Mellotronists

Howdy,

Some interesting tidbits from the neighborhood prog show inventory repair
shop...

I was able to reverse engineer the Sound Sales preamp circuit in the course
of troubleshooting #266 here. But back to that in a moment - I've been
quietly noting the numbers engraved/written on the various bits, and #266
appears to be another entity of unholy origin. The case and s/n tag say 266,
the keyboard says 210, and the preamp line box says 190. Wonder how many of
these mix-ups are out there? At least 3 I guess...

OK back to the preamp. The Sound Sales circuit board is a single PCB rather
than the usual tape head preamp/line amp pair found in most 'trons. The main
issue with this latest patient is the extremely low output, so I've been
poking around to see if the preamp has a blown something or other. In a
moment of quiet I sat with a blank piece of paper and pen and magnifying
glass (yes I am in my 40's) and drew out the circuit. Lo and behold the
front end (where the tape head coax comes in) is identical to the oringinal
Bradley 3-transistor preamp. The line amp is built around a single 741
op-amp. And the reason for the lower output was that somebody soldered in a
resistor between the preamp output and the top of the volume pot. Guess this
was intended for guitar amp usage? Anyway, removed the resistor, connected
the preamp out directly to the volume pot, and got normal 'tron output
levels. Another thing I noticed was that this preamp/lineamp card runs off
of +17 vdc and ground, rather than the +/-30 vdc shown in the original 'tron
schematics.

Looks like this machine was in the middle of an upgrade. The cheekblocks
aren't original and one of them needed to be cut down to fit. The Volume and
Tone pots have shafts that were cut way too low to fit the knobs on, gotta
replace those. The motor controller is a non-CMC, the one with the heatsink
fins going horizontal instead of vertical. More work ahead...

Regarding 'tron spotting... anybody notice what sounds like 'tron noises for
a couple of phrases in the movie "Shrek"?

- Gene