Talk back from the modulators and mod trans..

[ke7trp]

Been feeding the Globe king 500C with external audio for a year now.  No issues.

Today, I fed the Globe CHampion 300 with the same audio.  Same cable ect..  I get high pitched whine from the Modulators and the Mod trans. If i turn the audio way down this goes away.  This transmitter is one that you can always hear yourself inside the case.  Any tips to get rid of this? 

WIth a D104, I can run 125++ with no feedback. 

C

[K1JJ]

C,

Acoustic talk back can usually be cured by putting the unit on a piece of insulating material like foam rubber, etc and building a small enclosure around it lined with insulation.

My power transformer hums, so that's what I did and now runs silent and deep.

T

[ke7trp]

So insulate the Trans from the chassis?  I guess I can try that. Maybe I will just use the D104.  I tried all night to get that thing to work with the external audio. It just wont work.  I cant get the audio up enough before the high pitched noise comes in.

C

[K1JJ]

When I say insulate, I meant acoustically, though electrically insulated wud be good too...

The way you describe its sudden appearance, maybe something has gone bad.  Maybe something like the EQ is jacked up at the freq of the feedback or something like that which is boosting the feq that is feeding/talking back?

Maybe something has changed mechanically in the xfmr or mounting area to make it mechanically resonant at that audio freq?

Try to figure out what, if anything, has changed.

There's others here that will give suggestions too, so stick with it.

T

[ke7trp]

Nothing has changed or gone bad. The transmitter has alot of audio in it. Its a big case and the 809s scream. With the D104 its great. I was trying to hook modern audio rack gear up to the audio jack in the back. This jack bypasses the first audio tube.  Works perfect on the King.

C

[KB3DKS]

I would assume that your processing has a ballanced output. If so then float the shield at the output leaving it connected at the transmitter. Or try the reverse, float shield at the transmitter and tie at the processing.
 If the line is not ballanced then install the proper good transformers to make it so.
Sounds like you have a loop that is allowing some RF to get in the line.

Bill, KB3DKS in 1 Land

[N3DRB The Derb]

how much audio pow-pow-power are you getting out of those 809's at what plate voltage? I see some in my future.

[ke7trp]

I dont know off hand.  I have never measured the voltage.  However, Its Class C and from a 200 to 220 watt carrier, I can easier go 900 watts PEP on a calibrated lab grade meter.  I normaly run this transmitter "down" a bit to conserve it. About 100% audio.

C

[WD5JKO]

Today, I fed the Globe CHampion 300 with the same audio.  Same cable ect..  I get high pitched whine from the Modulators and the Mod trans. If i turn the audio way down this goes away.  This transmitter is one that you can always hear yourself inside the case.  Any tips to get rid of this? 

Clark,

   Have you tried the setup with the Champion loaded into a dummy load? If so, and it is quiet, then you are having RF feedback.

   Maybe the audio setup is across the room, or plugged into a different outlet creating an RF ground loop? Sometimes running a seperate ground wire from the audio setup to the rig helps where the seperate ground wire goes chassis to chassis. Better yet, use a twisted and shielded cable where the audio shares the twisted pair, and the shield bonds both chassis. Bonding both chassis together also creates a ground loop, so no easy answer. In extreme cases use a cable with two shields insulated from each other where one shield is connected at one end, and the other shield is connected at the other end.

   The audio return wire in the twisted pair could be terminated to ground at the Champion side through a small resistor, say 22 ohms. That way, any small potential between the two chassis will mostly be a small voltage across the resistor. Without the resistor, significant current might flow in the return wire, and magneticially couple that (mostly power line stuff) into the audio.

Regards,
Jim
WD5JKO

 

[ke7trp]

Yeah.. That is a possibility Jim.. I did Run the cable in diff routes and I did put RF chokes on the audio lead. Nothing seemed to help so I decided it was not RF.  I can try grounding. The Audio system is into the same socket as the champ. Maybe I will try things again today..

C

[W2PFY]

I had RF with one of my transmitter getting into the audio. The cure was unexpected but simple. All I did since my mike amp is stereo, change it to the other channel and the problem went away.

Does that problem happen on all bands or just one??

In my case it was 40 meters when I did the above change for the cure.

[W2PHL]

Check the coax jumpers between the tuner, antenna switches etc... An open shield on a short jumper can cause those types of problems. Running the rig straight into a dummy load is a great starting point.

Phil

[ke7trp]

Lots of great ideas guys. Thanks again for the help!  I am going to give it another shot today.. I will try into a Dummy load. I only tried 75 meters so far. 

C

[K1JJ]

Clark,

Yes, RF getting into the audio would be a different problem (different fix)  than audio acoustical feedback.

If it's RF, then what has worked for me:  After grounding/ bonding all audio equipment together well with shielded braid, then add a 500 ohm - 1K resistor in series with the audio leads at the plugs. Then bypass to ground with a .001 disc. (series resistor first, then .001 disc to ground, then the input)  Do the same for outputs of each piece of gear  if needed. This RC network will usually kill any RF getting into the audio. Do it in a few prime low level spots and it may cure the problem, assuming it IS RF getting into the audio. You will need to dial up your audio gain a little to compensate for the resistor losses.

T

[ke7trp]

Ok. I can try that. I have some extra cables around to test that.  I doubt its RF.. I can hear myself in the trans and tubes.  But you never know.  Going to test all this out after I clean the house and wash Toby the dog...


C

[Jim, W5JO]

This problem is pretty typical of Champions.  One thing it might be is the connect points on the bottom of the modulation transformer.  What they used is like all thread with nuts on the inside of the transformer and nuts on the outside.  Then they used another nut to fasten the wires going to a fro from the transformer.

The transformer is potted so access is difficult.  But be sure the posts are tight and not moving around.  The plate voltage on the 809s is near 1000 with 8.5 volts bias.  They do a good job modulating the finals.  I would hestitate boosting the base much.  The power supply is scant in this one, more so than the King and you could see excessive carrier shift if you drive the lows heavy.

[ke7trp]

I have solid state plugs ins. I am sure my Voltage is even higher.  I also just noticed that the Champ is plugged direct in and not on the variac. Maybe, I should move it over to the variac. I have a 30 amp unit down there. At 128 volts in, I am sure the fils and plate are way to high. I noticed the other day it loaded up to 240.  Maybe at the correct voltage, the match will be better. 

I am going to have to take it down for service soon anyways. The Meter switch is not making good contact on the plate current so its hard to tune.. I have to wiggle the switch to get a reading. The fan is getting noisey and needs a cleaning and oiling. The thing has been in daily service for nearly 2 years so its time to check tubes and such.  My Drive control is having to be tuned higher and higher as the months go by.

Do you see any problem with me setting the NC300 ontop of the king? Its going to block some of the cooling.  I have no other place for it. Maybe I will try to get some painted wood blocks to raise it?

Thanks for the help and conversation!  I am going to check the low bands.. The Sp600 is now playing three BC stations around 1440 so that means we got some Propigation..

C

[Jim, W5JO]

I would hesitate about setting anything on top of any piece of equipment.  I know you see it all time, but consider that caps have a temperature rating.  If exceeded, all bets would be off on longivity.  Resistors change value with increasing temperature.

If you want to B-B-Q your NC 300, put it on a spit and rotate it over a fire pit.   

[ke7trp]

Yeah.. Ok.. I am going to find a spot for it.. Should be a nice match up. This Grey NC with the king.  Going back on 75 meters tonight.

C

[Gito]

Hi C

When Rf gets into Your audio system it,s rectified ,as an audio signal and if it's in the right phase ,You got a positive feedback so if You modulated Your Transmitter high enough,that at  certain point it will oscilate .because of the feed-back.
How about changing the connection of the secondary Mod,Transformer,I meant interchange the connection.
for instance call it pin 1 and pin 2 at the secondary  Mod,Transformer.
Assuming  than the B+ is connected to Pin 1 and Pin 2 goes to the plate of RF tube,.
Now change it B+ is connected to Pin 2 And Pin 1 goes to the RF plate of the RF tube ( interchange the polarity of the connection).

Hoping the positive feed-back is changed as a negative feed-back.( changing the audio phase)
If that happens ,than the culprit is RFI


Gito

[ke7trp]

Good point Gito.   I can try to swap the modulator phase. Its a real pain on this transmitter though! 

Anyone have an easy way to tell if you are in phase?  I have tried watching the scope and its very hard to tell..  I have even sent a tone in, set the scope to exactly the line and then powered down and reversed them. I cant tell on my 500 which way is which!

C

[K1JJ]

Clark,

A sine wave is symetrical and will not show a biased polarity.(or so-called phase)  Your voice is the only way to align your audio system since you are tailoring it to your own voice. Other people will have different polarities.

Do you have a peak reading wattmeter?  Sock a continuous Yallo while switching polarity and look for the highest peak reading.

You need to put in a low level audio double pole, double throw toggle switch so you can switch polarity on the fly.

Another way to tell is by the "look" of your speech waveform. The shark fin look is correct for my voice, while the symetrical look is backwards. Again, this is for my voice and yours will probably be different. You can also axe someone on the air who has a scope and knows what he is doing. That will often give a correct polarity.

Another thing to be aware of is that sometimes your own voice may favor one polarity and then later in the day it will change. When I get up in the morning or have a cold, my waveform looks much different. There can also be days when your voice favors no particular polarity - they put the same power into each side. Also, sometimes a voice will have more asymetrical power in the lows than the highs or vice versa.  I believe all these reasons are why it can be such a difficult area to grasp and implement for some. But for many, proper polarity is very easy for their voices to determine and stays reasonably constant over time.

We once measured about 7 hams in a row using a modulation monitor and found asymetricity varied from 110% to 140%.  Mine was only 110%, almost symetrical, so that's why it varies so much - being close to even .

T

[ke7trp]

Ok.. Yeah.. Someone hear mentioned using the voice already.. One way, I got 50 more watts on the Bird 43p. I left it that way. I have no idea about this champion.


PROBLEM SOLVED!!!!!!

It was RFI..   Joe said something that made total sense.. 

I was using a XLR to 1/4 inch cable. This way, I coud plug the Audio technica Low to high impedance matching transformer in.  I realized that my DBX compressor/limiter has XLR outputs.. I grabbed a balanced XLR to XLR mic cable, Ran it from the DBX to the matching transformer, Then plugged that into the Champions back input plug.  BINGO.  No more Feedback and all the audio I want. 

I am running the Globe king with unbalanced cables and have for a year. I dont have this issue.  I am going to swap both transmitters so they run off the Audio rack, Balanced. 

Clark

[flintstone mop]

I think we're getting closer to your problem. What do you see on your 'scope with no modulation? No speaking in the mic? Should be the 2 graticule thick trace with some minor indication of mod from room noise. If you see major stuff going on in the display there may be ground loops or ultrasonic stuff driving the modulator out of its mind. Not to mention the mod transformer.

Fred

[ke7trp]

Nope. Spot on Fred.  Clean and clear. It was that non balanced cable that caused me all the Grief

C