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May 25, 2022, 06:10:25 PM

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Fender STC1 Chorus Pedal

Started by jammy5152, December 07, 2020, 01:17:11 PM

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Being new here, I'm asking for information in finding a schematic or a site for
a Fender STC1 Chorus Pedal...
Vintage 80's Pedal


All those early chorus pedals used a BBD chip (bucket brigade delay line chip) and are obsolete now.
If you want to service, :
The BBD chip has variants. Some with 512 stages, some with 1024 or 4096 stages. 4096 stages usually for producing longer delays (echo).

All the circuits using the BBD chip have these common components.
Signal path : An input buffer > low pass filter (20KHz) > BBD chip > low pass filter (20KHZ)> output buffer, mixer
Modulation path : A Low frequency oscillator (0.5 Hz to 10 Hz) > A square wave oscillator with 2 outputs with 180 deg opposing phase

The square wave oscillator IC drives the BBD chip. (one for odd numbered stages and other even numbered stages). This frequency determines the delay time. This square wave oscillator frequency is varied (modulated) with a very low frequency oscillator triangle wave (LFO), thus the delay time is also varied.
The instrument signal is buffered and passed to a low pass filter (around 20 KHz), then to the BBD chip. The output from BBD is passed through another low pass filter (around 20KHz). This signal is mixed with the original and buffered to output.   

Depending on the delay time, feedback and mixing methods, same circuit does the chorus, flanger, delay, echo etc.

Hope this is of little help.

Jazz P Bass

Nice run down on the BBD circuit, Mandu.


Just a small explanation of the name bucket brigade delay.
Imagine a line of 10 fire fighters. One side is fire and the other side is water. Buckets of water has to be moved from the water source by buckets to the other to kill the fire.
Since the fire fighters are in a line, when they receive a bucket of water, they can't deliver to the next person. Similarly when they deliver water to the next person, they can't receive water.
A BBD chip in similar, has tiny line of capacitors (say 511) inside representing the buckets. Electronic switches (say 512) are in between each capacitor and the  switches represent the fire fighters.
The odd numbered switches and even numbered switches are turned On and OFF alternatively from the square wave signals.
When the odd numbered switches are turned on, they transfer the charge from the left side capacitors (input side) to the right side capacitor. When the even numbered switches are turned on, they transfer the charged capacitor value to the next and so on.
At the output, switches 511 and 512 outputs are connected together so there is a continuous signal flow. Now there is signal at the output but chopped at the same frequency as the square wave.
Since this chopped frequency is 30 Khz or more, a low pass filter of 20 Khz is inserted at the output to filter out all higher frequency above audible range. If the portion of signal is fed back to the input to create a flanging or echo, another low pass filter is required at the input before the BBD.
Since the stages work similar to the bucket brigade fire fighters, hence the name bucket brigade delay line.
To prevent loss of signal (leakage of capacitors) during modulating frequency transition, the driver chip is interlocked internally cmos or ttl gates so the rise and fall of the wave forms never cross each other.

Jazz P Bass


Here's a freebie chapter from an analog electronics book I was working on back around 1997 or so. I do have the copyright on this (the actual piece of paper from the gov't), but am letting it go here because you guys rock!

This is missing a key ingredient for BBD circuits - the compander (NE571) that limits dynamic range into the BBD and then expands it coming out.

I've been pondering the idea of doing a 3-part Udemy class on analog audio electronics... one of these days.


Jazz P Bass


Great information in one place.
Philips produced TDA1022 and TDA1097 BBD chips
And there was SAD512 chip from some other manufacturer
These were originally developed for telephony applications.
Thank you very much.



The parts involved are also less common than they were, but Peavey made a chorus circuit for the Mace and some others, not using a BBD chip.  They made a delay line with discrete stages.