what is going on that causes backward swing with the amp on?
It swings forward just the radio no amp. And swing forward on a different amp
It swings forward just the radio no amp. And swing forward on a different amp
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backward swing
- Thread starter spyedar7
- Start date
Most cases it's too much radio drive with that one amp. Back off the RF power on the radio with the amp that back swings and see if that doesn't fix it.
Most amplifiers are not directly compatible with a stock legal 40-channel CB. The amplifier tends to exaggerate the carrier, and limit the audio peaks. There are two ways to fix it. One is to turn down the amplifier if it has a High/Low or H/M/L switch. This is the cheap fix, but not so popular. It limits the wattmeter reading. The other way is to reduce the radio's carrier wattage. If it's done correctly this will restore a balance between carrier and audio power and restores the forward swing that the radio had when run barefoot. But so much depends on which radio and which amplifier. They are not all created equal.
73
73
You can try different lengths of coax jumper cable.
It should be significantly less than 6 feet, or significantly more than 7 feet.
11ft, 4 inches of RG58 (with hard, clear center insulation) will often fix the swing on the amplifier.
You can coil up the jumper to keep it out of the way.
The high / low switch is a trick to switches out the impedance balance in the feed input of the amplifier.
This causes the watt meter to show slightly higher power, but causing the impedance to change greatly when modulated.
Switching the high / low switch to Low, switches in the designed feed balance of the amplifier.
In the old days, the amplifiers had this balance circuit, but didn't have a switch on it and it was designed by the transistor makers as part of the amplifier.
So if there is a high / low power switch, place it in Low and try some different length coax jumper, between the radio and the amplifier.
As the radio sees an impedance change while modulated, the radio protection circuit will reduce the power output of the radio, so it is important to keep the drive impedance balanced.
It should be significantly less than 6 feet, or significantly more than 7 feet.
11ft, 4 inches of RG58 (with hard, clear center insulation) will often fix the swing on the amplifier.
You can coil up the jumper to keep it out of the way.
The high / low switch is a trick to switches out the impedance balance in the feed input of the amplifier.
This causes the watt meter to show slightly higher power, but causing the impedance to change greatly when modulated.
Switching the high / low switch to Low, switches in the designed feed balance of the amplifier.
In the old days, the amplifiers had this balance circuit, but didn't have a switch on it and it was designed by the transistor makers as part of the amplifier.
So if there is a high / low power switch, place it in Low and try some different length coax jumper, between the radio and the amplifier.
As the radio sees an impedance change while modulated, the radio protection circuit will reduce the power output of the radio, so it is important to keep the drive impedance balanced.
We really need to know what amp(s) and the radio used.
Have not seen the op for several days now......
73
Jeff
Have not seen the op for several days now......
73
Jeff
If he is putting too much drive into the amp putting several feet of coax inline is going to do nothing to solve the problem.
We need more information.
73
Jeff
The drive impedance of the amplifier changes as the carrier is modulated.
A 4 watt carrier 100 percent modulated produces 16 watts of peak power.
Solid state amplifiers are supposed to have a resistor network on the input, to balance the drive impedance.
The solid state radio protection circuitry reduces the output power of the radio, as the SWR of the radio goes up.
This causes the reverse swing.
The coax jumper trick fools the radio protection circuit and allows it to maintain power as it is modulated.
This will usually result in forward swing of the amplifier output.
This doesn't address the problem of not knowing what radio, what amp, what drive power he is using.
We need more information.
You can keep repeating the coax will fix it without knowing what is going on with the OP's situation but unless he answers the questions it just a guess.
73
Jeff
Yes, I know.
Many of the current amplifiers often have a high / low switch that switches out the input balance circuit and causes all sorts of impedance mismatch with the radio, when this switch is switched to high power.
The radio itself is often reducing the drive power while being modulated, due to seeing an SWR mismatch with the amplifier.
The coax jumper should be shorter than 4 feet, or longer than 11 feet.
If everything is a good, a 3 foot jumper should work well, but if it isn't, the 11 feet, 4 inches of RG58 can help keep the radio protection circuit from seeing a high SWR and reducing the output power when modulated.
Yes, I know those high output power radios can causes all sorts of other problems with amplifiers.
A radio that's too small for an amplifier that's too big will ALWAYS sound sweeter (and swing better) than a radio that's too big with an amplifier that's too small. This has always been the perennial favorite comination that we're used to seeing. The answer is always to turn something down. Maybe the radio, maybe the linear.
Maybe both.
73
Maybe both.
73
This is why I keep a station monitor (Kenwood SM-220, Yaesu YO-101/901, Heathkit SB-614) inline....or have a service cart w/ a Tek 7603 mainframe available to roll around the shack to various spots. Can throw a spectrum analyzer plugin or vertical amp/timebase combo in and look at transmitted signals if any doubt.
Class D stuff is always run barefoot here but there are a lot of other radio/amp combos where verification of TX purity is important.
As far as amplifier input padders are concerned:
If done correctly they're always going to present a near 50 ohm load to the exciter. If done haphazardly or cheaply, on the other hand...
Class D stuff is always run barefoot here but there are a lot of other radio/amp combos where verification of TX purity is important.
As far as amplifier input padders are concerned:
If done correctly they're always going to present a near 50 ohm load to the exciter. If done haphazardly or cheaply, on the other hand...
Looking at the model recommended design amplifiers from the transistor companies, besides proper bias, they have an input circuit that has a network of resistors in it.
This because as an AM carrier is modulated, the impedance the radio sees changes and the input circuit resistors are there to minimize the changes in impedance that the radio sees, as its modulation goes up and down.
Besides poor bias in the amplifiers, often the recommended input circuitry is missing, or bypassed by a high / low switch.
The radio's protection circuit is sensitive to the impedance seen by the radio and will often reduce the output power as it is modulated, in an effort to protect the radio from increased SWR.
This results in the output power being reduced while modulating the carrier.
The input resistors are not there to reduce the drive to the amplifier, but are meant to maintain a steady impedance to the radio's output.
In an ideal design they'll do both.
The FL-7000 in my main shack is one example. There's a rear-panel switch which allows use with a high(er) or low(er) output transceiver, all the while maintaining proper load impedance and input linearity.
Of course, this costs money.