Replacing RM Italy KL-203P blown transistors with IRF 520's?

[45 North Florida]

Just wondering roughly what kind of output I should expect out of these in this amp? I'm not worried about a small power decrease if that's what I end up with. It's just a "helper" in my side by side anyways. I know, most will say I'm wasting my time on a pc of junk. It was free and I have a jug full of IRF 520's, and just want to replace them just for a tinker project.

 

[sp5it]

530's are better, but if you have 520 just replace.

 

[45 North Florida]

530's are better, but if you have 520 just replace.

I was kind of thinking the same since I have them already. 50/50 considering that I guess.

 

[doffo]

I havent had the best of luck doing this. i did buy some trim pot resistors so you can set the gate voltage. From what I understood, some IRF520s need a higher gate voltage so they turn on. I think one needed to "match" a set of transistors for the amp. Otherwise, I was able to use some 520 transistors taking a shot in the dark, but at least the amp worked again. Of course, doesn't quite make all the power it once did, but at least its better than a broken amp.

 

[9C1Driver]

The correct final is an RM3 but several threads here on the forum here show people using the 520 or 13n10. Not ideal but you can make it work. The kl40, 60, 200, and 203 used to be considered throw away amps. If you can fixit and make it work why not.

 

[Redbeard190]

I purchased three of those a few years ago one of them came out of the box doing 25 watts max. Replaced transistors with 520's a friend gave me.just to see how it responded. after that it would do over 100 watts only problem was SWR'S were through the roof. Was thinking about trying them on an amp that I know is working. Send some updates back.

 

[45 North Florida]

I purchased three of those a few years ago one of them came out of the box doing 25 watts max. Replaced transistors with 520's a friend gave me.just to see how it responded. after that it would do over 100 watts only problem was SWR'S were through the roof. Was thinking about trying them on an amp that I know is working. Send some updates back.

Redbeard190,

I can say the same results came out with me on this one. With 20 watts SSB swing in, I get around 100-120 watts out, which is satisfactory to me considering...but yes the SWR is horrible while on...1.6-1.8'ish.

 

[BC Coyote]

I can say the same results came out with me on this one. With 20 watts SSB swing in, I get around 100-120 watts out, which is satisfactory to me considering...but yes the SWR is horrible while on...1.6-1.8'ish.

only problem was SWR'S were through the roof.

One word...... Harmonics. Most of that additional power is being radiated on frequencies other than the intended one, hence the very high SWR. It could also be that the amp is self-oscillating. Either way, it's not good.....

 

[Shadetree Mechanic]

Probably just needs a little something on the input tune.

 

[Radioman56]

Virtually all the MOSFETs mentioned above, have a totally different Vgs (gate threshold voltage range) of 2 to 4 volts, whereas the Vgs of the "RM-3" MOSFETs is MUCH lower, of which the fixed-bias circuity of the RM Italy amplifiers were designed for. Especially if you want the amplifier to actually be / stay "linear" in its amplification curve.!

Using MOSFETs with a higher Vgs than the bias circuitry was designed for, will only produce MUCHO spurious emissions AND harmonics, and simply show-up on your watt-meter as a lot of "ghost / fake" watts

I had written a few lengthy responses to a RM amplifier "clone" builder in the UK, who was attempting to "peddle" his RM-3 replacements (at quite a profit-margin) compared to "the real deal" from the main importer of RM in America (as I know how little they had cost the guy in the UK) and is contained within the FaceBook Group/Page I created a while back.

Anyone interested (and has some type of FB account) can just click on the web-link (at the bottom) to read and understand the details concerning this subject, along with other web-links for other information, and where to get them.!

Regards / 73
Eric Klein
Klein Communications
( also as KY8E )

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