The AM Forum

I've been working on using my Xenyx 802 mixer to run my Shure SM57 Beta into my Ranger and have been running into a few issues with RF pickup. When I load the rig into the dummy load everything works just fine but when I switch over to my antenna and transmit a low pitched whine manifests itself in the audio. The modulation transformer makes a strange noise as well. The 802 only has unbalanced outputs and you have a choice of either balanced or unbalanced inputs for the mic inputs. When I read the manual it stated that the mic inputs are "electrically balanced",any idea what they're on about?

I've been reading a little bit about RF ingress into audio and one fellow had luck with using the balanced output of his mixer into his rig. The problem I have is that A) I only have unbalanced outputs and B) The Ranger's input is unbalanced.

Secondly confirm with me that shielded cable is absolutely necessary for the entire length of the run into the mixer. I ask this because someone at some point converted my Ranger to the two pin style connectors(like those on a Heathkit SB/HW series)and currently I'm just using my connector that I used from the mic I was using previously and so there is roughly a 1ft run of unshielded wire before it hits the cabling that runs into the mixer.

One thing I was perplexed about is that even if you use a balanced out from your mixer when you run through a matching transformer isn't that changing it over to an unbalanced signal? I'm just using a simple plug-in Audiotechnica line matching transformer for the time being. Do I need a 1:1 isolation transformer as well?

I've been around audio for quite some time but this area of it is new ground for me so any help would be appreciated. I hope it isn't too much of a bother for me asking but from what I've read each setup requires a slightly different solution and I didn't find much information regarding interfacing with a Ranger.


           - Mike

Mike,
I like your un-intentional pun here in your post:
"I've been around audio for quite some time but this area of it is new ground for me so any help would be appreciated."
 ;D
Anyway, for starters, here is a link with some nice info/tutorials that should be helpful.  Hopefully the guru's on this board will weigh in on some of the finer points.

http://www.rane.com/note151.html

http://www.rane.com/library.html#gpm1_2

Hah! You're sharp,good catch.

Thanks for the links. After pondering some bit so far what I'm going to end up doing is converting the mic input on the Ranger to 1/4" mono just so I can run shielded cable for my entire setup.

I'm still wondering if I need a 1:1 audio transformer. I was also discussing with my friend if it makes a difference if,from an unbalanced 1/4" output,I run a 1/4" balanced(TRS)connector to an XLR into my unbalanced Audiotechnica impedance matching transformer which will then connect directly to the input. We both thought that there should not be a difference and he felt that perhaps the balanced connector might have better shielding which would help. In short I'm wondering if running a balanced cable out of an unbalanced output would possibly cause for a greater chance of RF pickup.

I had the same problem with my Viking 2 CDC.
The audio input is high z so all I had to do is add a 620 ohm load resistor in the audio plug at the rig. This way the audio stuff sees 600 ohms and the rig is happy with a low Z load at the input.

For some reason I have a similar problem with an RCA ribbon mic. When using say a RE-11, EV-664, I don't experience any intrusion via RF. But when I use the ribbon mic, the audio squeals like a pig. Everything is balanced except the input on the Ranger. There it is unbalanced going into the grid of the 2nd stage of the speech amp.

I installed a 1/4" mono jack on the Ranger and so far everything seems to be okay. I had a rather frightening incident where when I first fired it up all of a sudden the 5R4 started arcing wildly. After some thought I came to the conclusion that when I reinstalled the metal shield that encloses the input jacks for the key and the audio one of the wires that ran through the shield must have been grounding out against it. I repositioned it and checked to make sure nothing was being pinched or touching the shield and it's fine now.

The only issue I have is due to my rather long run of cable going from the mix a small amount of RF gets in if the cable is too close to the transmitter. I'm going to procure a shorter run of cable in the very near future and this should help this.

@W3SLK - Are you running both mics through a mixer? Does your setup remain the same except for the mic?

@WA1GFZ - I'm using a simple Audiotechnica in-line matching transformer and so far so good.

Mike asked:
Yep, it runs to the same board and out to the audio gear. Its all balanced with that exception to the Ranger. I want to try it on my other rig. The speech amp, (Bogen PA) has a mixer built in so it would be a good measuring stick.

This may not apply here but...

R.F. feedback can be the result of common mode R.F. on the shield of the audio cable. Using balanced audio right up to the transmitter makes it easier to tame.

A balanced to unbalanced transformer near the transmitter allows for proper plumbing. It's not a bad idea to terminate the output of the transformer with a resistor that will provide a given load to the audio chain. This makes level measurement a little easier.

If using a 1:1 bal to unbal xfmr into the Ranger's high impedance mike connection try a 600 Ohm resistor. If using a 1:10 use 6K. The Ranger's mic input can be driven with a low source impedance typical of line level audio gear. Most likely there will be way too much level which is a good thing.

A balanced attenuator can be placed right at the last balanced part of the audio chain along with a polarity switch. If there's 20 db of extra audio the attenuator will take care of that and will also reduce the R.F. problem.

In stubborn cases the static shield (ground) of the balanced audio cable can be broken right at the transmitter. Famous hams like W3GL are using this method.

Another feed to consider is an unbypassed cathode resistor in the Ranger's speech amp somewhere. That will be a lower load impedance and higher level and the passband will not be influenced by the preceding audio stage. Just use a DC blocking cap and feed it unbalanced. Look at the schematic of the Junkston 500 for an example of this. IIRC it's the phone patch input.

   "Famous"?    No kidding...
 ;D ;D ;D ;D ;D ;D ;D ;D ;D

An issue that I am wondering about is the fact that the ouput impedance of the mixer is around 100ohms. The in-line matching transformer is expecting an input impedance of 600 ohms and has an output impedance of 50k ohms. If the Ranger's input impedance is around 1 meg would the impedance mismatch between the input of the Ranger and the output of the matching transformer cause an issue? Would the impedance mismatch between the mixer output and the matching transformer cause an issue as well?

Feeding a 50K mic input with a much lower source impedance should work just fine. It will most likely be necessary to place an attenuator at the Ranger's input or the first stage will sound like the guitar intro from Revolution.

Hm,so I'm still having issues with RF ingress.

The first issue is the fact that on 80m if I grab the microphone my audio sounds very distorted and low in volume. If I don't touch the mic and keep the mic cable off of any part of my body the audio sounds fine. I can't do much on 40m as the Ranger starts to oscillate way before I get to the required level of audio I need. If I unplug the mixer and only have the 1/4" mono jack connected to the Ranger with nothing connected on the other end everything is fine.

Both the mixer and the transmitter are plugged into the same grounded outlet.

I'm running a 1/4" mono from the transmitter to the mixer and then a balanced XLR to the microphone.

I'm going to try using a 1:1 isolation transformer and see if that helps. I just wish that my mixer had a balanced output on it...

   This has been going on now for two months and it's time to
    look at the RF egress problem from another prospective...

    You obviously have some RF in the shack.

 

Well I'm working with what I have at the moment.

I'm running a random wire antenna with a counterpoise. The tuner sits about a foot and a half away from the window with the wires running the rest of the length to the window. I can certainly try and move this so that the tuner is *right* at the window but I would like to have that as a last resort only.

In regards to what WA1GFZ said about using a 620ohm load resistor I will most certainly try this and see what happens.

Is it really causing that much of an issue that I am running unbalanced from the output of my mixer?

This is a perplexing problem for me and when I'm perplexed I ask questions so bear with me.

Moving the tuner should be the first resort.

What is the counterpoise?

Have you bypassed all the ins and outs on your mixer?

Do you even need a mixer or can you do without it until the antenna situation improves?

If you place an isolation transformer right at the transmitter and leave the static shield disconnected there you would have the identical setup Ralph is using.
'Twood be a good idea to go over to a center fed sky hook and all balanced interfaces in the audio gear.

I kind of like what Steve said. Work back logically. Try it out with say a D-104 going directly into the Ranger the way is typically would and see if you have RF issues there.

I moved the tuner to the window and little changed.

The antenna is 120ft long in an inverted L shape and is about 45ft above the ground. The counterpoise is about 250ft long and arranged low along the ground outside. It serves double duty as a mini beverage for AM DX'ing. I did attempt putting the microphone straight into the radio and it actually eliminated the feedback on 40 meters. It didn't sound so great due to the lack of EQ but it works.

I'm thinking with the tuner location not making a huge difference that the problem might lie elsewhere. I don't have any medium length runs of coax so I have a 50ft or so length of coax that I use to run from my radio to the tuner. It sits bunched up below the small footrest that the Ranger sits on. The distance between it and the Ranger is maybe three feet. I'm going to go up to my University's shack tomorrow and see if I can find a more appropriate length of coax. Perhaps the coax may be coupling to the tuner and all of that bunched up coax three feet from the audio feed point might not be doing favours for keeping RF out of the chain.

I'm thinking that I should be inferring something about the fact that when I hold the microphone the audio is distorted but if I keep my hands off of it the audio is fine. There might be a clue in that observation but I'm not seeing it.

I'm going to try to source a mixer with balanced outputs. In the interim,though,I'm going to play around with a suitable length of coax from the radio to the tuner as well as an isolation transformer.

Static shield? So on a mono 1/4" this would mean disconnecting the shield and only having the "hot" side run into the transformer?

@W3SLK - I've used the rig with both a Shure 444 as well as my current Shure SM57 straight into the rig no worries though on 80m with the SM57 I still have the issue of holding the mic causes distorted audio.

I'm not side stepping the fact that I would very much like a center fed dipole. This will happen in the near future.

This may or may not be of any use in your case, but I will pass it along anyway.  I had a really bad time ridding my homebrew transmitter of distortion caused by RF feedback.  After trying everything to get rid of the RF it finally came down to placing ferritt beads on each of the balanced audio leads just before they connected to the transmitter speech amp and making sure the transmitter was well grounded.  Good luck, I feel your frustration.

   120 foot end fed wire on 75/80 meters = "Voltage Feed"...

   Of course you WILL have RF in the shack with that set-up.  And yes, 40
   meters should  be  good to go as you have moved the high RF voltage
   half way out on the wire...

    If you can, add an additional 20/30 feet of wire out on the end of the 120 ft.
    and you should be golden on 75...  

    If that is not possible, a chunk of coil stock inserted some place in series  
    with the antenna wire (not in the shack) should move your high RF point
    down the wire, moving it away from the audio gear. In any event, ferrite
    beads and grounding the gear will help especially with trying to end feed
    the antenna.

    The "counterpoise" wire adds nothing to the long wire...

    And, yes, that big coil of coax is not helping.  Get a short section of
    coax to replace it.

    A chunk of RG-8X with the PL259's and adapters works well and is  
    a lot more flexible them RG-8U.  I push a KW+ through it between the
    final and tuner from a pr. of 3-500Z's

When I go to a dipole it'll be capable on 160 meters so I'll be able to be on every band with the help of a tuner.

I'll see if I can get some beads this weekend. I'll re-read the Ranger manual for grounding suggestions but what has worked for everyone else in regards to grounding a transmitter? Should I run a line from the transmitter ground to where the counterpoise connects at the tuner? I'm sure there are acres of articles online so I'll have a gander there as well.

Hey everyone,

I really appreciate the suggestions you've been giving me. I think I'm just going to end up going balanced from mic to the isolation transformer at the transmitter. My friend has a spare isolation transformer that he is willing to give me which is quite kind of him. In the mean time I've put the 620 ohm load resistor in line and it definitely seems to help both in modulation as well as the level adjustment of it. The question I have is that load resistor is spec'd for a 600 ohm output. Until I can secure another mixer with balanced output I was wondering what value resistor would work best for a mixer that has a 120 ohm output impedance? It just seems like my transmitter has an excess of low end and the impedance mismatch might not be helping.

Just to let you know I've done the following audio mods in the hopes of improving high end response:

- Clipped C53
- Removed C60(it is across the interstage transformer)
- Removed C66 and C68

I've done other modifications to open up the response such as increasing the size of C52 and C57.

All my Ranger/Valliant/Homebrew published doc's on this site use a 600 ohm input for the rigs. This is easily done by running the 12AX7 first stage grounded grid and driving the cathode. Very similar to the phone patch input that some of these rigs had stock. Go to the transmitter mods section and look up the Ranger, Valiant & 833 rig data posted here under my callsign. You have an audio mismatch that's causing your problem. That hi-z input is a pain in the ass. Once you get rid of it you can hook up a good audio chain and spend all your cash!
:)
Keith
WA1HZK

I have done this with my valiant and it works great.  No RF problems at all and VERY clean audio

Tom - AB3FL

Yesterday I went through the Ranger with my multimeter to check out all of the resistors. They all seemed fine and so I did a voltage test on nearly every tube and found that the 6CL6 buffer was lower than it should be with the plate receiving 250v on transmit and the screen receiving only 70v. The correct figures should be about 310v and 195v respectively. I traced the screen voltage problem to a 68k resistor that was reading around 120k. I replaced the resistor and put it back on the air and noticed that my buffer current was still low at a bit less than 10ma at 3ma grid current and 120ma plate. I'm thinking something else is up with the plate supply that is holding down voltages still. I'll do another voltage test tonight and see what the 6CL6 has now that one faulty resistor has been replaced. Anyone have any ideas,past stories of repairs,etc in this area of a Ranger or other Johnson rig?

Here is a strange one for you,though. When I received my Ranger someone had bridged the accessory plug in such a manner that 7 and 8 were tied together(correct),3 and 5 were tied together(correct)but 1 and 6 were tied together(huh?). I took a look at the schematic and saw that with the Ranger configured as such it looked like it was only using half of the modulation transformer windings,right? I brought the jumper from pin 1 to pin 2 and now it is modulating at a much higher level than before. From a quick look at the scope it looks like it is hitting ~95% positive which is a big improvement from before. I have no idea why someone would connect the accessory plug as such.

Yeah you're famous now. Number one. Here's a salute.

Okay,this is strange. I replaced the 68k and did a few tests. Once the rig is turned on the voltage on the screen rises up to about ~205 volts or so but then drops to about 100 volts in relatively quick fashion. The same sort of behaviour applies to the plate as well except in this case it rises to about ~310 volts or so and drops down to 220v.

In the schematic the voltage measured where R8 connects to R9(just below and to the right of the buffer tube in the schematic)is correct at about 320v. The voltage after R8 is about 100v even though I've replaced it and the new resistor reads 68k. The voltage where L5 connects to R9 is about ~220 volts once warm. I'm going to try replacing R9 and see where that gets me. It may be rising in value as voltage is applied to it. It is either that or something is loading the tube down somehow. Would it be wise for me to remove the tube and test the voltages with it removed provided I do not transmit and only stay in tune mode? Could C29,a 50pf mica,be leaky?

I have two ribbon mics and both are more susceptible to RF than a variety of dynamic mics, using the same input and physical proximity to things.

Here, though, the SM57 (Beta) that he's got is a dynamic mic with a metal case so it should not be unusually vulnerable.

Also, and I learned this from noted AMer Dave Wanger, K3ZRF,  on your shielded balanced cable, only ground the shield at the equipment end that's receiving the audio signal.

If you are running an end-fed wire, you are asking for problemz! ! ! ! !

An end-fed is very difficult to tame, and unless you take special precautions you will USUALLY have RF in the shack problemz!  (been there, done that)

You need to first put the tuna right at the feed point of the antenna. (keeping it as far away from the rest of the gear as possible.) then you need a GOOD ground system tied directly to the tuning network as close as possible as possible. (A remote tuner is the best way possible to do it.) Then run coass back to the transmitter or T/R relay.

Keep in mind that an end fed inverted L is basically a Marconi type antenna and still needs to play against a good ground or ground field. If not, EVERYTHING between the antenna feedpoint and the transmitter will be hot with RF. One single conterpoise probably wont cut it. It also depends on where the feedpoint ends up being in regards to the frequency of operation. (Are you feeding it on a voltage node, a current node, or somewhere inbetween?)
Like others have said, shortening or lengthening the wire may make a big difference.

I got so fed up with trying to tame my end-fed that I said screw it and changed over to a balanced antenna and have never had a problem since! !

Just my $.02 worth! !

I think my plan of action will be an isolation transformer,a better mixer and a balanced antenna.

What concerns me more now is this problem with the buffer circuit. I replaced both R8 and R9 in hopes of raising the plate and screen voltage on the buffer tube as they were both low at 250v plate and 70v screen when they should be 310v and 195v respectively. When I did this replacement the voltages stayed exactly the same and now my screen voltage on the multiplier tube has dropped from 98v before to 57v after which has caused my oscillator current to rise from an on spec 23ma to around 29ma.

What's strange is that the voltage where R8 and R9 meet is just fine at around ~330v but after R8 right where it connects to the screen is far too low at 70v or so. Could C27 be shorted to ground? It's getting to the point where I might just end up slowly rebuilding this transmitter,one section at a time,with new resistors and capacitors(it has been partially recapped as the micas and ceramics still remain).

+1

Sounds to me like you are trying to take the long way around the block!  ???  ???

My suggestion is to put up a properly fed balanced antenna. Set up a good station ground. ( A GROUND THAT IS AT RF GROUND! ! ) And send the transmitter off to someone that knows what they are doing to repair it. A Ranger is just not that complicated of a transmitter. It is just a Valiant that someone dropped in the dryer and shrunk a couple of sizes. If you are changing component values to get the operating voltages to come into line, you have another problem that you are overlooking somehow, somewhere.

You stated earlier that you audio changes if you touch the mic or get near the mic cable, that alone should tell you that everything in the shack is hot with RF.

ALSO KEEP IN MIND THAT  A DC GROUND IS NOT ALWAYS AN RF GROUND! ! ! ! ! ! ! ! !

 

I understand that. I mentioned in my first post that I would be planning to put up a balanced antenna. Through reading these posts I've been able to better understand the situation at my station. The reason why I am changing subjects is that I feel it is better that I keep my questions confined to one thread instead of starting up multiple threads which would increase forum clutter.

In regards to sending the transmitter off to someone who "knows what they're doing" there is a problem of the facts that the people who usually do such work aren't cheap(which,at times,can be understandable depending on the amount of work/troubleshooting that must be done),shipping such a heavy item is not at all cheap and finally I would much rather figure this one out on myself. The only way one can become better at troubleshooting and repairing vintage equipment is through attempting the repairs themselves.

My instincts will tell me when I am in over my head and considering how this problem is only afflicting the buffer/multiplier circuit I feel that this is within my realm to tackle. In addition to the point about finding someone who works on this type of equipment I have a Hammarlund HQ-129X that needs some help(that I feel is outside of my comfort zone)and it took me considerable time and effort to locate someone who would lend me a hand. There simply are not that many people around where I live who work on older equipment.  

- Accidental double post -

R U sure R.F. isn't messing up your voltage measurements?

I'm running the transmitter into a dummy load during the test procedure so I do not think RF would play a part. I did measurements on all of the tubes except those in the VFO compartment and,except for the oscillator screen and buffer plate and screen,they were all within spec.

I've been still working away at it. I tested the voltages at each tube and here is what I've found.


Default

    I just did some voltage tests on the pins of both the oscillator and buffer and here is what I've found.

    All tests done at 3ma grid current.

    6CL6 Oscillator

    Pin 1: 1.8v standby 1.7v transmit
    Pin 2: -14.30v standby -13.90v transmit
    Pin 3: 46v standby 44.2v transmit
    Pin 4: 0v standby and transmit
    Pin 5: 0v standby and transmit
    Pin 6: 327v standby 321v transmit
    Pin 7: 0v standby and transmit
    Pin 8: 46v standby and 44.2v transmit
    Pin 9: -14.30v standby -13.90v transmit


    6CL6 Buffer:

    Pin 1: 0.85v standby 0.83v transmit
    Pin 2: -5.47v standby -5.30v transmit
    Pin 3: 105v standby 99.2v transmit
    Pin 4: 0v standby and transmit
    Pin 5: 0v standby and transmit
    Pin 6: 237v standby 227v transmit
    Pin 7: 0v standby and transmit
    Pin 8: 105v standby and 99.2v transmit
    Pin 9: -5.40v standby -5.29v transmit

So it looks like the control grid on both 6CL6's has too much negative voltage on it. This negative voltage seems to originate from the junction of R45,R43 and C89 which is right outside of the 12AU7 keyer tube. I checked all of the voltages of the 6AL5 and 12AU7 and they're on spec. The interesting this is that at pin 7 there are 0 volts present but at the junction of R45,R43 and C89 there exists a negative voltage of about 7 volts. I'm trying to figure out where this negative voltage originates from as I need to lower it as this is where V3 receives the -1.75v needed.

Just curious, what type/brand instrument are you using for the measurements?

A Fluke 87 and a Simpson 260. If I get a weird voltage with one I cross check it with the other. I do know understand that both meters have a different impedance.