Old Fender Rhodes electric piano

[jordankersten]

Hello everyone! I have an old Fender Rhodes electric piano I am rebuilding and I have a question. It uses a stereo pan circuit which pans the signal back and forth between the left and right amplifiers. It does this with a simple oscillator circuit that drives two 2N3053 transistors (now replaced with NTE 128's) that switch two light bulbs back and forth which then control each channel via a photo-cell. When the light bulbs get switched OFF, you get a bit of a pop each time. This results in a slight popping or thumping that pans back and forth when the light bulbs turn off. I put 2 68uf capacitors between the collectors and emitters of the 2n3053's and this helped a lot. The popping is hardly noticeable. I am wondering if A) is that safe to put those caps there, and B) if so, can I put in a higher uf cap to completely rid of the pop. I am wondering if this is safe for those transistors. Also, should I be using polarized or non-polarized electrolytics Thanks everyone!

P.S. Here is a schematic of the entire pre-amplifier with the vibrato circuit: http://www.fenderrhodes.com/org/manual/index2.html

 

[jordankersten]

Very Sorry, I thought that was a direct link to the schematic. To get to the schematic, click on "schematics" and it is Fig 11-8 "Pre-amp for 80 Watt suitcase and super satellite."

 

[rbj]

i don't see any reason it would hurt the 2N3053 transistors (that are driving the light bulbs). it would cause a little reactive load to the transistors in the flip-flop oscillator that is driving the 2N3053s. it will reduce the amplitude of the vibrato for faster vibrato as you increase the capacitance. you could put a resistor between the caps and the bases of the transistors to maybe round out the vibrato waveform a little. that may or may not be desirable. you have to listen to it to see.

i'm more of a DSP guy now, but long ago, in another life (about the same time the Rhodes was designed), i was a sort of circuits guy. I've always thought the mechanical action of the older Rhodeses were a little hard compared to a Wurlitzer model 200.

best of luck,

 

[jordankersten]

Thanks for the reply! I have actually never played a Wurlitizer. Just my Rhodes and my B3...but playing Steinways on jazz gigs, the Rhodes feels good to me...I tend to like the heavier action.