Mesa Boogie F30
Would be nice to have some simple faults for a change.
This amplifier came in with a report that it was making popping noises. Upon power up, after about 10 minutes, it did indeed make a couple of small pops, then one large and went all quiet again.
With the chassis out on the bench, first signs were that there were no bad looking capacitors. Output valves were tested and all okay as were all the pre amp valves and phase splitter. It was now looking like we had a component breakdown. The problem here, is that the fault was very intermittent. Tapping the board would not bring on the fault, so no dry joints. Grounding the anodes of the pre amp valves showed that the fault was not in the pre amp section, as the pops still appeared. The output stage was isolated and no pops heard. After considerable head scratching, everything pointed to Mesa’s contour filter circuit. With this bypassed, the popping was not present. Scoping around the circuit showed random changes in the DC conditions. The circuit consists of various chokes, capacitors, resistors and 4 transistors, all fed via a -15VDC supply. Suspecting a transistor, each were removed one by one and replaced with similar types, even the last one a Darlington. Fault still was present. Heating and freezing would not bring on the fault. A bit of brute force was now needed. With the amp powered up, the legs of each component were heated with a soldering iron to see if the fault could be brought on. Typically the last component assaulted, a 15uF capacitor in the filter network, showed it’s colours and caused the popping noise when heated. Replacing this cued the fault. Very time consuming looking for a 1€ component!
