RM50 problem

[ricky]

Let me see if I can communicate this properly.....I am having what appears to be a tube issue in my RM50 and am not sure which tube it might be. It doesn't matter which module or which bay as it is the same guitar clipping sound. It is not a nice warm distortion but a definite clipping sound. It only happens when I have the master volume on the RM50 turned down and the volume on the module up past 12:00. so if I turn the module volume way down and crank the master it is a nice clean tone. It is not a nice gain-y tone at all...kind of blatty. I don't understand circuitry at all, but it seems that if it were a power tube it would break up when I run the RM50 master up high. And if it were an individual module I could easily diagnose that. Sometimes I also her a tube-ish kind of noise when I first turn the amp from standby to on. I accidentally turned the amp off w/o switching the standby off and it really made a big crackling tube kind of sound if that helps anyone understand this. Any ideas about this? thanks in advance.....

 

[suphuckers]

Ricky I'm only answering because it doesnt seem like anyone else is (at least right now... it is saturday night ya know LoL) and I dont know if I have an answer for you but... does it clip with clean modules too?

 

[ricky]

Funny you should ask that! So in the "A" slot I have a JF Dumble OD. In The "B" slot I have a JF Trainwreck. Both sides clip badly. I just put a JF SuperClean in the "B" slot and it does not clip at all and the Dumble OD in the "A" slot doesn't clip. Totally weird. So then I put the VaiAgra in the "B" slot and the clipping happens in both channels. I will add that the VaiAgra and the Trainwreck sound fine in my RM20.
I do have a reverb and delay in the loop as someone in the other thread asked about that.

 

[kc2eeb]

I would check the phase inverter tube. Standard is a 12ax7. I f you have a spare tube, swap it out and see if it makes a difference. A 5751 NOS tube woud be better here or even a 12at7 or a 12au7. BTW that's the same module set I use; an OD Special and a Wrecked.

 

[ricky]

So looking at it from the rear, there are two 6L6's then two 12ax7's in a row and then one tube behind the 12ax7s. which is the phase inverter?

 

[ricky]

I also looked online at manual. It says that the modules use a common input tube. Which one would that be?

 

[Mattfig]

I also looked online at manual. It says that the modules use a common input tube. Which one would that be?

The one closest to your input is your input tube and the one closest to the power tubes is the PI

 

[kc2eeb]

Mattfig,
I went to bed. Thanks for picking up the thread.

 

[Mattfig]

Mattfig,
I went to bed. Thanks for picking up the thread.

My pleasure..

 

[mrtatemyballs]

remove everything from the FX loop just to remove the possibility that the tube/pedals/cable are not the problem.

 

[ricky]

Still trying to get to the bottom of this. As I live in Montana, the nearest Randall authorized repair place is a long way from here.
I think you are right in that it seems to be related to the loop. It is intermittent, so that every time I think I have it figured out, it works fine. I am slowly trying to rule out what it isn't. It is not a module. At one point last night the amps power just cut itself in half. I messed with plugging and unplugging things in the loop and then it was fine. I notice that with something plugged into the loop, as I turn up the loop control there is an increase in volume and a rounder tone. I a wondering if this could be where the distortion is coming from.

 

[kc2eeb]

It sounds like you need something as simple as cleaning the pots and connectors. There is a product called DeOxit 5. To do a proper job you would have to pull the chassis, but it amounts to spraying a little in the pots and a little into each input jack. Also a SMALL amount on the edge of the module connections. You can plug a cable from the send into the return of the effects loop. Usually that will let you know if it's the switching part of the jack that's dirty since the amp well sound great with the cable connected.

 

[mrtatemyballs]

tap on all your tubes with the eraser end of a pencil with the amp turned on and decent volume. Usually a bad tube will have a very obvious rattle sound out of your speaker. If you dont find anything, proceed with the de-oxit treatment.

 

[ricky]

Still trying to isolate this problem. Last night I had an Erect and SLO-100 in it and it was fine for about 45 minutes. I was amazed at the Erect BTW in thatI had the Volume dimed and the gain real low and it had an incredible clean tone! Anyway, after 45 minutes it must have been good and warmed up and on certain notes it would get this blatty kind of distortion and almost always when I have the module volume dimed. I also notice this bad tube kind of sound when I turn the amp on or off....like a loud static -y kind of noise. It is driving me crazy as I have swapped out the 12ax7's....
I am waiting on my modified RM50 from Rob and then I guess I will have to ship this one off to him.
People on here have alluded to the PCB going bad on the RM50. What is that like when it happens?

 

[kc2eeb]

Earlier PC boards were prone to arching due to the clearance of the high voltage circuit board traces. When I changed the power amp circuit board on mine, along with the output transformer, I measured the actual clearances between the traces. They did conform to the MEC (minimum electrical clearance), .125" in this case, but, evidence of arching was present.
http://mtsforum.grailtone.com/viewtopic.php?t=12785&highlight=lots+pictures+rm50+repair
MEC are for an ideal situation, but, if there is any contamination, high humidity, near salt water, etc. a larger clearance is necessary.
Since you're going to send it to Rob, who will do an excellent job, just ship the amp and it'll be taken care of.

 

[ricky]

What were the symptoms of the bad PCB? Like I said, mine sometimes has this clipping deal but also makes a stat-icky crackling noise when I turn it on or off.
I am unable to send it off to Rob until I get my other modded one back. It is in process.

 

[kc2eeb]

In the beginning, the sound would get lower and crap out, like a fuzz box with dead batteries. As it got worse,you could hear the arching, buzzing. But, before jumping to conclusions, you really have to pull the chassis and do a visual, mechanical, and electrical inspection. If you are not qualified for working on high voltage circuits, DON"T do it. The Anode (plate) voltage is 505 volts, give or take, and can kill you. If you looked at the pictures in the link, that's where the arching typically takes place. Static or crackling can be a tube, bad resistor, cold solder joint, cap, etc.

 

[ricky]

Don't worry about me ever opening up my amp. I am far too stupid to know what is going on in there. It is weird that i did have a big volume drop last week though. it did the static thing and then dropped. I guess i will have to wait for Rob. i just hate spending the freight costs of sending a heavy amp across the country. Living in Montana does have it's draw backs....

 

[kc2eeb]

It's unfortunate you have to ship it that far, but, it's worth it to ship it to someone that know's what they're doing and actually cares about it. One thing's good, though, you'll have all the proper packing materials when you get the RM50 from Rob. Most important is don't use "plastic peanuts"
but do use bubble wrap. I've shipped and received a lot of heavy electronic equipment, including a 1000 watt Amateur radio power amp, (KC2EEB is my call sign) and "peanuts" will allow the amp to shift around eventually hitting the side of the box. I use at least 4-5 inches of bubble wrap on every side, and double box it with another 3-4 inches of bubble wrap around the inner box. Make sure there's nothing like a pedal or any AC cable or guitar cord in the amp.

 

[kc2eeb]

P.S. If we ever move out of New Hampshire, Montana on the short list.