ARP 2600 TZ-581 replacement?

[WeAreAs1 at aol.com]

In a message dated 3/7/99 5:06:19 PM, J.B. wrote:

<<Many years ago I had to replace the TZ-581 in my ARP 2600 keyboard (my then
girl friend, a keyboard player, borrowed the ARP and managed to plug the
keyboard in backwards -- the relationship ended shortly after). The TZ-581 was
no longer being made, so I got some NTE "equivalent. Since then, I've never
been able to get the low C to anything above -0.11 volts (like the obvious
0V).

1) Is the TZ-581 a germanium device?
2) In these circumstances, is it common practice to just piggyback another
resistor into the circuit in order to bias up the voltage?>>

I'm assuming you have the older one-voice keyboard, correct.  In which case,
you're talking about Q6 which is the keyboard current source.  

First of all, my old ECG substitution databook (I don't have the NTE book)
shows the TZ581 as a PNP silicon device.  I can't imagine that ARP would have
used Germanium parts for anything at that time!

Second, NTE parts are notorious for being out-of-spec, overpriced, and often,
just plain wrong.  And conveniently, all of the electronics stores have a no-
refunds-on-active-components policy.  Understandable, but SHEESH!  The rule
shouldn't apply to NTE parts.

Third - there must be some way to find replacements for those TZ581's...  I'm
sure counting on it, since I wish to build some cloned ARP modules.  I think I
have seen it listed in some parts supplier's stock lists somewhere - possibly
not in the USA, though.

It is quite possible that a garden-variety PNP such as the 2N3906 ot 3905 will
work just fine, at least in many applications - maybe not in that current
source, though.  It's worth a try, don'y you think?  All you'd have to do is
make sure to get the pinout roight when installing it.  The TZ581 reads BCE
going left-to-right, and the 2N3906 reads EBC, left-to-right.  You'd just have
to bend the leads a bit to get it to match the PC board, maybe use a little
insulating sleeve to be safe.

As far as playing with the bias, there appear to be a few resistors in that
circuit which could influence its range and ability to scale properly.  You
mention that it won't go all the way down to 0 volts for low C.  However, can
you still get the keyboard to scale into an in-tune scale (trimming with the
Scale trimmer)?

I wonder if maybe the S/H tracking opamp on the keyboard board was damaged
athe time of the incident.  Maybe it's putting out some kind of offset.  Did
you check there?  There's certianly not much else in the circuit that could go
wrong!

Mike B.