Amp power drop off to zero, buzzing

[Big Kahuna]

Troubleshooting an amp problem hoping one of you experts can help diagnose to save me some time.

Amp is a Magnum 2000 (Similar to the Cobra 350 2 x 2879's)

Amp works fine initially, but after a couple of minutes of use output suddenly drops a little bit, then drops to zero.

Relay seems to be working fine even when power drops to zero relay still keys and unkeys with radio.

Right before power loss there is a barely audible slight buzzing in the amplifier.

Once it drops to zero output if you unkey and then key again, sometimes it won't key at all, or if it does key it jumps up to normal output again, but within seconds the power will drop off again.

On SSB mode it will run longer before the issue occurs but same issues and on AM with deadkey it happens quicker and the buzzing is noticeable right before it drops to zero output.

No power drop on input wires, fuse is fine.

Issue occurs regardless of rotary switch power position or AM/SSB delay position.

https://i.ibb.co/4ggSp4d/Magnum-2000-Inside.jpg

https://i.ibb.co/QmLKkFV/Magnum-2000-Inside-2.jpg

https://i.ibb.co/cFSdtsL/Magnum-2000-Inside-3.jpg

https://i.ibb.co/V06VqyC/Magnum-2000-Inside-4.jpg

 

[ExitThirteen]

Sounds like a capacitor failure on the relay switching circuit, or the keying transistor has a leaky junction causing it to fail due to RF saturation.


~Cheers~

 

[Big Kahuna]

Any suggestions based on the images where to check first? The transistor in the front looks like it was possibly not original?

 

[ExitThirteen]

The transistor that located next to the bottom relay in the first image you linked, and the electrolytic capacitor right next to the transistor. The capacitor is black. Those two components can contribute to the dropoff of output, as it sounds like the relay is losing voltage to stay keyed down. Originally, I would have suspected the SSB delay capactior which should be located right behind the AM/SSB switch, but since it does it in all modes, I'd suspect the transistor and capacitor that are next to the relay.


~Cheers~

 

[ExitThirteen]

Even though it's a major pain in the butt, I'd check the underside of the PCB and make sure all solder joints are solid, and check the fuse as well, re-seat it if you haven't done so already, even though you did say the fuse is confirmed good.

It could also be that the power setting switch is arcing, it may need to be cleaned.

Last but not least, pull the cover off both relays and clean the contacts, it's never bad practice to do that occasionally.


~Cheers~

 

[dave457]

This dont look good

 

[Big Kahuna]

Yeah - that's what I thought looked a little ugly as well.

 

[Big Kahuna]

Even though it's a major pain in the butt, I'd check the underside of the PCB and make sure all solder joints are solid, and check the fuse as well, re-seat it if you haven't done so already, even though you did say the fuse is confirmed good.

It could also be that the power setting switch is arcing, it may need to be cleaned.

Last but not least, pull the cover off both relays and clean the contacts, it's never bad practice to do that occasionally.


~Cheers~

Awesome - thank you so much for the help, going to start with that transistor and ceramic cap and then work my way from there. I'll report back once I get them out and test.

 

[ExitThirteen]

Sure thing! Keep me posted.


~Cheers~

 

[AudioShockwav]

That looks like the original transistor went kaput and they did not want to remove the board so the cut it out and tryed to lap solder it to the legs left sticking through the board.
And I don't know what kind of solder they used...... acid core???
Need to get rid of that and clean it up.
I would do as 13 suggested and pull the board and make sure the leads are not hanging down and oxidised under there too.
73
Jeff

 

[Big Kahuna]

Got the transistor out and yes it was just soldered to the legs from the original transistor. It tested out ok 0.753 and 0.746.

In the process learned that my eyesight has degraded considerably over the last year and my ability to solder small parts in tight areas has also degraded. I could barely get the transistor out.

I really don't want to go through the PITA of removing the board or straining my eyes further so I'm going to put this one on the shelf until I get some sort of magnifying glass or just decide to pay someone else to mess with it which is the more likely outcome

 

[Tokin]

Got the transistor out and yes it was just soldered to the legs from the original transistor. It tested out ok 0.753 and 0.746.

In the process learned that my eyesight has degraded considerably over the last year and my ability to solder small parts in tight areas has also degraded. I could barely get the transistor out.

I really don't want to go through the PITA of removing the board or straining my eyes further so I'm going to put this one on the shelf until I get some sort of magnifying glass or just decide to pay someone else to mess with it which is the more likely outcome

Dont spend a bunch of money on some fancy magnifying rig. A simple visor style loupe from harbor freight works fine. My roomie works with glass fiber all day long, threading it thru eyelets for making medical sensors and thats what he uses.
Another thing that will help is go to the dollar store and try on some reading glases. For a buck a piece I buy them by the fist full. I have them stashed everywhere. Made my life a whole lot easier. Start out with the lowest power and bring something to read so you know what power is best.

 

[2NC995]

Perfect time to change it over to direct key

 

[Tallman]

I use these. I have seven or eight of these around the house. I like them with the light and the lens.
https://www.harborfreight.com/fluorescent-magnifying-lamp-60643.html

You had a very lazy technician work on that amp. A repair job done that way would get you fired in a New York Femto-Second in any of my shops.

If I can be blunt, I would tear the thing apart and look for every cold solder joint and bad assembly process I could find. If the company was cheap enough to let that pass I would not have much faith in the rest of the amplifier. No military trained tech would allow that to leave the shop.

 

[Big Kahuna]

Thanks for the suggestion on eyewear! Definitely disappointing getting old sometimes

I got the amp I’m a trade with some other stuff and it tested out ok so didn’t know about the issue, no idea who did the sloppy work to start with. I do like these amps but not sure I want to spend time on this one.