I had a sudden loss of all three channels. WTF? Was it related to power tube trials? Absolutely not.... I was able to get a guitar signal out to the power tubes by plugging the guitar directly into the return (I also used a dummy cord plugged into the Send as I was uncertain which jack was switched) and then turned on the FX loop with the switch on the back. Very nice clean channel alternative without any tone controls but I was able to narrow down where the issue was coming from. Note that I tried this after replacing V7 and V5 since I lost all three channels, I did get sound but and had to crank up the masters on all channels to hear anything. I did try rolling some tubes in V3 and V4 to determine if the tone and character could be manipulated with different 12AX7 tubes (all were used with other Mesa amps without any issues including the RA100 which is currently using the arrangement I wanted to try in the TC-50).
I was unsure what the issue was exactly but the symptom was pointing to the V5 12AT7 tube position. Change the 12AT7 on several occasions with all of the 12AT7 tubes I have in my inventory. No change. I guess there was a reason to take a look inside as I have never opened this one up yet (JP-2C has not been looked at internally either). Very small supply capacitors, I was expecting huge barrels but not. Perhaps 50W does not require the larger cans due to differences in current flow and sustained DC voltages required. There were also many unpopulated relay footprints, a few sockets without any chips. I am interested to find out what the 8 pin DIP are... I did not record the part number to look up. Before I bore you to death with what is inside, I will cover the other boring details about how I found the problem. Visual inspection, I found three suspect solder joints on the PCB. Overall the solder quality and workmanship is excellent. Only one resistor had a poor solder fillet on the top side of the board. A little flux, small amount of solder and no big deal to get that one off the list. The Input cable looked suspicious and resembled a solder joint to a wire that would not solder, either due to lack of flux action or contaminated copper core as that can happen with wire that remains exposed to humid conditions. (Not the case as California is not a humid location). A dab of flux and reflow of the solder joint cured that one too. The third was actually on the part that failed, it just had a brown spot on the solder joint that would not reflect light. First though was baked flux from wave soldering which did not get cleaned properly. What ever technique that is used to solder the boards, Mesa does an excellent job especially if it is lead free solder, I am impressed with the fillets on the top side of the board.
I went to reassemble the amp, inserted tubes and powered up. Hoping it was that simple. Nope. Same thing.... Time to send to Mesa for repair. However I wanted to take one last look at the preamp circuits and probe around with a multi-meter. I doubt it was a dc block capacitor between any stages. Thought it may be a JFET or relay... I focused on components around V5 as that is where I suspected the issue to be and found an open resistor. It was one that I assumed had a bad solder joint but it can be difficult to determine without doing a cross section of the assembly. Not what I have in mind as it was not necessary to go into forensics of the issue. R105, 39k resistor had a resistance measurement that appeared to be a capacitor charging curve before it went to open circuit. That was one.... what else is there? probed around until I was done checking all resistors on the preamp section (did not bother with the components on the power supply or one's that cannot be reached easily. If I did not find anything in view I would send to Mesa for evaluation and repair. ) This was not a job for my home solder iron, for one it is too big, and second may cause damage to the PCB if used for this type of repair (as it appeared that it may be connected to a copper pour which is a large area of cooper on the PCB used as a ground plane or as a heat sink, it was not but it could have been. Lucky to have a 1/2W 39K resistor available, carbon film type and suitable for high voltage I believe is rated for 350V. When I got home, reassembled the amp, powered up and heard nothing. Helps to plug in the guitar. All was good. Fixed and up and running again. The TC-50 is not as tightly packed as say a Mark V. There are many boards that are suspended by controls on both sides so the power tube board is hard to access if needed. I guess I have voided my warranty now that I had to repair the amp. I could have shipped it out to have a $0.02 part replaced plus the time it would have taken the inspection and review of warranty claim. In the end it would have been worth it as long as the cost was reasonable.
