[sdiy] CA3046 Heater / Substrate Question

[Nils Pipenbrinck]

Hi folks!

I'm working on my expo converter again. Did some traces of the Moog 
Prodigy Heater circuit. It works as it should, but long story short I 
want to heat with two transistors instead of just one.

Here is what I came up with, and I can't find out why it doesn't work.

   http://torus.untergrund.net/synth/heater.png

My part is in yellow.

So the circuit around Q1 and Q2 is directly from the Prodigy. R3 has 
been raised because I'm using dual 15V.

In my part I generate a small negative voltage using the LED and R5. 
This puts the base of Q3 at -1.8V and the emitter at roughly -2.4V. 
That's the most negative point in my circuit.

R6 allows some bias current to flow at any time and R4 limits the total 
power to about 280mW.

That should be fine, right?


The problem:
============

If I apply power the measurement I get from my temperature BE junction 
(not shown) directly jumps to about 500mV and the heat cycle never 
starts. I have doubts that this measurement is correct. Since I've 
messed around with the substrate connection I think this could be the 
problem, but I have no idea how to debug this.

If I just remove R4 everything starts to works as expected, albeit with 
just one transistor heating.

So, do I have to maintain some minimum impedance at the substrate pin? 
Do you have another idea what could be the problem?


Best,
   Nils