VFO and Amplifiers Testing


To learn about the VFO, read the
Circuit Details - Tesla Oscillator
before building this section.


The VFO board is tested without the Tuning Capacitor installed. This is done so the capacitor does not get banged around or makes Board 1 too heavy to easily complete soldering of the rest of the board.

The Tuning Capacitor will be installed at the completion of Section 3. Only a touch up of the frequencies will be needed after the Tuning Capacitor is installed.

Testing at this point makes it easy to correct mistakes on the VFO board that will be hidden after the Tuning Capacitor is installed.

The VFO is the most important part of the receiver and the most complicated with all its relays and three frequencies, so spending special attention to this section makes further diagnosis of the receiver easier.


Frequency Counter

A Frequency Counter, either your own bench Frequency Counter or the one you purchased with the kit, will be needed to test the VFO. A general coverage receiver with reasonable calibration can also be used by attaching 2' or more of wire to the "VFO OUT" connection within range of the antenna connection to the receiver. Make sure the signal from the VFO can be heard.

If you have purchased the LED Frequency Counter, this would be a good time to build the Frequency Counter kit. Go to Frequency Counter Construction page for instructions.

After the kit is finished, short the "Osc In" to "Gnd" on the board so it will work as a regular counter. Use the "VFO In" and "Gnd" to make the measurements. This setup is shown in Step 12 of the Frequency Counter Construction.

The following procedure will set up the receiver for the first 130 kHz CW portion of the 40 and 20 meter bands of the receiver. The 14 MHz VFO, with a 180 KHz range, will cover all the 30 and 17 meter bands.

See "40 and 20 Meter SSB Setup" if you want to tune the SSB portion of the 40 and 20 meter bands.

To have the 40 and 20 Meter VFO cover the entire band, see the Circuit Details, Tesla Oscillator, "Modifying Bandwidths".

Making Connections

See Making Loops for Connections on the Boards for making easy connections to the board for testing and completing the board.

Place these loops at the 12 Volts and Ground connections and at the VFO OUT and smaller Ground box. Note the picture below:

Be careful with polarity reversal. The 10mfd tantalums may blow up and some traces may be burned going to the 7805. Use diode protection if a mistake might be made in connecting the board to 12 Volts.

Solder a 1 amp diode, 1N4001 or similar, to the 12 volt main connection (band goes to 12 Volt connection) to prevent accidentally hooking up the receiver with reverse polarity. (This diode is not currently supplied with the kit.) The diode will block current from traveling the wrong way to the board.


Testing the LEDs

Connect a wire from the "Stabilizer In" connection to ground, or hook onto the tab of the 7805 as shown in the picture below. Without the ground connection the frequency will not be stable. The frequency should drift slowly in one direction, but not be varying quickly up and down - this is due to 60 Hz energy being picked up by the varicaps when they are not grounded or biased by 5 or so volts.

Connect a frequency counter to the VFO OUT and the small Ground connections. Connect 12 Volts and ground to the 12 Volts (or protection diode) and Ground boxes.

With the shop light shining on the board, all the LEDs should light. The frequency counter should be reading around 11 MHz, plus or minus 1 MHz.

If the LED next to the 10.455 relay is dim (one at the far left), point a shop light to the dark colored Photodiode and the LED should show the same brightness as the other LEDs.

The dark color of the Photodiode filters out visible light so it will not be as sensitive as the Phototransistor between the VFO Amplifiers. An incandescent light will easily have enough IR to turn on the Photodiode. Dim sunlight will not work and neither will LED Flashlights (unless they are InfraRed).

Measure the voltage at the large RF Choke, the text VFO should be right next to it, and check to see the voltage is near or at 8 to 9 Volts on both sides of the choke.

Setting the Frequencies

Cut a 1 1/2" piece of Black Shrink tubing supplied with the kit in Bag 6. This will be used to block light to the Photodiode and Phototransistor when needed to set frequencies.

First, the 14.000 MHz VFO Frequency

Place the black tubing over the Phototransistor located between the VFO amplifiers so both LEDs are turned off at the relays - 10.545 and 10.455. See picture above. Be aware that a strong light shinning on the back of the Phototransistor will also turn it on, so if the tubing doesn't turn off the LEDs cover the back side of the Phototransistor also.

Adjust the ceramic 2/8 trimmer to the frequency shown in the picture (14.468 or so). Adding the tuning capacitor will drop the 14.000 MHz VFO frequency by 400 kHz, so it is set 400 kHz high, When the stabilizer is used, the metal adjustment screw on the 2/8 ceramic trimmer is not grounded. When using a metal screwdriver, calculate how much it changes the frequency when you touch the adjustment screw and adjust accordingly.

Spread apart or squeeze the windings on the VFO toroid to help reach the frequency. Try to get the setting with the 2/8 ceramic trimmer near its middle position. Line up the slot in the adjustment screw with the capacitor pins, with the rotor plate on the end with the arrow. The rotor plate is the part of the top with the metallic color.

If you can't get above 13 MHz, remove one turn from the VFO toroid. If you wound only 18 turns on the toroid you will have to squeeze the windings together a bit. The toroid shown has 19 turns, which is the number recommended when using the stabilizer.

Accuracy is not important, just get close. When the tuning capacitor is installed and the board is in its final mounting position the frequencies will be touched up.

To immolate the stabilizer being connected, touch the "Stabilizer In" wire to the 5 Volt pin on the 5 volt regulator - the one on the left (toward the VFO) next to the 2.2mfd electrolytic capacitor. The frequency should raise 10 to 20 kHz. Place the wire back on the grounded regulator tab, the frequency should drop 10 to 20 kHz.

By the way, accidentally touching the "Stabilizer In" wire to the 12 Volt pin on the 5 Volt regulator will not do any damage.

Second, setting the 10.545 VFO

Move the black tubing from the Phototransistor and place it over the Photodiode near the 10.455 relay. The LED at the 10.545 relay should be on and the LED at the 10.455 relay should be off.

Notice the black tubing over the Photodiode at the far left bottom of the picture.

Set the frequency to 10.808 MHz or so. The tuning cap drops the 10.545 MHz VFO approximately 263 kHz. Again, not critical at all, just close.

Third, setting the 10.455 VFO

Remove the tubing from the Photodiode so both LEDs are on at the VFO. See picture below:

Set the frequency to 10.701 MHz. The tuning capacitor drops the 10.455 MHz VFO approximately 247 kHz. Again, not critical, just get the setting close.

To test the stabilizer varicaps, touch the "Stabilizer In" wire to the 5 Volt pin on the 5 volt regulator - the one on the left (toward the VFO) next to the 2.2mfd electrolytic capacitor. The frequency should raise approximately 130 kHz. Place the wire back on the grounded regulator tab, the frequency should drop back to 10.701 kHz.

When the Stabilizer is connected and is reset, the voltage to the varicaps is 5 Volts. So the same changes that are seen when connecting to the 5 Volt regulator will be seen when the Stabilizer is connected to the receiver. Final setting of the VFO is done after the Stabilizer is connected to the receiver.

Remove the "Stabilizer In" test wire when the frequencies have been set.

If all went well, then continue to the next section.

Diagnosis

No Output from the VFO

The most likely problem is a bad 2N5486. Check solder joints first and look for solder bridges. More information can be found at Troubleshooting Board 1, VFO.

MOSFET VFO Amplifiers

When an LED doesn't light at the VFO Amplifiers, check for unsoldered or bad solder joints in that amplifier. Almost all errors with the MOSFET amplifiers have been soldering mistakes.

If you need further help, go to Using the LEDs to Diagnosis Problems. This page goes into a lot of detail on what the LEDs show you when a mistake is made with a MOSFET amplifier.

LEDs at Relays do not turn ON

The LEDs at the Relays are indicators that the relay is getting power. Relay voltage goes through the LED first at the 10.545 relay and then to the coil in the relay. At the 10.455 relay current is turned on by the IRFU220 and photodiode, through the relay, and to ground through the LED.

10.545 Relay

Check voltage at the plus side of the LED first, this is the pin on the LED away from the relay. You should have 9 Volts.

If you have 9 volts, then you have a bad connection on one of the inner pins of the relay. If you find a pin that did not accept solder, clean the pin with a razor blade or very small screwdriver and resolder the pin.

When looking at the traces of the inner pins, one should be soldered to the ground plane and the other pin goes to the ground side of the LED - the one with the flat side on the bottom of the LED.

10.455 Relay

Have light on the Photodiode and check for 9 volts at the right pin of the IRFU220 as shown in the above picture. If no voltage, check for 9 volts on the center pin. If good, check the connections at the photodiode and 100K resistor. Make sure you have enough light to turn on the Photodiode.

If you have 9 volts at the right pin of the IRFU220, check the inner pins of the relay and the LED leads. Make sure the LED is mounted correctly. Flat side away from the relay.

Frequency does not drop when switching the 10.545 and 10.455 relays.

The outer pins of the relays connect to the switching parts of the relays.

The problem will be seen when turning on the relay the frequency raises instead of going down at least 100 kHz or so.

Check the outer pins of the problem relay and see that they have taken solder properly. Use a magnifying glass if necessary.

If the leads are soldered properly and it still doesn't switch properly it could be a bad relay. Happens rarely but there has been a bad relay or two. To test, jumper the outside leads and see if the frequency comes down. Email me for a new relay.

Frequency lowers but does not adjust

The yellow trimmers are stored in a small amount of oil in the warehouse. Sometimes they can be hard to solder. Check, clean the tabs, and resolder if necessary.

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Last Update: 4/21/10
Web Author: David White, WN5Y Free Website Translator