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...

Saturday, September 19, 2015

Matshushita's Revenge

I just finished swapping out the capacitors on the disk drive devices in a Commodore 720-D.  Since this is the 7th and 8th time I've done this particular process, I figured it would be useful to share some of the lessons and pitfalls I learned.



First off, this post is for anyone who has one of the following disk drives or computers:
Commodore SFD-1001 Disk Drive
Commodore 8250LP Disk Drive
Commodore 8296D Computer
Commodore 720D Computer
Commodore 256-80-D Computer
Any computer or disk drive with Matshushita 1MB DSDD 5.25" disk drives mechanisms.

If you own one of the above, you *must* change the capacitors in it, and soon.  The capacitors in them *will* leak, and they will damage the drive motherboard.  The damage gets worse over time, so the sooner the better.  If the above doesn't scare you, consider this: of the 8 drives I've checked, all 8 have had at least one leaking capacitor with some amount of drive board damage.

Here's a picture of what I'm talking about:


You can see the leak damage in the bottom left and the upper right.

So, to replace the capacitors, you will need the following:

  1. Soldering iron and solder
  2. Solder wick
  3. Alcohol and Q-tips for cleaning
  4. Small phillips-head screwdriver
  5. Flux pen
  6. Pen-razor
  7. One (1) 33uf 25V capacitor  (5mm tall or so)
  8. One (1) 47uf 10V capacitor  (5mm tall or so)
  9. Two (2) 4.7uf 25V capacitors (5-7mm tall)
  10. Two (2) 22uf 10V capacitors (5-7mm tall)
  11. Multi-meter with continuity checker



The first step is to remove the screws to the round magnetic wheel thing.  These screws can be very difficult to start, but slide right out once they are turning.  Just don't strip them trying to get them started.  Put some muscle in it!


Now lift off the magnetic wheel and set it aside.  You'll also want to remember to pull out the "spacers", otherwise they will just fall off and slide around.  Some drives will have two of them.  Some only one "fatter" one.



The next thing I usually do is de-solder the power-cable in the upper-left.  You can see the four pins of this wire in the picture above, being pointed at by the soldering iron.  Apply some flux and solder wick, and they will dry up.  When removing them, you'll want to carefully rock them back and forth a bit until they come out.


Now remove the remaining four screws on the board.  These are a BITCH.  They will start difficult, get easy for a few turns, then go difficult again.  They are harder to strip than the "wheel" screws, but still trouble.  Just make sure you push down hard and GO SLOW when working on them.  Remember that the longer screw went in the upper-left area.


Now you'll want to carefully lift the motherboard up from the LEFT side ONLY.  Underneath the right-side area is a little ribbon cable that can be damaged if you yank it out, and once it is yanked out or torn, it is impossible to get back in.  This is also why I don't just de-solder the ribbon cable; you don't have to remove it to do your work if you are careful.

The board may be "sticky" at first when trying to lift it.  Just keep rocking it around and lifting up on it at various angles and it will eventually come up.  Remember not to pull up on that right side though!  Below is a picture of the board lifted up, so you can see the little ribbon cable we are protecting:


You can see it just above the right-middle of the white wheel paper stuff.  Sometimes you will have lots of slack.  Sometimes you will have very little.  Just feel your way to how much you can move the board around without disturbing that cable.



The first time I did this, I made myself a little drawing showing the six capacitors I was replacing, what their values were, and which pin was ground.  This is because, unlike resisters, capacitors are polar, and it is important to put the correct pin in the correct hole.  In my drawing, I show each capacitor is a V shape that points to its value, and where one of the V legs has the traditional ground symbol.

You can also see that I documented the colors of the wires in the power-cable we removed, so that I would put it back in correctly.  It turned out that that was not really a problem, but well, safer than sorry, right?




Here are the actual locations of the six capacitors you will be desoldering.  No special tricks here.  I just apply some flux and soak it away with the solder wick.

The only problem I ever had was with a through-hole trace that was so badly corroded and covered with black gunk that I had to scrape it off with my pen razor a little bit in order to get any solder lifted off.  Also the metal trace was no longer secured to the board, so I had to treat it with kid gloves.  It sucks when that happens.


After desoldering the capacitors and setting them aside, I typically take a magnifying glass and a bright light and look at the through-hole traces where the new capacitors will go in.  Acid damage from the bad capacitors will often leave a black gunk on top of the metal, making it impossible to get a good solder connection to it.  When I see this gunk, I use the tip of a pen-razor to scrap the gunk off as best I can.  Make very light tiny scrapes around the edges of the through-hole.  It doesn't take any pressure at all to remove the gunk, and if you scrape *across* the trace, you might accidentally pull it away from the board, so I recommend scraping in a circle, very lightly.  Obviously, if you can see all the metal around the hole, this is unnecessary.


At this point you can start cleaning off the places where the capacitors were removed with some alcohol solution and q-tips.  You'll need several q-tips because of all the black gunk you'll lift, as well as any flux left from desoldering.


Once everything is cleaned, you can use your multi-meter to start checking to make sure the acid damage from the bad capacitors did not eat clean-through any traces.  If any traces were damaged, you'll have to repair them with wire.  None of mine have been quite that bad, but I have heard of ones that were, so be prepared.



Now you can put the new capacitors in.  As I mentioned above, capacitors are polar.  The side of the capacitor with the white stripe is "ground" and should match with the hole that is in the white-half of the circle in the above picture.  You can also consult my little diagram for the same information.

 
It should also be mentioned that this board lives in a very tight space.  If you purchased capacitors that are taller than the things around them, you'll need to bend them down.  I made this mistake in one of my early re-caps.  The capacitor was too tall, so the board sat a few millimeters higher than normal, which caused the board to scrape against the magnetic wheel once it was put back on, preventing the drive from turning.



Once the new capacitors are in, you can put everything back together.  Don't forget to re-solder the power-cable we removed in an early step!

Also, make sure the screws are very tight.  Yes, the 4 screws for the motherboard will be almost as painful going back in as they were coming out.  Again, go slowly and firmly.

When you are done, the only thing that should move around is that magnetic wheel, and it should spin freely.  In fact, you might want to give it a spin with your finger as a final step.  If it scrapes against the board, it means you need to lower that board (or raise the wheel) until it does not.

Now your drive is good for another 10 years (or more?)!









Sunday, September 13, 2015

Commodore 8296D: De-Deutsch-ification


Some years ago I picked up a Commodore 8296-D from a kind fellow aus Deutschland (from Germany).



If you don't know about the C= 8296-D, it is a computer similar to a CBM 8096-SK, but with built in CBM 8250-compatible dual disk drives and the Execudesk rom and software package.  The 8096-SK, if you don't know, is the new-"Porsche" style of the CBM 8096 computer.  And the CBM 8096, in turn, is a CBM 8032 with 64k expansion.  The 8032 is the 80 column version of the Commodore PET 4032-12.  If you don't know what the PET 4032 is, shame on you.

Since I loved the 8296 so much, it quickly became a member of my official PETting Zoo.
Several reasons eventually put it on my workbench.  One is to "re-cap" the Matsushita drive mechanisms.  They are notorious for leaking capacitors, and that notoriety is well-known to this collection.  Every Matsushita drive I have inspected has had at least one leaking capacitor.

Another task was to fix the internal roms.  The computer had been born in Germany as an english 50hz 220V Execudesk computer, but at some point had been converted to a German 8096-SK-D by replacing the Execudesk rom with a German editor rom, and the addition of a Paperclip-software chip, both on hand-written 462532 EPROMs.

The last task had to do with the internal fan on its magical power supply.  For some reason I cannot explain, the power supply works perfectly at U.S. 110V.  Nice for me!  However, the fan only operated at 220V.  A simple fix.

The first task was to recap the drives.  These would be the 5th and 6th such drives I've done.  This wasn't exactly a "new" task, and yet on the second of the two, I had a hell of a time with two of the screws on the magnetic platter. In fact, these were the first unturnable screws I'd ever encountered.   After researching the problem on the internet and trying several things, I had only made the problem much worse.



The last thing I wanted to do was take a drill to them in order to use a stripped screw remover, but that is exactly what it took.  Luckily only one of these drives suffered this fate, and I'll know more to avoid it in the future.

The second task was the ROM replacement.  Unfortunately, I had no spare 2532 ROMs.  So I ordered some compatibles off eBay, and received TMS 2532As, which I discovered, to my great annoyance, are unprogrammable by either my Prominade C1, the parallel port-based programmer on my PC 50-II, or my modern PC's USB-based programmer.  In frustration, I ended up re-purposing the Paperclip rom (I had others).  I burnt the english Execudesk rom, same as the one on my U.S. CBM 8296 computer, and plugged it in.



To my surprise, this didn't work.  The Execudesk menu was shifted too far to the left.  While that was only a mild annoyance on that particular menu, a similar problem made BASIC unusable.  I had discovered that there truly is a difference between 50hz and 60hz PET editor roms, and what I needed was a 50hz Execudesk.  Some research uncovered the fact that my U.S. Execudesk ROM was identical to the PET Editor 4.0 60hz rom, with the addition of the menu code at the end.  So all I did to generate a 50hz version was to add the Execudesk code to the end of the PET Editor 4.0 50hz rom.  In the end, I got my PET Editor 4.0 50hz Execudesk rom, and the computer was in business.

The last task was the simplest.  Find a 3 x 3 x 1.5 inch 110V fan, and swap.  Easy-Peasy.



The PETting Zoo is whole again!