Cobra 29 LTD Classic. low volume.

[Low_Boy]

I checked D5,6,7,24 all good. I have to get to work so I will be starting down the path of checking when I get back in. Thanks.

I do know I get the loud rohr that I should get when I touch pin 4. One of the reasons I knew the audio amp was good.

 

[Handy Andy]

Greetings!

I'm just going to throw 2 cents in here...

RX Audio Low
PA audio works - (i'm presuming Volume Control works this too)
C48 replaced...
Ok, I'm just gonna throw out this -
Did you check EXT Speaker out? Did it have all the volume?
Did you check the speaker itself - make sure that the braid is not shorting to case or the speaker wires are ok?
Mic 1 to 4 - on RX shorted or is it bad...that would kill audio for both RX and PA RX out too - so if PA seems to TX ok, check the RX side of things - with the PA mode on - with that mic and see if the JACK is bad, or the Pin 1 and Pin 4 routing to the radio has failed. Lots of times I've had the mic plug or jack get hairy on me and a simple twist will bring back audio - scratchy as all get out - but it tells me audio is trying to return somewhere in the mic cord...a tricky disassemble and reheat with good solder and looking for bad cracked joints and blobs - clean up AISLE 4 stuff - and see if that was the problem.
All the other stuff I hear from you tells me either a return (Pin 4?) or a Speaker Ground has goofed up. RF gain ok, S meter works too...Look for R51 it's a 3.9 ohm (big ceramic encapsulated resistor - and see if that's good too)
T2? Unless it's smoked and blown open like a fuse - PA would not work at all...so you can rule that out for now
Long shot? The horse to bet on there is the AF Mute switch from the detector, called TR13 ... if it failed shorted then it would kill all RX so you'd hear TX'd PA and mic audio from the PA, but little from the RX side of things when the PA mode is on.
In fact, if you've had any problems, anything from the volume control on would be suspect too...

I could go on...

Hope these ideas help!
Regards!
:+> Andy <+:

 

[OldTech03]

Hey, Andy glad to see your still kicking! Now I went down the logical troubleshooting trail with him because he said if he touches pin 4 it howls big time. That should mean it's all is well from the audio amp to the speaker. But all does not seem to be well with pin 4 getting a signal from the detector stage. Next, a good troubleshooting method would be to trace the signal from detector stage and follow the trail to pin 4. could also use an audio signal tracer nice to have when you are troubleshooting simple things like this.

OldTech03 alias Vintech52:

 

[Low_Boy]

Hey Andy. Welcome. Some of the first things I checked you mentioned. TR11,12,13 all good R151 Good. Volume control replaced with a used one but what are the chances that the two of them would act the exact same way. Two different microphones were used. Speaker in radio and ext. speaker all good. I checked for bad wiring at the speaker and Mic plug.

So now I will be tracing. I may study schematics tonight and trace tomorrow. I cut down two trees that blew over in a wind storm last week and I am out of shape. And getting older. I am hurting. It sure is a different sore and tired than driving all day LOL.
Thanks,

 

[Handy Andy]

Greetings!

Thx for the welcome, but I haven't fixed a radio here yet...

Low-Boy - On Cobra 29's - In RX mode Mics' the Pin 1 and 4 are shorted - with Pin 1 being the ground side. So Pin 4 is FROM the speaker (hot) and Pin 4 is where that power goes...to ground to make the circuit complete and you hear the RX.

...this step presumes the speaker and T2 and all associated supports are in place...mic wired right and no broken pins...

A quick check would be to "tap" the audio line, arriving to the VOLUME CONTROL from the detector (wire to the volume pot can work) with a test lead probe-tip and see if a buzz can be heard. then locate the wire from the pot that routes back towards the Audio Amp (W22 may be identified this way for color-coding purposes Or W42 if it's an LXLE model) and see if that buzz is LOUDER - if there is no change no matter which way the pot's "checked" - then that's the area of concern. Detector - thru pot - to Audio Amp W22 or TR13 whichever comes first...

From above and the FORMER message - if the RX still does NOT get thru the EXT speaker (low volume?) when an EXT SP is hooked up, then remember the internal speaker CONNECTS TO ONE PIN on the EXT Speaker jack, as a port, then when a plug is inserted into the jack, a switch contact then is pried apart and disconnects the INTERNAL SPEAKER (which has a wire going to the speaker from there)...then that can narrow down the problem even further...

T2 going bad? Wow...only on TX, and if he has audio TX ok?

He does, so then the SECONDARY of the T2 is suspect, but only if it's had power run thru it as shorted winding to ground kind of thing...That test check is below...

Low-Boy, unsolder the wires to the speaker, leave them free for the moment - disconnect the mic cord from the radio - and measure the resistance across those speaker leads (not the speaker itself - remove unsolder the speaker from those wires)

Should be infinite - check each lead to ground too - both should be infinite to ground and show about 4 to as much as 10 ohms ACROSS these two. That means the SECONDARY on T2 is good, and not shorted to ground...

If one of these two wires was showing ground - then suspect C49 or C50 or even the EXT SP jack as potential culprits...look for shorts and re-heat the pads - the EXT SP has a 3-rd leg to RETURN power back to the speaker. IF one wire is shorted, the other wire would have a few extra ohms on it...due to R51 (not the rear ant jack R55) having 3.9 ohms or so...some continuity checkers would not sense that so you'll have to pay attention to the ohmic meter readings...

Now, put the Mic cord back on that jack and see if your resistance ACROSS these has changed (speaker leads), hopefully not (like if someone had messed with the mic plug wiring and back panel wiring - accidentally shorting out one lead to ground THRU the mic cords' return)...then suspect one of your speaker wires is open to the rear EXT speaker jack - you have LOW audio - like leakage - there are two small caps C49 and C50 on the speaker line from T2 that are less than 0.047 (473) so they will cut speaker power - but if the wires are OPEN - then the caps provide an audio path to ground for the speaker letting the speaker work, but at a VERY LOW volume.

IF one of those CAPS are shorted - then all audio will get sucked thru it into ground and the speaker just sits there...or you get talkback all the time because of one of the caps being shorted and it's on the live open side off of pin 4 is shorted to ground...you'd hear a squeal like a banshee and you don't have that so....

Now you know why those speaker wires are color-coded so the assembly team doesn't have to figure al of this out for itself...

Low-Boy - re-check your work, I'm hoping the CB/PA switch is ok.

Reason? I've had a few in the "dung-eon" that had work that put voltage thru the front panel CB/PA switch - the wrong way and they would up a torched foil mess - but those are on windings that are all tied together at things like T2...

Hope the above helps - it may be a wire you put back the wrong way onto something you weren't expecting like a failed cap that got shorted onto something like a physical failure like the speakers' spyder cage shorted out to one of the braids going to the speakers core winding...

Lots of stuff - but are all that can be checked rather quicky to be ruled out and continue..

Regards!
:+> Andy <+:

 

[Handy Andy]

Wow I'm getting old...

T2 ok, Power supply Choke
Try T1 - speaker winding side...

C123 is power input filter cap

SIGH...

So again, I said T2, I meant T1 - so you just need to make sure the wiring is ok. But if you blew C123 - that may have a trace or two to those caps that are part of the Positive or Negative Ground protection - again back to C49/C50 and that region in that rear corner - it's tight but I hope it was not due to the C123 blowing up and now it's guts are fun-fettti back there, coating everything......

Hope this correction helps!
Regards!
:+> Andy <+:

 

[Low_Boy]

Old thread but I just got it working. After checking many more parts D4 was bad.

 

[Chevboy0167]

Thank you for the update Low Boy. It's good to see solutions are found and posted to possibly help others.

 

[Handy Andy]

Glad it was something simple ...

D4 the Detector...

Man we hit just about every device in the chain - except that one...

What threw me off of that trail was the fact that the S-meter worked.

Uses same coil - but different tap off of it.

D4 - is dedicated to the Audio side - all the Audio and ANL develop off of it - then uses another part of L9 to obtain operational performance - Same coil - just a different tap of the wind - to make the S-meter and Squelch "pop" open and operate properly on signals proportional to their signal strength - requiring AGC too.

The second L9 tap, that one the "D5" diode used, was also the same tap for D6 (Or D61 depending on Board revision) - which made AGC and Squelch work.

So if those parts were ok, it meant that the IF up to there worked.

Just "Audio branch" side used D4 into the ANL circuit.

Killed audio...

Graphic attached...

 

[Low_Boy]

Andy LOL. After I got it working I went back and looked through everything we checked and you are right that is about the only thing that was missed. I played with the PC122 and no luck. I will not talk about the old President grant I have been working on for a year also. LOL.
Thank You.

 

[Low_Boy]

Glad it was something simple ...

D4 the Detector...

Man we hit just about every device in the chain - except that one...

What threw me off of that trail was the fact that the S-meter worked.

Uses same coil - but different tap off of it.

D4 - is dedicated to the Audio side - all the Audio and ANL develop off of it - then uses another part of L9 to obtain operational performance - Same coil - just a different tap of the wind - to make the S-meter and Squelch "pop" open and operate properly on signals proportional to their signal strength - requiring AGC too.

The second L9 tap, that one the "D5" diode used, was also the same tap for D6 (Or D61 depending on Board revision) - which made AGC and Squelch work.

So if those parts were ok, it meant that the IF up to there worked.

Just "Audio branch" side used D4 into the ANL circuit.

Killed audio...

Graphic attached...
View attachment 26417

I did replace this with a black banded diode. The green stripe on the bad diode did get me thinking though. Possibly not as strong?

 

[Handy Andy]

Black Band refers to "Basic" or silicon.

You have to look at the packaging the diode was in.

The older diodes had a larger glass envelope - which usually indicated a special or specific application versus say a black epoxied or smaller fast switching glass encapsulated diodes like the Schottky or Silicon.

Germanium has a particular response curve that many newer Silicon's cannot rival. The smoothness of it's use as a detector and derive audio frequency from as well as it's own ability to mask (what appears as) a shot noise that a fast switch silicon tends to produce. "Sound"? Yes - like a clicking sound in some instances of high power swing carriers the AGC section just can't track fast enough without help (Schottky works for this type of switching action) due to it's low forward drop and lack of P to N intrinsic region (an insulating region) that a standard PN diode has that induces this effect.

But not so fast as far as removing the silicon for this novel newt of galena that is also known as "Cats Whisker" - ever pluck a cats' whicker? Pissed Puma - but in Radio land the Cat's whiskers were the cats meow when it came to mid-range to treble roll off - the clarity and smoothness is an artifact of it's slower switching times.

While the other markings were specific to application - usually "G" meant Germanium while the "Double Blue" referred more to a power handling issue - but in this instance both are Germanium types. So their performance was used to make the radio work with a type of performance that were idealized for AM and some SSB transceivers.

The NB section and it's diodes were also Germanium types to provide a response curve to help in recovery of the audio embedded in a noisy signal. The rapid rise of the spike is what helps the Noise blanker determine the level of attenuation it can apply to the spikes. The slower rise and fall (almost capacitive effect) of the detectors own properties across it's junction allowed the spike to propagate across an otherwise level playing field like a sore thumb or a Mole digging thru your lawn. The bump was used to trigger the attenuator.

So in some ways, many other uses for the "receiver mods" that many use like the 2SC2999 and the Schottky have their benefits to their drawbacks. Some of the better uses for the faster diodes are used WITHIN the radios response to these noise sources. Just - not the signal. Allow or let the signal propagate thru the path. But use the faster switching and lower voltage conduction and low-loss forward voltage drop properties to act upon the noise abatement and limiter sections the radio uses against the noise.

Sorry to rant. Er, well no, I mean, it's not a rant per-se. It's just there is a lot of audio applications the still use the Germanium based devices like the Diode and the Transistor to provide a level of sound reproduction and effects that Silicon cannot produce without extra support circuitry to mimic them.

 

[Low_Boy]

Not a rant, a learning experience for me.

 

[sonoma]

Handy Andy Rant, never. HE HE
I love to read his post