RM Italy KL 503 Crunchy

[Greg T]

You get what you pay for. Rm italy, c'mon now, really.

Well, that's a given. We all realize that. BUT my buddy is, shall we say, quite lacking in the coinage department? He's on the back side of 70 and post stroke, needs something to stay busy, and decent amplifiers today are nearly impossible to find without mortgaging the house. He had a nice running Texas Star DX-250 that was pretty clean and talked well until his radio decided to throw a 16 watt carrier into SSB. Tried to find him another one, new or used for a decent price. Not gonna happen in this economy. Most builders are not even building for lack of parts.
He had no choice but to grab what he could afford and now we just want to make it work.

 

[Handy Andy]

In order for the 503 to work correctly in modes, the thing is set up to be switched in between two type of inputs.

YELLOW ARROW
Hi-input - means it has carrier - so the power input would be too high for safely operating unless it's attenuated, that is the AM setting of the switch (AM/SSB) - what it is for...

RED ARROW
to help the user receive weaker signals...

Blue Arrow
turns on or off the amplifier (bypass)

That "cruncy" is from too much input into the wrong mode, or you'll have to install a dummy load resistor across the output of the radio to reduce the signal going into it from the radio but let's you hear from it.

May have to be a permanent fix install to do this...

 

[Greg T]

In order for the 503 to work correctly in modes, the thing is set up to be switched in between two type of inputs.

View attachment 47665​

YELLOW ARROW
Hi-input - means it has carrier - so the power input would be too high for safely operating unless it's attenuated, that is the AM setting of the switch (AM/SSB) - what it is for...

RED ARROW
to help the user receive weaker signals...

Blue Arrow
turns on or off the amplifier (bypass)

That "cruncy" is from too much input into the wrong mode, or you'll have to install a dummy load resistor across the output of the radio to reduce the signal going to the radio but let's you hear from it.

May have to be a permanent fix install to do this...

He only uses it for SSB. Switch is permanently engaged. AM mode is never used. I marked his mic gain knob so he knows where 10W pep ( Amp specs say max 12W pep) is so he can turn it down during SSB operation. He keeps it a little under that because I told him it doesn't doesn't sound good. I'm beginning to wonder if it's worth having a tech bypass the driver stage.

 

[NZ8N]

I used to bypass the driver stage in my Texas Star DX667's, making it into a 500, radio was either a Ranger 2950 or Uniden HR2510 at the time. I made it so I could restore the driver stage later if/when I sold it. Which I did quite often, as I would build another radio/amp setup when I got bored with the current setup.

 

[Handy Andy]

The 503 has variable power - and works like an attenuator pad, but it's not perfect.

It also has "switchable" class - AM versus SSB - so part of the problem is with that switch and to get that inherited delay so SSB works ok, but the AM side changes the class - the 503 does this via that switch - thru a set of jumpers on the board - but has to be set by you.

To use the SSB and only SSB;

By your symptoms - that "crunch" is two fold, one; not enough bias - so the start of the word gets cut off, on top of; number two, the PEAK of the consonant or vowel within the word making the radio signal swing peak - gets compressed or severely limited clipped - off.

So to me this sounds more like the Dynamics are what is causing this problem - signal strength and signal peaks are too widely spaced apart in power level for the thing to truly work right.

I'm not trying to preach but Swing kits', Sideband and MOSFET amps are not compatible in ALL areas - choose any two...Switches are optional.

So the amp runs in SSB, so the carrier kicks it into the middle to high range if you use AM mode with it on, because of the CLASS the amp thinks you want to use...

In AM you have the carrier beating down the amp's inputs already, in SSB the Amp "opens up" it's heart to hear your story and amplify it to the world - you have that vulnerability by Class of operation, in AM mode it's C or about 50% - again a limitation - even worse when you look at it...

Hello - Tin Man...
You're box is being replaced by Aluminum soon...
​

In SSB the Class AB sign is sure to exceed your expectations...

...So - o - o - o....
Until something relents - our hands are tied to help you...

He keeps it a little under that because I told him it doesn't doesn't sound good. I'm beginning to wonder if it's worth having a tech bypass the driver stage.

 

[Greg T]

The 503 has variable power - and works like an attenuator pad, but it's not perfect.

It also has "switchable" class - AM versus SSB - so part of the problem is with that switch and to get that inherited delay so SSB works ok, but the AM side changes the class - the 503 does this via that switch - thru a set of jumpers on the board - but has to be set by you.

To use the SSB and only SSB;

By your symptoms - that "crunch" is two fold, one; not enough bias - so the start of the word gets cut off, on top of; number two, the PEAK of the consonant or vowel within the word making the radio signal swing peak - gets compressed or severely limited clipped - off.

So to me this sounds more like the Dynamics are what is causing this problem - signal strength and signal peaks are too widely spaced apart in power level for the thing to truly work right.

I'm not trying to preach but Swing kits', Sideband and MOSFET amps are not compatible in ALL areas - choose any two...Switches are optional.

So the amp runs in SSB, so the carrier kicks it into the middle to high range if you use AM mode with it on, because of the CLASS the amp thinks you want to use...

In AM you have the carrier beating down the amp's inputs already, in SSB the Amp "opens up" it's heart to hear your story and amplify it to the world - you have that vulnerability by Class of operation, in AM mode it's C or about 50% - again a limitation - even worse when you look at it...

View attachment 47666
Hello - Tin Man...
You're box is being replaced by Aluminum soon...
​

In SSB the Class AB sign is sure to exceed your expectations...

...So - o - o - o....
Until something relents - our hands are tied to help you...

I understand what you're saying, and your description of the issues is dead nuts. The peaks are being clipped. AM is inconsequential in this case, as I said, the amp is never used on AM. The radio is a ranger 2980 with no major mods. I even reinstalled the limiter that someone had snipped. I suppose I could have him turn his D104 back a little, although his AM modulation is crystal clear with nary a hint of over-mod. I suppose cutting back to about 5 to 6 pep drive would be a good test, but, there goes the output. I guess we'll just make the best of a bad situation and try not to let the magic smoke escape. Thanx for the input! Appreciated.

P.S. Almost forgot: I did test this amp on my Yaesu FT-990 before I turned it over to him and I received "good" audio reports. So, I do know the amp is capable. Maybe just doesn't like his rig.

 

[ForestRunner98]

You get what you pay for. Rm italy, c'mon now, really.

Was looking at one of these or the 506.

What would you recommend in a 2 pill that does SSB? Hopper, Fatboy...?

 

[NZ8N]

There is no AM/SSB "Class", there is only AM Relay Delay (small, or 4.7uf cap) or SSB Delay (added 33uf capacitance to the RF sniffing keying transistor).

The Bias does not change from AM to SSB.

 

[Greg T]

There is no AM/SSB "Class", there is only AM Relay Delay (small, or 4.7uf cap) or SSB Delay (added 33uf capacitance to the RF sniffing keying transistor).

The Bias does not change from AM to SSB.

View attachment 47668

So, better to use power level 6 and drop input drive way down?

 

[NZ8N]

I was going to tell you to jumper this, remove that, etc... but it would be easier to just remove L12 (DC power input to the driver transistor), the driver transistor and C24, run a jumper wire from where C24 connects from L15, to L13. This will bypass the driver stage, and "should" work, might have an impedance mismatch, might not. I got out of repairing radios around 2000, long before the "pills" started drying up and they switched to MOSFET's.
There might be a closer part of that trace where L13 goes under the driver transistor, too hard to tell from their service manual and not having one here in front of me to eyeball.

I looked at the schematics again, after pulling L12 and the Driver transistor, C24, run the jumper from where C24 connected to L15, to where you pulled L12, closest to the Driver, which is a shorter run.

 

[Eldorado828]

Hmmm, I believed there's a thread around here somewhere that mentions just as or very similar to what NZ8N is saying. I recall reading up that thread for the good knowledge involved but it's been a good while. Had to do with bypassing that driver.

 

[Eldorado828]

I think this might be it!

https://www.worldwidedx.com/threads/rm-italy-kl-503-watts-vs-amps.171280/page-3#post-604733

 

[Greg T]

View attachment 47673

I was going to tell you to jumper this, remove that, etc... but it would be easier to just remove L12 (DC power input to the driver transistor), the driver transistor and C24, run a jumper wire from where C24 connects from L15, to L13. This will bypass the driver stage, and "should" work, might have an impedance mismatch, might not. I got out of repairing radios around 2000, long before the "pills" started drying up and they switched to MOSFET's.
There might be a closer part of that trace where L13 goes under the driver transistor, too hard to tell from their service manual and not having one here in front of me to eyeball.

I looked at the schematics again, after pulling L12 and the Driver transistor, C24, run the jumper from where C24 connected to L15, to where you pulled L12, closest to the Driver, which is a shorter run.

View attachment 47676

Thanx much!! As soon as i can get him to drop it off I'll give that a try. I'm hoping it will be a bit less sensitive

 

[Handy Andy]

To help...

Augmenting NZ8N's ...
This is my approach to these amps...
But I will mention, you forgot to re-trim for the other radio that may not have enough drive level to offset the push the Driver needs in both SSB or AM modes - the "low-level cut" you experience is from a improper bias trim - here's some details:

NZ8N's given us a better schematic to help you I dug up my notes...​

To help you, you adjust the amp in SSB mode using POWER trying to set level for best "audio drive"...

• don't worry about "clarity" TRIM1 does that...later step...

This does mean you'll have to set power to a given "best results" setting - so when you adjust this amp - best to locate and "turn - dial in" the Power knob to find the best "run" setting - and this becomes your baseline.

The knob tells you how much input attenuation you are using - following the schematic above.

Once you know this setting - R19 is changed to offset this new drive level - only you REDUCE the OHMIC value of R19 - right now? I'd set it for 470 Ω YELLOW - VIOLET - BROWN. Wattage rating about 1/2 to 1/4W - whichever fits that R19 footprint - don't need a BIG sized resistor - the PI-filter attenuator took care of that wattage INPUT...you are trimming down the input signal dynamics - then you'll do a final TRIMMING to adjust the Dynamics of AM and SSB for best results.

• There's tight clearance in there - so be cautious about the reassembly - if you wish, you can piggy back resistors ACROSS R19 to adjust and find the right ohmic values - R19 is a 6.8K resistor - so you have OHMS law on Parallel resistors working with you here.

This amp does use "Class C" but you trim to offset the C and AB effects you need to satisfy the user. To find the best trim point? Locate it's best guess on the POWER knob - then Using R19 - you find a value that let's you use this Best Setting on the HIGHEST POWER Setting on the POWER KNOB - you've reset the input of the amp to match more closely the radios output - and that will be your new MAX - changing R19 changes the INPUT level used for that DRIVER MOSFET.

Then raise/Adjust/Lower the R19 Ohmic resistor level to MEET that initial power setting curve - but on YOUR HIGHEST POWER setting.

You then (By Adjusting) TRIM this for best gain to Audio sound and drive quality once you're set R19 and POWER INPUT knob right. Trim handles the lower end setting of the dynamic range - then you can go all the way up on the POWER KNOB for the PEAK dynamics and the TRIM takes care of the lower Drive levels.

Now, I also know that some wildly swinging radios - like Galaxy, when turned down swing really well but too much in dynamic range for the input of this type of amp - that is where I mention the C21 and C22 removals - this gives you the range of signal RF drive above and beyond what R17 power load resistor does, when you remove one - YOU REDUCE DRIVE (stage gain) by a given amount - remove both, and you're at the LEAST GAIN of the stage can provide. This becomes your parachute if you needed to reduce the drive and swing ratios to make the amp sound better and swing tracks closer to the radios' own output.

NZ8N didn't want to go into details, I don't want to either, but the places I worked with push these amps onto the unsuspecting buyer - so they get wattage, but not the HIGHEST wattage - but they get the better sound because of the board supports this thru the TRIM on the DRIVER.

IF you run into problems. STOP - take a pic or two and post your situation - we can help you thru this - best to go slow and If you have ANY hiccup - supply us with a view of what you're up against so we can guide you.

 

[Greg T]

To help...

Augmenting NZ8N's ...
This is my approach to these amps...
But I will mention, you forgot to re-trim for the other radio that may not have enough drive level to offset the push the Driver needs in both SSB or AM modes - the "low-level cut" you experience is from a improper bias trim - here's some details:

View attachment 47678
NZ8N's given us a better schematic to help you I dug up my notes...​

To help you, you adjust the amp in SSB mode using POWER trying to set level for best "audio drive"...

• don't worry about "clarity" TRIM1 does that...later step...

This does mean you'll have to set power to a given "best results" setting - so when you adjust this amp - best to locate and "turn - dial in" the Power knob to find the best "run" setting - and this becomes your baseline.

The knob tells you how much input attenuation you are using - following the schematic above.

Once you know this setting - R19 is changed to offset this new drive level - only you REDUCE the OHMIC value of R19 - right now? I'd set it for 470 Ω YELLOW - VIOLET - BROWN. Wattage rating about 1/2 to 1/4W - whichever fits that R19 footprint - don't need a BIG sized resistor - the PI-filter attenuator took care of that wattage INPUT...you are trimming down the input signal dynamics - then you'll do a final TRIMMING to adjust the Dynamics of AM and SSB for best results.

• There's tight clearance in there - so be cautious about the reassembly - if you wish, you can piggy back resistors ACROSS R19 to adjust and find the right ohmic values - R19 is a 6.8K resistor - so you have OHMS law on Parallel resistors working with you here.

This amp does use "Class C" but you trim to offset the C and AB effects you need to satisfy the user. To find the best trim point? Locate it's best guess on the POWER knob - then Using R19 - you find a value that let's you use this Best Setting on the HIGHEST POWER Setting on the POWER KNOB - you've reset the input of the amp to match more closely the radios output - and that will be your new MAX - changing R19 changes the INPUT level used for that DRIVER MOSFET.

Then raise/Adjust/Lower the R19 Ohmic resistor level to MEET that initial power setting curve - but on YOUR HIGHEST POWER setting.

You then (By Adjusting) TRIM this for best gain to Audio sound and drive quality once you're set R19 and POWER INPUT knob right. Trim handles the lower end setting of the dynamic range - then you can go all the way up on the POWER KNOB for the PEAK dynamics and the TRIM takes care of the lower Drive levels.

Now, I also know that some wildly swinging radios - like Galaxy, when turned down swing really well but too much in dynamic range for the input of this type of amp - that is where I mention the C21 and C22 removals - this gives you the range of signal RF drive above and beyond what R17 power load resistor does, when you remove one - YOU REDUCE DRIVE (stage gain) by a given amount - remove both, and you're at the LEAST GAIN of the stage can provide. This becomes your parachute if you needed to reduce the drive and swing ratios to make the amp sound better and swing tracks closer to the radios' own output.

NZ8N didn't want to go into details, I don't want to either, but the places I worked with push these amps onto the unsuspecting buyer - so they get wattage, but not the HIGHEST wattage - but they get the better sound because of the board supports this thru the TRIM on the DRIVER.

IF you run into problems. STOP - take a pic or two and post your situation - we can help you thru this - best to go slow and If you have ANY hiccup - supply us with a view of what you're up against so we can guide you.

This is an amazing guide and I truly appreciate the effort that went into this post. I'm not a tech, but I am familiar enough with circuitry to do this mod. What is really a shame is that someone has to perform this much surgery on a brand new piece. They make these cheap amps, they should realize they will be used on cheap radios. My friend headed for Mayo Clinic this afternoon and I wish I could have had this in my hands before he left. I could have had it done for him by the time he returned. I'm going to print these instructions and keep them down in my shack. VERY nice work, Andy and I really appreciate it. Thank you!