. RD 112 50 - The resistor eater

Hi all,
I just picked up a Music Man 112 RD 50 that appears to be un-well. She's got very little power at startup but the power will increase over the period of several minutes, no clean head room (everything is distorted), and a little pulsating as the distorted note tails off. There is also a problem with the reverb in that there is no reverb added to the signal. The reverb volume does work and I can get the reverb "crash" sound by giving the tank a good thump. I put in some tubes that I know are good. That gave me a little more volume but no love still distorted. I also tried running the line out into another amp and still got a distorted signal (no I did not overdrive the input of the other amp ;)). Now for the "funky-ness", One of the original tubes was missing the "key" off of the base that lines the tube up in the socket, just a nice little bubble of glass hanging there. I opened her up and noticed a little carbon spot by pin 5 on a power tube socket but the 100 ohm resistor seems intact and test okay (although it is a little crispy on the outside ), there is another fried resistor on the direct out jack and last but not least there are two small blue caps just below the aluminium channel on the right side of the board, in between those is another fried resistor it appears to be R62 but I am really bad at schematics so this is a semi-educated guess. I did a quick check on the OT and measured the impedance from lug 8 on the sockets to the red wire from the OT and the legs measured 55 and 50. I replaced the 100ohm resistors on the sockets, the one on the line out jack and the infamous R62,put some good tubes in it and it fried resistor R62 again. Any ideas as to what I am up against? I am not an electronics expert and won't be able to do much tracing of components based upon the shematic, so unless it is someting fairly easy I will probably be selling the amp or parting it out. As a side note if you want an amp that has some nice overdrive, just remove R62.........
Thanks,
Tom

I don't remember there being any 100 ohm resistors ON the 150 power section. I glanced at the schem real quick and don't see any. Are you sure that's what they are? I can guess that it has driver issues on the driver board and that's controlling the bias current so that's probably why it distorts so bad. We will try to help if possible but it sounds like it needs to go somewhere for extensive poking. I remember there being different resistors than that first/

Mike - The 100 ohm resistors are R47 and R48 on the schematic. R47 and 48 are connected to a thin red wire that goes back to the board, which is in the same path as the resistor which I believe to be R62 (the one that keeps blowing). I also have some additional info. I checked the green secondary from the OT that goes to the output jack. According to Dan Torres' book it should give a reading of about .5 ohms when checked against the common ground, When I checked it, I was getting no continuity at all then I noticed that the thin green wire that goes from the output jack to the board had somehow come loose. I re-soldered the green wire to the board and now the secondary to ground reads about .4 ohms, which according to Dan's book, would indicate a 4 ohm tap. There is also some kind of dried substance the color of motor oil on the OT that looks like it may have leaked out. Are there any other tests I could run to rule out the OT? I would like to trouble shoot the items that I can before I turn it over to a shop and have them go over it with a silly scope and what ever other electronics voodoo the do.
Thanks - Tom

First things first. R47 and 48 are the bias splitter resistors in that amp. That means tha they are providing control voltage to the control grid on pin 5. Tubes out, power on, standbye off, check the voltage where r48 and 47 come from. If you can't see, check both sides of the resistor. Make sure you have sufficient control voltage. In this case, the control grid uses POSISTIVE voltage, not negeative. Make sure you have 30v POS coming out of the resistor. More on the oter side. The dried substance is usually shellac from the transformer. It might mean it was getting hot. I had the wrong schematic before by the way! Sorry. Check c36 to see if it's shorted through and has continuity all the time OUT of the circuit. Also, just for giigle, check d8 and d9. There's not much in the way of the bias circuit to cause a lot of currnt draw in it other than the tubes and the cap.

So with the generous help of a Music Man expert, I was able to get the amp running again. It is still not 100% but it is a lot better than it was. Here is what I have done so far. Replaced R47 and R48, (Mike - both are reading 32.4v on either side). I replaced C40 and C41 since they had some carbon spots on them from when R62 went the first time. I replaced R62 (which was fried) D9 which was bad and C36 just in case. Also replaced were C30 and C31 again, just in case. The amp now has some clean head room and is starting to sound like a real guitar amp. The current issue is a "farting" sound. If I set the volume around 5, roll up the bass, turn the volume all the way up on my guitar and hit an open low E, the initial attack results in a "farting" sound which cleans up as the string rings. If you were playing an all tube amp the "farting" would occur right about where the "sweet spot" is, that allows you start getting a little grit by increasing your attack. Also my reverb is still not working. I can get a "boing" by plugging either the input or output of the tank to the return, but when I hook the tank up properly, there is no reverb. I can control the "boing" with the reverb volume, so I would imagine it is either the send or the tank?
Thanks for the help so far. I am beginning to think I may be able to save this baby yet.
BTY - I have tried different tube sets and speakers/wiring, we can rule that out.

Farts-bad caps, bad speakers, too much bass. Reverb-sounds like a tank. The caps are getting old and that will tighten things up.

Any advice on the caps? Mouser has the 60 mf 350v(75-39D606F350HL6), but doesn't have a 2 mf 350v, just a 2.2 mf 350v.
Thanks - Tom

That's fine. If it's even CLOSE on that end, it's OK. Remember, ALL the electrolytics need to be changed. All those 20@25's need to go too. Mike.

It's Alive, It's ALIVE!!!
My 112 RD 50 is now putting out tons of sweet tone! I just replaced the electrolytic caps last night and that provided the icing on the cake. Many thanks to Steve for hosting this forum, Mike Kaus for your advice and Terry Loose for actually helping over the phone! Terry is "scary smart" when it comes to MM amps, and I would not hesitate to ship one off to him for repair.

Help - It made smoke this time!!!
The reverb was a bit scetchy on the old 112 RD 50, it would sound really lush then dry up. If I tipped the amp forward or back the reverb would start working again. Long story short after poking at the tank a bit I got the reverb back but noticed that the amp was breaking up (no headroom) at lower volumes. I swapped out the tunes and fired her back up. I got a loud hum then smoke.I have it appart and found some fried resistors but for the life of me I can't find them on the schematic. I have pictures that I can email, maybe somone can help me track these resistors down? Also there is a green wire hanging loose by the input jack, I am assuming that it goes along with the orange one on the tip but would like confirmation if possible.
Thanks,
Tom