Original HT-37 VFO Circuit
A couple of things before I start:
First, this is not my fault. The Radio Gods are to blame. I innocently tried to by an HT-37 tuning capacitor on e-bay, but the seller sent me the entire VFO unit. The only thing missing was THE TUBE. Clearly, that was a sign, right?
Second, this is a work in progress. That is why my diagram (below) is a bit ugly. I am looking for your input and advice on how I might do this better. I will understand if religious principles prevent some of you from participating in this endeavor.
I am trying to solid-state this device WITHOUT major surgery, and without adding any reactive components that would change the resonance or tuning range of the original. The original circuit tunes from 5 to 5.5 MHz and that is fine with me.
I started out by just sticking a J-310 FET into pins 1, 2, and 5 of the tube socket. I put 12 V on the drain and the thing oscillated right where it is supposed to. That was a good sign.
Here is what I have done so far:
Bill's initial solid state conversion of HT-37 VFO
Mechanically, my effort has been very simple. At first I tried to fashion a more serious male socket for the FET using two broken 7 pin tubes. This didn't work well.
So then I just ran three short wires up through the center hold of the tube socket to the connections for pins 1,2, and 5. I superglued the J-301 to the chassis and made some non-reactive connections: I put a 47 ohm resistor on the source, and a 220 ohm resistor on the drain. I grounded the drain for RF with a .01 uF cap to ground. I added a 100k resistor and a diode on the gate. Oh yea, I put a couple of ferrite bead on the FET gate lead. (See pictures below.)
Three lead up through the center hole
A rare look inside an HT-37 VFO
The original thermatron circuit has an output bandpass transformer, a 3900 ohm resistor and a coupling cap. I left them in the circuit, but they are not doing anything.
The output from the source of the FET looks pretty good. I can see some VHF on the trace, but I suspect this is from my FM broadcast nemesis at 100.3 FM (one mile away). On a receiver, I can hear some AF noise on the signal, but this may be the result of the RFI from THE BIG 100 -- WASHINGTON'S CLASSIC ROCK.
So what do you folks think? What else could I do, or should I do?
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