Cobra XL 350 Project

[ww981]

Hi, I recently acquired a Cobra XL 350 and it there are some weird things going on with it. I hooked it up the other day turned it on and I noticed that when I did, the receive on the radio would cut out, regardless of where the pre-amp switch was positioned - almost as if the pre-amp switch has zero effect. As soon as turn the amp off, the receive on the radio would come back. I opened the thing up to see if there was a glaring, obvious problem and I am not seeing anything specific. To be fair, I really have no clue what I am looking for other than discolored or burnt items. The amp has definitely been worked on as you can see evidence of soldering being done after it came off the assembly line. I figured what the hell, powered it back on and I keyed the amp up and it is definitely putting out some wattage, so the 2879's appear to be working soundly. I scrounged around the net and was able to find a schematic for the unit. I was going over everything and I noticed something odd to me. The schematic calls for Q1 to be a 2n3904 transistor and Q2 call for a 2n3906 transistor. I looked at the two transistors in question and they do appear to have been worked on post-factory, and they are both 2n3904's... Now maybe this is intentional and the way it's configured it may work as designed, but I found it weird that it's a different transistor than what the schematic calls for. Could/would that component change effect the receive portion of the amp? If not, any guidance?

I am excited to learn about this repair/diagnostic process, even though I am pretty ignorant, I am eager to learn. I appreciate any advice, thank you.
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Here are some photos of what I am working with along with the schematic I found.

I believe this is Q1

And I believe this is Q2, which calls for a 2n3906

 

[ww981]

Not sure if this video would help explain what I mean by it cutting the receive when powered on, but just in case, here it is.

 

[Dmans]

The schematic calls out Q1 as an NPN transistor and Q2 as a PNP.

Swapping to the correct (2N3906) transistor in the Q2 position may well solve your problem.

73
David

 

[ww981]

The schematic calls out Q1 as an NPN transistor and Q2 as a PNP.

Swapping to the correct (2N3906) transistor in the Q2 position may well solve your problem.

73
David

Ok, I will order a transistor kit and replace Q2 with the 2N3906 and report back what I find. Thanks for your help!

 

[ww981]

After reviewing the schematic more closely, it is not the same thing as what the Cobra XL 350 I am working with. Some things just don't line up. I ended up finding 3 transistors by the relays, not just the two on the schematic (besides the 2979's) and there are a few other odds and ends that don't quite line up.

Not sure where to go from here. I'll keep poking and prodding to see what I can find. Does anyone have one of these amps that can pop the bottom off and take a look at what the 3 transistors on the relay side are marked as?

Here is an image of what this amp has installed currently...

 

[Holydvr]

I have one that does the same thing. As soon as you turn it on, even not hooked to a radio it keys up. You can hear the relay click. I may be wrong, but I believe the keying transistor is popped on mine.

 

[Smokinone]

All 3 transistors are 3904 in this one of mine.

 

[ww981]

I have one that does the same thing. As soon as you turn it on, even not hooked to a radio it keys up. You can hear the relay click. I may be wrong, but I believe the keying transistor is popped on mine.

@Holydvr Sounds like the same thing happening with mine as well. When you turn the power on, does the power LED and the LED on the far right come on? Maybe @Smokinone can confirm if that is normal for both LED's to be on when unkeyed. The preamp light will come on when the switch is flipped on, but it has zero effect on the receive of the radio.

All 3 transistors are 3904 in this one of mine.

@Smokinone Huge thank you for that confirmation - that helps a ton. Even though it didn't get me anywhere near bringing this amp back to life, it is nice to know the correct parts are in there, and look to be set in the correct orientation. So thank you, and especially for adding the photos.

I'll keep playing around and seeing what I can come up with. If anything comes up, I'll be sure to report back.

 

[Holydvr]

You put new transistors in? Or tested them properly? Could be a relay contact issue also I would imagine. I have one that began to go deaf and I confirmed it was the relay itself.

 

[Smokinone]

@Holydvr Sounds like the same thing happening with mine as well. When you turn the power on, does the power LED and the LED on the far right come on? Maybe @Smokinone can confirm if that is normal for both LED's to be on when unkeyed. The preamp light will come on when the switch is flipped on, but it has zero effect on the receive of the radio.

The right hand light only comes on when keyed.

 

[ww981]

You put new transistors in? Or tested them properly? Could be a relay contact issue also I would imagine. I have one that began to go deaf and I confirmed it was the relay itself.

I replaced one of the 2n3904 transistors, the one closest to the front controls since a couple of the legs were poorly soldiered to the board. I might as well replace them all just for the sake of making sure they have clean connections and soldier joints.

I’ll also look into the relays, good thought.

The right hand light only comes on when keyed.

Thanks again for the confirmation.

Man, I’m really hoping I can get this thing back on the air. Cheers!

 

[ww981]

It’s alive! I got it back up and running, so happy about that.

So what I ended up doing was replacing the other two transistors, so all three of the 2n3904’s were replaced. While I was there, I decided to remelt the solder points of the two replays, all 8 pins on both relays. Once I did that, I brought it inside and hooked it up to a test radio and turned it on. I noticed that only the power LED came on this time and then I tested the preamp switch and that LED came on. I turned the preamp off and keyed the radio and “click” the relay sounded, the transmit LED light up and the wattage swung up. It dead keyed to about 50-60 I think, which is fine for me on the test radio. I unkeyed and the light went off and the relay clicked back. Such a good sound after fiddling with this thing for the last week.

Here’s a video demoing my test.



Now I’ll need to do some additional tests with the actual radio and see what I can get out of this thing and check for any hot spots.

A huge thank you to @Smokinone for the assist with the photos and confirmation of the transistors. And thanks to everyone else that chimed in to offer help.

Great forum here, glad to have found it!

 

[NY’s BADDEST WOODCHUCK]

Not sure if this video would help explain what I mean by it cutting the receive when powered on, but just in case, here it is.

I think you have the coax jumpers crossed the 259 on the back looking at the front of ampwould be all the way ilon the left in back of amp is for your radio and the one to the right is antenna

 

[nomadradio]

It's a little bit like "My car won't start".

One symptom, but more than one possible cause. Sometimes more than "just one" thing is wrong.

73