75A4 on 160 meters

[sndtubes]

Hi Guys,

I just started to use my 75A4 on 160 and was disappointed to find there were multiple spurs from AM BCB stations all over 160 meters.  It was worse, of course, when receiving with my 160 inv. L and much better using the K9AY loop.  The 51J4 does not have this problem.  Does it sound like I have a problem or is this what to expect from a 75A4?  BTW I don't live anywhere near any AM BCB towers.

[KB2WIG]

I can't directly address the  'A4 issue.

 I was able to copy an 810 Khz stations 2nd harmonic for up to 15 miles away. (WGY, Schenectady, NY). Got its's 4th (closer in) depending on the weather.....
You just never know.

klc

[ke7trp]

There is a post about using a balun on the reciever.  If you have a coax TV to antenna terminal device for your TV set. Try that.  It just might cure it.  Experiment on the hook up of the device. Ground the Coax side to the reciever in a test.

You can also order a BC band filter. There are lots of types and DB levels. Some are comercial units and some are made by hams and sold reasonably.

I have this problem here in AZ.  The Collins R390A hears the 50Kw Disney station that is less then 10 miles from the house. I can hear it out clear and clean to the 3rd harmonic.  There are spurs all over the place on that Rxer. Other hallicrafters recievers do not have this problem.

Its possible that something on your house or tower happens to be great for the stations you are hearing. 

The last thing I will mention is that it could be comming from the Transmitter INTO the RXer.  If you turn on the Transmitters beat or tune position, Do you hear the stations louder?  Try the RXer right into the antenna with nothing else inline for a quick test.

C

[K5UJ]

I have a 75A3 (has the same circuit as the A4 up to the mechanical filter) and I have no problems  with BC stations on 160.  I may be farther away from mine however.  I have a 50 KW station on 670 that's 12 miles away and two more 50kw on 720 and 780 that are 15 miles away.  I may be just a few miles too far for trouble thankfully. 

Incidentally, the last time I was near the 670 tx site, I spotted a ham tower with a yagi on top and it had to have been a 1/2 mile away from the 670 tower and I thought how does that guy operate?   If it were me I'd have to move.

[KM1H]

My 75A4 has a highly modified front end so its not a fair test comparison but it is also the best 160-80M CW DX receiver Ive ever used. Nothing bothers it but the TS-940 its used with does need a BCB filter. Ive a number of 5-10KW AM stations in 5-15 miles. All receiving is done with several Beverages.

The 75A3 is bone stock and has the early production front end and it does have a few birdies. I'll be modifying the front end soon.

Carl
KM1H

[k4kyv]

If I feed the quarter wave vertical directly to the antenna terminals of my 75A-4, I have BC birdies.  I threw together a 20 dB attenuator using 1/2 watt carbon resistors and that completely solved the problem.  The vertical tower was introducing too much RF into the front-end and overloading the rf stages and mixers.

The beverage does not feed enough signal for this to be any problem, nor does the indoor shielded loop.  With the dipole, the balanced antenna tuner has enough selectivity to reject BC stations.

Sometimes with the dipole as receiving antenna I get cross-modulation on AM when I am listening to one station and there is another strong station somewhere within 30 kc/s.  I can hear the modulation of the other station in the background under the modulation of the station I am listening to.  Again, the attenuator does the trick.

The overload problem is inherent to the design, with two mixers ahead of the i.f. selectivity.  It is common with about any double conversion receiver with the tunable i.f. design, since the first i.f. is tens of kc's wide.

You won't lose any signal no matter how much attenuation you insert between the antenna and receiver, as long as you can still hear a decrease in the background hiss when the antenna is disconnected, in which case the receiver is sensitive enough to hear the atmospheric background. That is really all the sensitivity you need, and is the reason why external r.f. pre-amps usually do not improve the s/n ratio when receiving weak signals. I have my attenuator located on the control panel of the station so I can quickly switch it in and out. I have an external "nuvistor" r.f. pre-amp, but took it out of service because all it would do was to make the S-meter go up several units without improving the audibility of the signal.

Ideally, a receiver would be single conversion and have only passive tuned circuits and no amplifier stages between the antenna and the 1st mixer. This is hard to achieve and still feed enough signal to the mixer and 1st i.f. filter so that the receiver is sensitive down to the atmospheric background noise

[w4fms]

Isn't the 75A-4 single conversion on 160m?  I use my 75A-4 with a GK500 on 160m.  I have the exact same problem here and have always attributed it to the receiver design.  No problems on the other bands.  I agree with Don..simply by using horizontal polarization on the receiver I was able to reduce the signal enough to prevent the imaging problems from BC stations.

Frank

[K3ZS]

I get crossmodulation on 75M AM, when there are a few stations on the air, using my Multi-Elmac PMR7.   Turning down the RF gain a bit gets rid of it.

[k4kyv]

Isn't the 75A-4 single conversion on 160m?  I use my 75A-4 with a GK500 on 160m.  I have the exact same problem here and have always attributed it to the receiver design.  No problems on the other bands.  I agree with Don..simply by using horizontal polarization on the receiver I was able to reduce the signal enough to prevent the imaging problems from BC stations.

Frank

Yes.  The double-conversion/tunable i.f. configuration is nothing more than a conventional single conversion superhet with a xtal controlled converter inserted between it and the antenna.  That conventional single conversion super covers the 160m range and the rest of the bands are converted down to it.  The rf amplifier and mixer simply can't take the signal level I get across the broadcast band from the 127' tower and radial ground system.

[ke7trp]

Don, Sounded good last night in AZ.

I think the largest factor is the antenna. My Inverted L blasts BC stations through some recievers.   On the Zep, This is really minimized.  When I put the elevated Ground radials on the L, The interferance became much stronger.

C

[sndtubes]

My 75A4 has a highly modified front end so its not a fair test comparison but it is also the best 160-80M CW DX receiver Ive ever used. Nothing bothers it but the TS-940 its used with does need a BCB filter. Ive a number of 5-10KW AM stations in 5-15 miles. All receiving is done with several Beverages.

The 75A3 is bone stock and has the early production front end and it does have a few birdies. I'll be modifying the front end soon.

Carl
KM1H

Hi Carl,

Which mods have you done to your front end?  I've been thinking of trying the W2VCZ / N0DMS mixer and front end mods.  I have 2 A4's  I'm gonna keep one fairly stock and use the other for a "guenia pig". 

[sndtubes]

If I feed the quarter wave vertical directly to the antenna terminals of my 75A-4, I have BC birdies.  I threw together a 20 dB attenuator using 1/2 watt carbon resistors and that completely solved the problem.  The vertical tower was introducing too much RF into the front-end and overloading the rf stages and mixers.

The beverage does not feed enough signal for this to be any problem, nor does the indoor shielded loop.  With the dipole, the balanced antenna tuner has enough selectivity to reject BC stations.

Sometimes with the dipole as receiving antenna I get cross-modulation on AM when I am listening to one station and there is another strong station somewhere within 30 kc/s.  I can hear the modulation of the other station in the background under the modulation of the station I am listening to.  Again, the attenuator does the trick.

The overload problem is inherent to the design, with two mixers ahead of the i.f. selectivity.  It is common with about any double conversion receiver with the tunable i.f. design, since the first i.f. is tens of kc's wide.

You won't lose any signal no matter how much attenuation you insert between the antenna and receiver, as long as you can still hear a decrease in the background hiss when the antenna is disconnected, in which case the receiver is sensitive enough to hear the atmospheric background. That is really all the sensitivity you need, and is the reason why external r.f. pre-amps usually do not improve the s/n ratio when receiving weak signals. I have my attenuator located on the control panel of the station so I can quickly switch it in and out. I have an external "nuvistor" r.f. pre-amp, but took it out of service because all it would do was to make the S-meter go up several units without improving the audibility of the signal.

Ideally, a receiver would be single conversion and have only passive tuned circuits and no amplifier stages between the antenna and the 1st mixer. This is hard to achieve and still feed enough signal to the mixer and 1st i.f. filter so that the receiver is sensitive down to the atmospheric background noise

Thanks, Don.  Actually I think I am experiencing the same thing basically.  When I switch to the K9AY loop (comparable to a beverage in terms of gain), the images mostly go away.   If I switch off the preamp in the K9AY controller, then they all go away.  Switching off the preamp will be similar to putting an attenuator in line.  I may also try a BCB filter or a tuner as others have suggested.  Thanks for all of the feedback, everyone.

[sndtubes]

Oh, incidentally, I just took the K7CMS AVC mod out of my A4.  It was awful.  I had all kinds of trouble with strong signals causing the AVC bus to oscillate.  This was only really noticable on CW.  It wasn't easy to detect on SSB or AM.  So if you folks are using your A4's on AM or SSB primarily you might not notice the problem. 

Then W1LSB published an article for an AVC mod in the current ER.  It works GREAT!!  I highly recommend it to anyone.  There is no AVC oscillation and it recovers so fast from standby, I can use it full QSK.  The slow AVC for SSB is really nice and smooth.  There are no "cross country" wires like with the k7cms mod, either.  It involves a couple of diodes resistors and capacitors.

[k4kyv]

When I switch to the K9AY loop (comparable to a beverage in terms of gain), the images mostly go away.   If I switch off the preamp in the K9AY controller, then they all go away.

Is your K9AY loop rotatable?  From what I have seen of the design, that might be feasible.

I get excellent results with the beverage, except for one problem; it is fixed direction to the northeast.  Many of the stations I work are in that direction, but I work other parts of the country, too.  I lease cropland and the only way I could erect a beverage in a westerly direction and stay out of the farmer's way, would require a transmission line over a 1/4 mile long!

The other problem is the direction the beverage points is aimed right at a connector road about a half mile away, with multiple power lines and a row of cracker-box houses, and I am constantly having problems with power line noise and other, more mysterious buzzies.  The crud often completely wipes out the beverage, while the dipole does not pick any of it up, but on 160 the vertical is all but useless for receiving most of the time due to electrical noise.  I am thinking of erecting a rotatable K9AY that hopefully could be physically turned to null out noise on 160.

My latest noise is something ungodly strong, that comes on intermittently, an hour or two at a time, maybe once or twice a day.  It doesn't sound like a power line leak, but more like some kind of digital hash or rough parasitic oscillation that blankets everything from just above the BC band to up around 8-9 mHz.  It has recurring peaks and valleys of signal strength, but no apparent discrete frequencies.  It sounds similar to the old TV birdies from tube type B&W sets of the 50's and 60's, but is continuous coverage instead of a buzzie every 15 kHz. Driving around the area with a radio doesn't seem to pin-point any specific location of the source.

[sndtubes]

When I switch to the K9AY loop (comparable to a beverage in terms of gain), the images mostly go away.   If I switch off the preamp in the K9AY controller, then they all go away.

Is your K9AY loop rotatable?  From what I have seen of the design, that might be feasible.

I get excellent results with the beverage, except for one problem; it is fixed direction to the northeast.  Many of the stations I work are in that direction, but I work other parts of the country, too.  I lease cropland and the only way I could erect a beverage in a westerly direction and stay out of the farmer's way, would require a transmission line over a 1/4 mile long!

The other problem is the direction the beverage points is aimed right at a connector road about a half mile away, with multiple power lines and a row of cracker-box houses, and I am constantly having problems with power line noise and other, more mysterious buzzies.  The crud often completely wipes out the beverage, while the dipole does not pick any of it up, but on 160 the vertical is all but useless for receiving most of the time due to electrical noise.  I am thinking of erecting a rotatable K9AY that hopefully could be physically turned to null out noise on 160.

My latest noise is something ungodly strong, that comes on intermittently, an hour or two at a time, maybe once or twice a day.  It doesn't sound like a power line leak, but more like some kind of digital hash or rough parasitic oscillation that blankets everything from just above the BC band to up around 8-9 mHz.  It has recurring peaks and valleys of signal strength, but no apparent discrete frequencies.  It sounds similar to the old TV birdies from tube type B&W sets of the 50's and 60's, but is continuous coverage instead of a buzzie every 15 kHz. Driving around the area with a radio doesn't seem to pin-point any specific location of the source.

I can electronically rotate the K9AY loop in 4 directions (SW, NW, NE, SE)  It's a pretty compact antenna compared to beverages, so it works well on my 1 acre lot.  Rotating it helps a little, but with the K9AY I don't have so much of a problem, anyway.  It's the full sized L that causes the A4 much heartburn.   I plan on putting up a 40 meter 4 square this spring/summer.  Unfortunately I don't have the room for an 80 meter or 160 meter 4 square. 

I'd bet your noise is coming from a neighbor's Plasma TV.  They are awful for broad band noise.  I know of a guy who actually bought his neighbor a new TV to get rid of the noisy Plasma set.  The noise is more like computer noise rather than the buzz from power line noise.  When I first moved in here, the power line noise from faulty insulators was as high as 40 db over 9!  The power company came out almost a dozen times to fix various insulators around that were causing me havoc.  Finally they all got changed.  There are still a few noisy ones that are somwhat distant.  If I swing my beam or direct the AY loop in that direction the noise comes up.  If you are having power line noise, your local power company should help you with it.  But from your description, I'd put my money on Plasma TV.

[KM1H]

Hi Carl,

Which mods have you done to your front end?  I've been thinking of trying the W2VCZ / N0DMS mixer and front end mods.  I have 2 A4's  I'm gonna keep one fairly stock and use the other for a "guenia pig". 

I did the mods in 1965-66 when at National and had all the gear I needed for alignments and performance tests.

Both mixers are 7360's and the RF stage is a 6GM6. NF on 10M is around 6dB and the NF is very noticable when using a loop and sometimes the Beverages on 160/80. Im considering a front end xtal filter or 2 which will really knock down the noise bandwidth.

At the same time I installed a SS hang AGC plus some circuit changes, cascaded filters, gain equalized, and cleaned up the audio. Its helped me to over 300 DXCC on 160 and many more on 80. It outhears a very modified TS-940.

I check it out regularly and its still holding up and the numbers beat anything Ive seen. I use CATV amp type signal density testing specs trying to simulate contest or huge pileup conditions.
If I was doing it over Id give the Pullen mixer a try and might do that to the 75A3. Ive done the Pullen to a few more basic boatanchors and Ive been very impressed.

Carl
KM1H

[K5UJ]

Carl,

Would you say the Pullen mod is worthwhile on a 75A3 just for AM ragchewing in an urban location or only something to undertake for dx chasing and other weak signal work?

tnx

Rob 

[KM1H]

Rob, if you are just interested in 160-40M and dont have overload problems then its probably unnecesary. The Pullen gives a huge overload AND noise figure improvement.

The 6CB6 RF amp is a dog, its a sharp cut off that they are trying to make it act as a semi-remote tube. I understand that late production went with a 6DC6 as used in the A4 and R-390A. If anyone has a late A3 schematic Id appreciate a copy. IMO Collins didnt understand AVC for ham receivers until the end of the S Line.

I discovered the front end step attenuator back in the 60's where it was used in the germanium SS HRO-500 and used 10, 20 and 30 dB steps. Ive built several with 6, 12 and 20dB steps and find the 6 or 12 to usually be more than enough for hollow state overload dogs to linearize the front end while still maintaining AVC action with the RF gain on full.

Carl
KM1H


 

[K3ZS]

7360's are a rare find these days, if that is what you need.

[KM1H]

That is why I suggested the Pullen. Depending upon the socket you can use a 6SN7, 6J6, 12AT7, 6ES8, 7F8, and many other cheap dual triodes. My choice for a 9 pin is the 6ES8. I have enough NIB 7360's should the A4 require it.

Carl
KM1H

[k4kyv]

I'd bet your noise is coming from a neighbor's Plasma TV.  They are awful for broad band noise.  I know of a guy who actually bought his neighbor a new TV to get rid of the noisy Plasma set.  The noise is more like computer noise rather than the buzz from power line noise. ...If you are having power line noise, your local power company should help you with it.  But from your description, I'd put my money on Plasma TV.

The power company here is pretty good at fixing line noise, if I find it and lead them by the hand to the source.  They don't have a clue how to locate noise sources themselves.

The nearest house in the direction of the noise is almost a quarter mile away.  Would a noisy plasma TV radiate a devastatingly strong signal that far? There are no power lines or other utility lines between me and the houses in that area to conduct the noise, just vacant woods, so I would have to be picking it up by direct radiation - definitely something that exceeds Part 15 standards.   I am at the end of the power line and the  houses in that area are on a different line. I can get a null using my rotatable loop out in the middle of the field with my portable short-wave radio.  I had planned to move the loop to another spot to see if I could triangulate the source, but it did not come on to-night. I tried driving around the area, but the noise seems to occur over about 2000 ft. along the connector road, and not get any louder as I pass by individual houses.