Cobra 2000 ssb tx dying out after a few minutes use . Am is fine

[wildwinds]

Hi,
SO the radio has been working fine since the last repair
Now i notice that after a few minutes of ssb use the tx audio dies away. THe tx indicator switches on ok but no rf going out, if i switch back to am the signal and modulation are fine. If i switch off the radio for a while the ssb tx comes back,
The ssb receive is working fine and not affected
I would be very geatful for any help.
best regards to all
jimmy

 

[Handy Andy]

Has the radio been recapped - when you say "Working fine since last repair" - I don't know what was done to it before...

But when audio "dies" like that - usually it means a part is on the overload, over-heat kinda thing. Meaning the old Chevy is on the Side of the road with the Hood up ... doesn't look good when steams' roaring out...

Could mean a switching transistor, or, depending on how it dies, the AN 612 can be getting more than it can handle and is heating up and then shutting down.

Now when I say "switching transistor" I'm referring to the MB3756 - if the Regulator can't feed a section it tries to keep others alive and just doesn't switch anymore, but that doesn't tell the PLL that - just that the PLL gets a feed of continuous power and if left to itself will think things are fine even though the radio is not.

Or AMC issues fighting with the ALC - causing the mod to just clamp off. When you switch to the AM mode, the ALC circuit is cutoff and AMC takes over...

Feel for heat in the parts in and around the AN612 - and by rear panel and MB3756 by side panel. Anything really hot signals you're overheating and radio is shutting down...

Or think, bad caps, and or work - referring to the previous repair...

 

[wildwinds]

Thanks Andy.
I will check out things heating up,
Where I have the scope on the base of tr23 when the tx stops on USB, no audio signal appears on the scope.
if i switch to am the Mike audio is there.
The collector voltage of tr 23 is a bit low and the base volt is 1.7, a bit high according to the schematic.
Thank you.

 

[sp5it]

Check output from AN612 on SSB.
Mike

 

[wildwinds]

Check output from AN612 on SSB.
Mike

Thanks Mike, I'll give it a try.
Is the an612 used for the am modulation also or just the ssb

 

[sp5it]

Just SSB balanced modulator.
Mike

 

[Handy Andy]

The AN612 takes your MIC audio like you have for your AM - but it "mixes" with the PLL IF to make the SSB mode work.

The AN612 just mixes the Audio to that IF generating a signal
that then gets sent to that IF Xtal filter than then pares down the carrier (IF) and the other half of the audio
(that other SSB sideband you don't want)​

So if TR23 seems weak...that may be the culprit. They suually get pretty hammered by noisy Microphones or hard driving outputs like the D104 - note the use of a TANTALUM C103 - it is a special part, but if it's going bad - it can drift off until you pause. But I don't think that is the true problem. For the "signal to die" may also mean TR21 - which routes the signal BACK to the Audio amp - like you would for PA - it removes the signal from the AN612 - for AM only. So when SSB dies, you'll need to look at TR21 too. C99 can get leaky but again not sure of other work done to it - but hope this helps you look for and locate the areas you have to focus on.

 

[wildwinds]

Thank you for clearing up the an612 role.
All the electronic capacitors are renewed.
The last repair was the replacement of tr 20. Nothing else changed.
I will check out the an612 and change tr21.
Nothing is heating up, as far as I can see.
Should I try an electronic capacitor in place of c193 as a test.
Best regards.

 

[Handy Andy]

Don't just "change TR21" check it - for there may be something else we didn't see.

I'm concerned that the Base lead of TR21 - that goes to TR31 which runs the Local IF - and so you'll need to check R113 and sometimes it was pulled in error because people though it was R131 part of the AMC limiter they have to remove to defeat the limiter. So if you've been brining it back to stok, check R113 too - it's a simple 10K but if it's value has changed or its now cracked in two - that can change the way the circuit operates.

If R113 is bad, or drifts off in values - as you test this - example -look for voltage to RISE in the BASE lead - it will then turn on and force the SSB signal to Die - when it's really sending it to the Audio Amp instead - so if TR21 is flakey, check R113 too - else you'll be throwing another transistor in their soon enough.

So then we will have to figure out what causes the BASE lead to go high - is R113 at fault or do we go to TR31 too and it's DIODE matrix mess? (This is just a possible scenario to think about)

Stay tuned...let me know what you find...

 

[Robalo]

i have another radio with this problem

 

[Handy Andy]

@Robalo - thanks! you're more than welcome to follow along - maybe some of this can help you...

@wildwinds - you mentioned something interesting that I need to ask you more about...

IT deals with any mods or changes, and are there any age related problems you encountered like those old caps.

I've seen resistors get flakey - literally - and break apart from age and -well- disintegrate under some heating of the solder pads underneath it. So I do know the composition and body these things have are so weak and tiny they can fail even while being reheated - even probed for testing.

I'm looking back at TR23. You mentioned 1.7 volts on the Base, so the concern here is the 22K (R125) versus the 8.2K(R123) and how both are working. That sets a "Mid point" for that transistor - so if the Base appears high, - I agree - yes it does, and why? So you'll need to check the resistors for values drifting off or open.

• Does the 1.7 volts appear in AM mode too?
• When SSB mode dies, what is TR21 showing on it's base?
• - it uses a different pairing but the same method of biasing R113(10K) and R112(1.5K)
• - only it is for transfer of Audio - not it's amplification - so the BASE is the switch and the Collector/Emitter is the transfer medium - it's set up to operate LINEARLY - it makes it look like a lower values impedance path (resistance path) and audio follow into the Rabbit hole and so the AN612 never sees the audio.
• - it's biased in a way so it doesn't amplify or attenuate the audio stream being sent thru it - just pulls it off the AN612's audio-in line and sends it to the Audio Amp instead.
• Is there a problem with the 7.8MHz IF section? Is it that TR21 is obtaining more voltage to turn on it's base than needed or is the Bias resistor network it uses just flakey- turning on the BASE and puling off audio?

I'm curious to know if other BASE leads in that area are also running higher than they should.

Hope these hints help!

 

[ExitThirteen]

Check to make sure the final bias current is properly set. If the bias on the final is way too high, it can cause this condition. It won't have this problem on AM because the final is not getting the full 13.8V at its collector. I've had this problem in the past where the someone had cranked the final bias trimmer pot wide open and it would slowly lose TX on SSB, while AM was unaffected. Let us know how you get on.


~Cheers~

 

[loosecannon]

Wasn't there a known issue with these radios losing TX and it was something like c95 gone bad?
LC

 

[Handy Andy]

10 Volt Blues? Now in Grey?

Perish the thought - or thereabouts...

Makes Sense...47uF 10V - kills AN612 power supply - but (*ahemn*) as owner claims - recently (renewed) repaired...

 

[wildwinds]

Today i have checked out the bias currents for predriver and driver. they were slightly high driver 67 and predrive 33 Ma. SO i set these to 60Ma and 30Ma..
I have looked at the resistors
R 131 Its there intact and reads ok
R 125 It reads correct also
R 123.
I wonder should i change these resistors in case they are changing value during use?.

i have checked the volts at the AN612
1....2.9v
2....3.3v
3....3.3v
4....0v
5.....5.8v
6......7.4
7.....7.6 and when transmit it changes to 4.4

I have not got around to checking the 7.8 input yet.

I think tr 23 needs to be eliminated from the problem. its b458, dont think i have any of those.
Its difficult to get the measurements, as sure enough when i sit down to it the radio works...

I did notice that TR26 measured Base .6 Collector 6.24 Emitter 4.02.....This is different to the service sheet.
Thank you all for the help..