[R-1051] Frequency Standard NTC Thermistor Retrofit CORRECTION

[David Wise]

In my original post I mistakenly said that Q7 is the 2N2907.  No, it's the 2N697, and I've corrected it below.

DW

________________________________
From: David Wise
Sent: Tuesday, August 07, 2012 10:22 AM
To: 'r-1051 at mailman.qth.net'
Subject: Frequency Standard NTC Thermistor Retrofit

I'm not done with this, but I wanted to let you know that it IS possible to modify
the R-1051 series Frequency Standard to use an NTC thermistor.

In fact, it's cheap and easy.

There's always somebody on eBay selling 10K leaded NTC thermistors for just a few dollars.
The one I got works fine.  You do have to modify circuit board A2A5A1, but it's easy and
reversible.  While working, refer to the A1 schematic, Figure 5-6 in NAVELEX 0967-LP-970-9010.

1. Connect a 3" to 6" stranded cable to the thermistor.  The cable and whatever
    insulation/strain relief you put on the joints must take full oven temperature.
    Try to arrange for the cable to be away from the crystal and the oscillator when
    A1 is reinstalled in the oven.  This will make sense after you have read through to
    the final assembly step.

2. At the other end of the cable, put a connector.  I used a 2-contact socket that mates
    with the popular 0.1"-spaced square pins.

3. Arrange the thermistor in the bottom of the oven so it's lying flat and out of the way but
    so the connector can reach within 1" of the upper left corner when viewing board A1
    on the component side, and seal it in place with JB Weld.  You may have to jig the
    cable to make the thermistor stay put.

4. Cut a piece of double-sided plated-through padboard, 1.0" long and no more than 0.3" wide.

5. On the padboard, put two resistors, 10K* (R902) and 9.1K (R913), and a connector
    to mate with the thermistor connector.  (As I said, I used 0.1"-spaced square pins.)
   The resistor leads at the top end of the board must not protrude through the board.
   Solder the connector pins to the bottom ends of the resistors.

    * Note: R902 and R913 may have to be selected, depending on component tolerances.
    The series combination of R913 and the new thermistor (which I call R813) replaces
    R13 (4.7K), and R902 replaces old thermistor R2.  R913 affects the transient response,
    and I'm still determining the optimum value.  Once R913 is fixed, R902 puts the
    temperature setpoint within trimpot R15's adjustment range.

6. Prepare the top surface of R15, and glue the padboard to it.
    I used cyanoacrylate.  JB Weld would work too.

7. Remove R13 (4.7K) and P2 pin 3 (which goes to old thermistor R2).

8. Run a wire from the top end of R902 to ground.  Ground is available at an
    unused pad next to the top pin of R15.

9. Run a wire from the bottom end of R902 to the left pad where R13 used to be.
    This pad also goes to where P2 pin 3 used to be.

10. Run a wire from the top end of R913 to the right pad where R13 used to be.
     This pad is +10V.

11. You're done.  Slide A1 partway into the oven, connect the thermistor cable,
     and slide A1 in the rest of the way.

As I said, there are still issues with transient response.  General Dynamics selected
R23 to control this.  In older A1's where Q7's (2N697) collector load resistor R24 is
2.2K returned to +28V, the nominal value was 1.2M .  Newer A1's with 1.2K returned
to regulated +15V have a nominal value of 820K.  Larger values tighten control but
increase ringing, while smaller values reduce ringing but degrade control.  R913 has
a similar effect: smaller values (making R813 a larger part of the total) give more
aggressive control but cause ringing or even sustained oscillation, while larger
values increase damping at the expense of accuracy.  With 6.8K (and 8.2K for
R902), the value I started with, the ringing takes over five cycles to damp out.
10K (and 11K for R902) settles completely after two cycles.
I'll try to make fiddling with R23 unnecessary.

Dave Wise