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RM Italy KL 503 Crunchy

A 40 watt dead key out from a KL203 is just a bit much.

RM Italy KL203P CB Amplifier | CB Radio Magazine

RM KL203-P Amplifier Review The KL203 has been one of those amplifiers I recommend to people again and again over the years. They are inexpensive, they put out a solid 100 watts, which is more than enough for most DX, and as long as you don’t overdrive them they can last for quite a RM KL203-P...
cbradiomagazine.com cbradiomagazine.com
 
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The bias does change in ssb mode. The transistors
NZ8N said:
There is no AM/SSB "Class", there is only AM Relay Delay (small, or 4.7uf cap) or SSB Delay (added 33uf capacitance to the RF sniffing keying transistor).

The Bias does not change from AM to SSB.

View attachment 47668
Click to expand...
slightly turn on in ssb mode to keep your voice from clipping. Then a delay is added too with a capacitor.
 
Look at the schematic again.
The RF output transistors (Driver and Finals) are "slightly turn on" when transmitting, there is no difference between AM (slight delay) and SSB (longer delay) on this amplifier.
When no RF is sampled on the input (RTX), TR2 is "relaxed", thereby causing TR1 through R23 to "clamp" the bias voltage feed to 0v to the RF transistors.
When TR2 "sees" RF coming in, this causes it to activate the relay, thereby also disabling the voltage going through R23 to TR1, allowing bias voltage to flow through R9, to the Zener Diode DZ3, then to the RF stages for biasing.
RM Italy KL-503.jpg
 
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As far as this amp sounding bad on SSB. Remove the 3.3 pf capacitor where the so239 is on the input side C34 & put a 15 pf cap in its place. Problem solved. I noticed my meter wasn’t responding to my words on SSB & it sounded horrible around town. A friend 20 miles away could barely hear it. Mine sounds perfect now.
 
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Mikey1966 said:
As far as this amp sounding bad on SSB. Remove the 3.3 pf capacitor where the so239 is on the input side C34 & put a 15 pf cap in its place. Problem solved. I noticed my meter wasn’t responding to my words on SSB & it sounded horrible around town. A friend 20 miles away could barely hear it. Mine sounds perfect now.
Click to expand...
Good info, @Brother DXer might find this useful.
 
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Chris Lawrence said:
View attachment 49069 View attachment 49069 You’ve probably gotten all the info necessary for this question but here is a bad pic for you to see. I bypassed the driver and it opened up the power a great deal. The kl503 is a terrible idea. The single driver gets overdriven by the radio and can’t drive 8 finals well. I think they have the Hd version now: don’t quote me.

[lQUOTE="Greg T, post: 774501, member: 42791"]Anyone have this issue with the KL503? My buddy just received his and I went to set it up for him. Instructions say 1 to 12 watts pep drive so I set up for 10 watts MAX peak and set the power knob on the amp on #4 so he didn't over power it for it's first run. Amp is peaking around 300+ pep. I went home to talk to him and the audio sux maggots. Esses are chopped, audio is crunchy, doesn't sound good at all. Wondering if we got a bad one or if it may be something else. He's running a 2980 with a D104. Without the amp the radio is clean clean clean.
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[/QUOTE]
If this bypass is done the SWR will be high. Mine was flat now it’s a 3 so I have to use a z100 auto tuner.
 
I bypassed the driver so only eight finals are being used. I took out the 3.3 pf Capacitor at the input so239 & put in a 15 pf. I have it in line with my icom. 30 pep on SSB now it does 325 watts peak. I listened to it & it sounds just like the radio does with amp off. Only thing seen is an increase on the s unit meter. Lovely amp after mod is complete. That driver is making it sound bad. Being the driver is gone you can drive it with the rated pep of one transistors. I don’t go over 40. Go on amazon & lookup My Sweety 50 amp 600 watt 12 volt power supply. It has all the protection you could ask for, adjustable voltage I set mine to 15 volts & @50 amps it runs this amp nicely. For $39 you can’t go wrong. It’s black in color.
 
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Mikey1966 said:
I bypassed the driver so only eight finals are being used. I took out the 3.3 pf Capacitor at the input so239 & put in a 15 pf. I have it in line with my icom. 30 pep on SSB now it does 325 watts peak. I listened to it & it sounds just like the radio does with amp off. Only thing seen is an increase on the s unit meter. Lovely amp after mod is complete. That driver is making it sound bad. Being the driver is gone you can drive it with the rated pep of one transistors. I don’t go over 40. Go on amazon & lookup My Sweety 50 amp 600 watt 12 volt power supply. It has all the protection you could ask for, adjustable voltage I set mine to 15 volts & @50 amps it runs this amp nicely. For $39 you can’t go wrong. It’s black in color.
Click to expand...
I will try the 15 pf capacitor as well on mine. Thank you for the report and that power supply looks tempting.
 
If this bypass is done the SWR will be high. Mine was flat now it’s a 3 so I have to use a z100 auto tuner.
[/QUOTE]

100% there is a lot of input swr with this mod unfortunately, I’ve tried tweaking the coils with no success so far.
 
CbRadiosPRONTO said:
100% there is a lot of input swr with this mod unfortunately, I’ve tried tweaking the coils with no success so far.
Click to expand...

Never experienced that after doing this mod. Fortunately, you can buy the HD version, now.
[/QUOTE]
Shadetree Mechanic said:
Never experienced that after doing this mod. Fortunately, you can buy the HD version, now.
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Presumably the HD version has no driver?
[/QUOTE]

That is correct. [edit] Actually, no. That’s incorrect. The driver is still there. I believe they’ve attenuated the input. I don’t understand why they just don’t lose it.
 
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