Cheap eBay 2m Amps

[VK1JA]

G'day All,

Cruising through eBay today (as you do...) I stumbled across some cheap 100w amps 'designed' for 80-170MHz.

http://www.ebay.com.au/itm/301315397810 ... 1423.l2649

My first reaction was to shudder and cringe, then I was curious, especially when I saw the price - ~$48 Aus with free delivery.

They seem to be based around the MRF186, and therefore operate in the 24-26v DC range, which appeals to me as I have an 80a 28vDC supply

One has to wonder what 4 of these through the required combiners and then through a LPF/BPF would be like. It may be a cheapish experiment to build one up and really see how they perform on the spec an.

A fellow on QRZ has listed some details and pictures, and it seems there's isn't much info that comes with the kit, a real DIY kit out of China ie. total lack of documentation

http://forums.qrz.com/showthread.php?45 ... fier/page2

Thoughts?

Jayson
VK1JA

[VK3ZAZ]

Caveat Emptor
Buyer Beware
Its only money

[VK4TI]

you need to read this informational , you get what you pay for http://www.w6pql.com/parts_i_can_provide.htm

[VK4DU]

VK4TI, can you sort out your coding/HTML/browser, PLEASE...

Not sure what it is, but when you try to quote a post, it does not work....

[VK1JA]

Lol, I wouldn't seriously use one of these amps, BUT if I had all the relevant test gear I would be interested in putting one through the ringer and test it to see just how bad they are. For 50 bucks it would be a cheapish experiment/test.

VK4TI - I'm well aware of Jim's great work, I've even bought parts from him, it's just a shame he doesn't offer a 400w version. No use spending the big bucks on a 1kw amp when we can only use 40% of it's potential.

[VK4TI]

Well with luck you might have the opportunity and wont that make a change to the way some stations sound
come on the new rules (fingers xed)

I imagine that cheap little amp will be just like a certain electric richards product ?

[VK2XAX]

Interestingly enough there is also a 100w UHF amp available too.

http://www.ebay.com.au/itm/100W-UHF-400 ... 0785857533

http://www.ebay.com.au/itm/100W-FM-VHF- ... 1461653712

It seems a few Chinese sellers have them but I can't seem to find the original source of the design.

So... being a punter, I'm ordering both to play with

cheers
Tim

[VK1JA]

not sure of the original design, but there is a web site printed on the circuit board of http://shop36884913.taobao.com/ if that helps any !

Also, the pics that have been posted on the QRZ thread point to this Chinese forum http://www.hellocq.net/forum/ but I haven't been able to find the thread in that forum that discusses these boards

I'll be interested in how you go with it, how clean the output is, how stable they are and if you need to modify the coax balun/stub (what ever you call them!) so it's tuned to 2m/70cm correctly.

[VK2XAX]

not sure of the original design, but there is a web site printed on the circuit board of http://shop36884913.taobao.com/ if that helps any !.

Ah yes the "Slowly Home Grocery Store" or so Google translate tells me !!

Also, the pics that have been posted on the QRZ thread point to this Chinese forum http://www.hellocq.net/forum/ but I haven't been able to find the thread in that forum that discusses these boards

Thanks I'll have a poke around, see what I can find.

I'll be interested in how you go with it, how clean the output is, how stable they are and if you need to modify the coax balun/stub (what ever you call them!) so it's tuned to 2m/70cm correctly.

Yes, it will be interesting. I'll be able to put them through their paces once assembled and, at the very least, I'll sketch out a schematic for them. I suspect they may be based on the "product notes" for whatever device they're using. I've seen lots of amps appear on the net lately that are nothing more than the manufactures test jig as published in the datasheet. Quote a few of the PCB layouts are now directly downloadable from the likes of Freescale etc. so you can roll your own.

regards

Tim

[VK5TM]

There are some comments in other forums about the actual PA transistor being supplied.

Some are obviously out of other equipment, coming attached to a piece of cut off pcb that you need to remove it from.

I tried digging around the hellocq site, but the Japanese translation's were anything but helpful in trying to locate the info on the PA design.
It looks very similar to a Motorola app note from some time ago, still looking for my old Motorola books to try and locate it.

[VK1JA]

G'day Terry,

Yep, spot on there. From all accounts what you get is basically a circuit board, a handful of parts in a bag, a second hand PA transistor still attached to their original circuit board and a handful of construction 'pics'. It certainly can't be called a kit. But for the price, really, what can one expect!

The 80-170MHz version seems to have some sort of filter network on the output - going from the pictures, while the 400-470MHz version seems to have no filtering on the output? I could be wrong here though.

[VK2XAX]

Hi all,

Both amp kits have arrived and, yes the documentation is lacking. I had to prompt the seller for it and he only provided a parts placement diagram and a couple o photos of completed units.

While the VHF unit has consistency in the photos, the UHF unit does not. Photos alternate between trim caps and ATC units on the PCB L on both input and output.

Both units have a 7 pole low pass filter on the output on the PCB. As to what the values of L are to work out what the filter looks like, I don't know yet, the only give you detail like 2 turns, 1.2mm wire on 8mm former. While that works out between 10 - 30nH dependent on whose math you use, the telling will be in the building

Both units use an MFR186/XRF186 - the VHF unit was a desoldered unit, the UHF unit is still attached to a chunk of PCB that they've obviously dremmeled out of some other amp.

The ATC caps are also second hand but everything else is brand new. The PCB's look very well made with some of the finest via stitching I've ever seen!

I prompted the seller for the schematics but he claims his supplier didn't give them to him either.

I'm going to tack issue with the second had parts since the sellers add insists that everything is "100% brand new"

Anyway, first up I've drawn out the schematic of the VHF unit as is shown below. I'll redo it again in DipTrace when I get some time.

If anyone recognizes it, I'd be grateful to know where it originally came from.

I'll post a schematic of the UHF unit soon, I'm only part way through that.

regards

Tim

[VK2XAX]

Hi all,

Here is the schematic of the UHF amp.

Notice the bias circuit is a little different. What doesn't help is that there are caps between the transmission lines in some photos, not in others and are trim caps in yet other photos

The photos also show an early rev of the PCB. I.e. in the photos there is a wire running from one side of the board to the other to distribute bias. On my board there is actually a trace.

Some photos show them taking the power out directly from the coax loop others show the low pass filter complete.

These amps have obviously gone through some sort of evolution, it would be nice to find their origin.

Its interesting that the MRF/XRF 186 is being used at such low frequencies, its also interesting to not that the recent spat of HF amps for the 817 , coming out of China, also use the 186!!
regards

Tim

[VK5TM]

Except for the bias circuit, it is almost straight off the datasheet for the UHF version

VHF version is similar.

[VK5ACY]

I don't like the bias circuit !

Even in the 'Gemini' packages I haven't seen both FETs ramp up bias identically.

LARCAN, NEC and others (esp.TV PAs) all use separate bias circuits for each FET in the PA devices.

"Your Mileage May Vary"...

[VK5TX]

I using a xrf186 in a hf amplifier I built. Pumps out around 50w on 13v. They use them because they are cheap.

[VK2XAX]

So anyway I've most assembled the 2m amp and tested the output filter to see what it looks like. Results below.

I've included a pic of whats in the box for references sake.

They supplied the wrong values for the ATC caps so I took them to task over that, they said the values were right and sent me a new parts layout to support ther case but that just made the errors worse.

I did a check of the 70cm amp and found that the ATC's were wrong there too.

I took them to task over that too, and asked for at least the right value ATCs.

After a bit of too n fro, there's a package in the mail to me

Not sure if its the missing values since their last email indicates it might be the whole thing again !!

We'll know in a week or two.

regards

Tim

[G5BBL]

Hi All,

I bought the 400-470 MHz Kit, wanting to get a bit more output from my FT857 for UK Contesting on 70cm.
Looks like a interesting project. a bit like a puzzle.
Has anybody built the UHF version built and got some output from it?

It seems that most of the parts, including the older version of the PCB were supplied in my kit with just the 5.1V zener and some ATCs missing or wrong value.
11 ATCs were shipped, 5 pcs 30pF, 3 pcs 6.8pF, 2 pcs 12pF and 1 pc 8.2pF
Means that the 2 x 100 pF and 2 x 470pf Output Capacitors in the circuitdiagram are missing (Thanks Tim - your circuit diagram is great help)
I can get some 1210 NPO 500V caps locally from Farnell though
4 pcs trimmer were supplied, probably just ok for the input circuit tuning but no way up to tune the output circuit.
I think I need fixed value ATCs or get some proper UHF TX trimmer caps (expensive)
Any feedback welcome.

73s Jan G5BBL

[VK2XAX]

Hi All,

I bought the 400-470 MHz Kit, wanting to get a bit more output from my FT857 for UK Contesting on 70cm.
Looks like a interesting project. a bit like a puzzle.
Has anybody built the UHF version built and got some output from it?

It seems that most of the parts, including the older version of the PCB were supplied in my kit with just the 5.1V zener and some ATCs missing or wrong value.
11 ATCs were shipped, 5 pcs 30pF, 3 pcs 6.8pF, 2 pcs 12pF and 1 pc 8.2pF
Means that the 2 x 100 pF and 2 x 470pf Output Capacitors in the circuitdiagram are missing (Thanks Tim - your circuit diagram is great help)
I can get some 1210 NPO 500V caps locally from Farnell though
4 pcs trimmer were supplied, probably just ok for the input circuit tuning but no way up to tune the output circuit.
I think I need fixed value ATCs or get some proper UHF TX trimmer caps (expensive)
Any feedback welcome.

73s Jan G5BBL

Hi Jan,

I complained about the missing and wrong value ATC's and the supplier is shipping me those values.

Don't give feedback to them until they've sent you the right values, you could even do the "request a refund" thing with eBay since all the adds for these amps claim "100% brand new" which they are patently not!. I pointed this out to my supplier hence the "new" or missing parts on their way to me. Hopefully they'll turn up this week. Just make sure all your communication with them is through the eBay system so you have a "paper trail".

I've not built the 70cm unit yet but my 2m unit is just awaiting the missing caps and me soldering in the transistor and coax.

As for ATC/ceramic RF caps, we just had the illustrious Wyong Field Day where a number of car-boot sellers had junked ex analog TV transmitters. I bought 6 transmitter boards for $50 and liberated more than $200 worth of ATC caps off the boards as-well-as stacks of 141 hardline, SMA connectors and a wealth of other miscellany to fill my parts bins

Oh and some big arsed heat sinks for my next QRO projects.

cheers

Tim

[G5BBL]

Hi Tim,

I also complained about missing values and the supplier responded with a component placement diagram for the UHF Amplifier.

It looks that the four output capacitors can be any value between 30p to 100pf, I plan to mount 2x100pF (not supplied) and 2 x30pF (supplied).

Elsie modeling shows less insertion loss when the caps are made around 470pF, the value you showed in your ckt diagram.

Looking forward to get some RF out from the kit soon.

73s

Jan G5BBL

Component placing 3392.pdf

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