KLV-550

[Oldtimer]

How can ya tell if a pill is bad in a KLV-550.
All I can get out of it is 275 watts.
I have tried five different radios that dead key 2 watts and swings to 15.
When I lived in NJ my friend got 550 watts out of his driving it with a Cobra 25 dk 2 watts and swing to 10 watts.
Thanks GOD BLESS

 

[Robb]

What does the SWR look like?
That would be the first place I would look.

I just did a little research on this amp.
It appears that this particular amp is prone to failure/damage if the SWR isn't nearly ideal.
Overdriving it - I don't think you did that - would also create damage. They are a bit sensitive - I guess. Never owned or used one myself. Just Texas Star and Palomars.

"...RM KLV 550 transistorised mains amplifier
A solid-state (transistor) amplifier which is powered directly from the mains.
Output power is variable in 6 stages, giving outputs from approx 50W to 300W (depending also upon input). There is a variable 26dB pre-amplifier.
The front panel also has a 13.5V outlet with a maximum current of 3A. This is suitable for powering a low power radio or various low power accessories.
This has the advantage over it's valve equivalent of giving instant output (no warming up) and requiring no tuning.
The disadvantage is that transistors are much less tolerant of a high swr and will blow if the swr is too high..."
-KnightsCB-UK

 

[Oldtimer]

My swr's are below 1.4 with the amp on.
Thanks

 

[Robb]

My swr's are below 1.4 with the amp on.
Thanks

Does the SWR get worse than 1.4 as you talk/modulate?

 

[Oldtimer]

Nope!

 

[TonyV225]

You wont see much more out of that amplifier I see between 300-400 if you know anyone looking for one I have one Ill sell I dont use it but I will say its a nice sturdy amplifier.

 

[Highlander_821]

I'd say there is nothing wrong with your amp. RM's model numbers are not indicative of the units' power output. (Shoot, they named their 2 by MRF455 amp the KL-400!!!!) My 550 with do about 250-260 on AM and right around 300 SSB. But it sounds like crap on SSB---either it is class C, or mine has some issue with the bias.

 

[bob85]

i agree with tony and highlander, you dont have a pill down, the kl550 only manages about 300w fm or ssb with 10w drive,

they sound poor on ssb because the antenna relay switches before the bias circuit, ssb audio starts with no bias at all and drops back to no bias between words and syllables,
the bias circuit is not stable when it is switched on been fed from the main unregulated dc supply that swings from around 30v at low drive levels to around 25.5v@300w,
when bias finally turns on it turns on too high but falls back as drive level is increased,
the onboard regulator circuit would be much better used as a regulated supply for the bias circuit,
the psu has nowhere near enough reservoir capacitance for an unregulated or regulated supply of that current,

i posted a quick fix a while back that allows the bias to turn on just before the antenna relay ensuring class ab operation at all drive levels and distortion free audio,
heres a pic where i tacked a variable resistor in to test the idea,

KL550 BIAS FIX pictures by ukmudduck - Photobucket

 

[Highlander_821]

Bob85, I searched for your post detailing how to modify the bias circuit for better performance, but couldn't find it. I see the pics you have posted, but is there any chance you could post the procedure again?

 

[bob85]

highlander,
you solder a variable to the two points in the pic then measure the bias voltage at the choke that feeds the input transformer, as you start to talk or make a noise you will see the bias kicks in after the relay and needs a good few watts to keep the bias turned on,
when the variable is set correctly the bias voltage will turn on just before the antenna relay kicks in at lower input levels and the class c distortion goes away

 

[liquidh8]

What was the value of the variable? Did you ever come up with a set value for that? I have revision 301 of the amp. Mine doesn't have R17 or C23, and R16 is jumpered? Looks like R-16 was used to drop the input power a bit before the power selector switch.

Mine sounds a bit raspy on power levels 4, 5 and 6 so i am told, I wonder if this is the issue?

THANK YOU!

 

[bob85]

the rev 301 shows it uses mrf422's which need more drive than the higher gain sd1407's used in my amp, r16-17 are input attenuation as you said and not needed for 422's,

no i don't have a fixed value you could drop in, its best to use a low ohm variable and set it just right to get the bias to switch in at lower drive,

if you have a bias issue its easy enough to diagnose with a multimeter on the bias choke L9 to see when the bias activates, it should activate at very low drive just before the main antenna relay kicks in, if the relay clicks before you see bias about .61-.62volt you are in class C at low drive levels heared as scratchy audio on the leading edge of syllables,

alternative no test equipment method is listen to yourself on another radio with headphones,
keyup on ssb and slow heavy breath from your throat with your mouth open at a constant level into the mic while slowly moving the mic away from your mouth ( sounds stupid but it works 100% when you get the hang of it ),
you should hear a smoothe heavy breather right up to the point the amp unkeys, if you hear a scratchy/crackling breather at any point in the test your bias is dropping out at low drive levels,

do the breath test with the amp turned off first to make sure the radio's biasing is not causing any distortion,
you must set the variable VERY close to the point the relay drops out otherwise your bias could be turned on all the time,


hope this helps.

 

[liquidh8]

It helps a bit actually. I put r-16 and r 17 and c23 in there, I am using 1407's like you. I have been getting the scratchy audio reports. I have a 5 K pot, but wasn't sure of what voltage I was looking for. It is so hard to tell if the meter is moving before the relay, lol. I have a fluke and an analog, which I was using. But I didn't want the make the resistance too low for fear of shorting something out. I was looking more for a general idea I guess, like 10-100 ohm, 100-500, 1k-5k, etc.

Thank you for the quick reply, you are the only one who seems to have "fixed" this when searching all over the net, lol. Even revision 500 doesn't seem to have much different in the circuit, so that one must have the same issue.

 

[bob85]

i have two 550's, a rev200 and a later rev, both had the same bias fault, i have no idea why rmitaly did not correct the circuit, their mobile class AB amps don't have the same issue,
i used the variable because it was much easier than pulling the board and playing with resistor values in the existing circuit,

the value is low, less than 100ohms if i remember correctly, the variable i used was the lowest value i had handy pulled from a parts radio but higher than i would have liked which made setting it more difficult,
i don't remember exactly what setting i ended up with, im pretty sure i measured it for tony or somebody else on here,

i use fluke, if you are struggling with the meter you can use the breath test to determine the bias is turned on at very low drive and the meter to make sure it turns off when you unkey, thats if you have a second ssb radio to listen on,
you can outmaneuver a slow reacting meter but you can't fool the human ear,

with the 550 fix you are not changing the bias level you change the level of drive needed to turn the bias on, when it turns on at very low drive the bias is too high around .62v on my amp but its not regulated so bias voltage folds back towards .6v at increased drive level,

good luck.

 

[liquidh8]

ok thank you, it doesn help some like I said. I have a slow fluke, so it is hard. I see in revision 500, they upper the value of c34 an c36, I wonder if upping them more would help?

Or adding a cap even right before TR8? Huh, just curious now.