It does not take a rocket scientist to replace a vactrol... Just a lot of patience in importing new ones.
Before reading the thread you linked to, my initial thoughts were also that it should be simple to diagnose since the whole mode has its own pcb. It should be ten minutes work to check if it is the vactrols.
Ps you don't absolutely need a Mesa tech, the schematic is available on www.tubefreak.com
Alan Ratcliffe wrote:
A qualified tech would also make sure that there isn't another issue that caused the opto to fail.
Said qualified tech would do well to first look up the Triaxis vactrol control patent - essentially they make sure of the resistor value by the use of negative feedback loop. It takes a while to figure it out without reading this patent ?
