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Negative carrier shift

R

Radioceo

I have a 5KW AM that's exhibiting 7 to 8 percent negative carrier shift with program audio. Both main and auxillary transmitters have this problem. No problem with either transmitter into dummy load. Carrier shift is noted on both transmitter output power meters, antenna base current reading and remotely located modulation monitor's carrier level. Antenna base current is one amp lower than normal 10.5 amps, without modulation. Transmitter RF output meters and normal base current have always agreed in the past.

I went to the site to address an unrelated problem and did not have a bridge, audio oscillator, or signal generator with me. I'll soon be going back soon with equipment. The ATU network is a T, with inductors in series legs and a capacitor in the shunt leg. Nothing appears to be heating up.

Any suggestions or comments?

Thanks
 
Do the lights in the building dim with modulation?

Anothe possibility with solid state rigs. Is there any DC voltage showing on the output of your processing?
 
Modulation is not excessive. I even cranked it down to 50% and still got about 3% shift on peaks. The main transmitter is a BE solid state running on single phase. The standby is an old Power Rock running on 3 phase. There's no light dimming with modulation. Keep in mind, that both run fine into the dummy. That would suggest to me that the problem lies somewhere from the output of the RF line switcher to the tower. All ground straps have good integrity.
 
Since it is doing it with two different transmitters, I agree that it is either something in the tuning/antenna or it's power. Get a scope sample of your power and see if it's holding up without dipping with mod peaks. If that's O.K. I would bet that you have a component that has changed values and the tuning is off to one side of the carrier. I've seen some pretty bizzare things happen if the tuning in the phasor (if you have one) or ATU is not right.

Good luck!
 
On the BE, has the reflected power risen? If so, something in the antenna system has likely changed.
 
And 'old Power Rock'. Sheesh <tm> I put I believe S/N 5 on the air. You could set it for whatever carrier shift you wanted within reason.. certainly you could get a bit of positive shift out of it by playing with the resting pulse width. Something ain't right in your antenna system. Get your bridge and oscillator and the original proof docs fropm when it was built, and go see what's changed. Put it back to the original at it ought to work much like the dummy load.
 
The BE does indeed show an increase in reflected but not really all that bad.

I had already placed a call to the station inquiring about the availability of the original antenna docs and there is a copy in the station's public file if not elsewhere. I think it will be very interesting to get the then and now comparison.

I've had to deal with 3 of the Power Rocks throughout the years. Challenging beasts for sure and dangerous as a cocked gun.
 
I found the radio different, but not particularly problematic. And I never saw anything particularly dangerous about it, although if you are used to playing with transistor radios, it or any other tube rig might hurt you if you aren't careful. The safties always worked on it, and policy was - and should be - two people present anytime high voltage is exposed. It had some no foolin' high volatge points. It also had the ability to run at less than about 1% intermod and very low harmonic distortion.. best sounding radio of its day. The plate modulated competitors didn't measure up at all. It did exhibit the Famous Fabulous Crystal Gayle Syndrome, but that was courtesy of Eimac rather than any inherent problem in the box itself.
 
littlejohn said:
I found the radio different, but not particularly problematic. And I never saw anything particularly dangerous about it, although if you are used to playing with transistor radios, it or any other tube rig might hurt you if you aren't careful. The safties always worked on it, and policy was - and should be - two people present anytime high voltage is exposed. It had some no foolin' high volatge points. It also had the ability to run at less than about 1% intermod and very low harmonic distortion.. best sounding radio of its day. The plate modulated competitors didn't measure up at all. It did exhibit the Famous Fabulous Crystal Gayle Syndrome, but that was courtesy of Eimac rather than any inherent problem in the box itself.
Click to expand...

The -15 kV was everywhere in that box. :eek:
 
That's why the pulse drive was on a piece of fiber, and why it wasn't rated to as high an altitude as the 820 series. But when you turned the first one on in a market, you kicked a$$ and took names with it. And if you didn't dehumidify, you could read the schematic by the corona glow at night, too.
 
I mis-read the dummy load part.
Yep, like they said, sounds like a bad, non-linear load. Besides the carrier frequency do measure every 10 KHz +- 40 KHz.
 
RE: Rock

I loved the way they used the fiberoptics. Clever way to work around others' patents. At the time, I found the folks in Dallas more technically-oriented than the agressive sales types at Harris in Quincy. Not to take away, however, from the real AM genius at Harris, Hilmer Swanson, who must have been involved in every new high-fallutin' AM modulation scheme.
 
littlejohn said:
I found the radio different, but not particularly problematic. And I never saw anything particularly dangerous about it, although if you are used to playing with transistor radios, it or any other tube rig might hurt you if you aren't careful. The safties always worked on it, and policy was - and should be - two people present anytime high voltage is exposed. It had some no foolin' high volatge points. It also had the ability to run at less than about 1% intermod and very low harmonic distortion.. best sounding radio of its day. The plate modulated competitors didn't measure up at all. It did exhibit the Famous Fabulous Crystal Gayle Syndrome, but that was courtesy of Eimac rather than any inherent problem in the box itself.
Click to expand...
I put serial numbers five and six of the Power Rock in service at WWDC. We were the first to see the Crystal Gale Syndrome. Steve Schott and the guys from Collins had seen my homebrew audio chain made of a triband using LA-4s on the low and mid and a Specrasonics 610 on the top band feeding a Mulitlimiter into a Loud and Clean Clipper (hand made by Grady Motes).
Before everyone critiques that chain, it is presented for historical reference only. I was only 24 and still in front of a long learning curve with audio processing.
They kept insisting that it was my "unorthodox audio processing" causing the trouble, until other stations started reporting the same thing.
That transmitter was indeed a flamethrower in both audio and special effects pyrotechnics!
 
We had an automated country format and Don't It Make MY Brown Eyes Blue was in the hot rotation. I called the factory, and bless them, instead of "It can't happen" someone - I believe Ken Branton asked me what we were playing. I'd thought it was random trips. He says "Waitaminnit, I think somebody else is seeing the same problem". Then asked me to swap the tubes, which I did and it ran all right. I think they eventually sawed a tube in half to find out what was going on. We were driving a tower with an 800 Ohm base impedance at carrier, and it used a simple T network. Jack Sellmeyer redesigned it into a two step network which broadbanded the coupling nicely and the thing sounded great. You could suddenly hear the lack of headroom in the DPS-1 and the cart hiss.
 
Problem solved. Replaced shunt capacitor in T network. All is working normally again.

By the way, I'm told that the Power Rock at this facility has for years been affectionately known as Old Nickola.
 
The one which resided in the development lab in Richardson was for a while known as 'Jack's Pet Rock'.
And even today, when it sees 50 Ohms +- j0 more or less, it is still capable of a competitive sound. Not a bad design at all.
 
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