The first step in diagnosing a problem with the DFD2 counter is to make sure proper connections have been made between the boards and the counter.
There are two connections to the board. One to the VFO and one to the Crystal Oscillator.
The following picture shows the connection to the VFO. See AADE DFD2 VFO Connection for more information.
For more information on the connections, see "Connecting the Frequency Counter to the Receiver"
Checking 445 kHz IF Offset
When you check your frequencies in the following steps, remember that they will be plus or minus 455 kHz, depending on the band you are checking. These offsets will be noted in the following descriptions.
The following will go over the proper setting of the 455 kHz IF offset, then the connection to the VFO and the Crystal Oscillator.
Have a set of clip leads so you can use it to ground the inputs to the frequency counter for testing the frequencies that are being input to the counter.
Or, you may remove the 74HC4046s as noted below. I found this a lot easier to do if you have not mounted the counter in a permanent place. Also, it will help you find a bad 74HC4046 chip. The bad one will show different readings than a good one.
Always double check that you have mounted the chip correctly in the socket before powering up the display!
The VFO and HFO inputs are the bottom two connections as shown above. The top one is the BFO and it should have a wire between the input and ground. Or, the 74HC4046 in the top socket, which inputs to the BFO, may be removed.
If you have not mounted the DFD2, pull off the display and remove all the 74HC4046 ICs. This is easier than trying to ground the inputs and more reliable. You will have to bend the .01 caps slightly forward to get a small screwdriver under the chips to remove them.
Check for 455 kHz at all band positions. If not, adjust the trimmer pots as follows:
First, make sure the reading is 0 kHz. This is done by turning the trimpots all the way counter-clockwise.
____First, turn the coarse pot (upper one) clockwise to 384 kHz.
____Second, turn the fine pot (lower one) clockwise to reach 455 kHz.
If you do not get 455kHz on all bands, or the 455kHz adjustments do not work, the PIC16C71 is bad.
Checking the VFO Input
Unhook the ground clip lead to the VFO input. Leave the ground clip lead hooked to the HFO (Crystal Oscillator) input.
Or, put a 74HC4046 into the middle socket on the right side of the PCB. This is the VFO input. Other sockets should be empty.
The frequencies read by the counter should be as follows:
Check your VFO frequencies to make sure they are the same or close to the above values under "VFO Freq". Measure the VFO frequency at the "VFO OUT" box between the VFO amplifiers.
If you are still getting 455 kHz, then the counter is not receiving the VFO signal. If the reading is varying a lot, then the counter is not getting a strong enough signal to read the VFO. Check for good connections, the 3.3pf cap connecting to the VFO and the counter shorting to ground, or raise the value of the 3.3pf coupling cap to 10pf or so.
If your readings vary between 74HC4046 chips, then one or all of the 74HC4046s are bad. I recommend replacing all the 74HC4046 and the 74HC153 chips. It is unusual to blow all the chips, but it has happened.
Checking the Crystal Oscillator (HFO) Input
Take the ground clip lead off the Crystal Oscillator output (HFO input to the AADE counter), and ground the VFO input to the counter.
Or, take the 74HC4046 out of the center socket and place it in the bottom socket. Top two sockets should be empty.
Check to make sure the Crystal Oscillator LEDs are switching with the band positions. When the 4.000 MHz Crystal Filter LED is on, the 3.547 Crystal Oscillator LED should be on. When the 3.547 Crystal Filter LED is on, the 4.000 Crystal Oscillator LED should be on.
The frequencies read by the counter should be as follows:
If you are reading only 455 kHz, the Crystal Oscillator signal is not getting to the counter. As above, check for good connections, the 3.3pf cap connecting to the Crystal Oscillator and the counter shorting to ground, or raise the value of the 3.3pf coupling cap to 10pf or so.
If both of the above tests show OK, disconnect all the ground clip leads. You should see the following readings in the table below for the display and VFO output.
Or, add a 74HC4046 in the middle socket. The top socket should be empty and the middle and bottom socket inserted with 74HC4046s. The BFO input is not used, therefore, the top socket can remain empty.
|Band||Display Reads MHz||VFO Freq MHz||DFD2 Top Jumper Volts||What it does||DFD2 Bottom Jumper Volts||What it does||Xtal Osc MHz||Crystal Filter MHz|
The easiest way to nail the above VFO frequencies is to set the VFO slightly lower and then use the main tuning cap to set the exact VFO frequency needed.
There is an offset of 2 kHz on the 40 and 20 meter bands and 1 kHz on the 30 and 17 meter bands. The 455 kHz offset should be 453 kHz on the 40 and 30 meters bands, and 457 on the 20 and 17 meter bands for the display to be exactly correct. Unfortunately, the IF offset cannot be changed through the band switching process.
A second offset that factors in is the exact "Series Resonant Frequency" of the Crystal Filter.
The capacitors in the filter that set the bandwidth lower the series resonance of the filter. For the 4.000 MHz filter, the frequency of the signal that gets through is actually 3.999 MHz. This means that the IF offset in the counter needs to be 456/454 kHz to get an exact reading of the Band's frequency.
The same applies to the 3.547 MHz crystal filter. The capacitors in the filter lower the series resonant to about 3.546 MHz. This also results in a 456/454 kHz IF offset in the counter to get an exact reading.
An easy way to correct these offsets is being investigated and will be posted here when found.
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