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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment. |
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25th Dec 2022, 10:46 pm | #1 |
Dekatron
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BNOS LPM144-25-160 linear amp help.
Hi.
I was recently given one of these old Linear amps as it was destined to the bin! It has two faults. 1..... The preamp has burned out, allegedly caused by a faulty relay that as yet I cannot confirm or otherwise, the issue is there is no circuit diagram with values and in this case the resistors are dust! If someone has one can they please photograph the preamp section with high resolution and upload it as a zip file or PDF so the forum software doesn't make it illegible. 2....... Although I've not tested this it is distorted on TX on both SSB & FM, rated output power is OK apparently, any pointers would be useful but I do suspect the PA bias is most likely the issue. Any help would be greatly appreciated. Even a scrap board with the preamp intact.
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28th Dec 2022, 9:59 am | #2 |
Dekatron
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Re: BNOS LPM144-25-160 linear amp help.
Hi.
Anyone able to help me with this or is this now a lost cause?
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28th Dec 2022, 11:13 am | #3 |
Heptode
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Re: BNOS LPM144-25-160 linear amp help.
Hi a poor copy of the circuit from George Smart may be he has other stuff, on his web site he details repair of a 144-100 Mick
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28th Dec 2022, 12:07 pm | #4 |
Nonode
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Re: BNOS LPM144-25-160 linear amp help.
There's some reasonably clear photos in this thread https://www.vintage-radio.net/forum/...d.php?t=157730
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28th Dec 2022, 1:06 pm | #5 |
Dekatron
Join Date: Apr 2011
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Re: BNOS LPM144-25-160 linear amp help.
So far as I can make out from Trev's photo....from left to right....68 Ohm, 12 or 120 Ohm ?, 68 Ohm...R34, R33 & R35 respectively on the schematic.
Lawrence. Last edited by ms660; 28th Dec 2022 at 1:35 pm. Reason: extra info |
28th Dec 2022, 1:29 pm | #6 |
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Re: BNOS LPM144-25-160 linear amp help.
It's likely tat the power amp areas are simply slight variations on a Motorola applications note for those transistors.
David
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28th Dec 2022, 1:42 pm | #7 |
Dekatron
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Location: Cornwall, UK.
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Re: BNOS LPM144-25-160 linear amp help.
Re:Post#5: Looking again, R33 could be 120 Ohms as that resistor network would then work out to 50 Ohms which would match the strip line.
Lawrence. Last edited by ms660; 28th Dec 2022 at 2:08 pm. Reason: extra info |
28th Dec 2022, 2:29 pm | #8 |
Nonode
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Re: BNOS LPM144-25-160 linear amp help.
This is the exact damage you would expect from TXing up the input while the power supply is off. Relays can't switch so the preamp output pad is fried.
If you are lucky the BF981 might have survived. As for the TX side distortion on SSB is probably bad biasing. Distortion on FM is probably down to something else entirely. |
28th Dec 2022, 10:35 pm | #9 |
Dekatron
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Re: BNOS LPM144-25-160 linear amp help.
Thanks guys
Now have the info I require. The BF981 Is shorted all ways, the resistors that are cooked are 2x 68r, 220r and the 1 watt 330r. The decouplers will routinely be replaced. The bias transistor has an open circuit B/E junction. Hoping to get a result once I've got a BF981, A genuine device is ordered.
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29th Dec 2022, 12:27 pm | #10 |
Octode
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Re: BNOS LPM144-25-160 linear amp help.
Don't forget the 2 x 1N4148 (or similar) signal diodes being used to clamp the input of the pre-amp. They've probably been damaged as well.
Dave
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29th Dec 2022, 2:35 pm | #11 |
Nonode
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Re: BNOS LPM144-25-160 linear amp help.
Maybe. On further reflection I think there could be some issue with the relay switching as OP stated.
The preamp seems to have a bypass relay so TXing into the unit with the power off should not have damaged the preamp pad. The RF would have taken the preamp bypass route instead. Obviously it hasn't... |
30th Dec 2022, 7:38 pm | #12 |
Dekatron
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Re: BNOS LPM144-25-160 linear amp help.
First thing to test will be the preamp on receive with no RF power from the transmitter.
Next of course is to manually check the switching of the relays prior to feeding it with any RF. The 1N4148 protection diodes oddly are 100%
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30th Dec 2022, 8:14 pm | #13 |
Nonode
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Re: BNOS LPM144-25-160 linear amp help.
Good. I expected that. I think they made the designer happy and that's about it.
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