BNOS LPM144-25-160 linear amp help. - UK Vintage Radio Repair and Restoration Discussion Forum

murphyv310 · 25th Dec 2022, 10:46 pm

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.

murphyv310 · 28th Dec 2022, 9:59 am

Hi.
Anyone able to help me with this or is this now a lost cause?

mickm3for · 28th Dec 2022, 11:13 am

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

Terry_VK5TM · 28th Dec 2022, 12:07 pm

There's some reasonably clear photos in this thread https://www.vintage-radio.net/forum/...d.php?t=157730

ms660 · 28th Dec 2022, 1:06 pm

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.

Radio Wrangler · 28th Dec 2022, 1:29 pm

It's likely tat the power amp areas are simply slight variations on a Motorola applications note for those transistors.

David

ms660 · 28th Dec 2022, 1:42 pm

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.

Jon_G4MDC · 28th Dec 2022, 2:29 pm

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.

murphyv310 · 28th Dec 2022, 10:35 pm

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.

G6ONEDave · 29th Dec 2022, 12:27 pm

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

Jon_G4MDC · 29th Dec 2022, 2:35 pm

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...

murphyv310 · 30th Dec 2022, 7:38 pm

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%

Jon_G4MDC · 30th Dec 2022, 8:14 pm

Good. I expected that. I think they made the designer happy and that's about it.

25th Dec 2022, 10:46 pm	#1
murphyv310 Dekatron Join Date: Sep 2006 Location: Kilmarnock, Ayrshire, UK. Posts: 5,421	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. Attached Thumbnails __________________ Cheers, Trevor. MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member

28th Dec 2022, 9:59 am	#2
murphyv310 Dekatron Join Date: Sep 2006 Location: Kilmarnock, Ayrshire, UK. Posts: 5,421	Re: BNOS LPM144-25-160 linear amp help. Hi. Anyone able to help me with this or is this now a lost cause? __________________ Cheers, Trevor. MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member

28th Dec 2022, 11:13 am	#3
mickm3for Heptode Join Date: May 2015 Location: Chatham, Kent, UK. Posts: 962	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 Attached Thumbnails

28th Dec 2022, 12:07 pm	#4
Terry_VK5TM Nonode Join Date: Oct 2010 Location: Tintinara, South Australia, Australia Posts: 2,339	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 __________________ Terry VK5TM https://www.vk5tm.com/

28th Dec 2022, 1:29 pm	#6
Radio Wrangler Moderator Join Date: Mar 2012 Location: Fife, Scotland, UK. Posts: 22,857	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 __________________ Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done

28th Dec 2022, 1:06 pm	#5
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	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, 1:42 pm	#7
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	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
Jon_G4MDC Nonode Join Date: Nov 2015 Location: Welwyn Garden City, Hertfordshire, UK. Posts: 2,015	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
murphyv310 Dekatron Join Date: Sep 2006 Location: Kilmarnock, Ayrshire, UK. Posts: 5,421	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. __________________ Cheers, Trevor. MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member

29th Dec 2022, 12:27 pm	#10
G6ONEDave Octode Join Date: Dec 2019 Location: Owston Ferry, North Lincolnshire, UK. Posts: 1,698	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 __________________ Quote "All is hyperthetical, until it isn't!" (President Laura Roslin, Battlestar Galactica)

29th Dec 2022, 2:35 pm	#11
Jon_G4MDC Nonode Join Date: Nov 2015 Location: Welwyn Garden City, Hertfordshire, UK. Posts: 2,015	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
murphyv310 Dekatron Join Date: Sep 2006 Location: Kilmarnock, Ayrshire, UK. Posts: 5,421	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% __________________ Cheers, Trevor. MM0KJJ. RSGB, GQRP, WACRAL, K&LARC. Member

30th Dec 2022, 8:14 pm	#13
Jon_G4MDC Nonode Join Date: Nov 2015 Location: Welwyn Garden City, Hertfordshire, UK. Posts: 2,015	Re: BNOS LPM144-25-160 linear amp help. Good. I expected that. I think they made the designer happy and that's about it.