Dave Richards AA7EE

August 11, 2011

VFO Voltage Regulation For the Norcal 2N2/40

This is not a project by any stretch of the imagination, just a minor mod.  The original plan was to post it to the Yahoo Group for the Norcal 2N2 but Yahoo re-sizes my pictures down and the detail won’t be visible afterwards, so I’m posting here.

Jim K8IQY’s 2N2 series of transceivers are brilliant little rigs. Designed originally for a contest sponsored by Norcal which specified that the design contain no more than 22 x 2N2222 transistors,  the first 2N2 was for 40M, though designs for 30, 20 and 15 followed, to the delight of many Manhattan builders. Then Norcal brought out a kit version of the rig for 40, 30 and 20 and the 2N2 odyssey continued, now in it’s 14th year.

No active devices other than 22 x 2N2222 transistors were allowed in the original design, and although subsequent designs after the contest was over (Jim won first place, by the way) did include different transistors to improve upon the original performance, there was one thing still that I wanted to change. That was the fact that VFO voltage regulation in the transceiver is by zener diode only.  When using the radio with a regulated power supply, this poses no problem, but trying to run the 2N2 from batteries causes the transmitter to chirp.  I have about 44AH of sealed lead acid batteries here in the shack with a charger permanently connected from which I run my other rigs. Anytime there is a power-out, it’s wonderful – all the lights go out, but my little rigs just keep on truckin’! I love the idea that the grid could fail, but I’d be able do a little CW on 40M for at least a few days :-)

In the Yahoo Group for the Norcal 2N2, Bob WB2SRF told us how he used an LM431 voltage regulator, along with a couple of resistors and a tantalum capacitor as a direct replacement for D9, the zener diode that regulates the tuning voltage to the main VFO varactor.  I was in. The messages describing the mod are here and here. You’ll need to join the group to view them.  The schematic for the mod is in the photos section of the group under the title “Norcal Voltage Reference”.

The capacitor was mounted underneath the board. It’s only a 16V cap. I’d have preferred a larger working voltage, but there is limited space under the board (Edit: Bob reminded me that a 16V cap is perfectly OK, as it is across the output of the regulator – don’t know why I didn’t think of that!) The leads from the cap are soldered to the holes that used to anchor D9 and protrude through to the other side by about a centimeter so that the rest of the assembly consisting of the LM431 IC and the 2 resistors can be soldered to them:

I used 1/8 watt resistors for the above-the-board part of the assembly, to save space. Here it is with one of the resistors attached to the LM431 device. When soldering things, I use any tactic I can think of to make the wires stay exactly where I want them until the solder has cooled. In this case, I stuck the small assembly into a packing peanut and weighted it down with a pair of long-nose pliers, so that it would stay in place while I was soldering it together.  Note that in this picture the 1K 1/8 watt resistor has been soldered into place, but not the 1.5K resistor:

One thing to note if you’re a Norcal 2N2 owner and are thinking of performing this mod, is that on Bob’s schematic in the pictures section of the Yahoo Group, the pin-out numbers for the LM431 may not correspond to the device you have; they didn’t for mine.  I just made sure I knew which lead was the anode, cathode and reference, and figured out the wiring from that. Here’s the final assembly installed in place of D9 on the board. These boards get a bit busy, so I’ve circled it in red:

The mod works well. I can now power my 2N2/40 from a 12V battery with no chirp on transmit at all. I did have to re-adjust the drive to the final as it was putting out less power on the batteries than when powered from the PSU. I re-adjusted for the QRP gallon (5 watts) and all is well.

There is only one more very small thing I’d now like to change for my 2N2/40 to be the perfect one band CW rig, and that is to install a switch for the RIT and replace the center-detent pot with a pot that has no center-detent. If I need to apply a small amount of RIT the pot won’t let me, as it keeps wanting to click back into the center-detent position. Being a little picky, this gets my goat, as I like to hear the station I’m listening to at exactly 500Hz. This one mod, if I get around to it, will turn a very-close-to-perfect QRP rig into an absolutely perfect QRP CW rig – from my point of view at least :-)

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2 Comments »

  1. Dave, Very Well written and great pictures showing the mod. I have performed this mod on all of the three band versions with great improvement, thank you again for taking the time to properly document this mod
    73, Bob Johansen WB2SRF

    Comment by Bob Johansen — January 16, 2013 @ 1:31 pm | Reply

    • - and thanks to you Bob for initially telling us about this mod in the Norcal 2N2 Yahoo Group. I made 2 other changes to my 2N2/40 -

      a) the substitution of Q20 for a 2N3904 and the changing of R75 to 2.2K as recommended by K8IQY and

      b) I added a 27K resistor in series with the red wire from the tuning pot to the board. This way, I was able to achieve a tuning range of 7000 – 7055KHz – perfect for my needs.

      Comment by AA7EE — January 16, 2013 @ 3:01 pm | Reply


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