The AM Forum

a few questions here.  I have this frequency display: http://www.pongrance.com/ddfc.html

I also have this preamp: http://home.att.net/~jacksonharbor/preamp2.htm

The  input impedance on the frequency display is too low so I added the preamp with FET input ~10meg.   Here is the problem.

I cannot find a really good point to get the HFO.  I have done the following:

1) wrapped wire around the 12at7 oscillator and connected that to the preamp. Signal level is too low

2) Connect the preamp to the cathode of the HFO (pin 3 of the 12at7).  This works, but the pulls the oscillator way too much

3) Connect the preamp to the cathode of the HFO Cathode Follower (pin 8 of the 12at7).  This also pulls the oscillator, but not as much.  Well, as I go up in frequency, it pulls more.

4) Connect the preamp to the grid of the 1st mixer (pin 7 of the 6cs7).  This still pulls the oscillator (wtf?).

Is there any point that I can connect this to that will not pull the oscillator?

Should I maybe connect to the plate?  Also what should the rf voltage level be?  On the cathode it is less than a volt.




Tom - AB3FL

The Pongrance digital dial is Fine Business indeed, especially for the price.   But not the most sensitive, as you noticed.

What's happening is the capacitance in the coax between the oscillator and preamp is loading it down.  Suggest you put a 5 pF or so cap in series with the coax at the point where you're tapping off.  This will make a capacitive divider and will limit the influence of the coax capacitance on the circuit.

When you tried wrapping wire around the osc toob, did you affix the other end to the chassis?  That *usually* works, if the wire is inside the toob shield.  Also try more turns of wire...