Geoff -
The four pins are:
1. Chassis ground
2. Switch, grounded to turn Calibrator on via S7
3. +12 VDC Supply
4. Antenna via S3
The Antenna output is coupled on the Calibrator via a 3.9 pF capacitor.
You may need to experiment with this to get good output without
overloading on the low bands depending upon the signal generated by your
calibrator. You can make a small cap like this by just twisting two
pieces of INSULATED wire together for two or three twists. One wire to
Pin 4, the other to your calibrator Output.
You would probably want to isolate the DC return for your calibrator,
using Pin 2 to ground it when the calibrator is on. Pin 1 would NOT be
used. The calibrator should be turned OFF when not needed.
73, Garey - K4OAH
Glen Allen, VA
Drake 2-B, 4-B, C-Line& TR-4/C Service Supplement CDs
<www.k4oah.com>
geoffrey mendelson wrote:
I recently got an SPR4 and while as a receiver I am very happy with
it, I always have been disapointed with the dial calibration.
The crystal calibrator seems to be way off and wanders all over the
place.
I looked at the radio itself and found that there is no crystal
calibrator in the radio (it was an option), what I thought was the
calibrator was random carriers and noise. (oops).
I just purchased from eBay a simple crystal calibrator. I'll have it
in about 2 weeks.
I looked at the schematic of the radio and there are 4 wires that come
from the original calibrator. One is ground, one is the signal to the
antenna section of the radio. The other two I really don't understand.
One goes to the on off switch for the calibrator and the other I'm not
sure about.
The calibrator I bought has 3 connections, ground, +9 to 12 volts DC,
and a signal out. Looking at the picture in the eBay ad, I probably
have to add a small capacitor from the output to the radio, so it does
not load down the input. Ground is obvious.
What do I do with the +DC? The schematic for the radio does not show
what actually is switched by on off switch. My guess is that the other
wire I'm not sure about provides the DC input, and the on/off switch
either grounds or "floats" a buffer circuit on the calibrator board.
This makes a lot of sense to me, as it would keep the calibrator on
all the time, but only injecting a signal when needed.
Of course, I could be completely wrong and the on/off switch turns on
and off the calibrator and I have no clue about what the fourth wire
does.
Any advice would be appreciated.
Thanks in advance, Geoff.
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