Monday, September 28, 2015

Two Things I'm Not Fixing Right Now

Sometimes I find a problem just isn't worth fixing, and sometimes the problem just isn't worth fixing by Me ... Right Now.

I recently went through two devices that were taken off the shelf for one reason or another, but both got re-shelved when I determined that my limited repair knowledge and resources were best used elsewhere.

First up was this Commodore VC-1540 disk drive.  It was just sitting lose with a pile of other stuff, so I decided to clean it up and properly box it, since it's 220V and not a drive I use every day here in the 110V U.S.

I also wanted to confirm that it actually had the original 1540 roms.  A disturbing number of 1540 disk drives were "upgraded" to 1541 roms for compatibility with the Commodore 64.

Fortunately, the roms were indeed the original 1540 roms, which give the VIC-20 both compatibility and a nice little boost in load-speed by default.  This youngster is a "true" and stock VC-1540.

The drive worked great as well, of course; the only problem I found was that, at some point in the past, the fuse-mount was cracked.  It was dangling loosely when I examined it.  I considered replacing the fuse mount, but there was still enough thread to secure it with a thin nut, and replacing would have meant tearing off the rubber insulation and/or desoldering the wires from the power supply.  Clearly I decided that this was good enough for now.

It's still a lovely drive, and since I know I won't be seeing it for awhile, I enjoyed giving it some consideration.

Next up was the Commodore 128D/81, which was originally taken down just for retr0brighting.  The entire lower half of the case was a pastel burnt orange, as were several keys.  You can still see that those keys could use some more work, but the rest of the case came out beautifully.

The problem is the same as it was when I first got it.  Neither the computer inside, nor the motherboard of the internal "1563" 3.5" disk drive, would boot properly.

I immediately dismissed the idea of repairing the computer at this time.  Here is a photo of the motherboard taken by the previous owner.

The MMU, video circuitry, and especially the all-important Z-80 processor board are a jumbled and hacked Mess.  Some day I may lay out a working C128 motherboard and start swapping out the chips from this thing to make sure that, circuitry aside, at least the chips are OK.  For now, however, there is no way I'm touching this thing.  Nope Nope Nope!

However, the 1563 drive struck me as an easy task.  It was only last month that I brought its direct descendant, the 1581, back from the dead.  During that fix I made sure I had every chip on the 1581 motherboard, so I was flush with parts.  NO Problem, Right?

At first it was simple.  I swapped out the CPU, no effect.  Then the 74ls241 (which was the bad chip on my 1581), to no effect.  I then went after the 8520 CIA chip.

Whoa nelly! See that little blue wire running from the 8520 to another point on the board?  That was the tip of the iceberg.  The 8520 on this board had several pins "clipped" and then re-soldered to the socket in *different pins locations* using more little blue wire.  In other words, that 8520 was going Nowhere without carefully diagramming how the schematic was being altered, and then de-soldering everything.  To try another 8520, I'd have to more-or-less ruin it by bending up pins and reproducing the hack job hidden underneath that innocent-looking little 40 pin chip.

It was a jungle under there! And I was too terrified to mess with it.

So, whether I'm waiting for knowledge or just courage, I put the lid back on this bad boy and sent it back into the sunset.  Some day though .. some day...

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