Pedal modification -- Pianocorder Forum

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Subject: Pedal modification

Author:
Carl Meyer

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Date Posted: 13:35:57 02/16/04 Mon

The pedal mechanism is second after the tape deck for problems.

Years ago I found that after using everything I could think of to improve the sustain operation the best was to change the capacitor c15 from 47 to 10 uf. I also changed r75 from 10ohms to a 50-100k pot.

I told Marantz-they weren't interested. I told Yamaha-not interested either.

Here's the story-the voltage to the solenoid is 180volts swithed on and off at a rate of over 20000 times per second.The average voltage to the load is dependent on the duty cycle. Time on versus time off.

Solenoids require more voltage to actuate than to hold. Any current flowing through the coil stores energy in the inductance. When the current is turned off, the energy must be dissipated before the solenoid will drop out. In order for this to happen a diode is connecter across the coil allowing current to flow until dissipated. If there were no diode the circuit would self destruct due to the voltage rising high enough to arc or blow out the transistor. This is called a "catch" diode.

If you've noticed, the Liberace tape has very fast pedaling and the sustain tends to float. This due to the excess current still flowing.

The surge (momentary high duty cycle) lasts a full second or so and so changing this capacitor reduces this to about 1/5 second. Still plenty to actuate the solenoid.

After this time the duty cycle reduces for hold in. To further reduce the current, the pot can be used to decrease hold in to only that necessary.

The procedure for adjusting then will be to set the added pot to max, adjust the surge control for reliable actuation, then reduce the added pot to get minimum necessary for hold in. Do this when the solenoid is hot since its power sags with heat.

I never use the o-ring cup damper. It's not reliable and adds to the friction. A clever approach to adjusting dampers and throw will add to the effectiveness of the sustain. Always use the minimum throw needed and use a good quality grease in the bearing surfaces.

Go to it.

Carl Meyer cmpiano@comcast.net

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Replies:

Subject	Author	Date
Re: Pedal modification	Carl Meyer	12:53:48 02/17/04 Tue