Heated expo-converters

[Grant Richter]

>    Yep, this was my problem.  I didn't have the 6.9V zener and was using 
> a LM336 5.0V!  Geez, how stupid!  Anyway, threw a pot in there to adjust 
> the voltage to the appropriate level, now I'm seeing some action!
>    The interesting thing I've found, so far, is that the op-amp doesn't 
> cycle.  It initially was in the on state (15V) but then after a minute, 
> the output voltage sank down to around 1V and stays.  I assume this means

> that the circuit has come to a nice equilibrium state.  Thankfully the 
> current limiting resistor isn't so bloody hot now, so maybe I'll try a 
> reduced resistance and see if the op-amp starts cycling more or if it 
> stays at an equilibrium state.

Ummmmm..... Tony's right. I built two versions of this circuit. The first
one used one quarter of a TL074 for the op-amp instead of a 741. I still
used the 0.01 uF cap. That one oscillates (the 200 Hz rounded square wave I
mentioned earlier). The oscillations also feed through to the VCO and I
couldn't isolate it. The second one I built moved the op-amp to the outside
and used the 741 as originally speced. That one comes to a nice DC level
without oscillation. I tried upping the cap on the TL074 to 0.1uF and
slowed the oscillation down but did not eliminate it. I'm thinking the main
difference is the slew rate on the op-amps (0.5V/uSec vs. 13V/uSec).

To summarize what I think I found out:

1. Don't substitute the 741 in the circuit.
2. Use a single chip package, not part of a multiple IC package.
3. If you can, run independent power supply wiring directly to this circuit
from the incoming supply rails.

The ETI article specifies 670mv at 20 degC and dropping 2.2mv per degC
above that. The Moog Source service manual has this to say (paraphrased):

a) (After the chip has completely cooled off) Measure sense transistor VBE
voltage with respect to ground (Vc).
b) Estimate room temperature in degrees C.
c) Subtract room temperature from 55 degC to determine temperature rise
needed to reach 55 degC.
d) Multiply result by 2mV (Transistor VBE decreases at 2mV/degC)
e) Subtract this product from cold reading (Vc) to determine hot reading
(Vh).
f) Adjust TEMP trimmer to obtain VBE reading determined above. Substrate
temp now adjusted to 55 degC.

I know the Source and Prodigy used substrate stabilized LM3046's for expo
converters as did the Serge NTO's. The Moog circuit is built upside down,
use 1/4 of a LM358 and a 47pF cap. Honestly, I swear I can hear a little
"trilling" in those circuits though, I always thought it was the servo-loop
cycling.

Grant Richter
Wiard Synthesizer Company
http://www.wiard.com/