Rex,

I'm sorry I have not responded about the calibration board questions.
It also brings up a point about seismometer calibration that I didn't
mention earlier, namely using a bridge circuit with the main coil to
introduce a calibration signal when a calibration coil is not available.
(or has been burned out). This assumes that the main coil constant is known.

Here is brief description of how the circuit is supposed to work; follow 
it on your schematic.  The calibrator is designed to work in a bridge
circuit with the main coil of an L4-C with the damping resistor installed
across it. Hence the bridge balance pot R23 is 5k ohms. The calibration
sequence is triggered by the "beep" of a 24-hour watch at pin 3; leads are
soldered to the beeper and the case inside the watch. Before the availability
of lithium batteries, the Hg watch battery was also trickle-charged by the
calibration board (the dangling diode/resistor to pin 2). The watch ground
is to pin 4. The 24-hour alarm is set for a night time hour; in a network,
different times must be used to avoid loosing all the data if a quake
should occur at a singular calibration time.

The beep is rectified by a diode and amplified by U12 to provide a pulse 
that un-resets the oscillator and the dividers via U5.. 

The oscillator U1 outputs 2khz; divider U2 outputs 200hz, 60hz, and 20hz.
Divider U3 outputs 1hz; the first 4017 U4 outputs 10 seconds, and U5 
sequences the switches of U11, a 4066 quad switch. U7 and U8 form a 
20-step sine generator, so sine frequencies of 10, 3 and 1 hz result 
from the switched inputs of 200, 60, and 20 hz from U2. Amplifier U13 
buffers the switch outputs and filters the sine wave; Amplifiers U9 and 
U10 buffer and invert the signal to drive the calibration bridge.
THe sequence is ended by U5 reasserting its pin 6.

After the trigger or the test switch is activated, the sequence switch first
applies a DC step to the calibration bridge for ten seconds, then turns it
off. A 100k pot R14 controls the DC amplitude. Then there is a 10 second delay.
The 20-step sine wave generator can be configured to have any two outputs
like 3hz and 1 hz (or other combinations). These are sequentially switched 
in to the calibration bridge for 10 seconds each after the DC step. The sine 
amplitude is controlled by a 100k pot R13. Then C2/C3 remove the DC offset 
and amplifier U13 filters it.. The oscillator frequency can be changed to
200 hz, which divides all the frequencies and times by 10 for use with
a long-period instrument. (C2 thru C9 need to be 10xed).

The bridge balance pot is set by clamping the seis (by laying it on its 
side) and adjusting the amplifier output for a minimum signal. The switch
manually starts the calibrator and holds it in the 3hz or 10hz sine wave
mode for this adjustment. The sine amplitude is easy to set in this mode,
but the 10-second DC step has to be set on the fly.

In standby the current drain is about 50 microamps. I'm not sure what 
the current is when it is activated, but I think it is less than 1 ma..

I used to have fun shopping for a set of 20 or so watches for the trigger
at places like Service Merchandise. The salesperson always wanted to
know what I was going to do with 20 watches, so I simply said that I
was going to bury them in the ground. They double checked my credit card!

I hope this explains everything. I can provide the artwork for the 2.5"
x 5.5" PC board (single sided) if anyone wants it.

Regards, 
Sean-Thomas

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