Optimist-80 update

VFO modification and more


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Some builders have reported trouble when trying to get the correct VFO coverage. A tuning range of 3600-3800 kHz is strongly recommended. Do not try to include the CW segment, 3500-3600 kHz, because the tuning linearity will suffer and the rig has no CW transmitting capabilities anyway.

I have been experimenting with two Optimist-80 transceivers. Unfortunately, all components have tolerances but I got almost identical results on both rigs. I found that the screen around the VFO tank IS necessary! Without the screen there was more receiver noise and also some VFO pulling when tuning the preselector through resonance.

My screens measure 19x28 mm (inside) and they are 20 mm high. They are made of 0.8mm PCB material. The inside corners of the box are carefully soldered along the entire height. A simpler and better alternative would be to make the box by folding a 20 mm wide strip of thin brass and solder just one corner, where the ends meet. Slip the completed box over the VFO tank components and solder it to the ground plane on as many spots you can reach without burning the components. The brass alternative is easily soldered on the outside. Leave the top open. The VFO frequency increased by about 23 kHz when I installed the screening box.

Component values for my own Optimist-80 VFO:

D2 = D3 = BB809

C11 = 330 pF
C12 = 470 pF
C13 = 330 pF
C14 = 470 pF
C15 = 470 pF
C16 = 18 pF

R14 = 12 k
R15 = 7.5 k
R16 = 21 k (22 k // 470 k)

RT2 = RT3 = 10 k, 15-turn, and RV2 = 20 k,10-turn panel pot, as before.

L2 = Amidon/Micrometals T50-2 toroid with 34 turns 0.4 mm wire covering 270 degrees (3/4) of the core.

VFO Comments:

With the components listed above, my Optimist-80 tunes 3600-3800 kHz with a tuning voltage from 1.49 to 4.85 volts across R16. The tuning was linearized by careful selection of R16. At the ends of the 10-turn pot RV2, the VFO is at 3600 and 3800 kHz respectively. 3700 kHz is exactly at 5 turns. The linearity is quite acceptable. Maximum linearity error is +3.4 kHz at 2 turns and -3.1 kHz at 7.5 turns.

The tuning voltage should be kept above 1 volt, preferably more, to have some margin for the oscillator amplitude. Use the component values listed above as a starting point. Set the tuning voltage at 1.5 volts. If mr Murphy is asleep, you will hit 3600 kHz right on the spot. If mr Murphy is with you, you may end up on another frequency. If you need to change the number of turns on L2, remember that a single turn corresponds to roughly 100 kHz in this case. Therefore, you may need to change the capacitance as well. Do not increase the capacitance too much. The varicap delta-C may then be too small to cover the desired portion of the band. If you want more than 200 kHz span, consider adding another varicap diode in parallel with D2 and D3 to increase the relative delta-C.

There is no technical reason why I made the winding cover only 3/4 of the toroid. It just had the right inductance so I glued it in place. It would be better to make the winding cover the entire toroid. That would improve the self screening properties of the toroidal coil because of symmetry. However, the screening box mentioned above does a good job and I have no problems with radiation from the local oscillator. Don't forget to put spacers between the PCB and the inductors. 1.5 to 2 mm is okay.

The VFO must have a good temperature stability even if the rig is operated at a constant ambient temperature. The class A amplifier generates quite a lot of heat during transmission periods. My rig drifts about 50 Hz down during a long transmission. The T50-2 toroid is specified to have a temperature coefficient of 95 ppm/C. It could be worth a try to replace it with a T50-6 toroid. The specification for -6 material is 35 ppm/C. Temperature compensation can be made by replacing some of the capacitance with ceramic units having a suitable negative temperature coefficient. Almost all inductors has a positive temperature coefficient. The varicap diodes, the tuning potentiometer and the voltage regulator all have some temperature drift. The voltage regulator, IC5, may be a source of temperature problems because the tuning voltage is derived from its output.


Output filter comments:

These following capacitor values gives good performance in my own Optimist-80. The inductors L3 and L4 are the same as before, 21 turns of 0.6 mm wire covering a T50-2 entirely.

C56 = 470 pF
C57 = 470 pF
C58 = 1500 pF
C59 = 220 pF
C60 = 820 pF
C61 = 56 pF


Overload indicator comments:

The sensitivity of the overload indicator is dependent on R30. Selecting a value for R30 is a matter of taste. I am completely satisfied with 82 k myself.


I am not chasing DX with my Optimist-80 but the distance record so far is G-land, about 1000 km from SM-land. My antenna is not very spectacular, a simple dipole about 3 (yes, THREE) meters above GND.

Good luck, happy QRPing and 72 de Johan, SM6LKM


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