Monday, August 24, 2015

The Commodore 128D Journey Ends

This is the final installment in my four-part walk through of repairing and upgrading my old Commodore 128DCR.

When we left off, I was trouble-shooting a problem with the section of the C128's address bus that touches the back cartridge port.  Several months ago, a lightning storm had sent a surge through the network wiring in my house, zapping a network cartridge in that port.

The four chips pictured above are the only four on that leg of the address bus, from left->right: Character ROM, 2k video ram, flip flop, and bus tranceiver.  The Char-ROM had already been replaced when I first started working on the computer, since it was strange character shapes that told me something was wrong.  It was a failed attempt to use a diagnostic cartridge in the cartridge port that told me that there was still something amiss in that area.

After narrowing down the problem, I ended up replacing the flip flop and tranceiver, socketing them as you can see above.  I then tried the diagnostic cartridge again.

The cartridge did boot this time, so those chip swaps were not in vain.  However, the cartridge would now lock up when it got to the Color RAM test.  This told me that the final unreplaced chip, the 3517B-15 2k ram chip, also needed socketing and replacing.

After doing that, the computer was finally able to run through an entire diagnostic check, finding no other issues.

I was finally able to get back to my upgrades!

The only upgrade I had not yet completed was a second sound chip for stereo sound.  I was going to use a SID2SID board (a pre-built PCB that saves a ton of time and dinky wires for second sound chip installation), but the board did not fit under the C128DCR's power supply.

I also gave piggybacking the sound chips a go, but the legs on those 8580 SIDs just weren't holding solder at the low temperatures, and those sound chips are just too expensive to risk higher temperatures.  For piggybacking, I had even constructed a small signal sub-board, and figured out how to get the filter capacitors on the second chip without soldering them.  However, like I said, piggybacking wasn't happening.

So I went back to the SID2SID board and basically attached an extension cable, so that the SID2SID board could be mounted elsewhere in the case.

To make the board easy to remove, yet firmly mounted, I chose to hot glue the board to a corner of the top of the metal RF adapter case, careful not to allow any short-circuits.  I also chose to use test-lead clips for some of the other necessary signals instead of soldering more wires directly to the board, including ground.  In the end, you can see that it's quite a mess there, but quick and easy to assemble and disassemble when necessary.

A quick test using a type-in program from the Commodore Programmers Reference Guide allowed me to confirm that all 3 voices on both chips were functional.  Since I had clipped the chip select to pin 7 of the cartridge port, the second SID would be found at I/O port address $DE00, which is typical.  However, a switch would allow me to release that port for other uses by shorting the chip select pin on the second SID to 5V (pin 3 on the cartridge port).

All of my upgrades were done!

In the last entry about this journey, you saw that I had actually removed the entire motherboard for the chip replacements.  So now it was time to put everything back together.

Once everything was reassembled, I ran it through one more pass of the diagnostic cartridge (even to the point of attaching the port-testing leads, not shown here) before putting the case top back on.

And that's all there is to it.  All shiny and clean and upgraded! She has:
  • All the stock features of the Commodore 128DCR, of course, including 128K of user ram, 64k of 80 column video ram, and 2k of C64 color video ram.
  • 2 new built in software packages in a switchable option rom, 
  • JiffyDOS kernal rom, also switchable
  • Second SID sound chip for stereo music
  • SuperMMU 128 for interaction with the SuperCPU accellerator card I'll plug back in
  • 4GB UIEC/SD internal compact flash card disk drive, complete with enable switch, as well as forward/backward button for quick disk-image mounting.
  • A thoroughly repaired address bus, including 4 new socketed chips.

I hope you found this useful!

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