The AM Forum

Hello Gents.

I am running rack gear on my Icom IC-718, and the audio is fed through the ACC jack on the back.

I have been running on SSB and I have not had any issues with RF in the Audio, but I am starting to get the itch to play with AM and I am now having issues.

No matter where I set the carrier power on the rig, I'm getting RF, I can hear it when I start adding enough audio to just start modulating the carrier, and I can hear the RF audio in the Carrier, even when I am not saying anything.

The Audio Chain is:
-MXL V57 Condenser or a modified Nady RSM-4 Ribbon Mic
-Behringer VX-2000 Mic Preamp
-Behringer PEQ-2200 Parametric EQ
-Behringer EX-3200 Ultraflex Pro
-DBX 266XL Compressor/Limiter
-Behringer MX-602A Mixer
-Rolls DB-25 Isolation box
...then to the radio through the Accessory Jack in the rear.

My audio chain is all Balanced XLR, except from the mixer output to the Roll DB-25 direct box.

So far, I have:
-Double checked my XLR cables to confirm they are wired correctly
-Added bypass caps to the mic cable input to the mic preamp
-Checked my grounding
....everything seems OK

No luck yet...

Anyone have any other suggestions?  ???

Thanks

With so many components in the audio chain, the first step is to determine where RF is getting in.  Start by closing the master level control on the mixer, if the problem is gone, the source is toward the mic input.  Still there, look toward the rig.  Work one component at a time to determine where the RF is getting in.

Are you using short XLR cables, or long ones coiled up?  If you find the location where RF is entering the audio chain, perhaps some clamp-on ferrites on the XLR cables or power cables might tame it. 

Instead of monitoring in the rig, place a scope on the output of the direct box, and you might find it easier to troubleshoot with lower carrier power.  The scope will also make it possible to validate whether changes to the configuration improve or degrade the situation. 

Too much equipment.

Thanks for the reply Rick


Yes, I have determined that the RF is getting into the audio chain, I shut down the master on the mixer last night, and sure enough, the carrier was silent.


The XLR cables interconnecting the rack components are as short as possible. The power cables are the ones that came with the units, and they are all run into a Datalink power conditioner on the rack.


OK, great Idea, I'll try that tonight.

Thanks for all the input.



Thanks for the input, but that doesn't help much....

Are you sure it is RF? Could it be 60 cycle? Not second guessing you Scott but it is common to introduce 60 cycle when you add a bunch of accessories like you have.

Mike

Thanks Mike, I think Its RF, but I could be wrong....

I PM'd you a link to a link to an audio sample.

Can you tell me what you think?

Let me ask this...

Is it possible to have RF with AM mode, but not SSB?

I have been running this setup for about a year on SSB and never had any RF in the audio complaints

Yes, I agree. You can have too much of a good thing.

In all things audio, less is always better. Use the minimum amount of equipment to do the job. You should need nothing more than a good microphone preamp, EQ, and limiter/processor, and that's it.

73,

Bruce

listening to the recording I made, the RF is into the signal before I even start turning the rack components on... it is getting in at, or before the preamp...

I think I'll build a new mic cable for starters....

No mention if you're running a linear. You could also try an RF sniffer (microamp meter with a 1N34 across the terminals and a short antenna hooked to the anode terminal) to sniff around your audio maze when you key the rig in the AM mode.

Hello there,

 Nice "Rack-o-Gear" OM,... :D.. First what is the location of Antenna, directly overhead..off to the side a little,... way back in the back yard..?...Further away the better.....Step one.

After yer Done sniffing around..."Separate the Gear"...Amplifiers and Antenna Couplers in their own area not directly stacked on or near the Transceiver and Audio chain...Near Field Effect here, OM..."Think near Field Effect"...before tearing things down move them... and monitor what changes...Does the Transceiver display a waveform distortion with the Mic Cable unplugged...ETC...

With that type of Stack you shud break the Audio chain often, an in between each component..if possible....ie gud isolation transformers.. Jensen JT-11-P...can get expensive but if yer gona play audio.. think isolation OM...gud rule of thumb...One Nice little item that may help...is the IHY IBox at the Mic input...Breaks the line and has Acceptable filtering...Think about it...http://w2ihy.com/ibox.asp ...it is OP buildable...have used it and It werks...copy the Skizmatic build yer own install internally at the input of the rig...

OK, Now, DC supplies...Never in the Immediate Vicinity of RF generators...Again separate move to the floor further away from RF the better...How is yer station AC supply Any filtering..?...Connections, clean nice an tight,... Check AC Ground..Clean Connections..?......all grounds Connected...

You have a very simple problem and it will have a very simple cure..you have to find it...these are some gud ideas and some things to consider...

Let us know what happens OM...

Something in the back of my mind is nagging that it may be that accessory jack or internal circuitry thereto.  Can you try through the "normal" mic jack for a comparison, Scott?

Dude, you tryin to stuff dolly parton's boobies into a 34 B cup. Do you know what freq response the Icom is good for?

and yes, I know I didnt solve yer question - but that impresses me as way too much audio chain. Take all that stuff out of there until you get rid of the RF, then add them back on one at a time and isolate the box giving you the problem. 

Don't neglect the possibility of RF getting into the equipment thru the AC power.  I connected up a cheapo mixer and in QSO with the HUZman, he asked if I had some BC SW station on in the background... long story short, RF getting into the mixer thru the AC power, mixing with some strapping BC SW station in the PS diodes.  Some .01/2KV caps from each leg of the AC where it entered the box to the chassis ground stud solved it.

Hi Scott,

I'm running the 718 with external audio also. I have the MXL990 into a MX-802.
I go directly from the mixer to the radio and put the nano compressor in the aux send/return loop.
The radio drive an amp to full power. No RF problems so far.

Ya might wanna try stripping down to the mic/mixer/radio barebones configuration and see ifn that helps. I wouldn't use the mic preamp with the 602. Just plug the mic straight into the board.
Which brings up a thought.
Are you putting the output from the preamp into a mic input or line level input?

Last night, I injected a 1kHz signal into the mic end of the mic cable....

Stuck my O'scope on the mixer output....

I could not see any noise on the 1khz signal when I keyed up the transmitter.

The Antenna is in the backyard, it is a vertical with the base about 25' up.

But I have been doing my testing with a dummy load in the shack, but the RF gets into the signal either way....



Thanks, I'll try that tonight


I am going to build a cable to interface the Mic jack with the rack today, but if that is the answer, I'll have to modify the radio to get and real frequency response out of it, the ACC Jack bypasses all of the "Voice Optimized" (aka DX'er Audio Heaven) Tone filters that the mic input circuit has.


I have been running the mic into the mic input (as apposed to the 1/4" LINE input of the preamp).

How are you interfacing the radio? the Mic Jack or the ACC jack?

Hmmmm,

Here's a thought some of you may not have considered.  It may be as simple as RF feedback.  Back in college in the 80s, I had a similar problem with an FT-101B (absolutly horrible phone audio on 160m).  It turned out to be RF feedback.  When I had my IC-703 Plus a few years back, I had to revisit the RF feedback issue again.  If I'm right, like the '703 Plus I used to own, the IC-718 is a single board radio, and like the IC-703 Plus, is descended from the IC-706 series.  Single board radios can occasionally have grounding issues.  On 40m, my '703 Plus used actually shut down if I transmitted on certain freqs.  I'd say re-inject the 1 kHz signal into your '718, but do it with the antenna hooked up, and then check your scope.  Also, check different bands.  On some bands I had no problems at all.  On other bands (specifically 40 & 15m), I had problems with RF feedback.  I never did completely get rid of the problem.  But, installing ferrites & chokes on the power and CW key cables, and a common mode current choke on the feedline (near the radio end of things), did help quite a bit. 

73,
Ellen - AF9J

Lotta gud stuff here for troubleshooting.
You can try to choke off the feedline from the vertical. Help folks....is it a 1:1 current balun that would isolate the aerial from the shack????
Verticals are usually problems with RF feedback.
You can also try wrapping AC supply cords or mic cords in those ferrite clamps sold at ShadioRack, if they still sell them.
Audio equipment sounds like a little much, but the proof is in the pudding. I, like the others, would be concerned that the processing is beyond the audio specs of your off-shore tranceiver.

Fred

Hi Scott:

My 2 cents worth:

Behringer gear does not have much in the way of RF filtering. If you open your mixer's master and the RFI begins, then it can be getting into

1) the mic (behringer mics are not shielded and having an active amp in it causes problems)

2) the mic preamp

3) the mixer.

One thing to try is run a ground strap from the transmitter to all affected gear (even the mic XLR at the mic) to attempt to bring all the gear to the same (hopefully low) RF point. You may need to filter the AC line as mentioned or the audio ins and outs in each box, or even put the offending item in another shielded box (drastic).

Being a processor nut, I have found that too much gain reduction (compression and limiting through many boxes) can make a very small amount of RFI very pronounced and difficult to resolve. I have solved it though in the past.

Hope all these ideas from everyone helps. Let us know what you find!

73,
Dan
W1DAN

The ACC jack, Scott.
AND.... it's all unbalanced.

http://www.w5efr.wz5q.net/index/pic/chain.jpg

He's got a website describing his setup fairly well.

http://www.w5efr.wz5q.net/index/pic/graph.gif

Frequency Sweep.

David Goncalves
W1EUJ

Hey David,

Yup, that was my setup... I might change it back to something like that...

The current setup is here:

http://riley-music.com/BowsStuff/SSB_Audio/W5EFRAudioChain.JPG

This is a better graphic of that same sweep, which I did after modifying the radio for 3.3 kHz on SSB:

http://riley-music.com/BowsStuff/SSB_Audio/3khz718.gif


Thank you for the information, Bud.
I'll keep hammering away at it.

Grounding too...

I'd monitor the output of the Rolls or the Mixer with the rig keyed into the dummy load, since you say the RF is there into the dummy load - key the rig and look at the output of the mixer/Rolls with the scope. Assuming the scope is good past the frequency of interest of course. Then work backwards.

Alternately, turn things on from the Rolls back and look for the RF to appear.

Imho, you might have had RF in ur audio all along, but on SSB with the usual inherent distortion and the ALC that is usual even inside a typical SSB rig, a little extra RF might not get noticed. Possibly. On AM it is much easier to hear this sort of thing.

                    _-_-bear


PS. A sweep won't show much in the way of RF getting in - since that is distortion. For that a look at a simple sine wave will show it up as a thickening of the trace everywhere or in one or two places... For AM you'll likely want bandwidth out to 5khz. although the Icom may or may not pass audio out there.

Is the rf online when the computer is connected, there's a loop there OM. You May Have Computer issues here..

Please explain this Jack.

Scott.... the line from the compressor to the mic input on the 602........ try pulling this out and run the compressor output to one of the line level inputs. These can be wired balance with a TRS 1/4" plug.

Here is what I did last night:
- Checked it out by running the audio into the Mic Jack, using just the Mic, Mixer, and Rolls Isolation Box... Distortion is still there

- Tore the entire $^#& system apart
- New ground system, single point connection (Audio Rack, TX, Tuner, AC Power Strip), and pounded in a new ground rod
- Cleaned up the power runs, AC and DC
- Cleaned up the Coax runs
- Rearranged the system to get the TX/Antenna Tuner further away from the Audio Rack

Then I started testing....

The system was about as simple as I could get it:
Mic-Mixer-Isolation Box-TX

The RF is still in the audio, and no Computer interfaced to the system.

I Scoped the signal coming out of the Isolation box, and it looks like the signal I was putting into the mixer, I can't see any distortion in it, it looks like a 1khz sine wave... maybe my 20Mhz O'scope isn't sensitive enough to actually see it...

Plug a hand mike into it (without disconnecting anything else) and see if you still have the problem.  If you still hav ethe problem, disconnect the audio chain.  If you *still* have the problem, focus on the power supply, coax interconnects, etc.

You mentioned a tuner in your last post that you didn't mention before.  You are running a vertical with the tuner in the shack?  Fed with coax?    Kind of a recipe for disaster there, OM, depending on what you're doing - ie is this a multiband vertical, say a R-7, and you're just trying to tune out a bit of SWR - or is this a base-loaded vertical where you're trying to match a 10:1 SWR?

Ok, I'll try that tonight... (where the hell is that hand Mic ??? ...)


Actually, I run the tuner (MFJ-949E) as more of a Antenna switch and dummy load. My Antennas are a 10 meter vert (base 25' off the ground) and a fan dipole (10, 20, 40, 75 meters). It is located next to the transmitter, and fed with about 2' of RG-8 from the TX.

I have never had to use the tuner as a tuner, my SWR's are pretty good (under 2:1) with the antennas I have.

did you say your ant is 2 ft away from our rig???? ???

I just wanna see Derb stuff them in a 34b ...  You could sell tickets fer thatun ... beefus

Guess he never heard of "NEAR FIELD"... (feeder is 2 ft of coax?)

Okay, I stand corrected...  How far from the actual antenna is the gear set-up? 

It still sounds like near field hetting into his system.  Works semi-okay on SSB but craps with steady carrier.

Read it again guys. The tuner is 2' from the TX. Try to be constructive here. Did your knowhow come naturally? Man!

Once I found that the mike cable shield wasn't working effectively for some reason.  -spiral coil type of cable.   
 
When I ran a wire from the mike shell directly to the input connector shell, the hum and RFI disappeared.

I think that an RF ground loop might also be the culpert. 
Looks like Star grounding has been already tried but might try it again with very little equipment.  The mike only needs the pre-amp. And is the balanced feed fed into a pre-amp stage on the transmitter that usually takes a mike input directly? i.e. is the audio fed into one amp stage too many? 

All kinds of things to try.  Sounds like if it happens into a dummy load then the preamp circuitry might have an open ground or open shielding inside the case.  Stranger things have happened. 

So as everyone says, isolate by removing everything. Then try another preamp directly into the transmitter without any other processing equipment.

My shack has 5 or 6 ft. of coax to a LC tuner into a single wire feeder to a windom; the tuner is right above the op. desk.  The single wire feed line radiates on some bands but no RF occurs unless something is not matched properly. ... this after finding the "no shield" shield mentioned above on the mike cable.

You've run into the bug-a-boo of a lot of hams having antennae too close to the rf generators.  We live in houses, not remote stations feeding a whole ground planed hill with five miles away.   One or more of you equpment internally may have an open ground or leaky shield from something as simple as a star washer on a mike input jack going open.  Hence the RF near field, even from signal going to a dummy load gets in the audio line.   

So get a different mike, a different pre-amp and see if the rf is still there. Then add back the components, (not so damn many ;you really don't need all of them) one at a time to see which is defective.  If not any then go for the wiring again.  Something will jump out in the middle of the night and you'll go "Eureka!"

Tonights Update.. With Audio!

OK, I picked up a Tripp Lite Ultra Isobar Surge Suppressor, as it has internal HF and VHF filtering.. just in case... Plugged in the Audio Rack and Power Supply for the rig...

No Joy, still RF'ing

So, I hook the radio to the dummy load 4 feet away from my rig (not grounded to the star ground), through the RF sampler. Sampler audio output to my sound card, O'Scope output to my O'scope. Put the rig down on 40 meters (well within the range of my 20Mhz O'scope) unplugged everything else, found the hand mic (brand new in the box) and gave it a shot...

http://riley-music.com/BowsStuff/718AudioProb/HandMicAccDisCont.mp3

No Joy...

I'm starting to think my audio rack isn't the problem....

Any ideas, oh Gurus of the AM?

OK, lotsa hands in soup..Listen to what Rick is saying he's right on the money... Good job on cleaning things up..Now start at the beginning...

No RF sampler No Computer...and no computer connections...Turn the computer off....On air testing with basic rig antenna coupler, antenna, jumper wire 3' or 6' then add components..find somebody on the air then add system in...at the point when distortion is found run a connector wire from the rack to rear of the rig....assuming rack is connected case to case...Ground run from station point to exit 2 inch copper flashing...inline Rf sampler shud be basic inductive type directly to scope..such as found in handbook..or on Garys main site here...I think it's there...or on huzmans..I forget where that's at now...it's real simple to build but try a different sampler...Hard ground the Scope directly...

Then Plug and Play...and it's VSWR Not swr's...LOL.....That cracks me up.... :D

I listened to that audio clip and it sounded fine to me, albiet a bit pinched.

Let's start at the beginning.  I guess we all presumed your concern was correct: that you're getting RF in your audio.

Your first post sez "I heard RF in the audio".  What makes you think this - on the air reports, another receiver in the shack, a modulation monitor, something funky going on with the transciever?

RF in the audio, on AM, will generally manifest itself as feedback and/or copious distortion.  I don't see that on the audio clip you posted.

Particularly with your last test - hand mike, dummy load - there's really no chance for RF to get into the audio unless the patch cable to the dummy load is bad.  I think the problem is something else.  Let's explore what you're noticing, and go from there.

Hello John,

Thanks for all the suggestions with chasing my problem.

I am starting to think my problem is not RF related to my transmitter. It seems that I can get this "beating frequency" or "motorboating" under my transmit audio even using a 4 watt CB I had kicking around here.

I ran a quick test last night with my Mic/Mixer/Rolls Isolation Box and piped it into a Uniden CB with my RF Sampler on the back, then into the dummy load.

I got the noise there as well.

Testing the Icom Hand Mic with the Icom, and getting the noise rules out the Audio Rack as the point of the noise source.

the only real thing these to radios had in common, besides the dummy load and sampler, is the Power Supply. I am thinking my NewMar power supply is suspect...


 

OK, where is the beating effect or noise coming from?  You say it's in your transmit audio.  What are you using to listen to your transmit audio?

I use this RF Sampler:
http://amfone.net/ECSound/RFsampler.htm

hooked to the back of the transmitter, and run the audio output to my Computer to record with Cool Edit Pro.

The audio sounded clip grainy to me and yes, maybe not too much rf afterall. Hard to tell.

what does it sound like as audio (granted, from the possibly tinny hand mike) before it goes to the transmitter and rf pick-off detector?.   how do the two compare?

Simple diode detectors without adequate f/b signal ratios 'to get over the knee into a quasi -linear region' tend to distort audio quite noticebly, e.g. a crystal set into an audio amp never sounds as good as the same crystal rectifying a larger signal with adequate resistive loading in an IF amp, thence to the audio amplifier. 

Your audio sounds very similar as that received in near field on a simple crystal set. 

So get a tape recorder going at a more remote site, say about half a mile from the house, then run your lashup and then playback the audio received there.  Might be a real eye-opener.  Might sound pretty good. 

Have you talked on-air to any of the guys on this site? 
I love problems like this, way fun and we all learn a lot. 

Hello Rick

Thanks for the reply....


This recording was just the hand mic, so the "before the TX" Audio is a bit hard to do. Tonight, I am hooking the audio rack back up and I am going to record it straight off the mixer, and then off the Detector. I'm am starting to wonder if I have been chasing a "non-issue" caused by my RF Sampler/Detector, making me think that my TX audio is bad, when actually, the bad audio is caused by the detector that is being used to input to the computer for recording...


Hmmm, that is pretty much the whole sampler setup!

Is there a better way to monitor your TX audio live?


That is also on the agenda for this evening... I just need to figure out how to set it up.


Nope, I have never talked "on-Air" in AM mode, with my little radio and low, subdivision requirement dipoles, I probably wouldn't even be a bump in the noise floor.

You're getting RF into your computer, not RF into your audio.  That RF tap is really meant to drive a set of headphones, not directly drive a computer sound board.  The computer, with all its millions of PN junctions, is gonna be quite sensitive to even modest RF levels injected into it.  If you want to drive the computer, you should insert a 600 :600 ohm audio transformer between the sound card input and the RF tap's audio putput.

I'm not really a fan of this sort of RF tap because it can cause ground loops and damage if you transmit thru it with no antenna at high power.  Dumb, yeah, but who amongst us hasn't done it.  Notice also that this design has NO capacitor isolation between the RF and the detector diode.  You're gonna build up a fair DC voltage at the audio output - for 20 watts at 50 ohms, lets figure P=V^2/R, 20=V^2/50, V=10V, half wave rectified, so about 7VDC, just the carrier!

Better would be a small transformer ie as what's in a SWR bridge - that way you're isolated from the RF.  For my scope pickoff I just use 1 turn of wire wrapped around the coax coming from the xmitter, then fed directly into the vertical amp of the scope (at my lake xmitter) or the RF sample output from a Bird wattmeter (my home QTH xmitter).

My mod monitors are fed with a 1000:1 capacitive tap on the transmitter.  This is good for a bit of ground loop action once in a while as well.

Your next step is to get that bad boy on the air, make some contacts, and get a few reports!  If you wanna do much AM at all you *really* need a scope, and not one of those junky Heathkit "signal monitors" either.  For 75 meters a cheapo 10 MHz scope is fine; for up to 10 meters you'll want a 50 MHz (at least) model.  A $5 hamfest special is fine.

The added benefit of having a REAL scope around the shack, besides being able to see exactly what's going on with your signal, is you can use it for troubleshooting as well!  Ifn' ya had a scope, you wouldn't have had to go thru all this torture  ;)

Thanks, I would like to try something else for the "On Air" recording input to the Computer... and cap isolate it... any schematic suggestions?



Yikes! I never thought of that...


Yes Sir, I do believe you are correct! I am going to put it on the air tonight, throw it against the wall, and see if anything sticks.. If I can find anyone that can hear my PW signal...

I HAVE a scope, a 20Mhz dual trace, but I never did see any noise on my Audio... because there wasn't any... ;)

Thanks John

Instead of feeding the diode with a resistor, try a capacitive divider.  Perhaps 10 pF/500V from the RF to a 1000 pF, which would be grounded.  Connect the diode to the junction of the 2 capactors.  Put a 100K resistor to ground on either end of the diode, a .01 to ground in parallel with the 100K resistor at the audio end of the diode, and feed the audio with a 1 uF or so cap.

RF in --------------------------------------  RF out
                                        |
                                       ---
               1                       ---   10 pf
 Audio   ---)|----|<----------|
                    |--       |        ---
                   /    |   /          ---   1000 pF
          100K  \    --- \    100K  |
          .001   /   ---   /           |
                   \    |    \            |
                    ---|--------------   <gnd

damn, johnny, you be the ASCII master.  :)

 ;D ;D

Yessum, them computers do make life interesting.....

Thanks for all the help, suggestions and other comments.

It looks as if I was chasing a red herring, caused by the diode detector I was monitoring my audio with. and ON-Air check I did confirmed that my audio was free and clear of RF in AM and SSB modes...

But I learned new things, got a new surge suppressor/RF filter for my AC power and got a new compressor out of the deal...

Thanks a lot to  everyone.

Not I need to find a quality way to monitor my on-air signal... I should build John's detector and try that...

don't forget the "Hammer Down and Happy" lighted on-air sign.  :D

(http://www.dribbleglass.com/images/billboards/cb.jpg)

Great, Bow!  Here's a case where the test equipment was causing the problem  ;D  Been there done that.

In any event that RF tap is a seat of the pants design, don't take it as gospel.  There are other ones out there that are probably better.  You could even slightly modify some old CB SWR meter and get good results - those things bypass the detected audio to ground, just reduce the bypass cap to .001 or so, and put in a series dc blocking cap to the audio.

Hey, cool idea...

\Now that I have an MFJ-259B, I have an old, well abused SWR meter that needs a new lease on life!

Thanks again John...