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RM Italy KL-503 Lost Bias?

buickid

304 'round the minigrass
Nov 22, 2012
221
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Kona, Hawaii
Friend gave me a KL-503 that has been giving him fits. His mobile sounded great on AM, but on SSB, with his S45 driving the KL, driving a TS Sweet Sixteen, he sounded awful. I can best describe it as choppy and crunchy. We thought it was the radio, but swapping in another S45 and also a 257, the problem continued. With the KL off and the radio driving the Sweet Sixteen straight, the station sounded fine. Sure enough, with the radio driving the KL with the Sweet Sixteen off, you could hear the audio distort, which I can only surmise was amplified further when fed into the Sweet Sixteen.

Anyway, I'm guessing the biasing of the KL has gone awry since it sounds great on AM. None of the resistors look burnt, and their values all check out (removing one leg from the circuit when values were far off). Are the little disc capacitors between the pairs of MOSFETs part of the biasing circuit? What other parts should I take a look at?
 

AudioShockwav

Extraterrestrial
Staff member
Apr 6, 2005
7,678
5,640
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Sierras Near Yosemite National Park
With the KL off and the radio driving the Sweet Sixteen straight, the station sounded fine.

Regardless of watt the watt meter says ( :D ) take the KL out of line and talk on the station the way it is.
The S-45 is fine driving the 16 and the added watts you see on the meter will never been seen by the Guy on the other end and the 16 will live longer and run cooler for your efforts.
If i remember right the 503 has a driver, and the S-45 is likely driving the snot out of the rm on SSB and the RM is likely driving the snot out of the 16 on SSB.


73
Jeff
 
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buickid

304 'round the minigrass
Nov 22, 2012
221
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Kona, Hawaii
Good advice for sure. I told him it was too much drive, but I guess the extra watts on the meter are always quite tempting. Still trying to fix this KL though, so I can use it as a backup or something.
 

mr_fx

Sr. Member
Oct 8, 2011
1,537
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Kansas City
yes the kl-503 does have a driver, I know a guy local that claims he bypassed the driver on his, but I have never seen it in person
 

9C1Driver

Sr. Member
Aug 13, 2008
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Good advice for sure. I told him it was too much drive, but I guess the extra watts on the meter are always quite tempting. Still trying to fix this KL though, so I can use it as a backup or something.


Try running a lower powered radio like a Cobra 148 into it. It will probably work fine. An S45 into a KL-503 into a sweet 16 is about the dumbest thing I have heard in a while. If the S45 is the new HP version with the RFX-75 then even worse yet.


Upon doing a google search for the KL 503 I found it has a driver stage which explains why the max rated input
(peak) wattage is 12 watts. Your friend is lucky that KL amp did not blow sky high. It might already be damaged.
 
Last edited:

dd18

Davo
Dec 30, 2008
245
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South East Asia
www.yeticomnz.com
s45 to kl503 to dx1600

Something has to give here..not surprised it sounds like shit.
S-45 30w PEP into the KL503 that is rated for 10w PEP input.
At 10w the kl503 will put out at least 200wPEP and that to me is way more than a DX1600 wants to rammed up its input.
A kl503 doesn't sound very good at the best of times, so go with the flow....piss it off and run the S-45 into the D1600 for sweet safe SSB audio.
No body likes listening to crap audio no matter how loud it is.
 
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buickid

304 'round the minigrass
Nov 22, 2012
221
118
53
Kona, Hawaii
Guys... I understand that completely. But that doesn't help me fix the amp. Its not going back in that setup, its gonna go in my modest pile of extra equipment.
 

9C1Driver

Sr. Member
Aug 13, 2008
3,696
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Guys... I understand that completely. But that doesn't help me fix the amp. Its not going back in that setup, its gonna go in my modest pile of extra equipment.

So the KL is damaged then? Have you tried running it with the correct input?
 

buickid

304 'round the minigrass
Nov 22, 2012
221
118
53
Kona, Hawaii
So the KL is damaged then? Have you tried running it with the correct input?

I would assume that it is damaged, it used to sound fine but something happened and it no longer sounds good. I can try putting it back together and run it on my Galaxy, but even when driven with the variable on the S45 all the way down, still sounded bad.
 

9C1Driver

Sr. Member
Aug 13, 2008
3,696
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Keep in mind that if your S45 is the version with the RFX-75 even at a 1 watt carrier the radio will still peak out at 40-50 watts. I would also turn the Top Gun Modulator off. You really need to test it with a low powered radio.
 

9C1Driver

Sr. Member
Aug 13, 2008
3,696
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Can you just cut to the chase, is it an S45 HP with the RFX??????? That will tell us ALL we need to know. (hint) You can't run an 80 watt radio into that amplifier since it needs 10-12 watts. Ya see what I'm saying????????????????????
 

buickid

304 'round the minigrass
Nov 22, 2012
221
118
53
Kona, Hawaii
Yes, its the S45HP, with the RFX75. Its likely damaged from all that power, yes. Yes, I see exactly what you're saying, but no, it wasn't me, I don't drive the piss out of my boxes like that.
This was given to me, and now I'm trying to fix it so I can keep it on the side as a spare. No, it won't ever be driven by that much power in my hands, the "hottest" radio I have is my 979, and soon a 2950.
 

Tallman

W9WDX Amateur Radio Member, KW4YJ EXTRA class
May 1, 2013
5,123
5,965
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Louisville, KY
Friend gave me a KL-503 that has been giving him fits. His mobile sounded great on AM, but on SSB, with his S45 driving the KL, driving a TS Sweet Sixteen, he sounded awful. I can best describe it as choppy and crunchy.
Can you scan the schematic and post on here? If it was the old KL-500 I could help you out. I have rebuilt at least 50 of those. There is a bias setup for those in which you set the quiescent current at 100 ma per transistor. Easy to do if you know the tricks.
 

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