Cobra 29LTD Classic - Delay from key up to audio

[KK4JW]

I have a Cobra 29 LTD Classic, and I am experiencing an issue that I've never came across. I need a little help, pointing me in the right direction to correctly repair the radio if possible. Everything on the radio works well except for a noticeable delay between when the radio will actually begin "picking up" modulation/audio from the microphone and transmitting it.

For example, if I key the microphone, there's about 1 second delay before the radio actually "gets audio" from the microphone and transmits it. I'm literally watching this on the RF meter, and have verified it by listening to a receiver tuned to the same frequency. If I key the mic and wait about a second before I start talking, it isn't noticeable. I'm usually pretty quick to begin talking the exact moment I key up though, and that's when I noticed this anomaly.

I'm wondering if there's a failing capacitor / transistor or something in the audio chain that could be affecting this? Has anyone else ever experienced this before?

Cobra 29 video

 

[brandon7861]

Does it happen in PA mode?

Sounds like the bias or supply of an audio amp is taking longer than usual to come up to voltage. If you have an analog voltmeter, you can probe the anodes of the capacitors in that area (C144, C94, C91 and C148) and watch how fast the needle snaps to its final resting voltage when you key. The slow one is the bad cap. They will have different voltages, its the speed at which it gets there that matters. Which one takes 1 second for the voltmeter needle to come to rest?

 

[brandon7861]

The video finally played (wouldn't earlier). At the very first part of the video, it looks like it keyed with a carrier and modulation worked, but then the next time, even the carrier took a second. I don't think it is in the (low level) audio section if the carrier is also delayed. I gotta think about this for a bit.

 

[guitar_199]

Does it take that long before the carrier show up?

Or is the carrier there RIGHT AWAY.....but the modulation shows up late?
I think that the transmit side is "shut off" when in RCV.... so it could be a delay in a mixer or some element in the transmit chain starting up....BUT... you would see that in the output. You would key up and have "nothing" for that 1 second delay...then it would all show up.

 

[KK4JW]

The video finally played (wouldn't earlier). At the very first part of the video, it looks like it keyed with a carrier and modulation worked, but then the next time, even the carrier took a second. I don't think it is in the (low level) audio section if the carrier is also delayed. I gotta think about this for a bit.

In the video, I delayed keying the mic while I was audioooing so you could see what it does. The radio definitely keys when I press PTT, it’s just the audio is delayed.

 

[KK4JW]

Does it take that long before the carrier show up?

Or is the carrier there RIGHT AWAY.....but the modulation shows up late?
I think that the transmit side is "shut off" when in RCV.... so it could be a delay in a mixer or some element in the transmit chain starting up....BUT... you would see that in the output. You would key up and have "nothing" for that 1 second delay...then it would all show up.

The carrier is there right away. As soon as I press PTT I get a carrier. The audio is just delayed.

 

[brandon7861]

In TX, what is the voltage at the anode of D6?

I'm gonna take a shot in the dark here. If that shorted (showing closer to 0v than .6v), it could keep C147 discharged via R125 and when the TX voltage from the TX switch goes high, that same capacitor holds down the base of TR12 momentarily. If TR12 was momentarily held low, TR11 would also stay off momentarily, and that would keep the mute transistor TR13 momentarily high (and shunting mic audio).

I did not try to figure out the time constant of that resistor puzzle so this is just a wild guess.

 

[brandon7861]

Same thing if C30 is bad i think

Edit: its the abruptness of it that has me thinking that darlington pair in the squelch circuit is involved...

 

[Handy Andy]

There may be more than one reason for the slow audio...

One type. A poor Audio amp capacitor or a Slow TX switch cap. (Read:Bad caps)

Another would be a VCO with a slow TX shift time (455kHz - search for it here on the forums)

To help one understand this effect, when a VCO coil is not tuned correctly - it might take the Varactor some time to step up or down to a voltage that makes the PLL sense a lock and which it is trying to locate the correct frequency - the PLL keep the radio from transmitting.

Take the "monitor" radio off a few channels and do this test, you may hear a chirp - which is the very thing I'm describing here - the radios attempt to force a RF keyup - but must wait for the VCO to catch up to itself to lock in.

 

[guitar_199]

Another would be a VCO with a slow TX shift time (455kHz - search for it here on the forums)

To help one understand this effect, when a VCO coil is not tuned correctly - it might take the Varactor some time to step up or down to a voltage that makes the PLL sense a lock and which it is trying to locate the correct frequency - the PLL keep the radio from transmitting.

Take the "monitor" radio off a few channels and do this test, you may hear a chirp - which is the very thing I'm describing here - the radios attempt to force a RF keyup - but must wait for the VCO to catch up to itself to lock in.

So, if I understand this right ... you would STILL see "immediate carrier".... but it would be 'off frequency' until the VCO catches up..... hence "the chirp"!!! and the modulation would appear once the "catch up" is done. Does that sound reasonable?

 

[brandon7861]

I need to reset, get this schematic printed 3'x3' (and taped together lol), and give it the colored pencil treatment.

Regarding the PLL out of lock condition causing the delay.. Wouldn't that stop the low level RF and not the audio? Seems to me there would be no carrier in that case, which we have, but I am not 100% sure yet. That is one thing the colored pencil treatment will sort out.

TX switching delay somewhere might make sense too, but again, the carrier is there, so the TX mixer is working, and that too is tied to the lock detect, so...

That brought me back to the mic amp/mute/squelch area. That's where I went to bed. I will go print this off and see if anything jumps out at me once colored in.

 

[doffo]

There may be more than one reason for the slow audio...

One type. A poor Audio amp capacitor or a Slow TX switch cap. (Read:Bad caps)

Another would be a VCO with a slow TX shift time (455kHz - search for it here on the forums)

To help one understand this effect, when a VCO coil is not tuned correctly - it might take the Varactor some time to step up or down to a voltage that makes the PLL sense a lock and which it is trying to locate the correct frequency - the PLL keep the radio from transmitting.

Take the "monitor" radio off a few channels and do this test, you may hear a chirp - which is the very thing I'm describing here - the radios attempt to force a RF keyup - but must wait for the VCO to catch up to itself to lock in.

I think I have a radio or two that chirps. Of course much older radios and never got around to recapping the radio.


I had one SSB mobile radio where the the modulation of the audio slowly rolled in when you keyed up. I planned to keep the radio so after new caps, that issue went away.

 

[brandon7861]

For those wondering where my post went, I deleted it because I said the euro schematic was wrong by the audio amp, but it was me that was wrong. I didn't see C79 (another reason I need to print these so big).

Anyhow, I am going to go print both schematics and color the traces so I can make sense of the symptoms.. Be back soon.

 

[guitar_199]

For those wondering where my post went, I deleted it because I said the euro schematic was wrong by the audio amp, but it was me that was wrong. I didn't see C79 (another reason I need to print these so big).

Anyhow, I am going to go print both schematics and color the traces so I can make sense of the symptoms.. Be back soon.

"euro schematic"

What is that?

And.... when it comes to printing big copies of schematics and marking them up with map pencils or markers.......

You mean...... somebody besides ME does that?????????

 

[brandon7861]

"euro schematic"

What is that?

And.... when it comes to printing big copies of schematics and marking them up with map pencils or markers.......

You mean...... somebody besides ME does that?????????

The schematic I was talking about is the "classic" schematic from CBTricks. It says Euro CB on it, assuming they drew it up in cad software because its not factory. I have stumbled across issues before, so I was preconditioned to find an error on it. I didn't see the little sideways capacitor in there, and with the ground trace and modulated audio power trace being horizontal and co-linear separated only by that cap, I initially thought the audio amp and mic mute was using the 13v final trace as ground. I now see that cap separates them, so it was my mistake. I was too eager to complain about the euro schematic.

As for the PLL settling issue Andy mentioned. I would assume it is possible to listen on a sideband radio during key and listen for a sweeping tone as the carrier settles on freq, but it will require the clarifier be turned to bring the carrier into the passband of the filter on the monitoring radio. That should prove/rule out an issue with the PLL. Nomad told me a similar trick listening on FM.