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KL-703 tuned input

D

Dr_DX

WDX-861
Jan 29, 2006
583
870
103
Putting this here because I couldn't find it anywhere. The KL-703 has a high input impedance when run on "high". Mine was showing 1.5 SWR on the low end of the band and 1.6 SWR on the high end. It is hard to read the schematic but it appears to be using a SMD 120pf to ground before the input inductor.

I couldn't find my stash of ARCO trimmer caps, so I had to use fixed Silver Mica caps. My target was to have the high end be better tuned. After experimenting, I came up with adding a 33pf 500v Silver Mica in parallel with the 120pf SMD. This reduced my input SWR to 1.2 on the low end and 1.1 on the high end.

Maybe someone will get some use out of this.

YMMV
 
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Dr_DX said:
Putting this here because I couldn't find it anywhere. The KL-703 has a high input impedance when run on "high". Mine was showing 1.5 SWR on the low end of the band and 1.6 SWR on the high end. It is hard to read the schematic but it appears to be using a SMD 120pf to ground before the input inductor.

I couldn't find my stash of ARCO trimmer caps, so I had to use fixed Silver Mica caps. My target was to have the high end be better tuned. After experimenting, I came up with adding a 33pf 500v Silver Mica in parallel with the 120pf SMD. This reduced my input SWR to 1.2 on the low end and 1.1 on the high end.

Maybe someone will get some use out of this.

YMMV
Click to expand...
Update:

I ordered these (.73 each):

And replaced the Silver Mica with one. I was then able to get ch 1~30 down to 1.0 and 31~40 down to 1.1
 
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linearone said:
I always found it interesting that cb amplifiers and their customers want variable power. Except for ldmos amps, ive never seen this on any commercially available amplifiers. People should control the amp with the radio not the other way around.
Click to expand...
Bingo! Nobody uses a real 50 ohm attenuator in a CB linear. They cost $20 to $30 for each 6 or 12 db attenuator and require heatsink mounting. However, switching them in line is seamless without any impedance shift. In the absence of doing that, I recommend the same thing as you, adjust output power by drive level.
 
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Dr_DX said:
Update:

I ordered these (.73 each):

And replaced the Silver Mica with one. I was then able to get ch 1~30 down to 1.0 and 31~40 down to 1.1 it looks like you've already achieved near perfection, I'll just point out that you can make small changes by bending the inductor coil. I believe this amplifier uses a tightly wound coil that will only allow you to reduce inductance by spreading the turns apart a little bit.
Click to expand...

Dr_DX said:
Update:

I ordered these (.73 each):

And replaced the Silver Mica with one. I was then able to get ch 1~30 down to 1.0 and 31~40 down to 1.1
Click to expand...
While it looks like you've already achieved near perfection, I'll just point out that you can make small changes by bending the inductor coil. I believe this amplifier uses a tightly wound coil that will only allow you to reduce inductance by spreading the turns apart a little bit. Sometimes you can do all of your adjustment at the coil, without having to change a fixed capacitor value. In any event, it looks like you've done a great job, this is just something to keep in mind for the future.
 
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Shockwave said:
Bingo! Nobody uses a real 50 ohm attenuator in a CB linear. They cost $20 to $30 for each 6 or 12 db attenuator and require heatsink mounting. However, switching them in line is seamless without any impedance shift. In the absence of doing that, I recommend the same thing as you, adjust output power by drive level.
Click to expand...
While I agree about controlling output with the input, the 503HD has a 50ohm 7dB attenuator in series on the main input and the 703 has two 50ohm attenuators for low and medium. Not that I will ever use them, I only run it on high and vary my input.
 
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Dr_DX said:
While I agree about controlling output with the input, the 503HD has a 50ohm 7dB attenuator in series on the main input and the 703 has two 50ohm attenuators for low and medium. Not that I will ever use them, I only run it on high and vary my input.
Click to expand...
Let's be fair from both points of view. The RM Italy uses 6 cheap metal film resistors that do contain noticeable inductance to perform this process. They are bulky and produce lots of internal heat when operated in carrier modes like AM or FM. Unlike attenuators specifically designed for this application.

Having said that, it it the best designed input RF power switch I have ever seen on a commercially made 10 & 11 meter amplifier. It does its job well, at reducing drive while maintaining input impedance.
 

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