Symptoms are almost non-existent LED brightness, and no sound.
Installed a battery and checked voltages. Battery terminal voltage dropped to 6VDC. The only way that can happen is excessive current being drawn.
Checked battery current and sure enough 110mA (about 15 times what it should be). Tried the pedal on a power supply - same issue.
Checked the internal voltages... the +5VDC rail was close to zero volts. Unsoldered the 78L05 regulator and battery current dropped to under 10mA (correct).
OK, so either the 78L05 is faulty, or something "downstream" from it has failed. Likely not the 78L05 because it has a 100mA internal current limit, and the 110mA is suspiciously like the 78L05 being maxed out.
I measured the resistance from the output pad of the 78L05 to ground. It measured 33 Ohms. That is exactly the resistance of R65 - part of the smoothing circuit that provides power to the BBD and BBD clock. Something down-stream of the smoothing circuit has gone short-circuit.
There are only three things on that power rail that could cause a short circuit - the BBD itself (unobtainium), the BBD driver (I have spares) or a 47uF filter capacitor C41.
I got lucky. It was a failed C41.
Replaced C41. Pedal now works flawlessly.
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I've laid out the thought process that I went through above. But why did I do exactly that? Well, I inspected the pedal closely from all angles with high magnification. There was no visible damage, broken wires etc. So it had to be component related. Very hard to do this inspection if the pedal isn't right in front of you. Pictures do not tell the story.
Once I was convinced it was a component failure, the next thing is to measure the voltages and currents. I have good test instruments and have fabricated test equipment like a 9V power "breakout" cable to let me measure incoming power supply current. All of this lets me run tests really quickly.
Once I found the excessive current I dismantled the pedal - took the guts out of the case to give me better access - then followed the chain of voltage reduction circuit elements until I found a reading that pointed me in the right direction. Then it was a matter of checking the schematic to see what could be causing the short circuit and removing those components one-by-one and rechecking to see if the short circuit had been resolved, all the while hoping it wasn't the unobtainable BBD.
Removing the components cleanly requires a good iron (I have a temperature controlled Weller) plus high quality de-solder braid.
A ten cent C41 component that I had in stock fixed it. I have two multi-compartment parts boxes with all types of capacitor in stock. Perhaps $1000 worth that I've collected over the years... If I need a specific value that I don't have, I usually order a pack of 10 or 25 to ensure I have that one if needed again in the future.
I resoldered the 78L05, cleaned the flux off the board with isopropyl alcohol, and reassembled the pedal. A bit of contact cleaner on the pots and the switch as preventative maintenance. Once back together a bit mor contact cleaner for the jacks and 9VDC socket.
The point of this story is this... The explanation above makes it look fast and simple to fix, which it was - on my bench. I compare that to how difficult it is when trying to do the same thing remotely. Weeks of back and forth email/chat and maybe never finding/fixing the fault.
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Oh no