I have been disappointed to discover that my receiver is easily overloaded. Also tuning the variable capacitor seems to have very little impact on the received frequency where as the adjustment on the front end seems to have a dramatic impact on the received frequency. Why? What measurements can I take and what can I learn? Well I’ll tell you my story. . .
****Warning everything below this line is my best guess, I don’t actually know. If you know I’d love to hear from you in the comments below.***
First of all I don’t have much equipment, but I do have an oscilloscope that Bill Ellis (N5TXN) gave me years ago. It is a 15 Mhz BK Precision 1472C scope. Unfortunately I don’t have any probes. . . Amazon 100Mhz scope probes ordered. While I’m waiting on those, what else can I do? First I used a dummy load to transmit 7.000 from another radio at 5W and used the variable capacitor on the front end to make sure I was peaking the signal in the CW end of the 40M band. I then used the tuning capacitor to try to tune off the signal. No luck. Then I keyed down into the dummy load and started spinning the VFO. I made it to 7.150 before the signal diminished in any way. Hmm. . . that just doesn’t seem right. Could my LO be on the wrong frequency?
How can I measure the frequency of the LO?
Many phone calls and emails to many friends. A couple of notable quotes.
- Tuning doesn’t matter if the receiver is overloaded.
- You can hook up your oscilloscope or frequency counter, but know that you are loading the circuit and the frequency is not the exact frequency in the circuit.
- Given your LO arrangement, there just isn’t much signal to measure.
I was advised to use a 1″ form and wind 10 loops of wire around the 1″ form for a receive antenna. I could then use this receive antenna to try to get a frequency count by using my MFJ-259B frequency counter OR use my oscilloscope and the antenna. I was also advised that this was likely a futile effort as the total energy in the LO circuit is unlikely enough to get a good reading. Turns out this was correct. . . The MFJ frequency counter gave me results from 400 Hz to 30Mhz and the oscilloscope could see a signal, but it wasn’t enough that I could actually count the waves.
I took a trip to the local Nortex QRP meeting and brought my little creation. I expressed my frustration that the tuning capacitor seems to have no impact on the tuning. After visiting with Dave Lear (NE5DL) and Joe Spencer (KK5NA) I became convinced that the issue was in my band spread capacitor. It’s not a large enough tuning value. When I arrived home my Oscilloscope probes had arrived!
I hooked up the oscilloscope probe to pin 6 on the NE602. I got nothing. I played with the RF gain pot. Nothing. I put my desk radio on 7.050 Mhz and keyed into a dummy load at 5W. Hello. . . I have signal. I guess that the LO runs at such a low current there is no observable signal until a threshold is met on the RF input. I then spun my tuning cap and the frequency changed as did the amplitude. The amplitude and frequency changed dramatically. I interpret this as confirming my hypothesis. The band spread capacitor needs to be adjusted to tighten up the tuning capacitor. As the tuning capacitor exits the tuned portion of the front end the amplitude drops. In my case, its REALLY fast!!
I think my next update will probably be finishing my final experiments with this receiver and start planning my next project. We are approaching the time of year where operating outdoors is nice so I may take a break from building until it gets to be so hot that operating isn’t fun and field day isn’t far away either!
I’m a relatively new ham. Got my license, joined a club and I’m having fun! I enjoy CW, pedestrian mobile, backpacking, and I dream of building radios at some point. At present I have a wife and daughters so my building time is limited.
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