Commodore 1520 Plotter X/Y Gear Repair Guide

Commodore 1520 Printer/Plotter

Commodore 1520 Printer/Plotter

NOTE! New replacement Alps gears for the Commodore 1520, Atari 1020, etc are now available! Click here to order your set.

You’ve just unpacked your new Commodore 1520. It was most likely sold as untested, as-is or condition unknown. You plug in the cable, turn it on, and it buzzes and grinds for a couple of seconds then the red LED on top flashes. Silence. You might be lucky and get a little movement on the print head.

While the diminutive Commodore 1520 Printer/Plotter looks as good as the day it was made over 30 years ago, one specific part suffers over time. Two actually. Inside, the Alps printer mechanism contains two small stepper motors. You can’t see them– they’re inside the silver platen housing. On the left and right side, the stepper motor shaft sticks out and has two small gears attached to them. These gears (most likely made of nylon) have shrunk in size and split apart under the pressure of the spindle. There’s now a gap in one of the 13 teeth and it binds against the opposite gear causing the grinding sound you hear at start up.

Commodore 1520 Gear
Replacements are available. Best Electronics sells via email order a set of 2 replacement gears for $15 with a $20 minimum. That’s likely as much as you paid for the plotter.

Commodore 1520 plotter X/Y gears in digital calipers, about 3.6mm wide.

Commodore 1520 plotter X/Y gears in digital calipers, about 3.6mm wide.

You can also repair the split with glue, but not in the way you might think. We’re not going to try to get glue in between the split as that would be nearly impossible due to their tiny size. What we need to do is minimize the effects of the gear shrinking in size. If you remove the gear from the motor spindle, you’ll notice the split closes. This is because the gear bore is slightly smaller than the motor spindle. It’s pressure fitted. You’d normally want this as it transfers the motor force into the gear.

But if we increase the bore size of the gear, the spindle won’t force apart the split any more. Unfortunately it now freely spins on the spindle. Using some super glue on the spindle, you effectively mechanically connect the gear to the spindle again plus hold the split “closed” from the inside of the bore without the risk of getting glue in the teeth.

Tools to fix the Commodore 1520 plotter gears (left to right: calipers, gears, tooth pick, 1/6" drill bit, super glue, needle nose pliers)

Tools to fix the Commodore 1520 plotter gears (left to right: calipers, gears, tooth pick, 1/6″ drill bit, super glue, needle nose pliers)

Here are the steps I follow.

  1. Remove the old gears from the spindles. Be careful not to damage the opposing gears in the process. You can very gently move the spindle towards the back of the case with a flat head screwdriver in one hand and slip the gear off with the other.
  2. Locate the split in the gear. Gently clamp the gear making sure the gap/split has pressure pushing it together. I used a small caliper to do this.
  3. Commodore 1520 X/Y gear clamped

  4. Using a 1/16 inch drill bit, BY HAND, slowly and gently drill the bore until the bit goes all the way through. Clean up any debris with a toothpick.
  5. Commodore 1520 X/Y gear drilled

  6. Note that one side of the gear has a closed, flat edge. This will now face away from the motor. Slip the gear on the spindle and test it. Hopefully the split should no be visible and the gears should rotate freely without any binding or even pauses. Remove the gear from the spindle.
  7. Commodore 1520 X/Y gear re-fitted

  8. Apply a tiny dab of super glue on the spindle motor and slip the gear on. With your fingers, roll the opposite gear back and forth. This will distribute the glue around the spindle, stop with the split facing up.
  9. Using needle nose pliers, gently close the gap if there’s one until your glue sets. Gorilla Glue says 30 seconds.
  10. Let it sit over night for the best bond.

It should be noted that many computer companies in the early 1980’s had a plotter that used the same Alps plotter mechanism. They are the Atari 1020, Tandy CGP-115, Oric MCP40, Sharp CE-150, and probably others. These instructions most likely will serve you well with those machines as well.

I’ve repaired about 5 Commodore 1520 plotters, a CGP-115 and two Atari 1020s using this method. Out of all of them, I had one fail again after heavy use. Your mileage may vary.

Some tips that will keep your plotter working longer:

  • Always remove and cap the pens when you’re done using it.
  • When you’ve removed all the pens, power off the printer with the pen carriage all the way to the right (in pen replace mode).
  • When you power it on the next time, it won’t grind against the left stop as it tries to find “home.”
  • If you don’t have the paper spool holder, a standard round pencil works perfectly.

As with all things you find online, what you choose to do to your own equipment is your own choice. I won’t be held responsible for anything you damage (including your equipment or yourself). Use common sense and do your research before attempting any kind of repair.

Let me know in the comments if you successfully repaired your plotter, have an alternate fix or know of a source for replacement gears.

Adding S-Video to an Atari 800XL

Recently I setup a few of the computers from my retro computing collection. Along with the Amiga 2000, Apple IIc, and Macintosh Plus was the Atari 800XL.

I’ve not used it all that much and thought it deserved a little hands on time. When I acquired it I also purchased a 5 pin DIN plug for the video jack on the rear to bring out stereo RCA audio (although it’s just mono) and s-video. I was puzzled when I plugged it in and turned it on to find that the video was only in black and white. I switched to the RF modulator and saw color so I knew the computer was working fine. I searched and discovered that some (maybe all?) Atari 800XL’s have provisions for s-video, namely separated luma (brightness) and chroma (color) but Atari neglected to connect the chroma to the plug. This explains why the image has no color.

I searched and was surprised that I didn’t find all that much information on the subject. One site called the seemed to have the most information but I found the directions confusing and images that were hard to see. I kept searching and found a clear forum posting on AtariAge (post #8), also summarized below.

  1. REMOVE C56 capacitor (if present)
  2. Lift C54 right-hand side, preferably by inserting an on-off switch (if you want to preserve composite out).
  3. On the underside of the motherboard, solder chroma-signal wire from R67/R68 junction, to chroma pin in video DIN port pin 5. Follow the cable pathway exactly as shown so the RF shielding fits back on.
  4. Adjust brightness/contrast/saturation on your monitor. Don’t underestimate this step as I did at first. It’s important and made all the difference in the quality of the final picture.

With those small changes, I now have crisp s-video output from my Atari 800XL to an LCD monitor. The picture looks fantastic!