[R-390] BCB "images" on R-390a - progress

[Tony Casorso]

Hi everybody. I believe I have gotten to the heart of the issue with the spurious BCB images that appear on some of our receivers above 1.7MHz and below about 3.4MHz. I have a simple experiment that will demonstrate this on your receiver and it would be interesting to see results from others who try this. First the causes I have found. There are three that I have seen here and a fourth that has been brought up as we talked about this before.

1. BCB station harmonics. The broadcaster is the cause. The station that was a problem for me in my previous threads changed ownership last week and the new owners are producing a second harmonic that is 45 db down from the fundamental. Since the fundamental hits over 90db on my s-meter, I still get a good signal at the second harmonic. If you don't have a spectrum analyzer, you can buy or build a simple BCB filter. If the filter does not reduce the signal at the second harmonic then it is not the broadcaster.

2. RF stage being overloaded by a super strong broadcast signal. The overload produces second harmonics in the RF stage. These will abate when a BCB filter is installed as long as the cutin frequency of the high pass filter is high enough. In my case the station was at 1650 so a typical filter that you buy off the shelf would not help much because it was only 3db down near ths frequency. I built one with a cutin at about 2.5MHz. A characteristic of this one is that you only hear the the offending station at the harmonic frequency rather than a mix of stations.

3. Same as 2, overloaded RF stage but you hear a mix of several stations on top of each other in the 2 MHz band. The cause is mixing of less strong broadcast stations with the super strong one in the RF stage. In my case I could hear the 1650 station and a 950 station simultaneously at 2600 KHz. This is the sum of 1650 and 950. The mixing products of all of the signals present in the RF stage will be heard. Mixing signals between 500 and 1700KHz will have first order products in the range of 1Mhz to 3.4MHz.

4. External mixing. I have not observed this but it has been well documented in the past.

Case 3 is easily reproduced with two signal generators and a BNC Tee. In my case I set one signal generator to 600Khz with 1KHz modulation and the other to 1.5Mhz with no modulation. I tee the outputs together (OK when the generator has 50 Ohm output with attenuation) and plug them into the receiver antenna jack. I tune the reciever to 600Khz and adjust the RF level for about 40db or 50db on the carrier meter to simulate a moderate to weak broadcaster. Then I tune the receiver to 1500KHz and adjust the other signal generator for 90+db on the carrier meter o simulate my strong broadcaster. Now tune the receiver to 2100KHz wich is not a harmonic of either 600KHz or 1500Khz, and you will hear the mixing product signal. Now you can reduce the 1.5Mhz signal until the mixing signal disappears and tune back to 1.5Mhz to measure the reative strength of the signal necessary to induce the mixing. On my R-390a, the threshold is about 77db. The nice thing about this experiment is that the choice of frequencies makes the harmonic content of your generator less important. Case 2 cannot be measured unless the generator harminics are way down.

Moving the bnc tee over to my R-392 I find that the threshold is about 10db higher. Maybe because of the 2 RF stage design. I have 26FZ6 tubes in the 392. I might try 26A6s just for fun. It would be interesting to look at an R-390 but I don't hve one.

As mentioned in earlier posts, the antenna impedance at the frequency of the strong station can make things worse if it is unusually low by loading the preselector and broadening it's response.

Tony