Vintage Synth Repair

In a message dated 9/20/05 2:48:14 PM, VallebonaPaolo 
<vallebona.paolo@... writes:

<<  Hello,


I am trying to design a homemade clone of an ARP Odyssey,

but I already had some difficulties in finding components.

Anyone has suggestions about where to find these please ?

- tempco resistors, wire wound 3% 1.87 KOhm, 3400ppm

- slider potentiometer 1Megaohm, 60mm

- 1MF3958 dual J-FET transistor

- 2N5459 FET, ORN: what does "ORN" mean here ? Orange ?

- 2N5459 FET, YEL: what does "YEL" mean here ? Yellow ?

- a good two-buses 3octave keyboard

and then it is not quite clear to me how to make the "pair"

in the "matched pairs" of transistors for the exponential

converters driving the VCOs: which parameters should match,

and how can I measure them ? >>

Paolo,

First of all, you need to join the Synth-DIY mailing list.  You will find a 
wealth of information and knowledge there.  I can give you a few tips regarding 
your project:

1.  Those are very unusual tempco resistors.  It's not easy to locate 1.87K 
tempcos -- it's much easier to find 1K and 2K tempcos.  However, one of the 
SDIY list members has recently had some made, and is selling them to list 
members.  I think the price is approximately $2.00 US  His name is CJ and his email 
is  c0r3dump23@...

2.  I don't know where you can find the 1Meg slider pots.  Can you use rotary 
pots instead?  They will last longer than sliders!

3.  Dual JFET's are very hard to find these days.  You might try Linear 
Systems of Fremont, California (http://www.linearsystems.com).  They have a few 
dual JFET's, but they may be a little hard to get.  The part numbers are:

2N5905  Low Leakage, Low Drift, Monolithic Dual, N-Channel JFET
2N5906  Low Leakage, Low Drift, Monolithic Dual, N-Channel JFET
2N5907  Low Leakage, Low Drift, Monolithic Dual, N-Channel JFET
2N5908  Low Leakage, Low Drift, Monolithic Dual, N-Channel JFET
2N5909  Low Leakage, Low Drift, Monolithic Dual, N-Channel JFET
2N5911  Wideband, High Gain, Monolithic Dual, N- Channel JFET
2N5912  Wideband, High Gain, Monolithic Dual, N- Channel JFET   
2N5912C  Wideband, High Gain, Monolithic Dual, N- Channel JFET

You may be able to download the data sheet for these JFET's here:
http://www.linearsystems.com/downloads.html#JFETsDataSheets

They also have a cross-reference document that may show the part number 
equivalent for the original ARP part.

Personally, I would not try to clone the keyboard sample and hold circuit in 
the Odyssey.  I don't think it's a very good design.  I would instead use a 
modern circuit that has better performance, such as a LF398 Sample and Hold IC.

4.   The 2N5459's are available at various places, and there are also 
equivalent FET's that you can use.  The colors (Yellow and Orange) indicate some sort 
of parametric selection criteria.  You may have to get a few FET's and try 
them until you find one that works best.

5.  3-octave 2-bus keyboards?  Get in your time machine and go back to about 
1978, then buy one from Paia in Oklahoma City.  Unfortunately, those keyboards 
are no longer available.  You may consider getting an old home organ and 
removing its keyboards for your project.  Anything from the 1960's to the early 
1980's should have a usable keyboard.  It is also possible to use a one-bus 
keyboard, but you would need to make some additional circuitry to electronically 
derive the gate/trigger signal from the same bus that carries the CV current 
(make a clone of the Moog Micromoog keyboard circuit)

6.  I don't know how to do the matching in the ARP VCO's.  Join the SDIY 
list!  Someone there will know how to do it.

Michael Bacich