Ashly SC-66A EQ

April 28, 2018

The SC-66A is a two-channel, four band parametic EQ. Very handy thing to have around. This unit is one from the studio that was acting up.  The output was noisy and it seemed to be clipping more than it should. 


When I opened it up to test it, one of the op amps started smoking, which made diagnosis a lot easier.  It actually had a couple issues.  One was the fried op amp, the other was the power supply; specifically, the power transformer itself was damaged and not putting out enough voltage to keep the +-15V regulators working properly. So I ended up rebuilding most of the power supply, replacing the power transformer, the regulators and the filter capacitors.  After that everything was running much more smoothly.


The original power transformer (left),

side by side with its toroid replacement



The fried op amp and a nearby damaged resistor.

The SC-66A uses these adorable little modular boards for each band of the EQ, all linked together using what seems to be an early version of ribbon cable.



Internal view of the SC-66A

Individual EQ boards all lined up, and the I/O board along the right side.



The only thing I really dislike about this unit is all the knobs using hex set screws.  It makes disassembly and reassembly a bit of a pain compared to knurled or d-shaft knobs.


Also worth noting that those rectangular "winky" in/out buttons are still made by C&K and readily available. When I'm servicing an older unit like this it's easy to assume that parts like that are long gone, but it's not always the case.

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