Thread: On again off again guitar amp

Sounds like a dry solder joint to me, does it die with a crackle or just fade out.

Dry joints usually crackle.

A crook capacitor could also be causing it but that will be harder to see.

Thanks, Cliff. I can solder a little, is the faulty joint hard to find?

A good joint should have a smooth gloss finish, while a faulty (dry) joint will generally have a matt finish and may show a visible flaw around the component lead.

Primary cause other than poor initial soldering is usually lead movement. This might be because the component gets hot and the metal in the lead expands while the body is restrained on a rigid heatsink, or it could be a large component supported only by the lead joints subjected to a lot of vibration from road travel etc.

If possible, invert the PCB and examine the joints on all components with leads larger than about 0.4mm dia. There is about a 60% chance that this will locate a fault. Then check the balance of the joints. If the problem remains, it may well be similar to the heated lead expanding condition mentioned above, except that the fault manifests itself as a lead fracture within the component body. The easiest way to locate the component is to use the unit till it dies then hit the power transistors etc one at a time with a freeze spray to cool the component. Any component that will allow the unit to work when suddenly chilled is suspect and needs to be replaced.

Otherwise pass the unit to an experienced techo for repair.

Not familiar with Cobra amps, but I build tube amps here in the States. Mostly old Fender circuits. Here is what I use to troubleshoot bad connections. Be aware, this is dangerous. There are voltages inside amplifiers that can kill you. Open her up and turn her on. Then, use a wooden chopstick to press one connection at a time. Usually, when you find the faulty connection, the amp will either come back to life or turn off, depending on it's starting condition, if you follow me. Once you have found the faulty connection, drain the large filter capacitors before trying to resolder the joint. Good luck to you!

what sort of amp are we talking about.

tube, transistor or hybred.

does it die totaly..... like no pilot light.... or the sound just doesnt come out?


If you thump it does it make a noise or come to life.?

I could think of a variety of things that could cause such a fault.....I'm sure there are ore than that.

As for dry joints.......I've done plenty of resolders and some dodgy joints you wont see till you hit em with a soldering iron.....If a piece of equipment come to me with dry joints or suspect dry joints I'll rework the entire PCB....where there is one there will be plenty more.
Lots of gear that seems to have been quite reasonably soldered, after a few years can often show up with dodgy joints.
Big power amps that have worked hard can real suffer from the heat, particularly "old localy made amps" where the soldering wasn't flash to strat with

Not just me but several of my associates have found a good rework often solves a variety of niggly problems....and particularly things that arent right but you cant put your finger on....graphic EQ's and compressors are clasic candidates.

One place I worked for a time...they had some cheap HIFi gear they used to do waranty service on.....it was polocy not to try too hard to locate specific faults in this gear....... "narrow it to a board & resolder the lot".... was the polocy......most of the faults disapeared that way & there was less come back.

I have no hesitation in a complete rework.... but I have the right gear & I'm reasonably fast.....without a good iron and some developed skills though it could be a bit of a burden.

lets hear some more, before we draw any conclusions though.

cheers