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Any ideas on adjusting the low power output of the KT8900. The output tranny has a max rating of 12V and specs are centred on 7.5 volts. Yet they try to run it at 13.8V?? I want to reduce the output to about 5 watts, save the transistor some grief and run outboard amps on VHF and UHF instead, but there is no adjustment other than high and low which are just about the same anyway!

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The circuit diagram for the KT9800 can be found here, along with the specifications for the RF finals here.

Looking at the specifications sheet, the transistor's maximum drain-to-source voltage (Vdss in the spec sheet) is 30V, so I would not be worried about the finals unless you're using more than double the 13.8V you are worrying about.

The Vgs has a maximum of 12V, but it looks like the main IC (although I might be mistaken here) uses a supply voltage of 3.3V, so I would expect that to be the approximate level of the input to the finals. Way within the 12V maximum, anyway.

EDIT: To try to answer the question as asked, there is no straightforward way of changing the power output, as the circuit does not look like it has any way of adjusting the level of the signal going into the finals. There are no variable components (e.g. a variable resistor) between the output of the RDA1846 and the final stage. So if the powers to be varied, it has to be done within the RDA1846 itself. And of course we don't have direct access to that because there is a control IC in the way.

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  • $\begingroup$ Thanks for that. I have been trying to find the source of the information I had on this rig. It was in a review I had seen and was evidently wrong! I can stop worrying about the voltage on the rig and maybe concentrate on reducing the RF drive within the external amplifier itself. I need around 100 Watts to get over several mountain ranges between myself and the nearest repeater. It is not just my set up.There is no TV or cellphone coverage out here either so everything is via satellite! $\endgroup$
    – R Johnson
    Jul 4, 2019 at 9:39
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I recently got a KT-8900 and I think I may have found the solution via a Facebook group. There is a tiny SMD potentiometer close to the center of the board that apparently reduces or increases power. DO NOT adjust the pot closest to the side of the radio - I've been told that this messes with the frequency offset or something of the like. You're looking for the one near the center of the board. It is VERY touchy, so make very minute micro-adjustments with a precision screwdriver. If it were me, I'd make the high power output somewhere around 10W, that way you don't burn out the transistor but you have some transmission capability.

If you want to REALLY challenge yourself, you can replace the output MOSFET from the A5M06 (~7W max) with the A9M15 (~20W max) from NXP. They have the same exact footprints and should be compatible and much more suitable in this application. It also goes by the AFT09MS015NT1. It's under $10 on Digi-Key so it shouldn't be too bad to replace money-wise. However, there is a large and annoying heat sink on the bottom of the radio, so be prepared to spend a large amount of time on this. It's doable but requires lots of time to do.

Oh, and here's a link to that group! I can't see it when I'm logged in to FB but I can if I'm visiting publicly:

QYT KT8900/Juentai JT-6188 Mini Mobile Dual Band Radio (US Group)

Hope this helps out a bit!

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