Realistic DX-302 Pulse Noise

[jpanhalt]

Got a used DX-302 off eBay Saturday. Nice physical shape; questionable performance. Antenna is only about 20 feet of wire between chairs, but I get a couple of AM broadcast stations and got a strong SW international station with music on about 13 MHz last night late. I suspect it needs tuning up and maybe some capacitors, but there is one annoying problem I would like to address first. When tuned to a station, regardless of AM or SSB, there is a pulse/beep about every second. The louder the station, the louder the beep. Between stations, there is no beep. Frequency of the beep does not change with tuning or the frequency to which the radio is tuned.

I read about an issue with the AGC, but that was described as affecting AM differently than SSB. I tried a 12V DC power supply from an PC, and that reduced the noise but didn't eliminate it. The voltage regulators (2, one for 9V and one for 5V) are simply a zener and transistor.

Just FYI, here is the schematic of the 9-V supply:

The seller says the beeping is due to not being tuned to the station properly. I have tuned my wits out on a few stations without success in reducing the beep. In fact, the better tuned (using the signal strength meter and loudness), the stronger the beep.

Comments?

Regards, John

 

[alec_t]

and maybe some capacitors

I share your suspicions, if the beep is not a design feature. Something is clearly going in and out of oscillation, motor-boating. A 1sec period suggests cap leakage current could be the culprit.

 

[jpanhalt]

ADDITIONAL INFORMATION:

I have never played much with editing sound files. I recorded slightly off frequency to show the sound alone and then right on frequency to show it still exists. The spacing is about 0.75 sec from looking at the pattern on audacity. Can't upload wav or mp3, so it is included in a zip file.

Maybe the sound will help you recognize the cause.

John

 

[JimB]

What do you hear when there is no antenna connected to the radio?

If the noise goes away when the antenna is disconnected, then the noise is not generated in the radio.

My first thought is that the brippp brippp bripp noise is coming from a switch mode power supply, probably something in standby mode or a battery charger.

JimB

 

[jpanhalt]

The periodic noise goes away when the antenna is disconnected. With antenna connected (or me touching the terminal) and every electronic device unplugged in the house, the noise is present. I did not just turn the devices off. I unplugged the computer, modem, router, a small radio in the kitchen, and the supply where the FOIS enters the house. I do not know of a single smps within 1800 feet that was plugged in. If it is a signal, shouldn't the strength vary with the antenna? Could just loading the antenna input, like with my body, be causing the noise? I've uploaded that part of the schematic. The poor quality is because it is a scanned copy off the Internet.



There is another thing I just noticed while testing reception of my DDS emf. At 1.000 MHz, there is a very strong "signal" that pins the signal strength meter. With sideband, I get a pure tune. On AM, there is nothing.

John

 

[JimB]

Do you hear the noise when the receiver is tuned between stations?
Do you hear the noise on all frequency ranges?
If you touch the antenna connector with your finger, it is not that you are "loading" the internal circuitry, but you are now the antenna.

The signal at 1Mhz ?
Was this the desired DDS frequency, if so, I would expect there to be a strong signal in the receiver as you describe.
When the receiver is set to AM (Amplitude Modulation) I would expect there to be silence when listening to this strong local signal.
When switching to SSB (Single SideBand), either USB or LSB (Upper or Lower SideBand), there will be a tone as the receiver is tuned through the signal.
This is normal, it is the BFO (Beat Frequency Oscillator) beating with the incoming RF signal to create an audio signal. (A poor oversimplified description!).

JimB

 

[jpanhalt]

Do you hear the noise when the receiver is tuned between stations?

No

Do you hear the noise on all frequency ranges?

Yes

If you touch the antenna connector with your finger, it is not that you are "loading" the internal circuitry, but you are now the antenna.

I am a short fat antenna. I figured my configuration wasn't that efficient.

The signal at 1Mhz ? Was this the desired DDS frequency, if so, I would expect there to be a strong signal in the receiver as you describe.

No, not at the DDS frequency. When DDS = 10 MHz, it behaved as I expected and was strong. The 1 MHz squeal occurs with the DDS off and pins the signal strength meter high.

When the receiver is set to AM (Amplitude Modulation) I would expect there to be silence when listening to this strong local signal.
When switching to SSB (Single SideBand), either USB or LSB (Upper or Lower SideBand), there will be a tone as the receiver is tuned through the signal.
This is normal, it is the BFO (Beat Frequency Oscillator) beating with the incoming RF signal to create an audio signal. (A poor oversimplified description!).

In reference to the above 1 MHz "signal", that is what I observe. In AM mode, it is almost silent. The very loud squeal is only in SSB mode (either one). The question is, where could such a strong signal come from? It is far, far stronger than the strongest Cleveland AM station or my DDS. I turned off my scope, frequency counter, and old Wavetek frequency generator to be sure it was not locally generated.

John

 

[jpanhalt]

After fiddling with it for two days -- I even tested it off a car battery away from buildings -- I gave up on the click. The seller agreed to take it back. I am probably going to be out a bit for one-way shipping, but that beats the cost of trouble shooting and possibly replacing a slew of capacitors.

I am considering a different approach to catching the frequencies I want in the range of 30 to 300 kHz or so.

Regards, John