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Marko 100f mobile amp high swr

Aug 8, 2018
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need a little help. i run a silver eagle 200 amp with no issues at all, swr are great on and off, on the external meter and in the radio. i put my friends Marko 100f in line and while ON the radio shows high swr over 5! the meter shows 1.3 when the amp is OFF the radio meter shows 1.2 so does the external meter. there is no way to tune the amp internally so my question is how do i fix this issue? thanks.
 
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Two ways to 'tune' the input of an amplifier to bring down that high SWR reading when it's keyed.

If it has adjustments, turn them for lower SWR reading on a meter between the radio and the amplifier.

If it doesn't have an adjustment, it's a bit trickier. Calls for changing fixed parts, usually capacitors in the input circuit.

Never have seen a Marko amplifier, so I don't know what you should expect to see inside it.

73
 
i took a pic. looks like there has been some shotty work done in it with after market finals. i dont mind playing around with it to learn.
 

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Also...look at the bead of solder just below the input capacitor in the photo. There should be a choke soldered from there to the input transformer.

This just might be all it needs. I’d be tempted to put a choke on and reflow the solder on the transistors and 10 ohm resistors, and see how it looks on the input.
 
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Also...look at the bead of solder just below the input capacitor in the photo. There should be a choke soldered from there to the input transformer.

This just might be all it needs. I’d be tempted to put a choke on and reflow the solder on the transistors and 10 ohm resistors, and see how it looks on the input.
can you explain this alittle better? i know my way around a solder iron just dont know what should go where
 
can you explain this alittle better? i know my way around a solder iron just dont know what should go where

Here’a a photo off the internet that shows the choke in place:

E218E967-2179-4DF5-8F86-7A36F2CDD576.jpeg



EDIT: I am curious - does the amplifier produce any power like this? I’m in no way an expert on this, but I imagine there has to be a bias of some sort in order for the amp to work. Perhaps the transformer is sitting right on the board?


EDIT AGAIN...It also looks like you may be missing a wrap on the input transformer. I think that will also mess with your input tuning. I suppose the transistor swap could have required changes, but it doesn’t look like whoever worked on it was that sophisticated.
 
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Here’a a photo off the internet that shows the choke in place:

View attachment 32162


EDIT: I am curious - does the amplifier produce any power like this? I’m in no way an expert on this, but I imagine there has to be a bias of some sort in order for the amp to work. Perhaps the transformer is sitting right on the board?


EDIT AGAIN...It also looks like you may be missing a wrap on the input transformer. I think that will also mess with your input tuning. I suppose the transistor swap could have required changes, but it doesn’t look like whoever worked on it was that sophisticated.
yes it puts out about 170 watts. so there is good output just high SWR. i got a few radio checks with it and people say it sounds good. i just dont want to risk the high SWR. i see its missing a wrap and the choke. i wonder if someone tried to tune it for a different band? i think i will start by doing a re-wrap and adding the choke. does the type of choke matter? i do agree that a lot of the soldering looks very armature and i have seen where they touched and melted other wires being careless. i am new to all this but can solder so i think its a good box to learn on since it does work. just want to figure out the tuning side of it. TY much for all your help.
 
yes it puts out about 170 watts. so there is good output just high SWR. i got a few radio checks with it and people say it sounds good. i just dont want to risk the high SWR. i see its missing a wrap and the choke. i wonder if someone tried to tune it for a different band? i think i will start by doing a re-wrap and adding the choke. does the type of choke matter? i do agree that a lot of the soldering looks very armature and i have seen where they touched and melted other wires being careless. i am new to all this but can solder so i think its a good box to learn on since it does work. just want to figure out the tuning side of it. TY much for all your help.

I think they used a 15watt pencil that didn’t hold enough heat.

I can’t imagine what they tried to accomplish with the wrapping, to be honest. They may have just damaged the original and then had to do less wraps because of the large wire. The original wrap was probably no larger than 20awg. Does this have 454s in it?

As for the chokes, most of the class C amps today just have a wire with 3 wraps around a 1/2 x 1/2, or 1/2 x 1/4 ferrite bead.

I’m sure if you hold out for some more advice one of the more knowledgeable members here can help a lot better than I can.
 
I think they used a 15watt pencil that didn’t hold enough heat.

I can’t imagine what they tried to accomplish with the wrapping, to be honest. They may have just damaged the original and then had to do less wraps because of the large wire. The original wrap was probably no larger than 20awg. Does this have 454s in it?

As for the chokes, most of the class C amps today just have a wire with 3 wraps around a 1/2 x 1/2, or 1/2 x 1/4 ferrite bead.

I’m sure if you hold out for some more advice one of the more knowledgeable members here can help a lot better than I can.
it has 2- 455s in it. im in no rush to fix it i will set it aside till i can do more research or have others chime in. you have helped out greatly leading me in the direction of the issue.
 
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Maybe that explains the changes? It looks like they originally had 454s in them, but then again, who knows as the manufacturer could have used anything at the time.

Anyway, good luck getting it sorted out.
that every well could be it !!! i know the finals where changed out, heck they might even be fakes its hard to tell these days. i just didnt want to screw up a radio with the high SWR , it does have output and people say it sounds great
 

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