Amateur Radio

Replacing broken RF finals QYT KT8900 and adding fan modification.

QYT Modding

QYT KT8900 has good form factor that fits in the palm of your hand. It’s a dual band radio 2m/70cm that supports dual frequency monitoring has an acceptable receiver capability for the price, good audio performance (loud and crispy). Supports FM radio broadcast listening moreover it has a built in repeater function that will work in combination of another QYT KT8900 or pair it with QYT KT8900D by just enabling the function on the menu system and adding a repeater interface cable which can be built easily with spare LAN cable, RJ45 connectors and crimping tool.

If it’s not for the faulty heat sinking then probably this radio will work longer than expected rather than giving up the magic smoke of the RF finals too soon.

Preparing the RF finals removal and replacement

Disassembling the radio to replace the broken RF finals reveals that the screw holding the heat sink is loose just secured by a piece of copper strip inserted to the metal spacer to float the heat sink above the RF finals.

The radio board showing the QYT RF finals

The QYT radio board showing the RF finals when the copper heat sink is removed, also shown is the screw holder with a piece of copper strip used to tighten the loose screw a bit. This copper heat sink is not enough to absorbed the heat coming from the finals and not even touching the aluminum body

Replacing the RF finals is moderately easy provided you have the right tools a hot air rework station, solder flux, a pair of tweezers. The radio board backside reveals a slightly bigger heat sink which probably is intended to absorbed the heat more effectively by coupling it on the radio chassis, but without using a heat sink compound this also is not too effective to properly address the heat issue.

A much bigger copper heat sink attached at the back of radio board to couple directly to the radio chassis, but QYT didn’t used a heat sink compound to properly transfer the heat to the body.

Time to replace the RF finals, apply the solder flux on the chip, heat the board just enough for the solder to melt at the bottom of the RF finals, remember that this is also attached to a copper heat sink below so heating it will take a little longer before you can see that a solder is flowing on the side of the finals. Once removed successfully replaced the finals with a new one. Good news for people living in the Philippines, the RF finals is now available by visiting your favorite electronics store in Gonzalo Puyat street in Sta. Cruz Manila price varies depending on the stores.

Shown in the picture is the intended aluminum heat sink replacement on top of the QYT RF finals I will attached it directly to the chassis and apply a heat sink compound when finished.

Replacing QYT RF Finals

Replace the RF finals with AFT05M. Preparing the RF finals to be replace. Look for the solder to melt and flow on the side of the finals.

Preparing the fan modification on the radio chassis

Attaching the fan on the radio chassis requires a bit of drilling and tapping to hold the fan securely at the back of the chassis. For QYT KT8900 you can see at the back of the chassis that they somewhat prepared a fan mounting holes, but they didn’t pushed through with the plan. On later versions of this radio a fan is now attached to help in cooling the radio.

At the back of the radio you can see four holes probably intended for mount the cooling fan. I also added an audio port modification here as this radio doesn’t have a way to attached a headset

Drilling the fan mounting hole with a drill step bit.

Preparing the screws holder by tapping a thread.

Tapping a thread for the cooling fam I used M3 tap bits for this. Also shown is the wiring going to the audio port this is attached to the speaker point on the board.

Putting it back together.

The cooling fan is ready for mounting on the chassis. The heat sink is attached firmly on the chassis. Heat sink mounting compound is applied at the bottom of the chassis and at the side holding the heat sink.
The cooling fan is now mounted on the chassis. The heat sink is attached firmly on the radio chassis. Heat sink mounting compound is applied at the bottom of the chassis and at the side holding the heat sink.

Finally the finished modification applied to the radio.

The QYT KT8900 radio with the cooling fan attached at the back, the old heat sink removed and replaced with a new one.

The final look and successful cooling fan modification.

QYT KT8900 fan mod
The QYT KY8900 with the cooling fan modification applied. An aluminum tape is applied to closed the gap on the fan so air suction will work more effectively inside the chassis

18 replies on “Replacing broken RF finals QYT KT8900 and adding fan modification.”

How did you solder the chip to the board, ? AKI, solder the bottom of the device to the lower heat sink? Or did you
just solder the tabs. ???

You need a rework station to be able to remove and re solder the component to the radio board. Sometime people use heat blower to do that if you have one check out youtube for reference in doing it with a blower. Cheers!

Hi Stan, you need a solder flux to put it back on the board. If you managed to removed it using a blower you can put it back as well with the same tool but you have to put a solder flux on the board so that you will know if the solder is flowing at the back of component once the solder has reached the melting point the flux will help your solder to flow on the board and at the back of the component. Tip the component slightly with a twizzer to align it on the board and let it cool down. You will damaged the component by overheating usually a part of it will pop or smoke so you have to move the blower around the component and be alert when the solder starts to flow.

I got the lower heat sink off by using a micro flame, as the irons would not heat the heat sink up evenly, it came off clean
There seems to be too little room to feed solder in from the top side at the same time keeping the heat sink
at flow temp. Unless you have a sneaky way of getting it in there?

Usually you just need to be concerned on top, you don’t need to remove the bottom heat sink unless you want to change it. Applying little bit of solder and solder flux before placing the device on the same spot and reheating it will fix it in place. Cheers!

that’s is a good reference but the important thing is when the solder starts to flow it is around 217°C, and you have 60 to 150 seconds to align it properly not exceeding peak body temperature < 260°C

So far this is what I have come up with.
Solder part in place from the top
Solder a small bridge copper stub to the bottom of the device extending through the PCB
Cut a notch in the lower heat sink, letting the bridge extend through the notch
Solder heat sink to the bridge
use a flame touch to re-solder the heat sink into place. Starting at it’s edges

My thought is I can work quickly and stay below too much heat for the device…??

Good day po, Sir ibig sabihin po dapat pong palitan ang heat sink ng final po? Ano po ang magandang Ipakita na heat sink po para doon?
Salamat po sa tutugon.
Maraming salamat po

hi @Bon Consular – heatsink will definitely help in reducing heat directly applied to the radio finals thereby lengthening its usable lifetime (the bigger the better, its up to you on how you can fit it inside the case though), however it will eventually fail as it is overdriven to work at 25watts output power when the actual datasheet specifies that it is only a 6watts component see here:

A 6 watt transistor is incorrect in this application. The correct final transistor is AFT09MS015NT1 available from Mouser for about $6. This is tested to 20W+ @870 MHz.

That is correct @Chuck the default RF finals supplied and installed on those radios are underrated when they first came out when I first created the article wayback 2018, now there are also 50W version of the RF finals which has similar mounting based on the original RF finals supplied but not sure if it would fit nicely or its a bit bigger will take a look if I get my hands on it. Thanks for commenting.

Interesting. For what it’s worth, I think the KT8900 radios are amazing. I’ve had mine for about 3 years of daily use in the car until the Final went out. It started to loose the CTCSS memory and when I checked the output, it was only transmitting at less than a watt of power and barely reaching 2 miles point to point. I was curious as to what the blue wire you installed on the board was?

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