Do thorough research on amp techs before trusting valuable ones to be repaired. And in the end, the chopstick method.
Don't trust recommendations from a friend even if the recommended tech has history of successful repairs. Never trust your gear to the same friend either just because he is a more experienced musician who has played longer and had more gear usage and repairs before your 1st repair needs occurred.
A moderately sad but true story lesson I learned that has put me on this path in the first place.
My 1st amp repair was from my 70's SS Sunn AMP combo that stopped working. My friend offered a friend's help to fix it for me. After giving it to my friend, I never saw the amp again. He said the repair guy said it was not worth the cost to fix it. So I basically chalked it up to a $100 first amp, basically not cost effective to worry about. Still wish I had it though. I could likely fix a burnt resistor or fuse or something simple.
Next was me needing to have my 1972 Marshall JMP 50 watter fixed due to it blowing fuses after a bit of play time.
Same friend, but I was able to meet possibly a different tech guy a number of years later that he knew and said could definitely fix it.
When I took it to him, he checked the tubes, said no good. Then checked a few more things with volt meter and said yup, the Power Transformer was bad. The amp had already had a newer Output transformer in it by the time I got it. To make a long story short, the amp got new filter caps, and the new Marstran Transformer I got for him to fix the fuse blowing issue.
Sadly despite me telling him to save my old parts, he says he threw away the transformer. Even sadder, I seriously doubted he had but could do very little to retrieve it. Then in a little time, I still had issues with the amp losing sound signal from the guitar thru the speakers. After the loss of my Original Marshall transformer, I was no longer confident he would do me right and fix the issue he was already paid to fix.
Flash forward a decade to about 2016-2017 and through meeting Relic61, DonP, and IvanH on here, I now had 3 fellas who had the belief and knowledge to talk me through the type of learning I started this thread about. Through their teachings, the stuff I learned on youtube, googling and through books, I was confident my amp had tested to where all components were installed and functioning as they were from the factory. After numerous run checks, it seemed like it was back in reliable shape and functioning as it should. So, I was very surprised and a little frustrated when I went to play it and soon into doing so, the signal faded out and back in again, then out....
This time, I went back to a basic tip I learned during all my learning from the guys, and you tube etc. Visual looks and probing and listening with the amp on and thru speaker. chopstick probing revealed the issue and gave me a free fix. ( I now suspect this was the issue all the time and not the power transformer at all ) No odd noises or change as I probed every wire, pin, resistor, cap etc inside the amp, until I got to the pins on the OHMS selector. Suddenly the amp made a noise in the speaker, then I could make the guitar ring out too. Turns out the old style Marshall selector with the removable plug in cap had worked loose enough that the electrical connection was intermittent to none.
I disassembled the female socket and gently squeezed the female brass ferrules tighter and reassembled the unit to test for reliable connections. It passed the test and has been working since 2017 trouble free.
