Diagnosing Amp Problem

that sounds to me like a tube problem.  one of the tubes in the gain stage is probably not getting full current under load.

Thanks guys.
 
I think my best bet is to get it to the repair shop.
 
One follow-up. If I need new tubes does it matter what brand I get (as long as they are the appropriate type)?

DerGeist


Thanks guys.
 
I think my best bet is to get it to the repair shop.
 
One follow-up. If I need new tubes does it matter what brand I get (as long as they are the appropriate type)?

talk to your repairman and see what he recommends if you need new tubes. 

This sounds very similar to my JCM2000 DSL-401. Bought it back in 2003. Somewhere along the way it started to fade out similarly. It would be playing fine then the sound and lights would start to fade out. Turn it off and it would generally come back. Bottom line, it was sporadic. I tried the usual home-repairs of resetting the tubes and such. It would work for a while but eventually the issue returned. Finally I took it to a respectable repair shop in the area in 2008. They did right and fixed it. Haven't had a problem since.
 
As an FYI, the repair notes were:
 
"Broken ground trace connections (JS1) repaired, some component parts connection resoldered, screws, jack nuts tightened, jack contactcs cleaned, tubes checked, unit cleaned and tested ok".
 
I am glad I took it in. This repair was beyond my knowledge. Oh, I should mention that I originally took it in without the distortion on/off pedal, and they called to have me bring that it in. Apparently they needed it to verify some stuff.

If you enjoy tracing the problem yourself, I would recommend a large magnifying glass (to look for broken traces and bad solder joints), a soldering iron and solder, and a bottle of freeze spray (don't spray it on the tubes). Also, some plastic chopsticks to tap on some components may be handy.
And, yes, search for similar cases!

Good luck!

Wilko

Shadow of The Wind


and a bottle of freeze spray (don't spray it on the tubes).

How does one use the freeze spray in diagnosing the connections? This is new to me.

It could be a tube or what Mike outlined. Before you take it to a Pro, you could try switching out the preamp tubes one at a time. But it usually is a bad circuit board component or a bad speaker output. It can be a real hit and miss investigation if you don't know how to trace it and don't have a schematic and the proper understanding.

ABSOLUTELY!  if you don't know what you're doing, don't do it.  it is dangerous and could kill you if you touch the wrong thing inside that amp!

You might want to post where you live so someone could make a recommendation, Good tube amp repair shops are rare. You don't want to just take it to any old shop who's field of expertise is toaster ovens.

I'm in Ottawa Canada. There don't seem to be a lot of repair options here. The ones that I can find don't fill me with a lot of confidence.

OK... I did say in post 9 that injury or death could result from the voltages present inside the amp. 450VDC is not uncommon. But it can not be emphasized enough, that if you are not sure, and don't know, don't go.

I have been hit by this kind of voltage and let me tell you..... it hurts.


All that aside. If you decide to have a "look see" yourself.


This can and should be done with the amp off and unplugged and the power supply caps discharged. A cap can hold a high voltage for weeks even when shut off if the circuit does not have a "bleeder" resistor installed so use a an insulated jumper between ground and a 1000 ohm resistor to discharge the caps fully.


 A cold solder joint can often be found by trying to wiggle the components and looking very closely at the place where the component lead enters the solder joint. If it moves, it's bad. Simply touching a hot soldering iron to that joint to melt the solder will fix the joint. If a part moves before the solder is cool enough (just a few seconds) a cold/bad solder joint is formed.

Places to look....  around the tube sockets and the capacitors especially. But all components are fair game.


This is done with the amp running and warmed up for a few minutes under a load to be sure it is hot enough.  So be careful.
Cooling spray is often used to cool a component that is in heat related failure. This usually works best on capacitors. Most other parts are NOT as affected by heat. A bad cap will cease to function correctly as it heats up. Cooling it quickly will often allow it to begin to function again..... if it does, you found the bad component. Shut the amp off and replace the part. Observe the polarity of the part if it is polarized.

A side note: the freeze spray is also handy for removing warts and skin tags.... way cheaper than a Dr.

Broken traces can occur just about anywhere in the amp.... but if it is occurring after the amp is warmed up, it is likely in a heated area, like around a tube socket. Do not spay the glass tube with the spray, it will cause the glass envelope to crack and the tube will then be bad. Parts that are under stress from movement, such as where a jack attaches to a circuit board..... some new amps are using this solder direct to the board method rather than having the jacks mounted to the chassis and connected by wires.

Bottom line: if you are not comfortable doing any of this, then don't. Look up a good tube amp shop in your area. Contact a local music store and ask if they repair or sub repairs out and if so to whom.



Best wishes