170W Chinese Amp

[Maximus]

Hi Necks. I've just built one of those eBay Chinese amps but with a Freescale transistor from Enigma.

For the first minute of powering up it easily does 150W with about 4W drive. After a minute the power slowly decreases to around 70W.
It isn't getting hot as I'm using a massive heatsink. The only thing getting hot is the 2x 25 ohm coax coils, which sometimes smoke. Would that cause the power to decrease when the coax gets hot? I'm going to try some teflon coax tomorrow when I can find it in the workshop.

Please see attached pictures for reference.

[radium98]

why not showing youre own amp pa

[3metrejim]

I had a repair job to do on one of these. It turned out that whoever had stripped the co-ax had cut almost all of the way around and through the outer shield (you need a good magnifier to check). I'm pretty sure that is what blew one half of the transistor. After making repairs to the co-ax and changing the transistor, I turned the voltage down slightly to reduce the power, and try to make it last a bit longer - better to drop the output by a few tens of watts then make the amplifier run full tilt. I'm pretty sure that the drive wasn't as high as 4W either, just over 1W as far as I remember and gave around 120W output.

[Maximus]

Thanks for your replies gents. I'll change the coax later today. Cheers.

[Maximus]

why not showing youre own amp pa

I'm embarrassed of my blobby soldering because I built it when I was drunk

Also the 25ohm coax melted and shorted internally so it blew the mosfet last night. I'll have to order a replacement on payday.

[rigmo]

I have made several and they work flawlessly. we went over this topic a long time ago. you should read the old posts so we don't get confused.
The mrf9180 only needs 700mW to put out 180W. your description says that you may have already damaged the mosfet. the scheme is not correct and so are the values!
Many have helped me, I will help you too! you don't just look at the wattmeter !!! That is a mistake. efficiency is key to know if your amplifier is tuned! on the contrary, it will burn!

[rigmo]

This pcb design is not good.

On photo is best you can get with this capacitance values..
TUNE LPF with nanoVNA!

1.5turn ferrite core BN43-202

coax wave length cut 25R 13.2cm
50R can be good 13.2cm or 26.4cm...

viewtopic.php?p=35138&hilit=mrf9180#p35138

[Maximus]

I have made several and they work flawlessly. we went over this topic a long time ago. you should read the old posts so we don't get confused.
The mrf9180 only needs 700mW to put out 180W. your description says that you may have already damaged the mosfet. the scheme is not correct and so are the values!
Many have helped me, I will help you too! you don't just look at the wattmeter !!! That is a mistake. efficiency is key to know if your amplifier is tuned! on the contrary, it will burn!

Thank you very much for your input Rigmo. Much appreciated.

OT, I haven't been on the site for a while. However I've just discovered a message from you asking about the LPF design on the Dutch RD15HVF1 transmitters. I'll try to PM you a picture of the schematics. I'm pretty sure it's the same as the previous RDVV/PKBEE designs if that helps?

[Maximus]

I do remember switching the supplied coax from the kit to 25ohm RG316 and it worked a treat on previous builds. I've found a load of RG316 so I'll deffo replace them with this stuff.

OT I found a load of RG412. Not sure if it's useful, but it's the gold teflon stuff. Forgot my mate gave me a roll of it lol

[rigmo]

Thank you very much for your input Rigmo. Much appreciated.

You Welcome, that's why we are here to help each other. to grow.

[radium98]

Good one Rigmo

[Maximus]

Thank you very much for your input Rigmo. Much appreciated.

You Welcome, that's why we are here to help each other. to grow.

Here you go mate