A follow-up to Richard Stibbons' posting on the Telektra expression
regulator.

The original Telektra literature speaks of "vibrating contacts" in
its expression regulator, but it is not really pulse width modulation.
The system operates more like the old car voltage regulators, where
the contacts are essentially variable resistors, continually arcing.
A feedback circuit in both cases varies the current to the magnet that
operates the contacts, keeping the voltage constant.

In the Telektra, the operating point of the feedback circuit is
controlled by resistors called by the expression perforations in the
(brass!) rolls.  There were two such regulators, one for the bass and
one for the treble.  The response time was quite fast, since there was
essentially no mass to move.

The problem with the regulator is that the constant arcing of the
contacts wore them out fairly quickly, even though they were made of
platinum.  A capacitor across the contacts helps this problem -- too
bad the Tel-Electric company didn't do this, as most of the Telektra
consoles seen today have dead contacts.

Bob Billings