In electronics heat is often The enemy. In valve amplifiers there is a tension between this basic rule and the valves themselves which get mighty hot and also the established design methods for amplifiers.
The problem is that heat damage caused by power resistors and valves eventually changes the circuit board material from a good insulator to a partial conductor. This damage is not reversible.
The burns do have to be quite severe mind you. You will usually see inner layers of fibreglass exposed, carbon particles. The only real cure is removal of the effected area. For small areas of damage the effected area can be drilled out but for larger areas a section may need to be cut out.
If you are having trouble with any of the following boogie symptoms then I may be able to help you out as I have spent more than my fair share of time on these metallic monsters.
Mesa Boogie amps are reveered by players for their unique sound. But for us Amp techs they can be a pain. Not only do they use unique to Mesa Boogie components but they also choose to hide circuit board markings so that diagnosis can be tricky they are helpful with schematics but they schematics are not easy to connect with the PCB in the amp as there are no component references.
Special Boogie Symptoms I have come across include:
Intermittent output, or rather occasional fading output. This is a common problem in amps usually caused by an unreliable connection somewhere in the signal chain. Usually such a fault will be either permanent or intermittent and respond to thump on the cabinet. With Boogie it is extra annoying as the intermittent nature is temperature / time related and of course it will let you down when you really need it to be reliable.
Bias light flicker – check the little red or green light on the rear when the output fades. Sometimes the output fading and the light flicker coincide together and sometimes not.
Channel switching strangeness including flicker, dim lights, no lights etc. Seems more noticable on footswitch but that actually it is usually the same with the rear switch.
These medium sized combos from Marshall should be very cool. They are packed with features (like multiple voicings, low noise pre-amp, fab speaker) all in a small box.
Unfortunately they are not so cool because they run so hot and they suffer from it. Look at the picture left. What you are looking at is a power valve that has had its label “fried” off it.
The cause of this problem is that the amps were originally biassed to run a little too hot and also the ventillation is limited with heat rising up but with almost nowhere to escape to.
One of the main problems can be the bridge rectifer shown below. This is black square on the right hand side of the picture. These components supply the pre amp heaters with the super quiet DC. Orignally Marshall fixed these directly to the boards and they get up to around 70deg celsius. The problem with this is that the heat so close to the PCB is enough to start melting the solder joints.
Later Marshall realised this and raised them off the board by a few centimetres, the impact was tremendous and reduced the temperature by about 30%. I go a step further and add a little heatsink to them too.
The symptoms of the overheating is faults like cutting out, channel switching problems and general weirdness. Once modified to remove these issues you have one of the most well thought out practical gigging combos out there.