NorCal 20 Ten Turn Pot Modification Information
This page describes installing a nonstandard (not the recommended 10K value) ten turn main tuning pot in the NorCal 20. This was originally posted to QRP-L April 23, 2000.
Subject: NC20 Ten Turn Pot Answer
Date sent: Sun, 23 Apr 2000 10:05:05 -0500
Last week I asked a question to the list regarding the use of a ten turn 5K pot for the main tuning control in the Norcal 20 rig. I wish to thank all the hams who responded to my question with ideas and suggested solutions.
After a bit of experimentation, I have determined that it is possible to successfully use other values of pots instead of the recommended 10K value. Perhaps others who do not have 10K pots readily available could use the information.
The NC20 tuning circuit uses a linear 10K tuning pot with a 8.2K resistor (R1) above the pot and a 3.3K (R100) below the pot. With a source voltage of 8 volts, the resultant voltage range across the tuning pot is 1.25 to 5 volts which is applied to U1 Op-Amp through a 100K resistor.
If the tuning pot value is changed from 10K to another value, both R1 and R100 values need to be changed to keep the tuning voltage range at 1.25 to 5 volts. In the case of a 5K tuning pot, the values of both R1 and R100 are divided in half. I simply paralleled another 8.2K resistor across R1 and a 3.3K across R100 to achieve exactly the same 1.25 to 5 volt tuning voltage. The 20 meter VFO frequency tuning range was unchanged from the pre-modification range. Other values of tuning pots may be used by adjusting the R1 and R100 values using Ohm's law calculations to keep the tuning voltage within range. Be sure your new tuning pot is a linear taper control.
The amount of VFO tuning frequency range is controlled by the gain of U1 Op-Amp and can be adjusted by the value of R9. The higher the resistance of R9, the greater the VFO tuning range. With R9 at 230K, I had a VFO tuning range of 89 KHz. With R9 at 163K, the VFO range was 67 KHz and 154K gives about 62 KHz. It is possible to customize the VFO tuning range for operator preference. Since I don't work CW above about 14.065, I selected 163K (230K and 560K in parallel) which provided smooth tuning from 14.001 to 14.068 MHz.
Increasing U1's gain also proportionally increases the RIT range. If you find that you have too much RIT, look at increasing the value of R6 and R7. I used 47K for both resistors and also paralleled a 2K resistor in parallel with VR2 to reduce the RIT to about 1 KHz for personal taste.
The NorCal 20 is a wonderful little rig. I wish to thank Dave Fifield for the design and NorCal for producing this rig. I have enjoyed working with it and customizing it. I hope others can use the info in this post.
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