R71a In The Fog

[ridgescan]

KFOG 104.5 FM loading up in the front end of my perfectly operational 71a with modulation burbles. Right around 17mHz it will move the meter up to s7 even with the antenna coax removed. Nothing on the r75, and only around the 12 meter band area on the Hallie. How friggin strong are they to do that? I have Sutro Tower about 1&3/4 miles from me.
Anyone have any thoughts? I figure based on this that they are probaly responsible for a good chunk of the hash I still suffer at this QTH depite having the Wellbrook.

 

[ratboy]

Are you sure it's actually 104.5 that's doing this? A lot of FM stations also simulcast on AM too.

This might be something similar to what happened to me years ago. A local station on 1560KHZ about 2 miles from me had some kind of grounding issue on their antenna, causing the high tension power lines that run near the station and my house to act like another antenna. The signal level was huge. I could hear it with an S9 signal without any antenna coax being near it. If I picked up the coax cable and was near any of my receivers, the meter would peg. I could hear country music twanging away in my phone, one of my TVs and a few times, my toaster! Without an AM BC filter I built, Hf was noisy all over and almost intolerable below 7MHZ or so. I found plans to a 2MHZ and up high pass filter and it solved most of it, my 71A was still bothered slightly by it, but using the attenuator helped. A call to the station might be of help. This could simply be old age and a solder joint or component failure in the 71A. I would take the top and bottom off, loosen the screws holding the PC boards down and retighten them. Don't go "Gorilla" on them, just snug them up. A lot of them weren't tightened up in the first place when Icom made the radios, they were famous for this back when the r71a was made. You would be surprised at what an iffy ground on a PC board can do.

 

[ridgescan]

Thanks for the great post RB. On a hunch I threw the stereo on and tuned the FM dial and KFOG matched up with the burbles on the 71a. I will act on your advice soon!

 

[nanZor]

KFOG 104.5 FM loading up in the front end of my perfectly operational 71a with modulation burbles. Right around 17mHz it will move the meter up to s7 even with the antenna coax removed. Nothing on the r75, and only around the 12 meter band area on the Hallie. How friggin strong are they to do that? I have Sutro Tower about 1&3/4 miles from me.

Holy cow! That close to Sutro - wow.

My first thought since you have already removed the antenna is to choke your AC power supply leads with ferrites of say #43 material to handle KFOG.

Can you temporarily run your rig from a battery with short leads - even something small like a 7ah 12v agm or the like and see if the problem still exists?

 

[ridgescan]

hey Hz you figure it's riding in on my power chord? My 71a is AC only.
Also I wonder if this is the cause of my awful Tinnitus

 

[dkf435]

hey Hz you figure it's riding in on my power chord? My 71a is AC only.
Also I wonder if this is the cause of my awful Tinnitus

Have a CK-70 kit around here someplace, but it is just a plate that holds the Molex connector in the blank plated hole on the rear and a DC power cord, many less issues and heat if you run the Icom receivers on a regulated DC power supply.

David Kb7uns

 

[ridgescan]

Have a CK-70 kit around here someplace, but it is just a plate that holds the Molex connector in the blank plated hole on the rear and a DC power cord, many less issues and heat if you run the Icom receivers on a regulated DC power supply.

David Kb7uns

Gee you got all of the Icoms huh?? Wow! Anyway, I appreciate the replies. Since this a sort of new phenomenon, I tend to lean towards Ratboy's thought about the PC grounding or bad joints. If it was the power line I would've seen this long ago. Either those, or KFOG increased their power/direction. I am going to look into that when I have some time.

 

[k9rzz]

Did this just start? I'd suspect an antenna connection or one of your ground connections gone bad. Clean 'em all up good just to make sure.

 

[ridgescan]

Did this just start? I'd suspect an antenna connection or one of your ground connections gone bad. Clean 'em all up good just to make sure.

Whatsup John-yeah I even get this with no coax connected. Also, I get this on the sx-88 up around 25megs. Starting to wonder about the power of these FM stations because again, I also get 104.5FM KFOG burblings around 24.5megs plus I get 97.3FM Alice around 22.75megs on the sx-88. Like I said, Tinnitus anyone?
I'm starting to think I am directly in their beam path or something if I get this on the old boatanchor without antenna too.
Nothing on the r75 though.

 

[nanZor]

Nothing on the r75 though.

Hmm.. plot thickens. Is the r75 running from a different outlet than your other receivers? I'm assuming you are using the big wall-wart, but maybe using some other supply, but still, is it on the same outlet?

Just another check - when you say the antenna is disconnected, it is disconnected at the rig itself, or is it just switched away with an antenna switch? Possible ground issues with a switch?

I know I'm grasping at straws now, but still ..

 

[ridgescan]

Hmm.. plot thickens. Is the r75 running from a different outlet than your other receivers? I'm assuming you are using the big wall-wart, but maybe using some other supply, but still, is it on the same outlet?

Just another check - when you say the antenna is disconnected, it is disconnected at the rig itself, or is it just switched away with an antenna switch? Possible ground issues with a switch?

I know I'm grasping at straws now, but still ..

I appreciate your replies Hz. This happens even when the coax is fully disconnected from the rig.
Interestingly, the r75 is on a different circuit out in the livingroom.
The bedroom/shack where the 71a and sx-88 are, is on an east wall facing Sutro Tower. The livingroom is on a south wall and the r75 is there.
So this thing is getting into the two radios without help from an antenna. When I get time I will move the r75 into this room and see if it too will be affected.

 

[nanZor]

Let us know. My thought is that one ac circuit may be doing some major rf-coupling, whereas another in the house is not.

OR, the wall-wart of the R75 is effectively choking it off, and the test would be invalid. I suppose you could test the 71 from another outlet with no antenna attached and see if you hear KFOG from that room...

 

[k9rzz]

or even run the R75 on car battery. Might help determine if it's possibly a problem with their transmitter.

 

[ridgescan]

Let us know. My thought is that one ac circuit may be doing some major rf-coupling, whereas another in the house is not.

OR, the wall-wart of the R75 is effectively choking it off, and the test would be invalid. I suppose you could test the 71 from another outlet with no antenna attached and see if you hear KFOG from that room...

or even run the R75 on car battery. Might help determine if it's possibly a problem with their transmitter.

Hopefully sometime in the next few days I will do what Hz suggests and I will post teh findings

 

[nanZor]

One other thing to consider - I don't know if the Wellbrook itself is getting overloaded from KFOG, but if so I wonder if you could put an fm-trap *before* the input to the Wellbrook amp?

I don't know if that is true, but maybe down the line consider throwing one of the old RS traps in front of it and see if there is any difference ... or obviously a PAR fm-trap. Something for the back-burner notes perhaps...

 

[ridgescan]

I don't think it's the loop since this happens with the coax disconnected from the radios. I even disconnected the main coax to the Wellbrook interface box here in the shack-I tried it all. I still need to do the radio trade-off between rooms.

 

[TassieJay]

I reckon you might be on the right track thinking that it's a nearby powerful FM broadcast station getting in to your radio. If you've removed all antennae and the problem still exists, it's unlikely to be an AM broadcaster that's causing the issue - long wavelengths need some sort of antenna to properly couple into a radio, unless you're literally on top of the transmitter. FM broadcasting on the other hand, with it's shorter wavelength can couple in to a radio without an antenna much easier.

Certainly do try powering the 71a by battery, and keep the power leads very short. If the problems disappear, then you might well be able to cure your problem by some split ferrite cores (VHF ferrite mix) over any incoming cables: power, audio patch leads, and a few over the antenna lead too.

If problems remain, then the FM signal is getting in to the circuitry directly. That will be harder to isolate & cure. One easy thing to check for is to ensure all the screws are snug - both inside & out. If a circuit board inside has a loose mounting screw and depends on it for a secure earth, that won't be helping. The next phase of isolation & testing involves progressively disconnecting internal sections of circuitry - typically IF stages are most susceptible due to their high gain - until the problem disappears, and then working out how you can add bypass caps or FM traps to bleed off the problem signal.

 

[ridgescan]

Hey TJ thanks for adding in! I still haven't gotten around to swapping the radios between rooms-maybe today. Been busy and honestly forgot.
It's definately KFOG FM 104.5. I verified. It also shows up on the old boatanchor. It's that strong.
But not on the r75 out in livingroom. I need to do the swap. It's an involved thing as the AC power chord unique to the 71a goes behind the desk (weighs about 300lbs.) and I actually need my stepson to get in here to help (not easy-he's a new dad). I have more than adequate muscle, but the vascular disability I have will not allow me to do stuff like this anymore
I will work on the PC board tightening-up too. It's not something that is inhibiting good use of the rigs-just annoying and curious-hence the thread.