RM KL500 low power

[eldorado303]

Hello everyone. I just got some new SD1446 transistors for my KL500, since the
last ones burned out. I replaced them, and all was ok, I tx'ed with 1W carrier on AM,
around 27megs and was getting some amplification. But I had an issue, the power
selector that goes from 1 to 6, had some small pieces ( that werent in place at the time )
that kept it from rotating for the other positions ( the switch has 12 positions ), and by
mistake I rotated one or two below the place where I think the '1' position is. And the
output power went down. But I was looking at the amp schematics ( rev 4.0 ) and it looks
like the switch is only a resistor selector, and position '6' should be 0 ohms, straight through
connection, so is it possible that I burned or shorted out these new transistors by rotating
the switch to some other position? Or maybe some other component maybe attenuating
output power? I tested one of the transistors ( I took it out of the amp ) and it still has an Hfe
of 130, but using the diode tester mode it showed that the collector / emitter connection
had a value of 0.520 or so ( and I think it should read O.L. ). Any help is appreciated.

Thanks in advance,

Joao G.

 

[vkrules]

No thats an input circuit. should have isolated the transistors. Most likely you fried a resistor or the finals in the radio itself if it went open circuit. Have a look and see whats common to all sw positions and start there also check to see if you have arced the switch contacts.

 

[eldorado303]

No thats an input circuit. should have isolated the transistors. Most likely you fried a resistor or the finals in the radio itself if it went open circuit. Have a look and see whats common to all sw positions and start there also check to see if you have arced the switch contacts.

hello vkrules. thanks for the help. the resistors on the power switch board, are all ok.
The finals on the rig are also ok. What do you mean to check whats common to all
the sw positions? As I told you, the sd1446, shows around 0.500 between collector and emitter. Do you think they may have gone soft somehow? Thanks, JG

 

[eldorado303]

hello vkrules. thanks for the help. the resistors on the power switch board, are all ok.
The finals on the rig are also ok. What do you mean to check whats common to all
the sw positions? As I told you, the sd1446, shows around 0.500 between collector and emitter. Do you think they may have gone soft somehow? Thanks, JG

The amp is working. I figured out what was wrong. A resistor that goes to the collector of the PA bias transistor was fried. It's a 1 ohm, 1/2W. I replaced it with 3 10ohm resistors in parallel, because I had no 1ohm resistor available, and I got the amp working. I still got one issue though, dont know if its because of the 3.3ohm resistor, but the amp only goes up to 100W output (with 15W in, on SSB ). I'm going to replace the resistor later, with a 1ohm, to see if it improves output, but any ideas about this are welcome.

Thanks,

JG

 

[eldorado303]

The amp is working. I figured out what was wrong. A resistor that goes to the collector of the PA bias transistor was fried. It's a 1 ohm, 1/2W. I replaced it with 3 10ohm resistors in parallel, because I had no 1ohm resistor available, and I got the amp working. I still got one issue though, dont know if its because of the 3.3ohm resistor, but the amp only goes up to 100W output (with 15W in, on SSB ). I'm going to replace the resistor later, with a 1ohm, to see if it improves output, but any ideas about this are welcome.

Thanks,

JG

Hello everyone. I am still having some problems with the amp.
I am getting around 80W on position 1, with 10W in, on SSB,
on position 2, I get around 100W, but I dont get more than 100W.
Switching to 3, 4, 5, and 6 only gets me around 100W, and I've noticed one
of the transistors gets hotter than the rest of them. I wonder what's happening.
Does anyone have any ideas about what might be happening, why I'm only
getting 100W out? Might the transistors be fake, or something else?

Thanks in advance,

Joao G.