Cobra 148GTL-DX/SS360-FM SWR meter/Calibrate issue

[Handy Andy]

A lot of times, the "Beep" - if it's been operated too many times, will actually burn out R185, that 4.7M or thereabouts - resistor. IT's a high value, so different times the chassis was "run" they changed the value of that resistor - so look for that "green band" as the multiplier and also it's around TR33...
Here's' a Snippet...


D71 controls the "peep" so if they disable the beep, D71 or that R185 - I'd look into figuring they may be bad.

C146 is a 10V 100uF - for a VERY LONG peep or a long duration "mute mode"

D69 supplied the power - so check that too...

 

[Christophe Barbe]

Thanks Andy for coming back to me, must be a failed component as the solder side is genuine on this rig. The "peep" orange wire is also as it was ex factory.
I'll have a look at these and come back to you with what i have found...

 

[Handy Andy]

I check in here , but - some people think I harass, this site everyday...

But I find discussing something I enjoy as a hobby, is far better than to discuss other things not pertinent to radio or communications in general.

I do find at times that posts of mine "disappear" for unknown reasons but then this is why I check in - to stay active and to also review my various approaches to posting methods to help answer questions effectively yet thoroughly to help the next one that may stumble into the thread looking for other answers to a similar symptom or problem.

Back to the issue of the "Peep" - there are two small value caps that are supposed to generate a burst - it's a design similar to a relaxation oscillator with a strong feedback loop (the 270K resistor for example) to provide linearity and some analog effect to the peep else it will act more like a "one shot" pulse maker than an oscillator.
.
So the biggest cuprits are those Electrolytic's, but as your finding - solder "shards" and other wild errors you don't normally see.

But then too, parts are older in the unit so if the radio seems to work for the pause, but no burst of tone, suspect then either TR33 or the oscillator portion itself.

Good luck!

 

[Christophe Barbe]

Hi Andy, you certainly do not harass people, rather, I learn from you and you help folks. Your presence here is very valuable IMO.
Back to the beep: I replaced every electrolytic connected with the beep, checked most but not all the resistors (ohmmeter and visually), no luck, same applies to diodes, they work. Visually no burned resistor. And nobody was there to cut one (I saw this in a Tagra Pacific II once).
I did not check yet: the 2 green small value caps and TR33 for example.
And symptom did not change: there is a roger beep (releasing PTT triggers it, it has its duration), the only problem is that it is "silent".
The circuit gets 8V, since it works, well to my understanding of things.
Could not be the 2.2uF, as the beep has its "duration", but i replaced it, it was a grey one, so a pretty old one, maybe it'd have never failed but who knows.
What i would just like to understand, is what actually makes its "voice"?
The green caps give its frequency, could they be the culprit?
If the beep actually is there doesn't it mean that TR33 works?
After all it's just a beep, but it's nice to "brainstorm" on it, at least I learn something in the end

 

[Handy Andy]

It's when you use CW mode, do you hear it?

The Green Caps, are those Polystyrenes/Mylar (think foil Birthday balloon material) - it's micron thickness - sandwiched between plates of Reynolds Wrap - you get the idea...

If they've gone flat, which can happen, they should be replaced.

I found this article at WIKI - under Relaxation Oscillators...

You can read a Wiki of Relaxation Oscillators Here ​

The concept is to use an R/C - cascaded into two stages - using the recovery moment the caps charge then release thru the resistors - as a inception moment - this moment can be sensed by placing a device used as a switch - in this case - A Transistor using a Grounded Emitter as common - in the above example, they use a Neon bulb - but in either case, the rate of charge is modified thru R - when the cap is charged - the current intake into this circuit thru R - drops, the voltage rises - and the neon tube "fires". The tube arcs - being the gas has less resistance as an arc - the cap discharges thru the bulb, and once the cap is depleted, the value of R is too high to "sustain" the arc. The Arc stops, gas resumes it normal high-impedance (insulator value) state and the cap accepts charge thru R again.

The Collector offers a charge, and so does the Base - to the parts in the circuit. Once a cap "charges" the voltage rise it has, is modified thru a resistor tied to one lead, the other is grounded. What triggers this is - the Base to Collector difference - changing by the caps charge rates - and so the transistor switches from off on to off - then back on again - and the cycle repeats - it uses the two 0.0047 (472) caps to generate that characteristic tone - while C143 controls the level of regeneration and the "length" of the oscillation moment - modifying the tone (a log function) while the other two are simply the "burst" - C143 helps sustain the burst as providing a regenerative loop.

That 270K value? That too is a feedback loop, but controls the circuit as a level of Gain the circuit would have. Else the Transistor can self destruct from the "duty-cycle" the R/C values place on the ability to generate that tone.

Is that what you wanted to know?