.
I am able to get a directory off my Pi1541, but I can't seem to load any programs.
It was about this time that I really noticed some random characters on the screen, so I have some other issues to take care of. But with a working CPU, I can use the diagnostics ROM to see what's not working.
 
So the system boots up. Things seem to work. I have one cartridge game that seems to work.
But when I do just a directory listing from my Pi1541, sometimes it works fine. Sometimes the directory list is incomplete. Sometimes there are spurious characters. It seems to be random as to what will happen.
Loading a program simply doesn't work.
Note that the problem seems to get worse the longer the system is on.

So I started doing some preliminary checks. I also wanted to test out the Hantek PC-Oscilloscope that I got for my birthday.

I found that U9 and U10 (74LS257 3-STATE Quad 2-Data Selectors/Multiplexers) get rather warm to the touch. All other chips seem to be as cool in comparison.
I'm curious if this is normal or not.

I did locate this page which says that those chips have a track record of going bad.

I had assumed that the strangeness in the display and such was a DRAM issue, but the link plus the heat from the U9 and U10 are making me rethink that.
 
Hi rlauzon,

Good finds so far! That plus/4 I mentioned before had very similar issues. https://www.vcfed.org/forum/forum/genres/commodore/1227343-plus-4-incorrect-characters-printed

It was absolutely a bad RAM chip. I had suspected the MUXes for the same reason first, they run hot. But seeing those bad bits suggests that memory is getting stuck and its just filling up over time.

Sometimes what you can do is piggieback a RAM chip on top of the soldered RAM. Trying that is easy, and sometimes it works (but often it doesn't). You should also be able to figure it out by looking at the activity on the address lines on the RAM with your o-scope. If you can't do that, then you're desoldering and socketing all of that RAM.

Good luck, and keep us posted!
 
The diagnostic cartridge arrived today so I plugged it in and gave it a shot.

Everything (that was hooked up) worked. It said the RAM was fine.

I think I'll hook it up to my real 1541 and see what happens.
 
More interesting stuff. The real 1541 works just fine.
I don't have too many Commodore disks and 99% are C-64-specific. But I was able to load (and run) some simple BASIC programs from the disk.
I loaded other programs but, as expected, they just locked up the Plus/4 - but I was able to list them and they looked OK.

So something with the Pi1541 does not sit well with the Plus/4.

So far, I've only located 1 issue with the Plus/4 and the Pi1541. Maybe I'll try to access the Pi1541 through the real 1541. If that works, at least I'll be able to create diskettes for the Plus/4.
 
As a test, I hooked the Pi1541 up in the device chain as device 9. I get the same errors.
So it's definitely the Pi1541 that's the problem. It's supposed to work with the Plus/4, though.
 
Update: My new Pi1541 showed up and it works perfectly on my Plus/4.

I don't know what's with the old Pi1541 that allows it to work with the C-64, but messes up on the Plus/4.
 
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