Output power drift

Everything technical about radio can be discussed here, whether it's transmitting or receiving. Guides, charts, diagrams, etc. are all welcome.
kevycorsa
who u callin ne guy bruv
who u callin ne guy bruv
Posts: 26
Joined: Sun Apr 12, 2015 2:32 pm

Output power drift

Post by kevycorsa » Wed Apr 04, 2018 7:04 pm

What could cause output power drifting from amp
All voltages are stable
Pll is stable at 1w
Mrf151 chip
This is the type of amp
You do not have the required permissions to view the files attached to this post.

User avatar
Analyser
tower block dreamin
tower block dreamin
Posts: 321
Joined: Fri Sep 26, 2014 9:11 pm

Re: Output power drift

Post by Analyser » Thu Apr 05, 2018 2:56 pm

Power drifting down:

Main cause, inadequate cooling.
Other causes, old/ dying transistor, input to amp reducing, components overheating (output end).

Power drifting up:
Bias design

s2000
tower block dreamin
tower block dreamin
Posts: 442
Joined: Wed Nov 26, 2014 1:02 pm

Re: Output power drift

Post by s2000 » Thu Apr 05, 2018 4:39 pm

What type of power supply is it? If it's unregulated then the output power can fluctuate quite a bit when the voltage goes up or down.

Is the mosfet properly touching the heatsink and heatsink paste used? All screws tight?

Have you got a genuine mosfet and not a counterfeit chinese device?

When you say power drift, how much is it dropping to?

kevycorsa
who u callin ne guy bruv
who u callin ne guy bruv
Posts: 26
Joined: Sun Apr 12, 2015 2:32 pm

Re: Output power drift

Post by kevycorsa » Thu Apr 05, 2018 6:39 pm

It's a meanwell psu 48v
When checked it was stable at 48v
It's defiantly a real chip and used heat transfer paste
It was working fine when last switched off a few weeks ago
Drifts from around 150w down to around 90w then back up to 110w and just fluctuating up and down real slow

User avatar
Analyser
tower block dreamin
tower block dreamin
Posts: 321
Joined: Fri Sep 26, 2014 9:11 pm

Re: Output power drift

Post by Analyser » Thu Apr 05, 2018 9:51 pm

kevycorsa wrote:
Thu Apr 05, 2018 6:39 pm
It's a meanwell psu 48v
When checked it was stable at 48v
It's defiantly a real chip and used heat transfer paste
It was working fine when last switched off a few weeks ago
Drifts from around 150w down to around 90w then back up to 110w and just fluctuating up and down real slow
Sounds like a fault somewhere, such as loose connection or a component on its way out (series capacitor, for example).

MC Spanner
who u callin ne guy bruv
who u callin ne guy bruv
Posts: 32
Joined: Thu Mar 23, 2017 12:01 pm

Re: Output power drift

Post by MC Spanner » Sat Apr 07, 2018 12:35 pm

What are you measuring it with - I have 2 power meters which claim to be able to measure up to 400W but don't like being left in circuit continuously for more than about 20 minutes at over say, 150W. The reading drops right down, I assumed the detector diodes have had enough. All fine again when cool. Do you have any other methods of measuring your power, or can you look at input power over the time period. Or field strength. Just a thought.

kevycorsa
who u callin ne guy bruv
who u callin ne guy bruv
Posts: 26
Joined: Sun Apr 12, 2015 2:32 pm

Re: Output power drift

Post by kevycorsa » Sat Apr 07, 2018 8:54 pm

I have 3 power meters and different rigs to try so it's not the meter it's self and it's not the pll input side ive tested that
It's somthing on the amp it's self

s2000
tower block dreamin
tower block dreamin
Posts: 442
Joined: Wed Nov 26, 2014 1:02 pm

Re: Output power drift

Post by s2000 » Sun Apr 08, 2018 5:15 pm

To fix the problem all depends on your electronics ability and what equipment you have?

If you are sure it is the amp, check the bias voltage on the gate of the fet with a multimeter. Is it the same voltage when its putting out 150w and when it's 90w? I.e is the bias circuit drifting to cause the output to fluctuate or is it stable? Technically bias is more concerned with current draw but I'd imagine you would see a difference in voltage on the gate if the circuit was faulty.

Also I am just wondering about your power supply, you say it has a stable voltage but what about the current? Hard to say without knowing the meanwell psu you have. I am just wondering if it has some kind of inbuilt current limiting which has gone faulty? Have you tried swapping the power supply with one from your other healthy rigs? If the current was limited by the psu it wouldn't be able to put out the specified watts.

Another possibility is maybe a dry joint on the board, you could try touching up the connections with a soldering iron.

You got any better pics of the whole unit?

Banus_radio
no manz can test innit
no manz can test innit
Posts: 103
Joined: Wed Aug 24, 2016 9:58 pm

Re: Output power drift

Post by Banus_radio » Sun Apr 08, 2018 6:18 pm

The problem you have is common on that amp.
To the far left of your photo there are 3 capacitors at the end of the stripline.
Rf travels through one which is around 220pf and there are two caps from the stripline to ground. It’s these capacitors.
Run the amp and feel how hot them caps are, there are cooking. I think H just used ordinary 50v ceramic discs.
Replace them with something a bit more heavy duty like atc or just high voltage discs. Pinch them in the your fingers and you will see the rf power change dramatically. If you cant find high voltage discs then use several lower value to make up the capacitance which off top of my head is around 600pf to ground and spread the load. (Yes is almost an rf short when u think about it) Hope this helps

Banus_radio
no manz can test innit
no manz can test innit
Posts: 103
Joined: Wed Aug 24, 2016 9:58 pm

Re: Output power drift

Post by Banus_radio » Sun Apr 08, 2018 6:23 pm

Another suggestion. If you’re using a cheap switch mode power supply it may be rf getting into it. The cheap eBay ones don’t like being near them amps as quiet a bit of rf is rejected by the output filter.

Add ferrite beads to your psu wires as close to it as you can. Also stretch the output filter slightly and see if that helps, u may see power increase.

User avatar
SundayGroover
who u callin ne guy bruv
who u callin ne guy bruv
Posts: 37
Joined: Mon Feb 02, 2015 6:57 pm
Location: Sunny Suffolk

Re: Output power drift

Post by SundayGroover » Mon Apr 09, 2018 7:08 am

Banus_radio wrote:
Sun Apr 08, 2018 6:23 pm
Another suggestion. If you’re using a cheap switch mode power supply it may be rf getting into it. The cheap eBay ones don’t like being near them amps as quiet a bit of rf is rejected by the output filter.

Add ferrite beads to your psu wires as close to it as you can. Also stretch the output filter slightly and see if that helps, u may see power increase.
Additionally, those horrible cheap power supplies often quote a given current rating but then tend to wimp out under any load approaching their rated maximum.

User avatar
teckniqs
proppa neck!
proppa neck!
Posts: 1985
Joined: Thu Aug 21, 2014 11:37 am

Re: Output power drift

Post by teckniqs » Mon Apr 09, 2018 12:48 pm

Yeah I'm interested what power supply you have. Should need at least 5 or 6 amps of juice.

kevycorsa
who u callin ne guy bruv
who u callin ne guy bruv
Posts: 26
Joined: Sun Apr 12, 2015 2:32 pm

Re: Output power drift

Post by kevycorsa » Mon Apr 09, 2018 1:11 pm

Right I've got a few things to go on now.
I'll get some better pictures of amp an psu and post up a video of what its doing later tonight.
I've got a feeling it's the capacitors like banus has mentioned.
It's not the power supply as it was all working fine for months on end untill it was left off for a few weeks, I even have another one of these amps what I have installed in place of this one just to have a working tx and that is working fine...

User avatar
Maximus
tower block dreamin
tower block dreamin
Posts: 345
Joined: Wed Jan 21, 2015 1:29 pm
Location: Where you don't need licenses

Re: Output power drift

Post by Maximus » Wed Apr 11, 2018 6:26 am

Totally agree with Banus_Radio. Check the capacitors on the input. Had the same problem and changed the values, then suddenly I had full power.

kevycorsa
who u callin ne guy bruv
who u callin ne guy bruv
Posts: 26
Joined: Sun Apr 12, 2015 2:32 pm

Re: Output power drift

Post by kevycorsa » Sun Apr 15, 2018 7:24 pm

How do I upload videos
Will see what's happening to amp

Cheers

Shedbuilt
no manz can test innit
no manz can test innit
Posts: 136
Joined: Tue Dec 02, 2014 11:03 am

Re: Output power drift

Post by Shedbuilt » Tue Apr 17, 2018 1:45 pm

If the PSU were current limiting, the PSU voltage would drop when it current limits. If the PSU voltage is stable, then it doesn't sound - IMHO, like this problem is caused by current limiting of the PSU.
As well as the things others have suggested, make sure the output is clean (no sporadic parasitics / spurii etc).

radium98
no manz can test innit
no manz can test innit
Posts: 225
Joined: Fri Aug 26, 2016 7:01 pm

Re: Output power drift

Post by radium98 » Tue Apr 17, 2018 4:49 pm

kevycorsa wrote:
Sun Apr 15, 2018 7:24 pm
How do I upload videos
Will see what's happening to amp

Cheers
full editor ans attachment or just upload on sendspace or zippyshare or mediafire and drop the link here

kevycorsa
who u callin ne guy bruv
who u callin ne guy bruv
Posts: 26
Joined: Sun Apr 12, 2015 2:32 pm

Re: Output power drift

Post by kevycorsa » Tue Apr 17, 2018 5:44 pm

https://www.mediafire.com/file/8xu9fkbi ... 200755.mp4

Does that work
Should be video of what's the amps doing

s2000
tower block dreamin
tower block dreamin
Posts: 442
Joined: Wed Nov 26, 2014 1:02 pm

Re: Output power drift

Post by s2000 » Wed Apr 18, 2018 1:11 pm

Yes video working. It looked like you had switched the rig on at the start (from cold), is that the case?

Just wondering, what are you using as a dummy load?

Have you tried checking the caps Banus mentioned?

Banus_radio
no manz can test innit
no manz can test innit
Posts: 103
Joined: Wed Aug 24, 2016 9:58 pm

Re: Output power drift

Post by Banus_radio » Wed Apr 18, 2018 3:00 pm

That spike you have at switch on, im guessing the amp is tuned to another frequency (even though its suppose to be a notune).
Your exciter is sweeps up across the band as the pll pulls it to frequency. Can we see a picture of your exciter?

Try this in order:
With everything powered on, using a piece of plastic pull the filter coils wider apart and see if power increases, if not then try pinching them together (not with your fingers or anything metal).. The first coil should make most difference.

Next, on the input to the amp the rf will pass through a decoupling capacitor which is probably 1nf, directly after that there is a cap to ground (probably somewhere around 65 - 90 pf ish), remove that cap a replace with a 0-65pf yellow trimmer capacitor. Tune it for maximium power, if the discs on the trimmer capacitor fully cover each other for max power then add a 33pf in parralell with it, then tune it again for max power.

Next, replace the caps as i mentioned earlier in this thread.

If this doesnt work come back to me as theres a couple fo other things you can check.

Post Reply

Who is online

Users browsing this forum: 1608cc and 3 guests