tirsdag 21. februar 2017

Transistors in the ETI Vocoder

Many of the transistors in the ETI vocoder are either hard to find or very expensive (or both). I am trying to figure out if alternatives are available.

For the internal excitation board, two BC182L (NPN) and two BC212L (PNP) are used.

Q1 is definitely replaceable, probably with a 2N3904. It only acts as a logical inverter together with R20 and R21. It is similar to the one found here:

http://www.cs.unca.edu/~brock/classes/Fall2012/csci255/labs/lab05.html

Not sure if we have to change the resistor values, but probably not.

Q2 is a voltage to current converter. I am not entirely sure how it is meant to work so I cannot say for sure right now what parameters to look for. The same converter circuit can be found in the analysis/synthesis section, so if one is solved, both are solved. I think looking at the exponential converter theory I've written earlier may solve some of it - the way it works is that the base is tied to ground while the emitter is connected to the output of IC8b which probably follows the envelope follower. D5 is similar to what I have in my Xonik VCA. R33 and PR1 probably adds a constant current to trim offness or similar.

Q3 and Q4 control two switches. Again, I believe they are considered binary - the output of the comparator is either high or low and the switches are also controlled by high or low voltages. We should definitely try 2N3904 and 2N2906 here.

Update: I've confirmed that Q2 is not depending on Hfe and may thus be replaced by a difference transistor (see my post on the ETI voltage to current converter). The BC557 has the same pin-out (CBE, PNP) and at least the 557B has a Hfe in the same region.

As for the BC182L, it seems that its pin out (ECB, NPN) is harder to find. However, the BC182 is still available from Farnell and RS Components (though not mouser). If the circuit board is redesigned, using a different transistor will be problem free.

BTW: Here is a nice comparison page for different transistors: http://www.edutek.ltd.uk/Transistors_NPN.html

As for the BF244 N-channel JFET, I have yet to find a replacement. It could possibly be replaced with a J112 (which also has interchangeable source and drain). I would have to look at the exact function of the transistor but I suspect that it is only used as a switch. The difference between J111/112/113 is the gate-source cutoff voltage, the zero-gate voltage drain current and the drain-source on resistance. The cutoff voltage for the BF244 varies widely, more than between the various J variants, as does the zero gate voltage drain between the versions (A,B,C) of the BF244. The vocoder article does not specify a particular version of the BF244 so this may not be very important either.


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