A universal probe interface.
Signal pickup probe for
DFD’s and Digital Dials
Makes almost any installation plug-n-play
To purchase click here or at bottom of page.
The probe has three connections, power, ground and signal
- The probe has two RCA jacks that interface to the DFD or digital dial
- Power and signal.
- Power ( use grey cable supplied with digital dials) is 9 to 18 VDC for a DFD
- or 9 to 18 VDC or 6.3 to 12.6 VAC for a digital dial.
- Use of the power connection is optional. It does not power the probe.
- It is intended to tap power from the radio for the DFD or digital dial.
- Signal (use black cable supplied with digital dials) is the oscillator signal to DFD or digital dial.
It can be built in one of three versions
The emitter follower interface
- has no gain but a high input impedance and low input capacitance.
- It is used to connect to the tuning gang of any radio, as shown below.
- The emitter follower version is also available without the power connection.
- The intent of this interface is minimum loading on the radio and isolation of the coax cable leading to the digital dial.
- The tuning gang interface works with most older vacuum tube radios.
- The tuning gang is the large air variable capacitors used for bandset and/or bandspread tuning.
- The connection is made to the section of the tuning gang that controls the local oscillator.
This is the tuning gang of my SX-100
The GIMMICK makes a very small (<1pF) capacitor so detuning of the local oscillator is trivial and the emitter follower isolates the coax cable..
- The tuning gang fixed plates (stator) is usually the signal and the moving plates (rotor) is ground.
- It takes about 15V p-p to use this interface up to 32MHz, fairly easy for a vacuum tube radio.
- The emitter follower version can be used to make direct connection to solid state radios.
- Replace the gimmick with a small capacitor for increased sensitivity.
| capacitor | signal@32MHz |
| 2.2pF | 15 Vp-p |
| 4.7pF | 7 Vp-p |
| 10pF | 3.5 Vp-p |
| 15pF | 2 Vp-p |
| 20pF | 1.8 Vp-p |
| 24pF | 1.5 Vp-p |
| 30pF | 1.25V p-p |
The tube shield interface
- works with most vacuum tube radios.
- The signal connection is a tube shield which you simply slide over the oscillator tube.
The tube shield should not touch metal, only the glass of the tube.
- The tube shield attaches to the signal point on the probe. It has a ground connection alligator clip.
- Simply slide the tube shield over the oscillator tube and connect the ground to a nearby chassis ground.
The amplifier interface
- has a gain of 4 but has a lower input impedance.
- It is used for the tube shield interface and for direct connection to solid state oscillators
- or the cathode of vacuum tube oscillators
It is not available without the power connection but you don’t have to use it.
When using the probe with a DFD or with a digital dial
- Solder a 100 ohm ¼ W resistor from the install the jumper on the power module board
- input to +5V as shown above. (there are one to three jumpers, one for each input).
- Sold only with a
- DFD1,2 or 3
- or
- existing customer
- Universal interface for
- with DFD1, 2 or 3
- $5.00.
not applicable to DFD4
Additional information is available at