A Special Thanks To WU3WJM

A special thanks to Om Rahul VU3WJM for making excellent modifications to the actual Bitx by Om Farhan and making the PCB for Bitx Version 3 PCB's for Main Bitx Board and linear.

Hope the details of linear pics posted will help all making the consruction of Bitx 3 version easy now.

Special thanks to Om Leonard also for his worthy contribution to the Bitx Version 3 by VU3 WJM Om Rahul

All the best

Update Linear Cum Power Supply Board Version 3 Bitx [By Om Leonard]

Linear Amplifier section

The version 3 linear amplifier board has 2 very nice additions. The first, the RF signal is switched between rx and tx by relays. This cures a problem I found with the Far Circuits board that I cured by adding relays. The other main addition is an AC power supply. It is setup to accept an AC input and has a rectifier and voltage regulator on board. This allows 13 volts to be fed to the bitx board for it's supply voltages and a higher voltage unregulated to be used on the IRF510. Above is the original board setup.

I made a few modifications to the board. The board looks like this. Power supply on the right and the IRF510 lower left on the big heat sink.

I wanted to use a 22 vac transformer that I had so I needed to use a 35 volt capacitor. I had a 6800 mfd at 35 volts in the junk box so I used it. I had to move the 2 - .1 ufd capacitors to get room for the big filter cap so I put them on the back of the board using some chip caps I had. The spacing worked out just right. Just to the right of them is the original pads for the 2 electrolytic caps.

I used 3 amp silicon diodes so I had to drill out the holes a little and then stand them up vertically. The 6800 ufd cap was hot melt glued to the board after soldering the wires for it into place.

After hooking up the power transformer,, I measured 30 vdc for the IRF510 and 13.6 volts for the bitx. The next step will be to test it after I complete the bitx. I'll be interested in the power out with the higher voltage.

Version 3 Main Menu

Updates Bitx Versio 3 By Om Leonard [Second IF Section]

2nd IF Section

Before we start installing components, now is a good time to install all of the wire jumpers on the top of the board. It will make testing easier as we will connect to the rx or tx line plus the 12 volt line to power the appropriate circuit during test.

This section is almost identical to the the RF Amplifier section so it shouldn't present any difficulties.

I found a board problem on mine. On the left picture, notice there is no right hand hole for the 100 ohm resistor in the lower left corner. It's easily fixed simply by soldering the right lead of the resistor to the wire adjacent to where the hole should go.

Your board should now look like this.

Lets test it!

Testing the receive side is a problem as there are many frequencies present after the mixer and when looking at them with a scope, it is confusing. Lets look at the receive side using a spectrum analyzer.

Here's some of the frequencies at the input to the crystal filter. It's a good thing that most of these will go away because they won't go through the filter. Part of the reason there are so many frequencies here is because we are overdriving the antenna input circuits with 10 millivolts input. Lets look at what happens when we reduce the input to the mixer. This is normally done by decreasing the RF gain, either with a manual control or agc on the rf amplifier.

This is a reduction of 10db of input signal. That made a big difference. The VFO harmonics are still there but a lot of the mixer products are drastically reduced. Remember, each vertical division is 10db and a reduction to 1/10th of the power. Lets reduce the input another 10db.

That almost eliminated all of the excess mixer products. This is a 20db reduction from the first picture. 20db is 1/100th of the original power or 1/10th of the original voltage so we now have 1mv input at the antenna connector. Under normal operation, that's still a big signal. Our receiver should see signals down to around .3 micro volts.

Version 3 Main Menu

Bitx Version 3 Updates By Om Leonard [First Mixer Section]

First Mixer section

This section is a little harder to understand. I've included a picture to help show the diodes placement. The colors below are the wire color connections if you use red, brown, and green wire. If you use different colors, substitute them for mine.

We need to wind the trifilar transformers. Use the small binocular cores and wind 13 turns of trifilar wire onto the cores. 1 turn is a trip through both holes ending up where you started. We need 2 transformers. I had red, green, and brown #28 wire. The cores aren't big enough to use larger wire. The original drawing called for #32 but the size doesn't make much difference as long as it will fit into the core. The different colors make keeping the windings straight easier. When pulling the wire through the holes in the cores, be careful not to scrape the insulation off. The binocular cores can have sharp edges. When you are done, it should look like this.

Now you can place them on the board as shown above. The final placement should look like this.

Notice the direction of the bands on the diodes. Make sure the leads don't short together.