> A J Omega Electronics MTP-1 32-note (plenty of room for expansion) has
> been purchased and will be used to drive solenoids to actuate the keys
> and valves.  The problem here is the solenoids draw 0.7A, and as the
> rated output from the MTP is only 0.5A, either amplification or relays
> will be required.  Short of purchasing new solenoids, are there any
> suggestions?

First suggestion: Contact John Wale!  He is very helpful in such
regards.

What you can do:

1. Drive the solenoids with additional (PNP) transistors, as Mike
Knudsen explained.

2. Lower the voltage of your relays and check whether they still pull
up reliably.

3. Usually, solenoids need a smaller current to keep them activated
than the initial current for "starting the pull."  Thus you can do the
following:

In series to the solenoid, add a resistor and a capacitor in parallel.
The resistor should have 2/5 of the resistance of the solenoid.  This
will lower the voltage across the solenoid so that it draws only 0.5A
instead of the rated 0.7A.  The capacitor must be large enough to
"kick-start" the solenoid with its 0.7A current (I'd start out trying
with around 100 uFd).  For a split second this will run 0.7A through
the MTP-3 -- but I would guess that with a nominal maximum current of
0.5A, the short 0.7A spike won't be a problem.

4. Usually in electronic circuits with ICs (as the MTP-3), the maximum
current is not strictly per output, but per IC (as the heat dissipation
must be done over the IC's area and pins).  So if you can guarantee
that only, say, half of the notes on one driver IC play, you might be
allowed to drive 0.7A solenoids with their full current.

Last suggestion: see first one - especially if you try (3) or (4)!

Good luck
Harald M. Mueller
Germany