***Need HELP with my amp***

[mac]

Ok. This forum is about pedals not amp repair. I apologize.

I have an intermittent problem with a Laney LC-30. After playing for a while suddenly a loud buzz, like a low freq square wave, appears.  No guitar signal, the noise has a constant volume level and does not respond to the volume pot. It is like plugging a small ac supply to a loud speaker.
I opened the amp, no signs of fried components, everything seems to be ok. The power tubes are orange, no red plates.
If I turn the amp off and then on after say 10 minutes it sounds as good as always, but again after several minutes the buzz starts.
There are no Laney support where I live. Techs at the store where I bought the amp told me that it could be one of the 3 ¿marshall? jacks at the rear, send, return and footswitch. They told me that they have the tendency to rust and create bad contacts causing a lot of problems. They adviced me to insert a plug on this jacks and see if I can force the problem to show up.
I could not force this.  My intuition tells me that this is not the case, it seems to be like a component failing with temperature, vibration or similar.

Any help is welcome. Thx.



mac

[markm]

Filter Cap issue perhaps? 

[mjones99]

Sounds like a cold joint on the power supply filter cap, try turning the amp off, drain the caps with a 1W 220K resistor to ground, I usually hold it with  an insulated plier, and reflow the joints on the filter caps.  Also when the amp is making the noise what happens if you insert the cable between the effect loop in out, or pre out pwr in jacks, if it does nothing to the sound its the caps, if it cures it, its the jacks. thats about 90% certain to be one of those two things.

[idlechatterbox]

I'd be really skeptical of the jacks-theory...

Just doesn't seem like jacks or jack connections could be doing that, since something's obviously ok when you first fire it up. Could be wrong though.

I had a tube amp (simple 3-tuber) that did much the same thing. It would sound wonderous for the first 10 minutes or so, then the hum would just sort of start "approaching," until the point that it was so loud playing through the amp was no longer an option.

I replaced all the old caps and the tubes. They needed replacement anyway, which might not be the case with your amp, but it definitely cured the problem for me. Others here have a lot more experince, but from what I've seen, it's common to blame the tubes when in fact it's the caps. They just do weird things as they heat up and cool down.


Good luck

[mac]

I never had problems with previous amps, maybe because I never mistreated them and replaced the tubes frequently. This means that I am not an amp guru  

Filter cap... yeah, it crossed my mind because it sound like I wrote above, like an ac supply connected to the speaker.
Bad solder or the cap is fried?

The questions is, can I leave the amp on while it is buzzing to test the send, return and FS jacks? If it is the filter cap, can it blow up something being the amp on?

... Wish to be an elect eng, not a physicist...  



mac

[mjones99]

I'd say its the caps almost certain, it wont hurt it much more than already is to try the cable in the jacks as it buzzes, just plug in both so the cord is connecting pre to pwr, that takes the switching part of the jacks out of the equation, which is where the problem is IF its the jacks, it will immedeately cure it if that is the prob, but most likely its the caps or the solder connection of the caps to the board or terminals, if the cord does nothing for the buzz, try reflowing the joints on the caps, if that dont do it replace the caps, if that dont do it replace the power tubes, if that dont do it replace the rectifier, if THAT dont do it replace the power tube sockets, the problem is in the power supply most likely, power amp less likely, or at least AFTER the preamp (effect loop etc.) even less likely, one of these areas though definitely, if anything in the pre was the problem the volume control would affect the buzz, so its in the back half of the amp.  Really on a tube amp this should be pretty easy to fix, you'll see.  Just remember the latent voltage in a tube amp power supply can KILL YOU DEAD QUICK...drain the power supply caps before working on it, also you can ask this question at the AX84.com site, on the BBS go to non-ax84 section and you'll get bunches of help in no time.

[petemoore]

  This should be on an amp forum, amp guts Can Kill anyone.
  You should learn how to repair tube amps safely before proceeding.
   It is like plugging a small ac supply to a loud speaker.
  That's exactly what you should hear if your filter caps are dodgy.

[mac]

Believe me or not the few answers here helped me more than threads at other sites. Thanks.

Today I discarded the send, return or FS jacks. I connected them directly but the noise showed up after a while. What I noticed is that it only happens at very high volume, +70 - 80%, so it is triggered by more power compsumtion or vibration, or both.
I have to check the filter caps. What confuses me is how guitar signal dissapears when this happens. I DL the service manual to have a better idea.

This should be on an amp forum, amp guts Can Kill anyone.
You should learn how to repair tube amps safely before proceeding.

I do not fear the caps, I know what they can do and how to work safely. Anyway thanks for your concern.
Pcb is another story. It is complicated, I do not have the appropiate tools to work on it. It is not a small mxr+ board. So I'm considering to drive 400km to Buenos Aires to have it repaired by qualified people.



mac