RM50 Combo Noise - electronics experts please chime in

[KillTone]

Hey All:

I have a RM50 combo that has become very tempermental. Worked great for a few years - now all kinds of problems with microphonic tube sounds and other noises - kind of an inconsistent static/crackling/gurgling sound.

Have replaced tubes and experimented with different V1 tubes. Some better than others but the "other noises" never go away.

Any ideas of what could be wrong?

Thanks,
Ron

 

[eedude]

Some of the amps in the MTS line have arcing issues. I know that I have read some threads here about it and I have also experienced it in my RM100.

So, you may want to try this

Remove amp from chassis. Set it stable on a table with guts up so that you can observe the guts. Connect amp as usual (guitar, speaker, etc). Now be careful here. Power your amp up and while it is making those noises turn off the lights in the room and carefully look inside the chassis for arcing.

If you do try this please report back and we'll go from there.

 

[KillTone]

Thanks eedude. This amp belongs to the other guitar player in my band. I sent your info to him. Hopefully he'll be up for giving this a try.

BTW - I am not sure that I have seen a report of a RM50 arcing - thought it was isolated to RM100's. Since this could be a safety issue, we'll probably give this a try.

 

[Daryl]

If it arcs dope it. Corona dope that is...
http://www.google.com/search?rlz=1C1GPCK_enUS354&sourceid=chrome&ie=UTF-8&q=Corona+dope
That's what Trace@Voodooamps told me.

 

[wesarvin]

Ok, I just got a used rm50 combo today. It sounds amazing. But--

I'm having the same noise issues. I've tried replacing preamp tubes, but to no avail. The noise sounds like an intermittent crackle/gasp, like someone jiggling a cable. It also has a buzz that lessens whenever metal is touched on the amp. I checked the power outlet with a tester and grounding is good, but it seems like a grounding issue on the amp.

Any ideas?

 

[m0jo]

WARNING: Don't touch the PCB when it's opened, even with the amp plugged out and turned off!!! The capacitors store a surge that can be very dangerous, even lethal!!

Besides that, yes, good advice

Did anything else change in the rig recently?
New pedals, using the loop or even only a new adapter?

Wesarvin:
Did you try the amp without anything else attached? (just guitar > cable > amp?)
Also: are the module screws screwed in? If they're not the module might not be making a ground connection.

 

[wesarvin]

I changed the IEC cable and the ground buzz issues went away.

However, the intermittent static/crackling is the same, and even makes the noise when nothing is plugged into the amp. I've switched modules and made sure they're in. It makes the noise no matter what channel it's on. The tubes look a little corroded on their pins. Do you think a little DeOxit and wiggling will fix it? I've done a little more reading recently and dirty sockets might be the issue. It's making the same crackling as an amp that I owned years ago that was stored in humidity. This rm50 has a little bit of rust on screws and other metal parts.

 

[wesarvin]

Here's an update:

I cleaned all of the tube sockets with deoxit. Didn't help.

I replaced all tubes, one at a time. Didn't help.

I pulled the chassis and noticed a good deal of dark brown corrosion around the v3 tube socket on the circuit board. I used deoxit followed by 95% isopropyl to clean the corrosion. STILL didn't help!

What to do now? I purchased the amp from Guitar Center used (maybe that was the mistake), but I don't want to go through the hassle of shipping to return it. If it's a simple soldering fix, I'm comfortable doing that work myself.

 

[m0jo]

Here's an update:

I cleaned all of the tube sockets with deoxit. Didn't help.

I replaced all tubes, one at a time. Didn't help.

I pulled the chassis and noticed a good deal of dark brown corrosion around the v3 tube socket on the circuit board. I used deoxit followed by 95% isopropyl to clean the corrosion. STILL didn't help!

What to do now? I purchased the amp from Guitar Center used (maybe that was the mistake), but I don't want to go through the hassle of shipping to return it. If it's a simple soldering fix, I'm comfortable doing that work myself.

Cleaned the jacks (including loops) and plugs on your cables?

 

[wesarvin]

Yep.

What's bothering me is the amount of brown around that v3 socket. It looks like someone spilled a beer in there and it arced. Either that or they had the speaker load set wrong (it was set at 8ohm, even though the speaker is 16ohm, verified by the label and a meter).

 

[m0jo]

Yep.

What's bothering me is the amount of brown around that v3 socket. It looks like someone spilled a beer in there and it arced. Either that or they had the speaker load set wrong (it was set at 8ohm, even though the speaker is 16ohm, verified by the label and a meter).

AFAIK a wrong speaker load really only causes damage when it's really offset or if the amp was played at full blast for long/many times. 8<>16 might not be good for the tone, but I don't see it causing these kind of problems immediately.

If there was arcing there could be a carbon path left behind, causing problems now.
I thing this can be solved, but you'd need a tech for that.
Maybe you could make a pic and show it here I bet someone here can make a good judgement from that.

 

[wesarvin]

I already cleaned the gunk off, so the only picture would be a squeaky-clean pcb.

Do you know what would have caused that corrosion on the v3 socket? It was localized there, with only minor dust and gunk elsewhere. I do have to say, that the "corrosion" nearly crossed from pin to pin, with less than a millimeter in between.

 

[Jaded Faith]

Tough to say without a picture of it before cleaning. I know that the solder jobs on those sockets are questionable at times, so the spilling thing is not that likely.

There is absolutely nothing wrong with that impedance mismatch you describe. A 16 ohm speaker can safely be played with the amp set to 8 ohms all day. Those types of mismatches are even shown and suggested by Mesa in their manuals as safe and providing alternative tonal options. Three of my four gigging cabs are 16 ohms and my Mark V can only do 8 or 4. It sounds fantastic pushing the 16 ohm cabs from the 8 ohm out.

 

[wesarvin]

Thanks. I'm actually interested in your Dumble mod once this amp gets fixed!

Here are pictures of the board around v3, both above and below. The bottom is before cleaning. It just looks like dust compared to the brown corrosion on the top of the pins. The top is after cleaning, but you can still see a minute amount on the pins. I also included a picture of the area around the modules where it looked like something splashed and rusted.

I tested all of the resistors in the area and they looked like they were working. R20A is the only questionable-looking resistor on the whole board, but it tests fine with the multimeter.

http://dl.dropbox.com/u/23844329/IMG_0338.jpg
http://dl.dropbox.com/u/23844329/IMG_0337.jpg
http://dl.dropbox.com/u/23844329/IMG_0339.jpg
http://dl.dropbox.com/u/23844329/IMG_0341.jpg
http://dl.dropbox.com/u/23844329/IMG_0342.jpg
http://dl.dropbox.com/u/23844329/IMG_0343.jpg
http://dl.dropbox.com/u/23844329/IMG_0345.jpg

 

[Jaded Faith]

Your pics look really clean and nothing in them suggests arcing. My gut feeling was corrosion on the effects loop jacks, but it sounds like you addressed that. I'll brainstorm a bit and see what else I can think of.

 

[wesarvin]

Will it make that noise even if nothing's plugged in? I just cleaned those along with retouching the solder joints on the tube socket. I'm about to hook her up again and test.

 

[wesarvin]

Ok. I don't want to jinx anything but...

She's quiet as a mouse! When cleaning the loop jacks with deoxit and a q-tip, I got some gunk. It could have been the jacks, could have been the joints (they looked REALLY questionable before I cleaned them, as the whole area of the PCB looked a little gunky), but all I care about now is that it works. I'll put the chassis back and play it for a few hours and keep you updated, but thanks for all of the help.

 

[m0jo]

Ok. I don't want to jinx anything but...

She's quiet as a mouse! When cleaning the loop jacks with deoxit and a q-tip, I got some gunk. It could have been the jacks, could have been the joints (they looked REALLY questionable before I cleaned them, as the whole area of the PCB looked a little gunky), but all I care about now is that it works. I'll put the chassis back and play it for a few hours and keep you updated, but thanks for all of the help.

Seems good, the loops have been the culprit with this kind of problem before.
That's why I specifically asked about them

The switching jacks in there can get oxidised or dirty and cause problems.
I've had this in several amps.

 

[KillTone]

Hopefully your problem is solved. Mine however continues. The amp has been at a Randall authorized tech for three months and the guy cannot figure out why it makes a microphonic tube sound.

We have assured that the tubes in the amp are all dead quiet in other amps. With mid gain and higher at moderate volume the thing just squeals. It can be minimized but to totally stopped if we grab V1 with a cloth.

Still, the source of the problem is a mystery.

 

[m0jo]

It can be minimized but to totally stopped if we grab V1 with a cloth.

This is interesting, could it be that the tube socket is loose?

Also: does this tube have a cover? (the metal tube with a spring(!!) in it). Maybe tha can be the culprit or a contributing factor.

Just brainstorming here. It has to be possible to fix this problem.

Another idea: I've heard of the possibility to have a microphonic transformer.
I'm not sure if this is actually true or if the symptoms match, but it's worth a mention so you can check it out.