Need help with a Heathkit DX-20.

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NP4ZB:
 Hi to all.
I would like to hook up a external VFO
to use it with Heathkit DX-20. I
would like to know which is good VFO to use the DX-20?
and how to connect it to the DX-20?

Thanks.

73's de NP4ZB

WA2ROC:
I don't believe the DX-20 has an accessory power socket (usually an octal socket) on the rear panel.  This being true, you'd need a VFO with it's own power supply or build a supply for something like a Heath VF-1 or HG-10.

The power supply inside the DX-20 is already close to its maximum load, so I would not try to power an external VFO from it.  You'd also have to destroy the chassis or case by running the power wires between the 2 units.

Several others come to mind like some of the Lafayette Radio units which have built-in power supplies.

AB2EZ:
Referring to the schematic:

There is a 220uuF capacitor between pin 1 (cathode) of the 6CL6 oscillator and ground. When using an external VFO, rather than a crystal, you will need to add a .01uF capacitor from pin 1 to ground. You might want to install a switch for this purpose. If you do not plan to use a crystal, you should also remove the 22uuF capacitor between pin 9 (grid) and pin 1 (cathode) in order to reduce the capacitive load on the external VFO. The VFO should be connected between pin 9 of the 6CL6 and ground (capacitively coupled to pin 9 with a .001uF capacitor). You could use a plug inserted into the crystal socket for this purpose.

Assuming that you use about 3ft of shielded cable (with around 20 uuF of capacitance per foot) between the VFO and the DX-20, you will need a VFO that can deliver around 5 volts (peak) into a capacitive load of 93uuF (if you leave the 22uuF capacitor in place). At f= 7MHz, 93uuF corresponds to an impedance of about 244 Ohms.

If you use a 1.5 foot cable, and if you remove the 22uuF capacitor, the impedance will increase to 554 Ohms

Stu

NP4ZB:
 OK that is Great. Thanks for the information guys.
Now I'll look and see if can buy one soon.
I'm trying to build a vintage look to my shack. I love
vintage gear. I have a Viking Valiant that I received
from  a SK estate. I also need to know what will be a good
vintage receiver to use the heathkit Dx-20?

73's de NP4ZB

gerry_w1id:
As someone else stated, the DX-20 power supply is not really capable of supplying power to an external device. The DX-20 manual on page 33 states that a VFO should be powered from another source. Even if you could, regulation is very poor which would undoubtly lead to chirps. So the best thing is to use a VFO which has an external supply built-in or build an external power supply for one that does not. Another very important thing is to connect a jumper across the RF choke in the cathode circuit of the oscillator or it won't work with a VFO. On one of my DX-20's I managed to fit a slide switch near the oscillator tube which can be engaged to short out the RFC for VFO operation and remove it for crystal operation. There are many VFO's that I know will work well with the DX-20. THe VF-1 while drifty and not very mechanically secured is one but I prefer the Eico 722 and the Knight V-44 which both have built-in supplies. Incidentally, the V-44 happens to be the same height as the DX-20. Regardless of which VFO you choose, the DX20 is a very reliable transmitter and so basic, it is very easy to fix should something go wrong. The only thing is I always feel apprenhensive sticking my hand inside the darn thing to change crystal so a VFO relieves my anxiety!

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