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Old 2nd Jul 2019, 10:53 pm   #1
GSBX1220
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Default B.N.O.S 2m amp

Hi folks

I know the old microwave modules never gave quite the stated full out for the specified input. I know the one I had years ago gave the high 80's but dropped to around 80 as it warmed up.

Fast forward a bit, and I now have a B.N.O.S lpm 144 10-100. These had a reputation for putting out the rated 100w with the specified 10w drive.

However, the one I have is only puttting out 55 with 10w drive. I guess I right in saying that's in no way right.

I think this amp has been 'got at' it has a thermal switch with associated components very crudely fitted and the resistor below the bottom relay beside the coax input feed does not look factory as it's too big for the component spacing on the board. It measures 160 ohms. (Photos attached) third photo shows rough mods.

I wonder if it has been modded to work with a larger input drive.

Cheers
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Old 3rd Jul 2019, 9:42 pm   #2
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Default Re: B.N.O.S 2m amp

I'm now under no doubt that this amp has an issue. I drove it with around 15w which is apparently the maximum drive limit fir these amps according to the bnos instruction leaflet I found on line.

Output power does not rise with the increased drive.

Anyone have any experience in these amps?
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Old 4th Jul 2019, 7:38 am   #3
G4YVM David
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Default Re: B.N.O.S 2m amp

I have experience of these and it's all bad. I always failed to fix them. VHF linear are a total mystery to me I'm afraid. Good luck.
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Old 4th Jul 2019, 9:23 am   #4
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Default Re: B.N.O.S 2m amp

One MRF247 will be doing well at 100W - see data sheet which says 75W at 12.5V.
It might be possible to just reach 100W on a 13.8V supply.

Is it the correct device for that amp? Is there any bias on the base?
If not it is working Class C and it will not be linear anyway.

What does the green writing around the output device say?
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Old 4th Jul 2019, 9:38 am   #5
Peter.N.
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Default Re: B.N.O.S 2m amp

I am actually using one and it works very well but it seems to shut down if you overdrive it. I haven't actually connected a power meter to it but the LED display is lighting to about two thirds of the way along.

It has a very good built in pre amp, my FT 2800 is as deaf as a post but works brlillinatly with the amp.

Peter
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Old 4th Jul 2019, 9:44 am   #6
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Default Re: B.N.O.S 2m amp

It might be helpful to the OP if you knew what the transistor is in your one Peter.

Good that it works well for you. The RX side will be pretty hot for it's day - we can see it is BF981 equipped.
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Old 4th Jul 2019, 10:55 am   #7
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Default Re: B.N.O.S 2m amp

Is that thermal switch triggering a thyristor to short the supply if itgets too hot.

looks like a proper bodge.

is this of any help ?

https://www.george-smart.co.uk/2017/...10-100-repair/
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Old 4th Jul 2019, 11:06 am   #8
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Default Re: B.N.O.S 2m amp

I would say very helpful Steve - now we know the transistor is correct.
It also gives a good view of how the bias circuit was in the original state.
A good find.
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Old 4th Jul 2019, 1:34 pm   #9
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Default Re: B.N.O.S 2m amp

Hi Guys

Thanks for the replies. Yes that mod is a bodge of the highest order. With regards to the writing on the board, I cant read it. it seems to have arrows pointing to various components with comments, probably referring to any changes made. Its a bit like my handwriting, only legible to me!

most of the VR pots have been turned, as evidenced by the tamper proof seal paint not ligning up bewtween screwthread and pot body. I tried to peak up the output using the variable caps. no joy there.

Understood on the device providing 75W at 12.5v. Im definately running at 13.8V on TX measured inside the amp, so no volt drop over the power leads. With my 10W drive level I should ge to that output power, if not exceed it. As said before, upping the drive to 15W makes no difference.

I think I'm on a loser with this as I think I need to unpick someones bad knitting before I can get an idea whats going on.
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Old 5th Jul 2019, 8:21 am   #10
Steve G4WCS
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Default Re: B.N.O.S 2m amp

Thinking about it I've a palomar 140 watt jobbie that's not been finished.

It was pulling 20 amps but only putting 75 watts out. I had eyed the relay up suspiciously but perhaps it also needs alignment?
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Old 5th Jul 2019, 9:59 am   #11
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Default Re: B.N.O.S 2m amp

I will try and have a look at the transistor today if I get five minutes.

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Old 6th Jul 2019, 1:36 pm   #12
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Default Re: B.N.O.S 2m amp

PA transistor.
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Old 6th Jul 2019, 2:29 pm   #13
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Default Re: B.N.O.S 2m amp

Nice to see one that is as it left the factory Peter! All your adjustment seals are intact.

Seems like the OP's unit has an extra socket added with the extra thermal switch and transistor or regulator wired to it. Might be a fan switch output or something?

Between the sets of photos, and with the circuit from M1GEO page, it is not hard to work out the bias circuit of the main transistor. There is single temperature sensing diode attached to the transistor flange driving an OpAmp and Emitter follower finally applied to the base through one of the large ferrite bead chokes.

The other identical choke is in a series LCR feedback circuit, collector to base, presumably for better stability at HF where maybe it was a bit too keen to oscillate without it.
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Old 6th Jul 2019, 5:06 pm   #14
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Default Re: B.N.O.S 2m amp

It works very well so I'm not touching it. Even had a job getting the screws out.

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