Members orneon Posted July 31, 2014 Members Share Posted July 31, 2014 Hi all I have a problem with my JVM210 head and I hope you guys can help me out. The problem is that when i turn on the amp, and after a 5 min. warmup in stand by, I play for about 15-20 min. and then suddenly the the sound goes off and it's replaced by an electrical noise witch is comparable to grabbing an unplugged guitar input lead, only louder. The noise doesn't change if I play the guitar or not, if I unplug the guitar, if i change the the volume or the channels on the amp. I took it back to the store were i bought it from and the tech there couldn't find what's wrong with it. I have the amp for about a year now and it's only played in bedroom levels and not very frequently. The thing is that I live in Crete (a Greek island) and it's very difficult to find a good technician. Has anyone here had the same problem or something similar to it and if so what did you do to fix it? Thank you PS. Please excuse my poor English Link to comment Share on other sites More sharing options...
Members WRGKMC Posted July 31, 2014 Members Share Posted July 31, 2014 So you say the original symptom has returned after a year of use? Sounds like a bad solder joint to me. a cold solder joint can make intermittent connections, work find sometimes and when the tubes heat up it can cause the electrical connection to fail. First I'd check the color of the tubes when the amp is turned on and the standby switch is kicked on. If you have a tube turn bright orange, its an indication you have a bad tube. As the tube turns orange it will hum. You will need a new matched pair of power tubes, and then you have to figure out why they went bad. New tubes require biasing and its not something a laymen should attempt without supervision. One cause can be from having a bad speaker impedance match. Marshalls have critical standards on impedance ratings. You have to have the impedance selector set to match the cab and vice versa. Since this is an AC hum, I'd immediately focus on the grid resistors, power caps, and tube socket connections. The tubes have power resistors connected to them that do heat up and a weak solder joint at the tube socket wont withstand that heat for long. I'd pull the chassis, remove the tubes carefully, then flip it so I can see all the connections. I'd wait a day to make sure the caps have fully discharged or even use a resistor to fully discharge all their voltage. They should discharge on their own but its good to be safe. Then I'd use a soldering iron and electronic grade solder 60/40 rosin core solder, reheat any suspicious connections in the power amp section and give the connections a fresh dab of solder. (You only need to get the solder to reflow and have a nice silver look. You don't want to blob the solder on or you can wind up with shorts) I'd check everything connected to the power tubes and the big power caps. You'd also check and see if any of the resistors are burnt and if any of the caps have the dimple bulging out or see any signs of the white electrolyte leaking out. This is a newer amp so I wouldn't expect an amp to have bad caps yet but you still check these things because caps go bad quicker when they aren't being used then if you use them daily. The preamp tubes shouldn't cause a heavy buzz. You could always remive them and see if the symptoms go away. You could swap the 12AX7's around as well. I'd also inspect the glass of the power tubes. Make sure they don't have any milky look to them on the inside of the glass. That's an indication they are cracked and air got in. When on they usually have a little blue on the sides which is normal. I'd also clean the sockets with non residue contact cleaner and let it dry well. The cleaner is pure alcohol and flammable so you don't want to power it up and get a spark while its still wet. Non residue cleaner should never be used on the volume pots, so don't get liberal and spray everything with it. Pots need pot cleaner which has mineral oil in it to clean and lubricate them. Zero residue will cause them to make noise and crackle when turned. I doubt the amp needs the pots cleaned at this point and you should only do it when its needed. You only have a few times those pots can be cleaned so you don't want to reduce their lifespan. From here if the problem still occurs you have to use test tools and troubleshoot it. If the problem is intermittent, it can be difficult to diagnose and repair. Just beware. Tube amps use voltages or above 500V and the power caps can store that voltage. Never work on the amp with it plugged in ajnd if you don't feel comfortable working on them find a pro or at least someone who will guarantee the work. If they screw it up, thay can pay the freight back to the factory for repair. Good luck. Link to comment Share on other sites More sharing options...
Members orneon Posted July 31, 2014 Author Members Share Posted July 31, 2014 The problem is in the poweramp section. After alot of digging, I found out how to check the the signal chain and after connecting a headphone set in the preamp out, i could listen the guitar playing even though there was no signal through the speakers, only the loud electronic "buzz". I could listen to the guitar signal playing even in standby mode, which means that the poweramp section is bypassed. Is it time to change the power tubes or is it something else? Can I swap the power tubes or even leave a pair at a time to see if it's them or i will damage the amp? Thank you Link to comment Share on other sites More sharing options...
Members orneon Posted August 1, 2014 Author Members Share Posted August 1, 2014 Hello again Problem solved!!! It was the V5 (phase Inverter) tube. I found a spare Sovtek 12AX7WA, put it in there and all went smooth!!! Thank you very much for all the tips. Cheers Link to comment Share on other sites More sharing options...
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