Bl**dy Valves

normula1

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......

rico

Surely the complaint is about the circuit design?

Hope you get it sorted!

Phil_aka_Pip

You ought to expect valves to have a limited operational life. I'd say the problem was the layout and the physical design, not the circuit.

normula1

It's all sorted now, the real complaint is that the amp is a pain to work on, and that I should have tried swapping the valve earlier.

ICBM

It certainly isn't Mesa's finest hour from a design and build quality point of view.

Phil_aka_Pip

I turned off the idea of Mesa after a mate bought a Boogie and one of its channels failed about 3 weeks later, so the shop gave him another one. That failed, and so did the third one. After that he bought a Carvin and never had any trub with it.

DJH83004

I do agree the Studio 22 is a difficult amp to work on, you have no choice but to un-solder the cables to spin the board over. I think from memory I had to swap out an opto, and they are on the opposite side to the line of soldered connections so was just able to get the board far enough over. But as IC says not their finest piece of design. 

Chester

Phil_aka_Pip said:

I turned off the idea of Mesa after a mate bought a Boogie and one of its channels failed about 3 weeks later, so the shop gave him another one. That failed, and so did the third one. After that he bought a Carvin and never had any trub with it.

Wow - that's interesting .. but unusual IME. I've owned 7 Mesa rigs and never had anything go wrong outside valve related issues, and a spring reverb pan which needed replacing. Always found them bombproof .. but apparently bast*rds to work on if they do go