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Surgeon needed for RM Italy KL503HD

TimmyTheTorch

Active Member
Dec 11, 2022
101
57
28
Northeast Wisconsin
Well it appears I may have inadvertently overdriven this amp with my toned-down Anytone AT-5555N II one too many times. AM still seems to work okay (from what little I use it on AM) but SSB is a bit of a crusty, busted-up, garbled mess even when driven by just a watt or two.

Although I have a soldering iron and lots of other sharp/pointy tools, I also have zero experience with electronic repairs. I think the prudent thing for me to do is find a reputable person that can get this amp working properly again. I'm not looking for a souped up, melt the coax tune, just working like it should work is all.

Anyone know a tech that does this type of repair work but refrains from treating newly sent-in devices as a lab device on which to conduct various nefarious experiments?
 

Eldorado828

8-2-8 in the Lonestar state
Feb 21, 2016
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Do you a forum search and you'll find there's some threads pertaining to just your issue. It may be something you could do yourself.

If not, there's others here that might know of a good surgeon.
 

TimmyTheTorch

Active Member
Dec 11, 2022
101
57
28
Northeast Wisconsin
Do you a forum search and you'll find there's some threads pertaining to just your issue. It may be something you could do yourself.

If not, there's others here that might know of a good surgeon.
Yes, actually having a go at fixing this myself is starting to sound like a good idea. If it's already broken/inoperable, I can't do a whole lot more harm to it if I exercise some common sense.

I'll decide after the battery test tomorrow. If it's still kaput on SSB when running off the battery I'll begin the (hopefully not a) snipe hunt for details on what parts to replace and suggestions on how to replace them.
 

TimmyTheTorch

Active Member
Dec 11, 2022
101
57
28
Northeast Wisconsin
I just finished testing the KL503HD by running it from a fully charged, 700 CCA battery. It was connected to a stock President McKinley radio with the stock mic and I was listening to the output via headphones connected to my Anytone Q5N2. All of the previous bonding/grounding was still in effect.

Like when the amp was connected to the PSU, AM sounded fine. I tested it from about 0.5 watt up to about 3 watts input and modulation at about 80 percent.

Things changed when I switched to SSB. It did not sound terrible when input was at about 0.5 watts and the amp set between 1 and 3 although the TX light would only come on if I nearly hollered. When I increased power to 1 watt or above and/or increased the amp setting to 4 or above the garbage appeared. It went from bad at 1 watt/setting 4 to worse at 1 watt/setting six to OMG I apologize for submitting the public airways to this travesty at above 1 watt/ setting 4 or above. It also seemed that the TX light was barely lighting up when it was spewing forth the garbage.

I'm scouring this site and others to help narrow down if this may be an issue with one/all eight RM3s and/or the one RM4 or something else. I did remove the bottom cover despite it saying it would void the warranty but really, at this point, I would not have the stones to ask the seller or RM to warranty the problem. I did not see anything obviously physically damaged/popped/scorched so I just put the cover back on it and sat it on the shelf.

Any tech savvy person out there feel like a KL503HD repair challenge? Or maybe I can just take it with me to the range this weekend.
 
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Dmans

Sr. Member
Jan 22, 2017
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A U.S. stocking source for IRF-520's. I have dealt with Mouser many, many times. Good service always. At the price of these, replacing all of the mosfets in the RM is a no brainer. You may want to seek out biasing options for this amplifier. (it does clean up the SSB output) I've seen several threads on the forum addressing this.



Edited to add I am not familiar with the RM line of amplifiers to know whether or not they are biased from the factory for proper SSB use, but should be worth exploring the option.

73
David
 
Last edited:

Eldorado828

8-2-8 in the Lonestar state
Feb 21, 2016
2,823
5,112
523
The Lonestar State
I have now ordered the IRF520s. I figure I will need some type of heat sink paste too. Hopefully the small-tipped soldering iron and attachments/solder suckers will work for this.
Yep thermal paste will be needed.

This is what I use, it keeps well for years and only use what you need.

 

Dr_DX

Well-Known Member
Jan 29, 2006
354
418
73
What I use for thermal grease:



BENEFITS AND FEATURES: Grease-like consistency; opaque white in color; zero bleed; very stable at elevated temperatures; excellent thermal resistance and high thermal conductivity; efficient thermal coupler; effective and positive heat sink sealers and heat transfer agent. The product offers high thermal conductivity, virtually no bleed or evaporation over a wide operating temperature range. It will not harden, dry out or melt after 1,000 hours at 200°C. Meets Mil-C-47009 and Mil-C-47113.

MAJOR APPLICATIONS: Mounting transistors, diodes, rectifiers and resistors. Thermal joint compound for any device where efficient cooling is desired.

And IMHO, hands down the best "manual" solder sucker. Period. I don't even need solder wick with this. Even works on double sided PCB's (like CPI) which are a major PITA.



Good Luck.
 
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Dr_DX

Well-Known Member
Jan 29, 2006
354
418
73
I have this soldering kit. Hopefully it will do the trick. I also have several additional soldering irons but they are quite large and would put a real hurt on on the board.

It's going to be good for me to learn how to do this myself because I am pretty certain it's going to happen again.

Where to start.

As I said before, this is the worst possible "HD" amp configuration I have ever seen. You do realize the amp is a KL503 low drive amp (1 driving 8)? Component for component, it is identical. The ONLY difference is the value of the input "flameproof" swamping resistors is lower so more RF bleeds to ground.

Check out the 503HD manual. The schematic in it is the 503. Because it is the same. The amp was designed for a stock, legal CB radio with 4 watt AM carrier modulating 90% and 12 watt SSB - MAX.

BTW, the 503 schematic shows that "RM4" is BS. The schematic shows it to be a RM3 - just like the other 8. Proof?

Here is the "parts" list from the 503:

1674694863439.png


But on the schematic it shows MSFT1 = RM3

1674694985723.png


In the picture of the 503HD insides you can see the row of 9 Mosfets. The driver is the one on the far left. You can also see the 9 (count them) flame proof swamping resistors used to bleed RF to ground so the Mosfets aren't over driven.

In the picture of the 703 (straight 16) , you don't see any swamping resistors because it doesn't use flame proof resistors that don't maintain "50 ohms" to reduce RF, it uses RF resistors (the 2 white things in the bottom center that are mounted to the heatsink) that maintain a 50 ohm impedance in the RF input path.



Did you remember this:

1674695450528.png


Did you ever run the radio with full output when the amp was turned off? Your AM and SSB put out more than 50W PEP with the RF power turned all the way up.

I know a lot of folks here think Scott is a radio god but shame on him for pushing this amp as a HD option to go with the radios he sells. The 703 is a true HD amp and much, much better suited to work with the high power dual final amateur radios he sells.

Whoever the Italian engineer was that decided to use this design as a "HD" amp must have been hanging out with Hunter and was stoned out of his/her mind. He/She/They should be fired.

If you are dead set on using the 503HD, then do your self a favor and bypass the driver and drive the 8 Mosfets directly. It's not hard to do and we have some amp builders on here that could tell you where exactly to make the bypass.

Good Luck!
 

Attachments

  • KL503HD_English_Manual.pdf
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  • KL 503 schematic ver 1.2.pdf
    661.4 KB · Views: 4
  • 503HD-5-500x500.jpg
    503HD-5-500x500.jpg
    55.4 KB · Views: 10
  • Inside-600x342.jpg
    Inside-600x342.jpg
    60.1 KB · Views: 10
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TimmyTheTorch

Active Member
Dec 11, 2022
101
57
28
Northeast Wisconsin
Where to start.

As I said before, this is the worst possible "HD" amp configuration I have ever seen. You do realize the amp is a KL503 low drive amp (1 driving 8)? Component for component, it is identical. The ONLY difference is the value of the input "flameproof" swamping resistors is lower so more RF bleeds to ground.

Check out the 503HD manual. The schematic in it is the 503. Because it is the same. The amp was designed for a stock, legal CB radio with 4 watt AM carrier modulating 90% and 12 watt SSB - MAX.

BTW, the 503 schematic shows that "RM4" is BS. The schematic shows it to be a RM3 - just like the other 8. Proof?

Here is the "parts" list from the 503:

View attachment 62353

But on the schematic it shows MSFT1 = RM3

View attachment 62354

In the picture of the 503HD insides you can see the row of 9 Mosfets. The driver is the one on the far left. You can also see the 9 (count them) flame proof swamping resistors used to bleed RF to ground so the Mosfets aren't over driven.

In the picture of the 703 (straight 16) , you don't see any swamping resistors because it doesn't use flame proof resistors that don't maintain "50 ohms" to reduce RF, it uses RF resistors (the 2 white things in the bottom center that are mounted to the heatsink) that maintain a 50 ohm impedance in the RF input path.



Did you remember this:

View attachment 62355

Did you ever run the radio with full output when the amp was turned off? Your AM and SSB put out more than 50W PEP with the RF power turned all the way up.

I know a lot of folks here think Scott is a radio god but shame on him for pushing this amp as a HD option to go with the radios he sells. The 703 is a true HD amp and much, much better suited to work with the high power dual final amateur radios he sells.

Whoever the Italian engineer was that decided to use this design as a "HD" amp must have been hanging out with Hunter and was stoned out of his/her mind. He/She/They should be fired.

If you are dead set on using the 503HD, then do your self a favor and bypass the driver and drive the 8 Mosfets directly. It's not hard to do and we have some amp builders on here that could tell you where exactly to make the bypass.

Good Luck!
I just want to clarify a few things.

1. Assuming I am able to get this amp working again. I will not be using it with the Anytone or any other similar radio. I learned my lesson. Hopefully it will be usable with my little President McKinley for when I want to mess around with that combo. Or I will sell it since it is not fit for its original purpose and invest in a 703. Or upgrade the antenna and/or tower.

2. Regarding Scott's Radios, I did not buy the amp from him nor did he recommend it or any other amp to me. He does not sell amps nor does he particularly like them at least based on my interactions with him. I did buy the Anytone Q5N2 from him and did let him know that I was hoping to use it with the "high drive" KL503HD. Several times he explicitly cautioned me against this saying that although he is not that familiar with the specs of the amp, even though I would be able to turn the deadkey wattage down quite low that the radio very well may swing higher than the amp would like and it may have bad consequences. Being the type of person that sometimes likes to push boundaries and take a few chances, I did so in this case and paid the price.

Others that have the same/similar setup were more fortunate/luckier than me but their time will probably come. This mistake/setup was 100 percent my decision, nobody else, even after getting professional advice from Scott's not to do it. And yup, I also touched a lot of hot burners as a kid etc.

And yes I was very aware of the 50 watt bypass limit and did not run the radio full blast with the amp off. I do have at times at least some common sense and grasp of basic black and white specs. ;)

I find very few people that can look at the KL503HD specs and be certain exactly how hard they really can run this amp without damaging it. From 1 to 35 watts means 1 watt deadkey and 8 watts swing is a common thing I've read over the past few weeks.

I've built some performance related car, snowmobile and motorcycle engines over my lifetime and more than once I took it just a bit too far... e.g. a bit too thin of a head gasket for an tiny bit more compression then piston meets valve. guess that was a bit too thin. Countless hours swapping jets in carbs... too rich... too lean... Oh, cam profile is a bit too aggressive or not aggressive enough. Then fuel injection enters the scene along with all the fun of building fuel maps that give great performance while keeping the engine from grenading. And tuners, lordy there's another world of fun trying to extract bits of data from encrypted OEM ECUs etc.

I'm at the bottom of a big learning curve with a lot of this radio-related electronic gear and I fully expect to make mistakes along with progress. I just try not to make the same mistake twice.
 
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Dr_DX

Well-Known Member
Jan 29, 2006
354
418
73
@TimmyTheTorch

Glad to hear that a lot of things are clearing up for you on your base. Hopefully, you will be able to find out the issue with the amp. Also, the bad jumper was a good catch.

The Scott comment wasn't about you per se. Here is his YouTube video about the 503HD from 10 months ago:



If he doesn't recommend the amp now, he might consider removing the video. People might get the wrong impression.

Just my .02
 

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