Hi & Need Help with a RD112-50. Thanks!

TM1

Hi folks! I had a number of MusicMan's over the last 41 years. My first was an HD130 head and I then got an RD100 and used it all over the country playing. so pretty familiar with MusicMan's. I actually went to the original factory in 1975 with a friend of mine who had a problem with his. they fixed it while we waited!! Anyway, I've worked on a few over the years. I mainly work on old Vox, Marshall's, HiWatt's, Fender's Gibson's and also upgrade and mod alot of the "reissue" amps.
I have a 50 watt RD in my shop that's giving me fits. It has a nasty hum. I replaced most of the high B+ caps and swapped out the Main's tranny and O/p tranny, no improvement. I usually like to start with the voltage supply and then signal path, but on my "O" scope I get a nice wave form but a nasty hum line thru the signal. You can hear it causing an actual physical buzz in the O/P/ tranny when the signal level goes up in the amp. Other makes of amps I've worked on in the past that have had a similar issue I've found to either be a filer cap ready to go or the Main's tranny has a winding that's arcing. So since I dropped in a new Mercury Magnetics Mains tranny, it's still there, tried swapping in an output tranny, still there. This was after I swapped out the 35 year old filter caps right off.
This chain of events led me to look online and was happy to find you all!
I'm glad your here and hopefully someone has encountered this issue with this model and can point me in the right direction.
Thanks!

inertian Tue, 01/10/2017 - 16:44

I've used Mouser and Newark Electronics for amp parts over the years.

What diodes do you need, I may have some spares I can send you?

inertian Thu, 01/12/2017 - 08:07

...Newark Electronics? Also eBay often carries the diodes, that's where I found mine for a good price.

How many of each do you need? I may have spares I can send you.

wmrowley Tue, 01/24/2017 - 12:47

My RD112 50 Hums only when reverb switch is engaged. tSounds like a similar situation as above. I have just done the basics of cable contacts. I noticed that it happens when the footpedal is disengaged as well. There doesn't appear to be any grounding wire for the footpedal in the cable. Just curious about that? What can cause this? How is this fixed?

mgriffin155 Wed, 01/25/2017 - 14:37

There should be 2 center conductors and 2 shield conductors in the foot switch cable pair. One cable for each switch in the foot switch module. An unshielded cable for the reverb foot switch can introduce hum in the reverb summing circuit. The shield prevents the center conductor from acting as an antenna. Does the hum go away if you disconnect the foot switch cables from the amp? -mgriffin

TM1 Thu, 01/26/2017 - 22:10

Sorry guys, My wife's Mom passed away the 14th. She lived 5 minutes from us and she would have been 91 in early March. She was a real sweetheart! She liked to smoke pot andeven did LSD a couple of times..lol!! Then there was NAMM, then the Vintage Guitar Show and now I'm wacked out with Bronchitis and I have gigs both Saturday and Sunday. I need to go to Kaiser first thing tomorrow..I haven't been this sick since 2015..
Thanks for all your input and help.. I have all the diodes I need. I suree wish that whomever made these pc board had done a better job. I have a Temp. controlled iron but have issues with the tracing lifting up..

TM1 Fri, 02/17/2017 - 17:28

So I've changed all the diodes and IC's & still no difference. I wonder if one of the little transistors are bad? They're the ZTX109's.
I've had to deal with my Mother In-law passing and then NAMM and then coming down with a bad case of Bronchitis..all in the last 3 weeks..
Anyhow, Thanks in advance!

TM1 Mon, 02/27/2017 - 16:41

Anyone have any thoughts on this? Basically I don't understand with new Electrolytics, new IC's, new diodes and a few resistor change outs what's causing this issue.
Here's a shot of the scope showing the hum issue. I'm ready to throw it against the wall.. I'd rather fix it but am at a loss as to what to do.
Thanks!

Images

lmv Wed, 03/01/2017 - 18:41

Hi there,
let's go back a few steps. Can you post scope pictures of what comes out of the reverb send plug using the same input signal as used in the scope photo you just posted?

Cheers,
Lars

nash-kat Fri, 04/13/2018 - 11:46

I have 3 of these right now, ALL have the same issue and it looks like it may just be due to TOO MUCH GAIN in the Limiter Chanel. I changed the preamp tube to a 12AU7 and the noise went away... It was WAY TOO LOUD and unusable with a 12AX7 or a 5751 which is what I was using, with the Vol and Gain above 5, the noise sounds just like 60 cycle HUM. One of the 5751's a vintage Sylvania was even VERY Micorphonic! I mean you could tap on it and it sounded LOUD as knocking on a mic in a PA system.
The tube change to a 12AU7 may be an easy fix that will at least get the amp back to useable, it worked for me on 3 amps, including 2 early ones and a later 83' "Mid Shift".
If the source of the hum isn't found then a patch or change in the Gain and/or Vol controls,
or a trim pot on one or both of them,
or a mod to make the Preamp Tube less HOT!, might work...

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