I picked up another C4P machine

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Steve Gray
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Joined: Mon Oct 06, 2008 1:54 pm
Location: Markham, Ontario, Canada
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I picked up another C4P machine

Post by Steve Gray »

Hi,

I just picked up another C4P machine yesterday... not working. I'm hoping to fix it up.

It has the standard basic in rom, cassette operation, the 502 cpu board and 540-Rev B1 video. It has a 24K ram card that has no model number, however on the underside there is what I believe is a logo, which looks like a fancy "O" and a small "i" (but NO "S"). I have no idea who made it.

Connected between the CPU and VIDEO boards is a floating circut board wrapped in duct tape for insulation. It is connected to a rotor switch via 4 wires. On the board is a LS90,4040, and 4013. It connects to J1-pin1 and U3A pin 1 (dot clock) of the video board and 3 locations near the 6850 chip. Could this be a tape or serial speedup mod?

I am currently documenting the connections before powering it up, but I've been told it produces video garbage but won't reset to prompt.

Steve
C4P working, C1P working. 600D Replica working, C4P+D&N floppy not working. 505 board, 610 board, Mittendorf board, TOSIE hacker board need testing, PicoDOS disk untested.
Steve Gray
Posts: 321
Joined: Mon Oct 06, 2008 1:54 pm
Location: Markham, Ontario, Canada
Contact:

Re: I picked up another C4P machine

Post by Steve Gray »

A followup... the report was correct. The machine boots to the normal "garbage" screen but won't reset. I pulled out my original C4P (floppy system) and now it's doing the same thing. So now I have two machines to repair... My machine was working a few years ago when I last tested it, so it shouldn't be that hard to fix, and hopefully the new machine is a similar repair...

Steve
C4P working, C1P working. 600D Replica working, C4P+D&N floppy not working. 505 board, 610 board, Mittendorf board, TOSIE hacker board need testing, PicoDOS disk untested.
dave
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Re: I picked up another C4P machine

Post by dave »

Cool. I'd start by removing the mod, and the RAM board, and powering up with just the video and CPU cards, positioning the CPU on top so you can access it with a probe. What kind of test equipment do you have?

Dave
Steve Gray
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Joined: Mon Oct 06, 2008 1:54 pm
Location: Markham, Ontario, Canada
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Re: I picked up another C4P machine

Post by Steve Gray »

dave wrote:Cool. I'd start by removing the mod, and the RAM board, and powering up with just the video and CPU cards, positioning the CPU on top so you can access it with a probe. What kind of test equipment do you have?

Dave
Good ideas. I only have a meter and logic probe (that I havent used in many years).

Some good news... my original C4P is now working after re-seating some chips. I have a nice little 6" LCD screen I'm using as a monitor. 64-columns comes out pretty clear on it.

Steve
C4P working, C1P working. 600D Replica working, C4P+D&N floppy not working. 505 board, 610 board, Mittendorf board, TOSIE hacker board need testing, PicoDOS disk untested.
Steve Gray
Posts: 321
Joined: Mon Oct 06, 2008 1:54 pm
Location: Markham, Ontario, Canada
Contact:

Re: I picked up another C4P machine

Post by Steve Gray »

Followup... When I substiture my original C4P cpu board into the machine it boots normally. 24K board is removed. So it appears to definately be a problem with the new cpu board. I might try some simple chip swapping.... 2114's, 8T26's.

As an aside, the components near the J3 connector differ significantly between the boards. The new board is missing all 4 transistors and even one chip. It also has 2 small connectors where 2 of the transistors would go.

Steve
C4P working, C1P working. 600D Replica working, C4P+D&N floppy not working. 505 board, 610 board, Mittendorf board, TOSIE hacker board need testing, PicoDOS disk untested.
Steve Gray
Posts: 321
Joined: Mon Oct 06, 2008 1:54 pm
Location: Markham, Ontario, Canada
Contact:

Re: I picked up another C4P machine

Post by Steve Gray »

Another quick note: I looked through the OSI board archive and discovered that the 24K RAM board I have is identical to OSI's 527 board, however of course missing the OSI labelling and copyright.... some IC's and switches are also missing from my board.

Steve
C4P working, C1P working. 600D Replica working, C4P+D&N floppy not working. 505 board, 610 board, Mittendorf board, TOSIE hacker board need testing, PicoDOS disk untested.
dave
Site Admin
Posts: 710
Joined: Tue Sep 09, 2008 5:24 am

Re: I picked up another C4P machine

Post by dave »

WIth only a logic probe, you can fix the computer, but you have to be more of a detective than if you had fancier equipment. Start out by probing the CPU clock lines to make sure there are pulses. Unlike the 505 card, the 502 gets the clock signal from the clock divider chain on the 540 board. I don't think any of the 540 boards actually have a trace for this, so it's a wire jumper from one of the counter IC pins to the bus pin. A 540B may need to be modified (by adding the wire jumper) to provide the clock. Check the 502 schematics to see what bus pin this comes from. Then make sure you jumper the right 1Mhz or 2 MHz signal from the video divider chain to that bus pin. If you want to put a clock directly on the 502, you can get a little oscillator module for about a buck.

Then check the address and data lines. Look at the enable/direction lines on the 8T26's. Look at the ROM chip enable lines.

I remember the 527 card. It required a separate power supply when fully loaded. Did your new C4 come with two power supplies installed, one for the memory board and one for everything else? When I got my 524 upgrade, the extra power supply was part of the package. Nowadays, you'd be better just piggybacking a 32K chip directly on to the 502 board and removing all the 2114's. You could use A15 or inverted A15 (U28 pin 10) as the chip select, depending on the ram chip.
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