My Mesa Studio 22+ started to make a horrendous crackle on the lead channel and the amp had briefly gone quiet a couple of times recently so over the weekend I found the time to open it up and have a look. I'd done the lead drive mod authored by Boogie themselves a few years ago as it had way too much gain for me and to reduce it even further, I'd also been using an ECC82 in the first valve position which in this amp effectively acts a boost. This era of Boogies are famed for the loop jacks getting dirty and causing drop outs so a good clean of them was also on the cards.
First thing though was to try swapping valves out and as the crackle remained, out came the chassis for further inspection.
Rant...... I'd forgotten how much of a right royal pain in the arse this amp is to work on. All the components bar two are on the underside of the board inside the chassis and the Boogie service manual component view is a reverse image i.e. white on black of the board view taken from the component side. So not only are you working blind from the components, the image of what component is where is reversed too and you cant see where the underside PCB traces are going to..
To even get the board out a few wires e.g. the reverb tank return need to be unsoldered as they're not long enough for the board to flip over.
I discovered the anode resistor for V1 was way out of spec and I managed to swap it out without fully removing the board.. happy days or so I thought. After putting it all back in the chassis, first replacing the plastic stand off between the power valves that holds the board in place and stops it moving when putting the power valves back in which decided it had had enough of the best part of thirty years heat and cracked, making sure there were no disconnected or shorts on any of the trailing wires, I fired it up.
A light buzz from the speaker but nothing else. argh.... triple checked the wiring, went away for a cup of tea... still nada. well apart from a very tiny signal if the amp was fully cranked up. Plugging into the FX return was fine, and a jumper cable in the loop made no difference.
Fired up a signal generator and scope and signal was fine up until the input of V3 which is the mixer stage for the dry and reverb signal and loop send. After testing as many of the components as I could in situ from the bottom of the board I pulled out the dry mixer resistor as that seemed to be reading off.
Out of the circuit the resistor read fine. At which point I had an "I wonder" moment... swapped V3 for a known good one and lo and behold the amp fired into life... I'm guessing that the valve was on its way out as per the loop briefly dropping occasionally and pulling it out and back in was the final nail in the coffin.
Bl**dy valves......
Comments
Hope you get it sorted!
Seriously: If you value it, take/fetch it yourself
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Seriously: If you value it, take/fetch it yourself