Mesa Boogie Triple Rectifier 150 Watt Solo Head
Have I done something bad in a past life?? The third Mesa Boogie this week. This animal came into the shop today. Lifted it onto the bench and thought that I was getting weaker in my old age. I knew they were heavy. I looked in the back and there was a massive toroid transformer bolted to the inside of the case. The chassis said it was for 120VAC, so must have been a non-European model and the transformer was a step down. An isolation transformer not an auto transformer.
Reported fault was that the clean channel had very low gain. Indeed testing it, there was hardly any gain on the Clean channel at all, but a sort of breakthrough as the volume was increased. The preamp valves are common to both Clean and Drive channels and as the Drive channel was fine, I guess these valves were all good. Fearing the worse that there was a fault around the switching matrix and or relays.
A real pain to remove the chassis with the isolation transformer fitted, so that had to be unwired and removed first. 6 x 6L6Cs all lined up in a row like little proud soldiers. Both rectifier valves had been removed so it was running on solid state diodes. Do players really hear the difference between 2 parallel rectifier valves and solid state?? With the chassis out, testing now could begin. Most of the tone and channel changes are achieved by LDRs (light dependent resistors) driven by this switching matrix. The faults here can be a real headache. After much messing about and time wasting, the input signal was found to vanish through the tone stack of the Clean channel. Now the first thing to check are the tone controls. We have seen many times that controls can be damaged as a result of being hit or knocked. Eventually, it was found that the Treble potentiometer had an open circuit track. This pot replaced and happiness was restored to the clean channel.
Squeeze the chassis back in along with its isolation transformer. Test for half and hour. Slip into the surgical support and life the amplifier off the bench.
