Compatibilty problems with 300w amp & LPF?

Elopid · Post by **Elopid** » Wed Apr 27, 2016 1:27 pm

Hi there all,

An expensive problem, as some will be aware I recently purchased the 300w amplifier on eBay from Spanish seller fmworldcorp.
http://www.ebay.co.uk/itm/FM-Broadcast- ... 1788370942?
I have two 500w rated low pass filters from Broadcast Concepts and I'm wondering if there is a problem using these LPF boards with this particular 300w amplifier because the first time I powered it up I drove it with less power than recommended on the eBay seller's chart and I noticed the reflected power was very high and almost as bad as not having the dummy load connected. (FWD: 200w with REF: 180w approximately)

So I switched it over to the other identical LPF shown above, powered it up and as it quickly reached up to around 30w there was very sudden bright flash and it all went Pete Tong!
After this happened I spoke to someone who advised me that there is a LPF designed by the same people specifically for this 300w module and I should have been using that for this amplifier, I never saw or heard anything about this being mentioned before is this true?
The recommended drive for the module for the desired frequency according to the chart was between 4.0-6.5w but closer to the higher end, so I set the PLL to just 3.0w. The amplifier was DC powered at 48.2v.

Also, will the Broadcast Concepts LPF be OK to run on the PCS 300w module? I'm sorry for all the questions as I'm new to the higher powered stuff and need to make sure I get this all correct.

With special thanks, ELOPID.

Analyser · Post by **Analyser** » Wed Apr 27, 2016 1:56 pm

A low pass filter if done correclty should provide 50 ohms to your PA and you shouldn't need to buy a specific filter made by the same outfit. If you're getting that type of mismatch then something is basly wrong, like a shorted coax or something going to ground where it shouldn't. I noticed before in one of your pictures it looked like you have RG174 coax which is black and plastic, and in my expeience is a nightmare when you solder it because the inner shield tends to melt and short the earth. Is that what you're using, could there be a short?

Elopid · Post by **Elopid** » Wed Apr 27, 2016 2:50 pm

Yes that's what I thought, plus we'd heard that the Broadcast Concepts was a good one to go for.
The first thing I checked for was a short and it was fine.
When I was given the PCS boards they had RG58 coax poorly joining the LPF to the amp module, this was changed straight away.
The 300w amp module is now linked with the same piece of heavy duty coax as on the right hand side which came with this board:

Could it be there was an issue with the coax cable I linked them with as this is some kind of critical part of the matching stage with the 100w Chinese amplifier it's designed for? Is there a certain way you'd recommend how I connect the 300w RF out in to the LPF board?
Thanks, ELo

s2000 · Post by **s2000** » Wed Apr 27, 2016 4:45 pm

Hi Elopid, does seem strange that you tried two lpf boards and got the same results. I can't actually see that last image you put up. I'm just wondering if the fault is on the amplifier board itself? Did you try it without the lpf connected to a swr/dummy load?

If you have another low powered rig (test rig), it may be possible to test some of the parts you connected to see if they are faulty. For instance you could test the lpf boards by connecting the test rig to a swr meter and then connect the lpf with dummy load on the end. I think that would work and if the lpf is faulty you will see some reflections.

I found having a low powered test rig handy, great for tuning aerials, fault finding and also for testing setups before you connect the high powered rig.

s2000 · Post by **s2000** » Wed Apr 27, 2016 4:57 pm

I just read the last part of your post Elopid. If I am understanding you right, its possible the coax from the 100w chinese amp boards may not be of 50 ohms impedance. I know that you can buy all sorts of different ohm coax for combining etc...

Again you may be able to test it with a test rig and swr meter into 50 ohm load.

Analyser · Post by **Analyser** » Wed Apr 27, 2016 5:01 pm

That particular piece of coax is 50 ohms and shouldn't be a problem. If you want to eliminate the coax you can put the boards right next to each other and use a tiny wire jumper to connect them (don't forget the grounds).

Analyser · Post by **Analyser** » Wed Apr 27, 2016 5:10 pm

I have a sneaking feeling this will end up as a fault with the MRF151 board, I'm sure I read somewhere that one had been bought with one of the MRFs soldered in backwards so there can't be much quality control going on. The low pass filter is just a few coils and caps so is pretty hard to cock up, plus you've tried two of them.

Sent from my FTJ152B using Tapatalk

Elopid · Post by **Elopid** » Wed Apr 27, 2016 7:32 pm

s2000 wrote:Hi Elopid, does seem strange that you tried two lpf boards and got the same results. I can't actually see that last image you put up. I'm just wondering if the fault is on the amplifier board itself? Did you try it without the lpf connected to a swr/dummy load?

If you have another low powered rig (test rig), it may be possible to test some of the parts you connected to see if they are faulty. For instance you could test the lpf boards by connecting the test rig to a swr meter and then connect the lpf with dummy load on the end. I think that would work and if the lpf is faulty you will see some reflections.

I found having a low powered test rig handy, great for tuning aerials, fault finding and also for testing setups before you connect the high powered rig.

Hi there s2000,

Yes I did find it very strange, I ensured it was all set up nicely so it should have worked fine.
No unfortunately I didn't check it first before using the LPF.
Yes I have got another lower powered TX around which I will use to check these filters, I also have a friend who has a network analyser at work so I will get them tested.
But in the meantime I have a 150w pallet amplifier laying around with SD2931 that has got no LPF so maybe I will run that at low power say around 40-50w and have a little test with it now to see what's going on.

I appreciate your help, ELoPiD.

Elopid · Post by **Elopid** » Wed Apr 27, 2016 7:37 pm

Analyser wrote:I have a sneaking feeling this will end up as a fault with the MRF151 board, I'm sure I read somewhere that one had been bought with one of the MRFs soldered in backwards so there can't be much quality control going on. The low pass filter is just a few coils and caps so is pretty hard to cock up, plus you've tried two of them.

Sent from my FTJ152B using Tapatalk

Hi Analyser, yes I think this may be possible as it doesn't appear to have been tested with RF because there are no signs of solder down on the tracks for the RF in and out on the board, just signs of solder for the DC power from when he's set the BIAS up.
But I'll get the filters tested first, that way I'll have more idea because the way the reflected power shot up like that was very worrying.
I wish I'd tried it without the filter connected first and set the PSU to a lower voltage.
Next time I will set the driver to 1w and the PSU to the lowest voltage it will allow.

Kind thanks for all your help, eLOPiD

RF-Head · Post by **RF-Head** » Wed Apr 27, 2016 8:45 pm

I had a few of this amps to repair
A customer bought a few from ebay and didnt work or giving max 250W and running realy hot !!
Don't spend your money on new mosfets
Look at the things they sell on ebay and at the 500W lpf

'

sinus trouble · Post by **sinus trouble** » Wed Apr 27, 2016 11:45 pm

Hello Elopid!
Im not a fan of the yanks, but broadcast concepts are usually sh*t hot! unlikely that its assembled incorrectly? you can easily test for shorts with a multimeter, even whack it up to the Megohms and still get open circuit if everything is good!
I do feel for ya bro! them transistors are not cheap! I don't recommend it but if you do decide to replace the transistors? make sure they have the same Vgs group letter printed on the top corner! its crucial for matching devices!

nrg-uk · Post by **nrg-uk** » Thu Apr 28, 2016 4:26 pm

That low pass filter is only rated for 150 watts

RF-Head · Post by **RF-Head** » Thu Apr 28, 2016 6:21 pm

Lol this lowpass filter can handle 600-1000W

from the same seller as the 300W amp
http://www.ebay.co.uk/itm/FM-Broadcast- ... xyLN9SjxYf

s2000 · Post by **s2000** » Thu Apr 28, 2016 6:46 pm

RF-Head wrote:Lol this lowpass filter can handle 600-1000W
from the same seller as the 300W amp
http://www.ebay.co.uk/itm/FM-Broadcast- ... xyLN9SjxYf

Blimey that filter don't look brilliant for a supposed pro product of that power rating. I have never built anything at that kind of power but would assume its best to have proper screening in between the coils, wind them clockwise/counter clockwise and have them at 90 degrees? Also the transmission line in the coupler looks a bit thin for 1000w handling! I have a 300w stripline coupler here and the trace is double the size of that lol

Maximus · Post by **Maximus** » Fri Apr 29, 2016 11:32 am

Sounds like a bad mismatch or biasing problem. The LPF should work fine, even if you are overdriving it to a certain extent.

Or a dodgy transistor.

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Elopid · Post by **Elopid** » Sun May 01, 2016 8:37 am

nrg-uk wrote:That low pass filter is only rated for 150 watts

Hi there nrg-uk,

Thank you for your reply, it's very interesting you say that. Who is it rated at 150w by and where did you get this information from? I have taken a close up picture of one of them for you, I will be testing them both during the week to see if there's any problems with them.

As you can see they are rated at 500w from the manufacturer.

Thanks, ELoPiD

Elopid · Post by **Elopid** » Sun May 01, 2016 8:39 am

RF-Head wrote:Lol this lowpass filter can handle 600-1000W
from the same seller as the 300W amp
http://www.ebay.co.uk/itm/FM-Broadcast- ... xyLN9SjxYf

Hi RF-Head,

Yes I too saw those on eBay and thought that the coil's look a bit on the small side, they look smaller than the coils on the 500w Broadcast Concepts LPF boards I have and they are very expensive.

Elopid · Post by **Elopid** » Sun May 01, 2016 8:42 am

Maximus wrote:Sounds like a bad mismatch or biasing problem. The LPF should work fine, even if you are overdriving it to a certain extent.

Or a dodgy transistor.

Sent using Tapatalk

Hi Maximus,

Thank you for your message, I made sure I was under driving the transistors by around half of the drive. The transistors are 2x MRF151 in a pair.

Thanks, EloPiD

Analyser · Post by **Analyser** » Mon May 02, 2016 10:20 pm

Just to add my 2p into this thread again. The low pass filter in the pictures is fine, even for 500w. If you put too much power in to a filter either the coils get really hot or the capacitors explode, it won't cause a mismatch within a few seconds and blow the PA.
Broadcast concepts stuff is quite good but looking at the pictures of the Spanish stuff I'm not that impressed.

Maximus · Post by **Maximus** » Mon May 02, 2016 10:21 pm

Elopid wrote:
Maximus wrote:Sounds like a bad mismatch or biasing problem. The LPF should work fine, even if you are overdriving it to a certain extent.

Or a dodgy transistor.

Sent using Tapatalk
Hi Maximus,

Thank you for your message, I made sure I was under driving the transistors by around half of the drive. The transistors are 2x MRF151 in a pair.

Thanks, EloPiD

For sure. I'm far from an expert.

If you've got a good soldering iron, why not remove one of the transistors.

Like I say, I'm not an expert, common sense would probably say swap the things about.

Sorry if my replays are slow and sporadic lol