RM Italy KL203 troubleshooting

[arinkuddy]

So I am new here, and just started diving into repairing my lk 203 amplifier. The amp is putting out less than 4 watts, actually almost 0 watts. It sorta worked when it was new but not well. I picked up a dummy load and a MFJ-822 wattmeter. I am using a Uniden 980 to drive the amp. My CB is putting out 4 watts on the money and swinging about 18-20VOLTS deadkey. I believe I have all the tools I need to troubleshoot this (scope, DMM, etc), but I am not exactly sure what I should be seeing on some parts.

I redrew the schematic in KiCad, so I could make better sense of it. I have made some labels to explain some sections.

So what should I be seeing on the gate of the mosfets? Now its swinging between like -3 and +8 VOLTS, which seems low. It looks like DZ1 is clamping the voltage to 20VOLTS so for the heck of it I removed D3 and D4 to remove the voltage clamp out of the circuit, and it didn't change.

The mosfets are not shorted, but I am suspecting they are bad. I cant find anything else obvious. Any recommendations on what to replace them with? IRF520N?

Any help is appreciated, I'm mostly doing this to learn about RF circuits. Thanks

 

[Tallman]

Check the keying circuit first. I could not open your file but I do have schematics here.
Found schematics. TR1 is the transistor that closes RL1 relay. It also provides the base bias to TR2 which then provides gate bias for the 4 RM3 mosfets. These amps do not like high drive input power. Less than three watts is best. Check the voltage on the center tap of t2 and the voltage drains of all four fets.

 

[arinkuddy]

Check the keying circuit first. I could not open your file but I do have schematics here.

So I forgot to mention that I did that first and it seems to be ok. The relay is firing, and I checked and TR1 is pulling the collector to gnd as it should. and checking DZ2 there is about 7.5 volts dc sitting there when keyed up. I even pulled the relay and checked that and it seems ok.

You would need kicad to view it.

here is the PDF, I just redrew the schematic from online so it was easier for me to look at.

 

[Tallman]

The collector of TR2 applies voltage to the gates of the RM3's when it is turned off. If is open the FET bias is applied. When it is on there is no bias to the FET's. Put the diodes D3 and D4 back in before damage is done.

 

[Robb]

Different mosfets may require a different gate turn-on voltage. What is the marking on the original part? May be better to replace the original exactly if possible. If in doubt, either replace them outright or test each one.

BTW, swing watts are not dead key watts. Dead key is is a function of AM mode, in which an unmodulated signal is fed into the keyed circuit. Peak Envelope Power - or 'PEP' - are 'swing watts' are modulated watts. Had me going there for a moment; but I know what you meant.

Did you double-check your KiCad schematic - just for giggles?

Best to give the amp around 1 watt/dead key - rather than to try to drive the snot out of it.

 

[arinkuddy]

The collector of TR2 applies voltage to the gates of the RM3's when it is turned off. If is open the FET bias is applied. When it is on there is no bias to the FET's. Put the diodes D3 and D4 back in before damage is done.

I agree with that, that's what I measured as well. I think the damage is done, but yes I'll be putting them back. I just wanted to rule out the clamping circuit

 

[arinkuddy]

Different mosfets may require a different gate turn-on voltage. What is the marking on the original part? May be better to replace the original exactly if possible. If in doubt, either replace them outright or test each one.

BTW, swing watts are not dead key watts. Dead key is is a function of AM mode, in which an unmodulated signal is fed into the keyed circuit. Peak Envelope Power - or 'PEP' - are 'swing watts' are modulated watts. Had me going there for a moment; but I know what you meant.

Did you double-check your KiCad schematic - just for giggles?

Best to give the amp around 1 watt/dead key - rather than to try to drive the snot out of it.

Ha they ground off all markings I checked.

Yeah I was just measuring voltage with the scope, 18 volts on AM.

Is there an error with the schematic? I tried redownloading it and i opened it fine, if thats what you meant.

Yeah I do need to do that... Not sure how to turn down the Uniden 980 tho

 

[Tallman]

The FET's are RM3 from a KL-203 dealer, I do believe that the IRF-520's would work better than the RM3.
Most of us just throw those away and buy another. It's cheaper that way.
If you use the IRF-520 should work out fine. For the original part:

http://www.hyelectronics.com/KLV-Power-MOS/productinfo/T-RM3/

http://www.digikey.com/product-detail/en/vishay-siliconix/IRF530PBF/IRF530PBF-ND/811781

 

[Robb]

Dunno if the impedance requirements are different from mosfet to mosfet part #. But if so; the xformer windings/impedance may become a problem. Surely someone must know the correct replacement part. Schematic doesn't say? Woof.

Looking at a picture of the circuit board I noticed a single fixed disc cap. Might be fun to put a variable in there to see if a better match might be had - for giggles . . .

 

[arinkuddy]

I got some IRF-520 sitting in a digikey cart for about $1 a piece so its still worth it for me. Maybe I'll get some extras and try building an amp of my own design.

I'm going to pull the fets and test them out of circuit.

How do you turn down the 980?

 

[arinkuddy]

Dunno if the impedance requirements are different from mosfet to mosfet part #. But if so; the xformer windings/impedance may become a problem. Surely someone must know the correct replacement part. Schematic doesn't say? Woof.

Looking at a picture of the circuit board I noticed a single fixed disc cap. Might be fun to put a variable in there to see if a better match might be had - for giggles . . .

which cap are you talking about, C15? if i get it running i would give it a try

 

[Robb]

Don't have a scheme ATM.
Found this pic; thar she blows . . .

 

[arinkuddy]

View attachment 18148

Don't have a scheme ATM.
Found this pic; thar she blows . . .

yup thats it


soo this amp has the fancy 88 mosfets in it

 

[BigKahuna]

If you have time to fix it great - but I agree with Tallman, my time is money so I'd just pick up a new one for $60. Ever since I sold my bigger amps and recently sold my 4 pills I've only been using KL203's and they are the best bang for the buck.

The key with the Kl203's is the carrier input on AM - as Robb mentioned keep it right at 1 to 1.5 watts and you're golden. You can swing the max on the Uniden 980 and you should never have a problem. On SSB they can handle any single final or dual final radio without issue. I've used the KL203 with just about every dual final radio there is on SSB including the new Lincoln II and I've never had a KL203 issue. Low SWR, Low AM input, and you shouldn't have any issues.

 

[miketa_us]

I seem to be having the same issue as arinkuddy, difference here is I've replaced all 4 blown mosfets. Had on hand new STP80NF55-06's and installed those after comparing datasheets with the IRF520. I measure 1.9Vdc at the gates which is below the 2v minimum Vgs turn on voltage.

Being new to mosfet RF amps I have a couple of questions.

If the gate voltage isn't high enough as in my case and RF is applied, is it normal for the SWR to be very high?
Secondly, would I be correct to try and increase the gate voltage above 2Vdc by either swapping DZ2 from 7.5V to a 9v or 10v zener? or adjusting the R3 value in the voltage divider to bring up the Vg?