140 deviations from schematic (140A?)

Tech Questions regarded the Wurlitzer Electric Piano

140 deviations from schematic (140A?)

Postby Rhodesplyr on Wed Jul 13, 2011 9:47 am

I'm a professional keyboard tech, and I've got a piano in the shop with an amp that deviates from the commonly available 140 schematic (dated July 10, 1962) in a few circuit details. The model/serial number plate is missing from the piano. There are a couple of clues that this amp might follow the 140A schematic, but it's definitely NOT a 140B.

All these differences are definitely part of the original build; they are not later modifications.

There is a 10uF capacitor attached to the junction of R40 and R41. It connects to a 33k resistor. The 33k resistor connects to the junction of R34 and C13. These components appear to replace the NFB path of R51/C31 and R35 shown on the 140 schematic since those NFB components are not present.

There is a 22k resistor between the pickup input and the 0.1uF coupling capacitor to the first transistor stage which does not appear on the 140 schematic. It is not between the capacitor and the first transistor stage as it is on the 140B.

R41 and R38 are both 270 Ohm/5W resistors, and there are 1k and 1.5k carbon composition resistors wired in parallel with R41 and R38, possibly an attempt to tailor the bias of the output transistors. These extra resistors would bring the 270 Ohm resistors closer to 220 Ohms, the value shown the schematic.

C30 is 0.047uF, not 0.0047uF.

It's not absolutely necessary for me to have the schematic, but I'm curious. I've searched every other Wurlitzer Electric Piano resource I know of online. I often like to know what version I'm looking at so that, when doing a rebuild, I can make sure that I'm following the best version of a particular model--and not a temporary design change that didn't work out well.
Rhodesplyr
 
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