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Music Man RD112 Fifty problem


Posted on Sunday, September 19, 2004 - 05:39 pm:   

Model #: 112 - RD - 50
Chassis #: 1650 - RD

I just purchased this amplifier used. I was told by the seller that it had a problem with the "volume dropping out once in a while." He wanted $200 for the amp, which I thought was a bit expensive considering it could have a serious problem. However, after seeing/hearing one of these in a live setting a couple of months ago, I knew I had to take a chance and buy it.

After about 15 minutes, the volume drop issue reared its ugly head. It dropped in volume and got very distorted. It basically makes the amp unusable for live situations until this gets resolved. Hopefully with help from the internet I can fix this amp, because so far, it really has me thinking that perhaps I need to sell my Peavey Classic 30 instead of just re-selling this amp (I already have an offer of $350 for it, with the volume issue and all!). But right now I really want to try to get this amp fixed because I love the tone (when it works) and I absolutely adore the looks of it. The features, believe it or not, are awesome too:

A LIne Out
An AC receptacle on the back of it! That is way cool. I simply plug the amp into the wall, and plug my Boss power supply into the amp's AC. Awesome. Why don't more amp manufacturers do this?!?
Awesome footswitch with LED.
Bright switch

If anyone out there can help me fix this volume issue, please let me know. So far I think I have it down to the speaker being the culprit. At least, I hope it's the speaker. After the volume drop occurred, I replaced the 12ax7 tube (I don't have any 6L6's to replace the output tubes with right now). The tubes all *look* (for what that's worth) new/in good condition (matched groove tubes). The 12ax7 replacement didn't work, problem was still there. So then I plugged in an extension cab into the 8ohm extension speaker output. The sound went back to normal in the new speaker. At first I could hear some slight distortion when playing a low note on the E string, though that went away within a minute or less. A few times I pulled the extension speaker jack to switch back to the internal Mesa speaker, and the distorted volume drop followed suit, so I'm assuming that the speaker is in fact the culprit. I certainly hope it isn't the output transformer as I don't know how much one of those things would cost for this amp, even if I could find one.

Tomorrow when I have time I will replace the stock speaker with either a modern Jensen that I picked up a while back for a cheap extension cabinet that I never used. I hated the sound of the speaker, but it wasn't broken in and either way if the speaker is the issue in this amp then I'll gladly pay for a Weber or other high quality replacement.
Steve Kennedy
Posted on Tuesday, September 21, 2004 - 10:44 am:   

From looking at the schematic and applying your symptoms I think I might know what your problems is.

The extension speaker jack has a "switched" contact on it to the hot-side of the output transformer. When you are not using the extension jack, this switched contact is supposed to provide the output transformer connection directly to the other speaker jack, which is the primary output jack.

If these switched contacts are bent, corroded or both, you will get a crappy connection that can result in bad sound qualities in the speaker. Plugging in the extension cabinet "fixes" the problem because it is substituting a better connection than the one you have when the extension speaker's plug is removed.

The primary speaker output jack MUST be used as the primary output because removing the plug from the primary jack will short the output transformer to ground.

Here is a test... with the amp off, disconnect your internal speaker plug and plug the extension cabinet into the normal speaker jack (NOT the extension jack). Turn the amp on and see if this extension cabinet can be made to replicate the bad sound heard in the amp's own speaker. If so, then you need to clean and retension the switched contacts on the speaker output jacks.

You might also notice than pounding your fist on the amp above the area of the speaker jacks may also "turn-on" or "turn-off" the problem as it may well be affected by shock or vibration.

If you always hear the problem in the amp's speaker and never hear it in the extension cabinet, then it could be a speaker problem.

If after playing around with the amp for awhile does not provide any conclusive evidence then you may have an intermittent component or wired connection inside the amp.


Posted on Wednesday, September 22, 2004 - 10:39 am:   

Thanks so much Steve for the advice. After getting a chance to play with the amp more, I think the problem was/is a combination of really shotty volume pot and the "clean/limiter" switch. Primarily I think the problem was the switch. The other night, after posting my initial question, the problem occurred again. I then switched the clean/limiter switch to limiter and BAM the volume jumped back up (which was scary because I had forgot that the volume of the limiter channel was all the way up and my wife was sitting not 5 feet from the amp at the time!). So I *think* the problem was from that switch being corroded or whatever. The problem hasn't happened again. That said, last night after playing it for a while the amp lost most of its volume, but in a different way, like a dirty pot would do. Sure enough, I touched the volume pot and the amp started working again. It happened a few times after that, but each time just fiddling with the volume knob got it back. Today I purchased some Caig De-Oxit D5 and plan on shooting all of the pots and switches with it soon. Before doing that though, I was wondering if there are certain parts I should switch out while I have the chassis out? The filter capacitors, and resistors, etc? I read somewhere a while back before purchasing this amp that it is best to replace the integrated circuits that are in it. I'd like to replace everything that might be potentially bad so I can get this amp to tip-top shape and then decide whether it is a keeper for me or not. Right now the clean channel is the best I've ever heard, but the "limiter" channel is just plain nasty to my ears - real fuzzy like distortion. That wouldn't be an issue if it weren't for how nasty the clean channel sounds when I run a distortion pedal (Boss DS-1) through it. It sounds so ugly when overdriven (either though the limiter channel with no pedal or through the clean channel with a pedal) that I think it might be a component going bad. The clean channel, un-pushed, is still so good I'll most likely keep this amp anyway. Could you give me some advice as far as what parts to replace and where to purchase them? FWIW, I do have basic electronics knowledge in terms of safety as I had some experience/schooling when I was younger on it, and I have successfully built a tweed champ head from scratch (via a kit with instructions, so it was just following the directions). Thanks in advance for any help you can provide.
Steve Kennedy
Posted on Monday, September 27, 2004 - 04:22 pm:   

I would just clean things up internally, clean/treat all control pots, switches (if you can get some cleaner into them) and clean & retension all the input and output jacks.

Leave the chassis outside the cabinet (but connected to the speaker) and test out all functions. While you are in there, measure all the power supplies to see if they conform to the voltages stated on the schematic. If all look good, I wouldn't just start replacing any parts unless they have actually failed.

I have never own an RD50 so I wouldn't even know if the amp is sounding normal or not! I'd have to hear a known working one. If it doesn't seem to be obviously malfunctioning, then I'd leave it alone unless I learn different.