Music Man Amps

In my experience, any zener diode with the right specs will work. I have even used a pair or Radio Shack 1N4742A 12v zeners in series (for 24v) to test when I suspected bad diodes in both 50 and 65 series amps. You can still get the exact correct ones from any of the major electronics supply stores, at least here in the US.

Clean out all the dirt and dust, treat the pots, tube sockets, IC sockets, jacks and other contact points with electronics cleaner (DeOxit, CRC QD, or similar). The amp will also need a full recap (replace all 12 electrolytic capacitors), but it should at least work fairly well for right now.

The RD50s are probably my favorite amp, the right balance of size and sound. Plus you can mod them to improve the Limiter/Distortion side, add a Mid Shift (if yours doesn't have it), even a Raw control that removes the tone stack for a volume and gain boost.

Good luck!

Here's what you need to recap an RD 50:

2 x 60mfd 350v - replace with 2 x 100mfd 350v
1 x 2mfd 350v - replace with 1 x 4mfd 350v
5 x 20mfd 25v - replace with same
2 x 150mfd 50v - replace with same
2 x 400mfd 25v - replace with same

Total of 12 caps replaced.

The upgrades were recommended by Terry Loose, and he knows of what he speaks...

For the 100mfd caps, get the physically smallest sized ones you can find. The larger ones fit, but its a tight squeeze.

Also, you can always use a higher rated (larger voltage) cap, e.g., a 400v or 450v replacement for a 350v original.

Note that some RD 50s use 2mfd 25v caps on the 12AX7 tube for Distortion, some use 20mfd 25v. See what yours has and replace accordingly. This would be the 13th cap to replace.

Finally, some of the later (1983 on, Rev E version with Mid Shift toggle) use 10mfd 25v caps in place of the specified 20mfd 25v. When I have encountered this I replaced them with 20mfd 25v and the amps all sounded great. I posted about this elsewhere in the forum, maybe someone can shed some light on what if any difference this makes.

What is the date code (the white label on the right rear chassis under the top back panel) on your amp? Do you have the Mid Shift switch? Is it a 110 or 112 model?

Be safe and have fun.

Opinions vary as to the "best" way to replace caps. Some people - I call them "purists" - will unscrew and lift up the board, unsolder the old caps, and install the new caps in the original way. This is a lot of work, and, IMHO, stresses 30+ year old connections and solder joints. Plus its easy to screw up the traces.

What I was taught to do is:

Mark the orientation of each cap on the board. I use a black marker and put a dot to indicate the negative side.

NOTE: the negative lead doesn't always go to ground!

Clip the old cap out as close to the cap body as you can, leaving as much old lead in the board as possible.

Gently straighten out the leads so they are perpendicular to the board. Clean them with an electronics spray (DeOxit, CRC QD, or similar). I also lightly scrape the leads with a small needle nose pliers to remove any remaining crud and insure they are still firmly attached to their solder joints on the reverse side of the board.

Make a small loop or coil on the leads of the new cap so they will fit neatly over the old leads. I wrap a few turns around a jewelers screwdriver to make a coil, this allows the cap to stand off the board a bit for better airflow. You can bend the old leads out a bit for a tighter mechanical fit inside the new lead's coils

Using a 40 watt variable soldering iron set about halfway, I quickly flow solder inside the new coil surrounding the old lead, joining them.

Sometimes the old lead will heat up too much and literally fall through the board. Then you have no choice but to pull the board up and either resolder the old lead back in place, or install the new cap's lead through the board as originally done.

Check and double check your work, then check it again before you power up!

Your amp dates from June of 1981. You'll probably find an earlier date marked on the board with the builder's initials.

BTW, a great speaker for the 112 is the WGS Reaper HP, their version of Celestion's G12H Anniversary.

I've been a MM fan since they first came out, but really started learning about amps in general from a friend who built custom amps professionally. Another friend was looking for a small, loud, clean amp, and I told him the RD 50 was what he wanted, at least on paper. To prove the point, I tracked one down that was beat up, but the price was right. Unfortunately it was pretty much DOA. I did some online research and Terry Loose's name kept coming up as "the" guy who knew these amps. On a whim, I sent him an email, and much to my surprise, he responded, and then called me! Walked me through the design, troubleshooting, repair tips and techniques, etc. Got the first RD 50 up and running, and when fellow guitarists heard it, wanted one. I've been inside maybe a dozen of them, plus a bunch of the 65 series amps. Exceedingly well built, and usually simple to fix once you figure out what's gone wrong.

There are many smart and knowledgeable people on this forum who have helped a lot of people with their amp issues. Happy to repay some of what was graciously given to me.

The coils could be tighter, but you get the idea.