Nothing was wrong with the old, stock rectifier on my bike, but since I heard the old rectifier tended to go out, causing the battery to die and strand you somewhere, I decided to replace it with the more reliable, superior (and much cheaper) Radio Shack rectifier. I also heard that it's more efficient, resulting in brighter lights and a better spark.
First Attempt (Don't do it this way):
So far so good. I got the old rectifier out and attached the new rectifier to the bracket. I was stoked because it was so much smaller. This was important because the new Ni-Cd battery pack I made is longer than the regular lead-acid battery and wouldn't fit anymore.
Getting ready to install the new rectifier.
I connected the 4 rectifier connectors to the new rectifier and connected a multi-meter in VAC mode to see if the alternator was still outputting AC voltage. I kicked the kick starter and saw some voltage. So far so good...
Taping up the connectors. I wanted to keep the eyelet terminals so I could go back to the old rectifier if the new one ever happens to go bad somewhere out in the middle of nowhere. Thus, I connected a female spade terminal to an eyelet terminal. (The Radio Shack rectifier has male spade terminals.)
New rectifier installed in the bike. Dang it, the connectors I made stick out too far, so the battery still won't fit, not even the regular lead-acid battery anymore.
However, I did successfully start the bike, so everything should work if I just get the connector-sticking-out-too-far problem squared away.
I just ordered the CK-1 connector kit from vintageconnections.com, so I should be able to construct new connectors, and this time, I plan to put a wire between the the female spade terminal and the eyelet terminal. Then, hopefully things will fit.
Second Attempt (Done Right!):
I found out electrical tape is probably not a good idea because it will come loose with all the heat and vibration. Luckily, the Vintage Connections terminal set arrived, so I could do things right.
This time, instead of disassembling the old rectifier, I'm disconnecting it at its bullet connectors.
Here's me making the new connectors for the new rectifier. On my bike (1969 CT-90 K1b), I needed the wire that connects to the green wire (ground) to have a female bullet connector, while all the others need the male bullet terminal. The other end of each wire needs to be a female spade connector, and I got a pack of six of those at the hardware store.
Note the brand new crimper tool that came with the Vintage Connections terminals set. It's very important to have to make good crimps. I used to use pliers, but those crimps were never good. I'd have to add solder. BTW, don't forget to put the insulation onto the wire BEFORE crimping on the terminal. I forgot and had to push them on afterwards, and though that's easy with the males, it was almost impossible with the female.
Also note that I decided to provide an extra connection to ground by attaching a wire to the center post of the rectifier. This probably isn't necessary, but I read that an ungrounded rectifier is a bad thing. Better safe than sorry. Luckily, I already had a bunch of eye terminals that I used. (See below for how that's done.)
Now all the wires have been created and connected to the new rectifier. I carefully bent the male spade connectors it came with so that when installed on the bike there'll be room for the battery. Even if you have the room, I think it's safer this way. If the wires are sticking straight out, they might rub against the battery and eventually break.
Here's me removing the old, stock rectifier with the new rectifier nearby waiting to replace it.
I took the front and back plates off the old rectifier but left it otherwise intact. This way, if the new one ever fails and I need to go back to the old one, it will be easy to do, even on the side of the road.
Even though a lot of people say you don't need a heat sink, I figured I might as well use one, so when I connected the new rectifier to the old boomerang-shaped mounting bracket, I put one of the plates from the old rectifier in between. Even better would be to apply some heat-conductive gel.
I used the other plate in front just as added protection for the wires. It's probably not necessary, but I figured the battery might be shifting around in there. I'll be riding on some pretty bumpy trails. (Because of this, I plan to put some padding around the battery, probably bubble wrap.)
The bolt I used to mount the new rectifier to the boomerang-shaped bracket had to be around 1 and 1/8 inches long (not including the head). I only had 1 and 1/2 inch bolts that were the right diameter, so I cut one with a saw. (Holding it in a vise made that process much easier.)
Now the new rectifier is hooked up. The green wire connects to the - (negative) terminal on the rectifier, the red and white wire connects to the + (positive) terminal, and the pink and yellow wires connect to the ~ (AC current) terminals (order not important).
Finally, the new rectifier is mounted where the old one used to be. Now all that's left is to arrange all the wires in there so nothing is in danger of getting frayed, and then sticking the battery back in.
I took it for a ride up the nearby mountain, and everything looked good. I kept checking the temperature of the rectifier and NiCds, and they remained cool. Looks my 1969 CT-90 is ready for the 21st century.
Here's a link to another Radio Shack rectifier upgrade pictorial that has things I didn't put in. http://autos.ph.groups.yahoo.com/group/ct90/photos/browse/d92c
Note: In these pictures, you can see that my battery is a set of 6 D-size NiCds in a bunch of Radio Shack battery holders. I have since learned that using Radio Shack battery holders is not a good idea because the springs might get too hot, lose their strength, and cause a brief open circuit which will blow out all your light bulbs.