crt – Ussher Press Daily

Re-capping a Commodore 1702 monitor

I’ve got a Commodore 1702 monitor and it’s been flickering a lot lately. It’s high time for a re-cap. I’ve got a cap kit and am ready to go.

I opened it up and this is what it looks like on the inside. Note: it has a lot of capacitors, so I’ll have to do this over several sessions as soldering is a laborious process. 🙁

If you’re doing a re-cap of a 1702, here’s a good diagram of all the capacitors. I bought my cap kit from Console5.

Some quick notes about opening up the Commodore 1702:

There a bunch of screws in the back. After unscrewing all of them (including the ones near the rear inputs), store them all together somewhere. They’re all the same.
There are three screws in the front. Store these together. They’re different from the back ones.
Pull apart the front and back plastic frame carefully.
Next, discharge the cathode ray tube for safety. Please look up how to do this online. It’s very important as you don’t want to zap yourself. The CRT can store thousands of volts even when unplugged!
After discharging the CRT, slowly pull the CRT neck PCB off the neck tube. You’ll need to slowly wiggle it backwards to get it off.
Remove the flyback transformer “suction cup” from the top of the CRT.
Unplug the ground wire from the neck PCB.
Unplug the speaker wires.
Unplug the three-wire input (sorry, don’t know the term for this) coming from the CRT into the main chassis PCB.
Unplug the two-wire input input (don’t know this term either) from the CRT going into the main chassis PCB.

After doing the above, you can separate the chassis PCB from the CRT itself. This is what you see in the picture above.

For me, the next step is to de-solder, remove, and replace each capacitor from all the boards. I’ll do this one by one so I don’t get lost. (Removing all of them at once would lead to confusion when replacing them. I’d have to constantly look at the diagram!) I’m not sure yet if it’s better to unscrew the chassis PCB from the frame or if I can just work on it directly. We’ll see.

More updates coming soon!

Monitor Chassis Removed

After a lot of reading up on safety precautions and watching a lot of YouTube videos on CRT monitor discharging, I took the plunge.

First, I made a discharge tool. I took a flathead screwdriver and soldered an insulated wire to its shaft. I took the other end of the wire and attached an alligator clip to it. I soldered this end as well. I wrapped both soldered connections in electrical tape. I neglected to take a photo of the thing, but here’s a diagram of what it might look like if you used alligator clips on both ends of the wire:

I delayed discharging the monitor for as long as I could just because I was chicken. Finally, in the middle of the afternoon, I figured I should do it. I clipped the alligator clip to the frame, put my free hand in my pocket, and then slowly inserted the screwdriver tip under the anode cup. I have to admit it was frightening.

I slowly inserted it closer to the leads of the anode and next thing you know, I can feel it hitting the leads. And yet, no pop. That was it. I discharged the monitor uneventfully.

I was a bit underwhelmed and also a little scared that maybe I did it wrong, so I tried again. Still, no pop. I decided to go for it and pry the anode out using the screwdriver. After a few seconds, I got it off. Nothing to it!

I made contact with the hole at the top of the tube to verify there was no more charge. I found none at all, so that was it. The last time I turned on the monitor was a week earlier, so I suspect it discharged itself in that time. I’ve heard that the Wells Gardner K7000s don’t really hold a charge for very long, so that’s likely part of it. Anyway, it was overall much easier than I thought. As this page says,

It’s a little frightening the first couple of times, but the thrill wears off.

In the end, here’s what the cabinet looks like without the monitor chassis:

Here’s what the chassis looks like after removal:

A better view of it:

The next step is to inspect the board, fix bad solder joints, replace the flyback, and replace the capacitors.

What I’ve learned about CRTs in a week

It’s been about a week since the last update. Since that time, I’ve asked a few experts about the display. A few folks have mentioned replacing the “cap kit”, the HOT, and possibly the flyback transformer. Others mentioned checking the monitor chassis. A week ago I had no idea what any of this meant. Today I do.

Here’s what I know:

First of all, the monitor chassis is the printed circuit board on which the monitor components live. This sits underneath the CRT, the cathode ray tube, and larger display tube. Usually, if these old displays go bad, it’s because of a bad component on the chassis.

If you want to fix the monitor, you typically need to order new parts and replace the old ones on the chassis. I’ll get to the parts in a minute, but first a word about working with the chassis. The chassis is hooked up the big display tube itself, what we typically think of when we think of TVs. That big tube holds a lot of charge in it. To work with the chassis, you must remove it from the tube, but before you do that, you must safely discharge the monitor. If you don’t discharge it safely, you may end up discharging through your body, which could either be just painful or possibly lethal. There lots of good tips online on how to do this, so be sure to read up on it before attempting any repairs yourself.

What a “cap kit” refers to is a a new set of capacitors. If you know what type of monitor you have, you can order a new set of capacitors, or “caps”, for it. Capacitors look like little cylindrical batteries. They will typically expand or leak over the years, so it’s a good idea to replace them. If you inspect the chassis, you may find brownish-yellow goop around the base of capacitors. This may mean they have leaked. Other capacitors may also have a bulge at the top. These may also be bad. From what I understand, replacing caps will typically fix issues with bad keystoning in pictures and bad vertical or horizontal alignment.

The HOT refers to the Horizontal Output Transistor. This component is often placed near the flyback transformer (see below) and is usually situated beside a heat sink. I’m actually not sure what problems result when an HOT is bad.

Finally, the flyback transformer takes the input voltage for the monitor and essentially steps it up to several thousand volts. This voltage is applied internally to the display tube. When the flyback is bad, I think the display doesn’t even show anything. You also won’t see any “neck glow” (i.e. the cathode ray tube itself won’t glow when powered on).

There are also integrated circuits (i.e. chips) that may be bad on the chassis. They may also be bad on the “neck board”. That’s yet another circuit board that hangs off the back of the CRT.

That’s a quick summary of all the parts I know of at the moment. Hopefully it will be useful to someone doing a search online…