In response to Mr. Pete Knobloch's inquiry in yesterday's MMD, we
have copies of the original Pianocorder System schematics and circuit
description for sale here.

Mr. Knobloch is right on the money when he says that troubleshooting
the Pianocorder System is a dicey proposition.  It is incredibly easy
to fry the entire system with just one slip of the probe.

We build new power supplies for the Pianocorder System here.  Without
any load on any of the three voltages produced, one should read +5,
+12 and +170 VDC.  There is a pot on the top of the power supply for
adjusting the +12 volt section.  There are no adjustments for the +5
VDC and the +170 VDC portions.

It's well known that the 170 VDC return is carried on the 5 VDC return,
and must not be forgotten when one is troubleshooting these systems.
I can remember Bill Zabel, the former owner of Z-Tronics, Inc., which
invented and produced the "Piano-Encoder" which Mr. Knobloch wants to
use, suggested using a "floating 'scope" to troubleshoot the
Pianocorder System's electronics.  This involves plugging one's
oscilloscope into an ungrounded 110 Volt AC line.  However, whether
floating or not, misuse of any electronic diagnostic instrument can
instantly destroy the System.  And, of course, there is a major,
potentially lethal shock hazard with the higher voltages involved

As a matter of expediency, whenever I went on a service call where more
than a malfunctioning tape deck was involved, I just substituted known
good components for suspect ones.  Once the bad component was replaced,
the System sprang back to life and the customer was delighted

Each of the two different types of driver board involved can be quickly
checked for logic integrity with any good digital volt meter.  For
32-note driver boards, a DC resistance of around 4.5 megohm across the
chips' +5 VDC and ground supply says that the board is OK.  For 16-note
driver boards, 2.8 megohm is about right

One bad IC in either type of driver board will markedly reduce these
readings.  Where all of the ICs have been fried, readings of 29 ohms
are found.  The latest style of end driver PC boards now has IC sockets
to make chip replacement very easy

The later series of driver boards use TIP 47 transistors for a greater
margin of safety.

Regards,
Bob Baker