Thought I'd try one of these.....

[SOTS 87 7]

conew_p1050673.jpg

Thought I'd try one of these out. Comes in kit form at £25.99 from China.
Very surprising result, output device is a MRF 186 which I thought was a UHF device, but seems to work very well on band 2.
With a better LPF on the business end, about 2watts in and mounted on a good heatsink, This little fella don't even get hot and delivers about 100 watts out.

[SOTS 87 7]

Oh yeh, forgot, it operates on 24 volts at around 5 - 6 amps.

[teckniqs]

It looks like your typical 300w pallet amp for devices like BLF178 etc but with smaller output coils, did it include the said 100w transistor or did you have to purchase it separately? ...I see there's no device in the picture.

[SOTS 87 7]

Belive it or not, yes it did. That's why I was quite surprised at how cheap it was. It has mostly SMD components so its not for the faint hearted.
Here's the ebay item no: 191367693367. It's a thought.

[SOTS 87 7]

That was the finished board before it was mounted on the heatsink. The output device was fitted last after setting the bias voltage.

[teckniqs]

Just bought one

[SOTS 87 7]

OK, funny that, I've just bought 2. going to pair them up into a Wilkinson combiner. See what happens, maybe 200 watts at a push?
I'll let you know the result when they eventually get here.

[SOTS 87 7]

Incidentally, I've been running the first module for about a week without fault at a steady 100 watts.

[teckniqs]

With how much drive and on what (rough if you don't want to specify) frequency?

[teckniqs]

What's the readings on the analyser saying?

[midlandswest]

dont wanna sound dumb here looking on eBay pictures and also many amps I've seen the 3 coils on the output filter are 3 turns just wondered why you went with 2?

[pjeva]

dont wanna sound dumb here looking on eBay pictures and also many amps I've seen the 3 coils on the output filter are 3 turns just wondered why you went with 2?

Depends on coil diameter, but from picture I would say it is LPF with cut off frequency about 300 MHz. But it can be easily reworked...

[Shedbuilt]

Depends on coil diameter, but from picture I would say it is LPF with cut off frequency about 300 MHz. But it can be easily reworked...

I'd say you're about right with the 300MHz figure. The way the inductors are formed and installed, it looks like the spacing would probably work with appropriate value inductors turned the other way; then just change the caps in the LPF for appropriate values.

[SOTS 87 7]

dont wanna sound dumb here looking on eBay pictures and also many amps I've seen the 3 coils on the output filter are 3 turns just wondered why you went with 2?

I modified the LPF 1st & 3rd coil to 4turn and the second to 6 turn and the caps 22pf, 2x47pf then 22pf before the output. I found the 2 turn coils a bit odd for band 2.
About 1.5 - 2 watts drive at 87.7 MHz. Not done any readings as such yet, seems ok into a 50 ohm load.

[teckniqs]

Looks like there's plenty of these about, found another seller knocking them out for £30

http://www.ebay.co.uk/itm/100W-FM-VHF-8 ... 3367&rt=nc

[Shedbuilt]

dont wanna sound dumb here looking on eBay pictures and also many amps I've seen the 3 coils on the output filter are 3 turns just wondered why you went with 2?

I modified the LPF 1st & 3rd coil to 4turn and the second to 6 turn and the caps 22pf, 2x47pf then 22pf before the output. I found the 2 turn coils a bit odd for band 2.
About 1.5 - 2 watts drive at 87.7 MHz. Not done any readings as such yet, seems ok into a 50 ohm load.

Put simply, it's not designed for Band 2. It's a broadband amp, which can work up to well above Band 2, and the LPF is presumably intended to cut off somewhere above the maximum frequency of the amplifier. As pictured, the LPF is no real use for Band 2, since it would probably let the 2F through unattenuated, and probably not doing much at 3F. Here's one for Band 2, from Stephen Moss.......

[SOTS 87 7]

No, you are correct, its not designed for band 2 but could easily be modded to work perfectly well (or should I say correctly) on there without issue.
It just seemed a very good starting point for not a lot of money.

[Shedbuilt]

Yes. I'm not disagreeing with you, but I get the impression you think I am. The comment about it not being designed for Band 2, was in answer to you asking why they would use 2 turn coils in the LPF; ie because the LPF is designed to cut off much higher. As long as the transistor works ok in thy design, and it's stable it should just be a case of changing the LPF, which is why I posted the LPF. The component values could be used in the LPF section of the Chinese board - with appropriate component ratings etc.....

[LAZY]

Can someone explain why the 1000uf capacitor is conected on the input power supply?

[SOTS 87 7]

Yes. I'm not disagreeing with you, but I get the impression you think I am. The comment about it not being designed for Band 2, was in answer to you asking why they would use 2 turn coils in the LPF; ie because the LPF is designed to cut off much higher. As long as the transistor works ok in thy design, and it's stable it should just be a case of changing the LPF, which is why I posted the LPF. The component values could be used in the LPF section of the Chinese board - with appropriate component ratings etc.....

Hell no, I fully agree with you, the point I was trying to make was that it could be a good little amp that don't cost a lot of money. Only drawback is SMD components if your not so confident with soldering. The thing that threw me was using the MRF 186 which is essentially a UHF device, but it appears to work fine.