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

[sp5it]

Put oscilloscope probe to an612 output and check how it looks when radio transmits on ssb and when stops.
is signal ok or not?
Mike

 

[Handy Andy]

SP5IT - you may be onto something...

@wildwinds - take a look at those cables that route Signal from around the AN612 to TP3/TP4 - over by L14.

I have a hunch you may have a bad signal cable - causing AN612 to thermally shut down - you don't feel it unless you're right on those pads of the AN612. Pin 7's output may be going into a kinked cable - - check it when you can.

Check that cable on both ends for broken frayed wiring that can potentially short out that line. If it isn't soldered right - then you will have problems.

 

[Robalo]

An612 voltage are ok pin 7 always drop went key the mike

 

[wildwinds]

Thanks Robalo that's good so.
As it's raining today I get to do another bit on the radio.
So I have changed tr21, I dont see any improvement.
Also I have checked pin 7 of the an612
It has 7.8Mhz during am transmit.
I can't see 7.8 on the Tp 3.
What should I be getting at the tp3.
, when I touch the probe to it the speaker buzzes.
I will check out the cables later.
Diode 40 has the 7.8 on the banded side.
...thank you.

 

[Handy Andy]

You're supposed to get the 7.8 that is the radios COVERSION section for the SSB.

That's a good sign,

But it needs to still be there - when your radio "dies" in SSB mode.

What I was looking for was, to see if the CABLE that comes out of the AN612 and into the TP3/TP4 section - was working. Not damaged or pulled up from the board.

I'd like to rule out the AN612 but we still have te problem so we need to got to TR21 when this "Dying" occurs and measure it's BASE voltage.

Now, as far as "dying" we still have to see what occurs, when that SSB mode quits - what happens.

What to look for?

• - what you get on that line at TP3 - if it still produces 7.8MHz.
• Look at BASE of TR21 - it's voltage.
• Our next step after this post - then we need to look further up the chain towards TR31
  • that section "turns on" in TX - IN AM mode to provide the 7.8MHz signal.
• But what is causing the dying - What does the BASE - goes back to the TR21, it's BASE voltage doing?
• TR 31 may have a problem holding up - somehow a trigger is occurring...killing SSB signal
• TR31 routes 7.8MHz - BUT IN AM MODE - and keeps AUDIO OUT OF AN612 thru TR21 Tells it to turn on and send audio elsewhere,
• USB, and LSB - those COILS and Trim CAP only work for a specific mode.
• THE ABOVE BULLETED LIST IS BASED UPON IF TR21 CAN BE TURNED ON BECASUE OF SOMETHING FURTHER UP THE LINE - ONLY DEPENDENT ON IF TR21 IS DOING THIS...

We also need to look at if any mods to the CLARIFIER were done, like any cut traces on the TX line. IF the Open Clarifier had work done to it - how was it repaired?

• This is a BIG IF
• - Either the rewire or something on the TX line now feeds into that control line from TR31 so what while you're keyed, something tells that TR21 to turn on
  • - or is there really by what you said, the AN612 is working correctly or is it - again to look at Signal.
• - so something while you're in SSB mode - is triggering - but is it TR21 turning on or the AN612 shutting down?
• You mentioned TR26 - that is the PA amp.
• Here's a thought - because if TR21 - when removed the radio still dies, remove TR26 and reinstall TR21.

Read on...this is complex...

R113 and R112 are involved - if you say the resistors are good, then we still need to look up the chain back to TR31 if TR21 turns on - you don't have to swap it yet - just pull the TR21 transistor and keep radio in SSB mode until we find the problem causing the dying issue

• If the signal still dies, then put back in TR21 - there's nothing wrong with it - we need to find out what you said about TR26.
• TR26 is a PA amp...

But hold here, until we can reliably recreate the Dying - issue and make sure we know if TR21 is turning on or not. Because if it does, it's rerouting audio out of the AN612. Removing TR21 just keeps it in SSB mode (stay in mode) and the AN612 works like it should. Or unless it dies - then we need to find out if TR26 is ok is there really a problem with the AN612...? Or something more exotic.

Then we go back to square one...

 

[Robalo]

If he removed tr26 he not going loose the amc and the Alc ,radio still working right

 

[Handy Andy]

No, but if he's got issues with the PA amp transistor - TR26 may somehow be coming up or tuning on, not sure of the wiring if any was repaired in this...

The above is some thoughts as to why I'm asking this stuff...

It's because I had to track down a dirty CB/PA switch caused by an operator spilling (whatever) onto the front dashboard of his Cobra 142 - since it's a similar board - to see something "die" see above...

• Lite - BLUE - DIODE matrix of the MODES - Keeps 7.8MHz IF working
• GOLD - TR31 if in AM mode - turns on and sends power to TR21 forcing Audio out of the line to AN612 - and sends it to the Audio Amp instead.
• GREEN - TX Power GOOD LOCK switch to let Radio go into TX mode - routes out to CB/PA switch looks at Pin 5 of MIC socket if in CB - else OPEN in PA - turning off TX mode not matter what LOCK says. PA works independently but USES PIN5 of MIC socket
• RED - 8 Volt Constant branch - powers MIC amp and PA Amp strip and AN 612
• Dark Brown - PA Amp - note lines for MIC - although Audio arrives SEPARATELY - it does SEND audio thru same line to Audio AMP as if in AM Mode (TR21 inclusive)
• Lite - Brown - SHARED audio output line from both Mic AMp Thru TR21 and PA Amp directly

You have two places where Audio gets sent into a SHARED line. TR35 handles TX mode when LOCK is good but looks at PIN5 of the Mic socket - one side goes to the MB3756 while one heads to the switch - it has to go to the CB/P{A switch to turn of the TX side but now matter what it runs the Audio Amp. TR35? If it can't send power thru to the CB/PA switch - what does happen? The line from the SWITCH goes to PIN5 of the MIC socket - this turns off the TX side but what does happen when the switch is dirty and yet "held up"?

Now if the AN612 is working - the output of Pin 7 goes here...


It's why I had you ( @wildwinds ) check a cable - for the AN612 also taps this line - and THE FREQUENCY COUNTER DOES TOO! So if there are problems in the cabling - the Frequency counter might not work - but TP3 would show this as really low signal output on an O'scope. - you get a frequency so I can only hope it's strong enough - and if that was the case, you'd only have really low SSB output but would still hear the audio.

Now for the AN612 to the FT3 route, or that shrink-wrap mess of Xtals and Coils - that can still send signal to it, but if the output of the FILTER is bad - IC5 won't see it - so SP5IT is asking what you see on Pin 7 of the AN612 - that Balanced Modulator.

My thoughts on Audio and Dying -

The PA Amp could "latch" and send audio out killing all the MIC amp line because it SHARES the Mic Input from the Dynamike control. So if PA amp came up or if there is an OPEN line or bad CB/PA switch (read dirty internal contacts or harness pinching and/or wiring insulation breaking) I can see where the CB PA switch can be a culprit - when the contacts are not fully thrown in one direction or another - can cause a "Hang up"

I've seen this many cases that the Front Panel got jostled by the user - or just plain dropped, and now the Faceplate to Chassis - making alignment and screws with pins detents and alignment holes nearly impossible to put together because it's warped and the switch toggle levers won't line up to the outline - in short - a dropped radio is a big mess...

Now I'm not saying his radio is dropped, just the Control Panel Faceplate to Switch alignments may be an issue on the extreme ends of each side of that panel. So when you throw the CB/PA switch, the level does not complete it's entire throw so contacts are left barely connected. I've seen Base radios that you can tap a control and it fades out, taps again and it comes back - these types of symptoms are hard to track down easily and quite stubborn to repair because it takes so much time and effort in disassembly and reassembly and alignment all at the same moment to secure the proper tension and spacing so all switches and knobs work with no interference with each other or in their movement.

Just some of the Joys of being the guy others head to when they're tired of it...

 

[Handy Andy]

Now since these questions are coming up...

Next are AMC and ALC controls -

The easier way to check ANY of this, is to pull R131 out of the AMC amp section or just plain remove the limiter TR24 and see if anything stops - my guess (suggestions) doesn't count.

 

[wildwinds]

hello there,
ok so today i have removed tr21 and am operating the radio without it on ssb.
i has not gone off yet but i will try it after for a while and see how...if... it goes.
i have been tracing the yellow cable.


also there is a .5v signal of 7.8 at pin 3 of the an612.

the radio is transmitting and its on frequency audio quality is not great though.

I am not ready to look at the tr26 just yet..
Thank you all for the help its mush appreciated

 

[357magnum]

Wild , So Sorry you are having problems w/ your 2K & I hope you get it fixed soon ! But I can't thank Andy, Mike , & Rabolo for their input ! NP's w/ mine yet , but what a file I'm making of info ! 73 , Leo

 

[wildwinds]

Hi Leo, well I think the guys on here are amazing.
They put so much of there time into these repairs.
This cobra is doing strange things today..with r21 out. And it has died again just now, but there is now a good 7.8 Mhz on tp3....
For me it's an education in these posts on here.
Wonderful forum..

 

[guitar_199]

Just a tip of the hat to all of the "help providers" here. I too am copying this thread out to a word doc to later "consume" the nature of a good troubleshooting spree!!! Thanks.

 

[Handy Andy]

Ok, caught this...

Put back in TR21...

We will have to go further back.

When you said Audio is not great, TR21 should not have affected the QUALITY or QUANTITY of audio in SSB modes - so something is amiss...

A good 7.8MHz signal but SSB dies... ok, means that TR31 and the Diode Network are next to check...

They provide that 7.8MHz and the SSB offsets - so on we go...

There is a TR32 in there... it's a 2SC1675 - small signal amp...

If it's having trouble making waves - as in RF, one or both SSB modes it simply detuned the 7.8MHz crystal - for they are in series with it - even though they are all pigg-ied together on the same line - only one powers up in thie SEPARATE modes...only exclusion? L31 has extra inductance L32 added to it.

The separate modes are isolated by 3 diodes

• - D41 for AM
• - D40 for LSB
• - and D39 for USB

They steer their signal to the 7.8MHz making it resonate at a slightly higher or lower frequency

Note CT2 - and how it's placed in the circuit - it is a capacitor - so it ADDS capacitance to change - in this case RAISE the resonate frequency - USB.

This may indicate that you have more that one frequency resonation going on at the same time, or TR32 is not "ringing" well and so it "quits" and lets the main 7.8 resonate Xtal frequency thru.

There is also the effort of what FL3 - that big shrink-warp part near the PLL does - that is the Xtal filter for the 3 bands AM, SSB - USB and LSB.

The file is attached - there is a photo of a 3-stage filter too to help you understand the concepts of using coil and then Xtals - but the Xtals server as RESONATORS for the passing of SPECIFIC frequency bandwidth that is slightly tunable by the use of the coils arranged in the filter.


The other two add inductance as a means to lower resonate frequency - or set it to mid-point

 

[wildwinds]

just got back to the radio and its not working, unusual as it usually works from cold.
I think theres a problem with the 7.8. Signal from tr32
When i left the radio earlier the 7.8Mhz was at tp3 strong.
However now it is not at tp3.
There is a small.25v signal at pin3 an612 7.8Mhz
nothing at pin 7 an

Its frustrating as it differs from earlier,the 7.8 is at c65 but not the cable side...

REading the post i see mention of having a low power signal on ssb, i did notice that when it works the signal was about two watts on ssb.

BY the way Andy the radio has never been droped ,it has been well minded. But perhaps i might like to drop it from a height soon....only joking of course. I have reinstalled the tr 21 and the AM is working fine again,

What could be torturing the 7.8mhz
Best regards

 

[Handy Andy]

just got back to the radio and its not working, unusual as it usually works from cold.
I think theres a problem with the 7.8. Signal from tr32
When i left the radio earlier the 7.8Mhz was at tp3 strong.
However now it is not at tp3.
There is a small.25v signal at pin3 an612 7.8Mhz
nothing at pin 7 an

Its frustrating as it differs from earlier,the 7.8 is at c65 but not the cable side...

REading the post i see mention of having a low power signal on ssb, i did notice that when it works the signal was about two watts on ssb.

BY the way Andy the radio has never been droped ,it has been well minded. But perhaps i might like to drop it from a height soon....only joking of course. I have reinstalled the tr 21 and the AM is working fine again,

What could be torturing the 7.8mhz
Best regards

So how is TR32 doing?

And you'll need to look back at the IF signal arriving to AN612 - there MAY BE two caps on Pin 7



Now you'll learn the frustration of techs worldwide deal with...

Not everything matches up in VALUES used - there will be differences between Schematic and ACTUAL Board values...

If C65 seems to be getting good signal - remember propagation of signal thru a network of cables - like routing of Coax thru a installation - you have certain ways the cable can be routed - and only routed this way for otherwise the signal degrades due to reflections present between the two points that jumper has to connect also goes thru areas of stronger signals that can - if the cable going thru the mess is not properly shielded and routed - degrade performance simply by proximity to a leaky cable.

So we need to make sure we have a strong signal arriving TO AN612 from TR32 and FROM AN612 to FT3.

The two most frustrating efforts in dealing with boards are - whom changed what...

For example, you may notice C98 - should be 7pF and an NPO - but note the value of C207 - some boards don't have that high of a value - do you even have a C207 on your board?