Amplifier help!

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XXL
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Amplifier help!

Post by XXL » Sun Aug 07, 2022 10:29 pm

Iv got a blf278 amplifier that I’m having a nightmare with. It runs for a few mins and then drops in power slowly, from 250w to 150 and then I unplug it before it blows. Iv checked that it’s seated properly, the heat sink runs warm, the current and voltage stay the same when it’s dropping, All caps on the filter are porcelain type. iv no idea what it could be.

XXL
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Re: Amplifier help!

Post by XXL » Sun Aug 07, 2022 10:30 pm

I also tried running a fan on top blowing on the ceramic case which made no difference.

Albert H
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Re: Amplifier help!

Post by Albert H » Mon Aug 08, 2022 2:24 am

It's probably a thermal problem. The metal flange of the device needs to be absolutely flat against the heatsink. In some cases, you might need to scrape the surface of the heatsink so that you can get good thermal contact. Years ago, all the "Fischer"-brand heatsinks were lacquered, and this caused endless problems. I used to chemically strip the face of the 'sink that the transistors were going to contact.

Blowing air downwards onto the ceramic case of the device is going to do nothing useful!

You should get an infra-red thermometer, so you can do non-contact temperature measurements of your PA transistors. Don't rely on sticking your finger on the top of the device - if you have RF coupling into your finger, it'll feel warm even if it isn't! Also, one slip and you could get a nasty RF burn. At a couple of hundred Watts, this could be fatal (depending on which part of your internals gets cooked!)
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XXL
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Re: Amplifier help!

Post by XXL » Mon Aug 08, 2022 11:02 am

Yeh I don’t touch it while it’s on. I unplug it then immediately touch it. I might try sanding with 1000 grit so as not to cause pits in the heat sink. And try again. It’s definitely flat against the heat sink and iv tried a thin amount of heat sink paste and lots of paste, both made no difference.

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Re: Amplifier help!

Post by jvok » Mon Aug 08, 2022 1:17 pm

How big is your heatsink? Could just be the heatsink isn't big enough.

Are you using any ferrite cores in your input or output matching networks or lpf? If so they might be getting too hot and losing inductance.

Also maybe you've checked this already but keep an eye on your supply voltage to make sure its not the psu dropping out under load. and check you're still getting full input power from your driver, otherwise you could be blaming the amp when the problem is elsewhere.

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Re: Amplifier help!

Post by XXL » Mon Aug 08, 2022 2:05 pm

I’m using rg316 50ohm coax loops on the output (after the 2 25ohm loops) but it’s thinner than what I’m seeing online. Maybe it could be that. And the only ferrite iv got is on the input and the wire going to the 48v pad. It’s basically the same setup as the mrf186 amps.

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Re: Amplifier help!

Post by jvok » Mon Aug 08, 2022 4:09 pm

Check if they're getting hot, if they're not hot then that won't be the problem.

My first guesses would be transistor overheating for some reason, psu dropping out or driver dropping out.

Could also be a cracked solder joint that goes open when hot. If you can't track it down to anything else might be worth going through and reflowing every joint.

Albert H
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Re: Amplifier help!

Post by Albert H » Tue Aug 09, 2022 4:28 am

I've had thinner types of coax melt with 300 Watts!
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Re: Amplifier help!

Post by XXL » Fri Aug 12, 2022 2:29 pm

:( Still having issues with this. Iv attached a photo this time. Iv been ripping parts off left right and centre trying to work out what’s going on so it’s not very neat now. The ferrite on the input does get very hot, so does the resistor. Everything else seems fine. On this photo the heat sink is actually in water to rule out the heatsink getting too hot. It was Luke warm when it started dropping in power again. Also it does have thermal and swr protection but it’s disabled atm.

Image

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Re: Amplifier help!

Post by XXL » Fri Aug 12, 2022 3:16 pm

Also I forgot to mention I’m using this as my dummy load. It only takes up to 250w so I haven’t gone over that with this amp yet. But I can’t even get it stable at that atm.

Could it be this that is causing it ? I keep it in water when testing to keep the temps down.
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Re: Amplifier help!

Post by thewisepranker » Fri Aug 12, 2022 3:54 pm

It's highly unlikely that that resistor could dissipate anywhere near 250 W for more than a few seconds without exceeding the maximum junction temperature and failing. I can't find any data available for that exact part number as Florida RF's server throws an error, but a similar part number has no thermal data available (as usual).

If we look at a BLF278, which I've chosen as it's fairly similar in size, construction, aspect ratio and is a flanged device with two fixing points. Its thermal resistance (junction to case) is 0.5 K/W, so the junction temperature rise is 125 °C, plus ambient is 150 °C. You're well outside the safe operating area (which is provided on other Florida RF datasheets). And bear in mind the BLF278 has about 1.5 times the flange area of your resistor so you're probably looking at closer to 1 K/W.

You might say, well, the datasheet and the sales guff says that it's a 250 W resistor so what are you about? Well, this is a common theme in electronics - the way the manufacturers achieve their unrealistic power dissipation claims is to submerge the whole lot into a liquid that boils at 25 °C. This is why you'll see lots of semiconductors with large power ratings such as TO-220s that are supposedly able to dissipate 50 W, with a limitation that the case is maintained at no more than 25 °C.

Buy a proper termination such as a Bird, they're not that expensive vs. how many transistors you'll blow up and the time wasted with DIY loads that aren't up to the job.

In the meantime, have you tested the DC resistance of your load resistor, and have you got any way of testing the RF performance of it?

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Re: Amplifier help!

Post by XXL » Fri Aug 12, 2022 4:15 pm

So it could very well be my dummy load dying and I’m blaming the amp ? The load resistance is 50ohms (think that’s what you mean) it’s an RF resistor. And no way of testing it without just running rf through it. Tbh it was advertised as a 250w handling rf resistor and I just bought it but only ever used it up to around 150w until now.

87to108
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Re: Amplifier help!

Post by 87to108 » Fri Aug 12, 2022 5:33 pm

XXL wrote: Fri Aug 12, 2022 4:15 pm So it could very well be my dummy load dying and I’m blaming the amp ? The load resistance is 50ohms (think that’s what you mean) it’s an RF resistor. And no way of testing it without just running rf through it. Tbh it was advertised as a 250w handling rf resistor and I just bought it but only ever used it up to around 150w until now.
Well if you want to see can a 50 Ohm dummy load survive 250W approx dissipation without involving a transmitter! , you could put 110Volts across it (doesn't matter DC or AC) such as from the yellow 'building site' transformers. If you can measure the current too, all the better. (If its going high resistance when it heats up for some reason current will drop, current should be 2.2 Amps approx - 110V - 242 watts of heat )

Obviously be careful.

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Re: Amplifier help!

Post by jvok » Fri Aug 12, 2022 11:55 pm

Do you have an inline swr meter that can handle the full 250w? If so put that in line between the amp and dummy load then run it for a few minutes. If the swr rises about as fast as the power falls off, then you know the problem is the load. If the swr stays close to 1:1 then your problem is elsewhere.

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Re: Amplifier help!

Post by XXL » Sat Aug 13, 2022 11:31 am

Yeh the swr doesn’t change at all when the power goes down.

Shedbuilt
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Re: Amplifier help!

Post by Shedbuilt » Sat Aug 13, 2022 6:39 pm

If you haven’t already, monitor the drive to the amplifier, and the DC power supply voltage to the amplifier - as it warms up, drops off etc.

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Re: Amplifier help!

Post by nrg-uk » Sat Aug 20, 2022 1:51 pm

Heatsink to small that heatsink your using has a thin top plate and there from China

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Re: Amplifier help!

Post by XXL » Sat Aug 20, 2022 2:29 pm

What size would you use then ? The one I did have it on in the picture is pretty big, but iv found it’s not so much the size that matters but the way you cool it. A small heat sink is with thin fins seems to stay cooler than a big chunky one with big fins.

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Re: Amplifier help!

Post by XXL » Mon Aug 22, 2022 2:55 pm

Dont suppose anyone can help me fix this for some money ?? Iv no idea wtf is going on. Current doesn’t drop at all when it drops from 250 down to 100w and stays there. Drawing 7a exactly at 100w. Driver is supplying 1w and the rd06 doesn’t drop either. Only thing I can feel getting hot after unplugging is the porcelain caps after the output and the binocular ferrite for the transformer input.

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Re: Amplifier help!

Post by Albert H » Tue Aug 23, 2022 3:40 am

The heatsink is too small, and the dummy load isn't up to the job.
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