The RM80 is at it again

[JKMV12]

Fortunately this doesn't seem to be that severe (though sounds can be deceiving). Here is the issue:

Just today my speakers started producing a low-mid level "thump" noise. Both speakers produce it (i can feel the vibration on both cones). The volume controls have no effect on it. I have both of them all the way down and it still happens. It happens at irregular intervals; they are 1-10 seconds apart. It seems like the longer the interval...the louder the next thump. The presence and density knobs seem to change the tone of the thump (i.e. more bass or more treble). The guitar is not plugged in at all. The only thing attached to the amp is power.

My first guess would be a capacitor somewhere because it sounds like a pulse of energy being released. So a capacitor that can't hold its charge anymore?

I took the chassis out to check the inside but i found nothing obvious. I powered it up without the guitar in and didn't notice any arcing or abnormalities while in operation. I did find that the V1 tube was a little microphonic (no squeal...just a loud ring when its tapped. it goes away but is more pronounce than the other tubes). I got a good tube in there but it still makes the "thump" or "pop" like i described. Any suggestions/ideas?

Thanks,
John

 

[kc2eeb]

A thump or a pop can be a sign of arcing or shorting. Pull the tubes, starting with the input, V1, and see when or if the thumping stops. Continue with each module, then the PI and then the output tubes. With the chassis out, you can tap on components with an insulated stick, and see what happens.
You can check coupling caps for passing any DC with a multimeter.

 

[JKMV12]

A thump or a pop can be a sign of arcing or shorting. Pull the tubes, starting with the input, V1, and see when or if the thumping stops. Continue with each module, then the PI and then the output tubes. With the chassis out, you can tap on components with an insulated stick, and see what happens.
You can check coupling caps for passing any DC with a multimeter.

i forgot to mention it happens with both modules. that should mean that it is likely an issue with the board itself? it also doesn't start right away when i turn on the amp and put it in play. It seem to take at least a minute for it to start.

The tech said that there was some arcing underneath the board when he inspected further. He said that the board was designed with the HT too close to ground at the power tubes. He fixed it but i think he also said that there was a chance that it would come back and need a more permanent fix. Could this be the case?

now what exactly do you mean by "pull the tubes"? My instinct is that you mean to change out the tubes one at a time to see a change but literally it sounds like you are saying to take them out of the signal path completely. i don't want to misinterpret. :?

The power tubes are fairly new Sovtek 6L6's and i doubt they are bad (i really hope not). I will see what i can deduce as far as the tubes go.

So for the caps i just set my multimeter to DC mV and measure across the coupling caps? I haven't looked at the board hard enough to know where everything is...do you know where the caps of interest are? I know what coupling caps are and can probably deduce but number would help.

I was hoping that i was done having issues with the amp...about 3 years of good service with me plus however long it held together for the original owner...now it has to give me all sorts of problems. I know it is inevitable but it picked a really inconvenient time to fall apart. It seems like every time i turn around there is a new problem...not just with the amp. My laptop has been screwy lately as well, among other things. It sucks when things break all at once. :evil:

Sorry for the rant...Thanks for your help

 

[kc2eeb]

http://mtsforum.grailtone.com/viewtopic.php?t=12785&highlight=lots+pictures+rm50+repair
This is work I did to my own RM50. Show your tech what I did cutting out the burned portion of the board and filling it with epoxy. What I mean by measuring the caps for passing DC, is with the amp running you should see no DC passing through the cap. What I meant by "pulling the tubes" was to take them out and leave them out as you progressed from the input to the output. The popping sound was caused by the arcing of the HV.