NorCal 40a/Sierra mod (long)


From: Phil, AC6LS (
Date: Fri Nov 29 1996 - 14:22:21 EST

Well I just got the ok to post the following mod from Ron, KU7Y.

(Ron's article from QRPp(Jun 95) between the arrows)
NorCal 40a Variable Bandwidth Xtal Filter Mod.

The NorCal 40a is a very nice radio. One of the things I like about it
is that you can do so much with it. I have added an AF volume control
(which Wayne keeps saying I don't need!), and a keyer. The keyer is
based on the Curtis 8044ABC chip. So far, so good.

Good radio and I could have left it just like it was. But what the
heck, why not look for things to keep making it better?

Wayne Burdick E-Mailed this idea to me awhile back and yesterday I got
around to trying it. What is it? How about making the crystal filter
a variable bandwidth filter? Take four resistors, three diodes, one
capacator and mix with a little solder, wire and a few minutes of your
time and presto, you have a nice working variable bandwidth filter in
your 40A. I haven't measured teh response of the modified filter but
judging by the seat of my pants I would say it goes from over 1 Khz to
about 100 Hz. In the wide position you can hear both sides of a signal.
In the narrow position all you hear is the station you are listing to.
Wayne is using a similar system in his Sierra with good luck.

Parts needed: 3 - MVAM108 Varactor diodes. (I got mine from Newark
               3 - 100k 1/8w Resistors
               1 - 10k Linear Taper Pot
               1 - 0.1 uF capacitor

Now comes the fun part. You will need to drill a hole in either the
front for rear panel. I chose the front because there isn't any more
room on the back of mine.

1. First remove C10, C11 and C12. These will not be reused. Use care
    in doing the removal. This is a high quality board but with enough
    abuse it could be damaged! If you have a hard time getting things
    off boards, cut the leads and then remove the lead. Clean the holes
    with a solder sucker and or solder wick.

2. Install one end of each of the three 100k resistors where the
    removed capacitors connected to the crystals. Connect the free end
    of these resistors to the wiper of the 10k pot.

3. Connect one end of the 10k pot to ground. I used a piece of wire
    from the pot to the ground hole of C12.

4. Connect the 0.1 uF capacitor between the wiper and the ground end
    of the 10k pot.

5. Connect the other end of the 10k pot to the regulated 8vdc.
    I connected a wire from the pot to R20. Use the end of R20 that is
    facing away from the front panel. DO NOT connect to the R17 side.

6. I chose to mount the diodes on the bottom. Mount them where the
    original capacitors were located. Connect the cathode to the
    crystals and the anode to ground. Use the picture in the 40a manual
    to orient the diodes correctly.

Do the normal checking for solder bridges etc. Then fire it up. I took
the time to realign the whole rig. Then I started playing with it.
WOW, what a rig. I leave the filter wide while tuning and calling CQ.
Then, while in QSO, I narrow it down until it seems right! The more
QRM the narrower I go!

Enjoy the rig and give the thanks to Wayne, both for the rig and the
mod. Atta boy, Wayne!

72's, Ron, KU7Y

P.S. There are 2 filters in the Sierra. To make shure it is wide enough
     to listen to ssb you need to add one of the diodes to the second
     filter also. Wayne has all the details. Just e-mail him for the
Hold on before "flooding" Wayne for info...
having done the mod to both radios, let me add these comments...

the mods written up for the NorCal versions, should work on the
Wilderness versions also. Should also mention that the NW8020
shows the same mod as an option.

40A- I got the 8vdc from U5 (near D7) and the ground from the hole
      between C48 and J3 and mounted the pot on the back panel.
      (no room on my rig, I have Jim, W6QIF's freq display installed)

Sierra- For the 8vdc, I used the top "lead" on S1 (rit) (front pnl mnt)
         The capacitors are C7, C8 and C9

         The "extra" crystal filter is X5, just after the IF amp (U5)

This is very easy and works fine. The last thing I need to do, is
find a knob for the pot.

Have fun
72 de Phil AC6LS

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