Cobra 29LTD Classic - Delay from key up to audio

[brandon7861]

I think we can rule out the PLL and TX switching. Testing this new video option to explain why I think that. I will take a closer look at the audio section and squelch/mute after lunch.
View attachment VID_20240215_125109967.mp4
If the PLL was taking time to settle, it should still be getting modulated even if it was off frequency a bit. But the meter needle snapped into action as if something was muting it momentarily.

 

[KK4JW]

You guys are unbelievable. First, I need to thank all of you for your help, assistance, and advice. I'm going to dive in this evening and start poking around with my meter and get some measurements. Hopefully with your all's help I can get it working properly.

 

[brandon7861]

Sounds good! I am going to hold off on further speculation until you get a chance to check a few things. First step IMO should be check if it happens in PA, if not, that eliminates any problem in the AF amplifier section. Gotta do this methodically or we will be chasing our tails.

 

[brandon7861]

I know I said I was gonna hold off, but i found something that I think makes sense. Again, I will go the video route unless someone tells me not to.

View attachment VID_20240215_172427139.mp4

 

[KK4JW]

I know I said I was gonna hold off, but i found something that I think makes sense. Again, I will go the video route unless someone tells me not to.

View attachment 67652

Where the squelch knob is positioned has no effect. I also hooked a speaker up to the PA on the back, switched to PA, and I'm getting audio out of the speaker with the same delay.

Yes. Delay on the RF audio and also a delay on the PA speaker audio.

 

[KK4JW]

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),

I think that may be the smoking gun. I just checked it and it's 0.105v in TX. Heating up the iron right now to replace it and see what happens.

Edit: Replaced D6, no difference. I am seeing a small bump in voltage on the anode now though, up to 0.200v in TX.

Edit 2: I probed C30 and on the right side in TX (side that D6 is on) and I get the same 0.200v that I get from D6 when in TX. If I probe the other side of C30, nothing.

 

[brandon7861]

I think that may be the smoking gun. I just checked it and it's 0.105v in TX. Heating up the iron right now to replace it and see what happens.

Edit: Replaced D6, no difference. I am seeing a small bump in voltage on the anode now though, up to 0.200v in TX.

Edit 2: I probed C30 and on the right side in TX (side that D6 is on) and I get the same 0.200v that I get from D6 when in TX. If I probe the other side of C30, nothing.

Nope, I was wrong thinking there would be 0.6v there, I apologize for that. I was thinking in DC terms, but once the RF arrives on the scene, it will drop some. Take a look at my last video.

Edit: just saw the PA/squelch comment above, let me go ponder it again

 

[KK4JW]

Nope, I was wrong thinking there would be 0.6v there, I apologize for that. I was thinking in DC terms, but once the RF arrives on the scene, it will drop some. Take a look at my last video.

Edit: just saw the PA/squelch comment above, let me go ponder it again

Just noticed something..

I'm only seeing the delay when I FIRST press PTT. That's the ONLY time it happens. If I keep PTT pressed and go silent, and then audio again, the audio is right there like it should be. That really makes me feel like there's a capacitor in there somewhere that's slow to come up, but stays energized only as long as I'm in TX.

 

[brandon7861]

Just noticed something..

I'm only seeing the delay when I FIRST press PTT. That's the ONLY time it happens. If I keep PTT pressed and go silent, and then audio again, the audio is right there like it should be. That really makes me feel like there's a capacitor in there somewhere that's slow to come up, but stays energized only as long as I'm in TX.

I was thinking that was the case.

If it is happening in PA, I think that suggests somewhere in the audio section. I need to go look at the printed schematic again.

 

[Handy Andy]

Perhaps the best way to "find out" is to locate the Audio Mic amp in the radio.

The parts where the TX audio is amplified before it goes to the Audio Amp.

See this thread...

Cobra 29 ltd classic not transmit

The watt meter reads 4 watts but the modulation meter jumps a little when I key the mic then drops back to 0..

www.worldwidedx.com

Why? Because if it is the TX switch - the delay can be due to a slow keyup and only works when the PLL says it's "good" and the IF both the 10.240MHz and the PLL IF from the VCO side (L22 LO and L24 10.240MHz Xtal) are close enough together to "lock" but remember that might exist at the PLL, but when you try to make a radios' chassis follow this - the LOADING the chassis parts can do, does affect the ability to make the RF propagate to the right parts at the right frequencies.

So you may need to do some locating of the MIC amp - and see if there is a "lag" or sag in the power to the Mic amp transistor - then you'd know - you have to trace back the line to the TX switch and or the failed part in the line causing this condition.

 

[brandon7861]

Remember when I was complaining about the Euro CB schematics, yea...
So in the euro schematic, the mute transistors collector is in the wrong spot. I have been sitting here trying to figure out how it can mute RX and not PA, well now I know. BE CAUTIOUS OF THOSE EURO CB SCHEMATICS.

 

[brandon7861]

So I overlooked something else. The 2.2Ω resistor that pulls down the mic bias when the key is pressed. Dang it. So there is TX switching in the mic amp.. Check C66, it's probably leaky.

 

[Handy Andy]

Don't worry, for grins and giggles, there may be times where the INPUT cap may be too large to work the Audio Amp properly - so if it doesn't squeal, but the LAG-TIME generated from the cap charging due to the power it is supposed to block from one stage to another - is enough to block audio from overcoming the inrush effect that cap plate charge - change does. So there's a pinch-off effect similar to a squelch mute that happens - but you'd be aware of the effect by how the audio volume - rises as well as a pinch effect - decreases - when the audio returns.

Midland had several radios that used a Quad op amp that also was AGC and squelch amp that was designed to "flood" a twin-T cap as the rise in voltage applied to the caps to pinch off the incoming signal - as a means to quench the audio signal passing thru those two caps (in series end to end) with the tied-end to each other was the one that the Op-Amp flooded with 8X to pinch off the audio from the Detector control to the Audio Amp past the volume control - and of course it used Mylar/Polystyrene's.

So were only to check (mic amp stage voltage) to see how fast the amp gets gas to run, the accelerator part that causes the hesitation is only when it first gets used - so to me this just might be a leaky cap that let's a transistor stay pinched off until enough time has elapsed to recover the audio - so there may even be a resistor that drifted off in value making this a slow start thing - because power can't drop to open the Mic amp

 

[brandon7861]

I just realized something else, the TX voltage there disappears when the key is pressed, so the cap at the emitter of the mic amp should be dropping in voltage to turn on the amp. The mic amp is kept off by keeping the emitter high. I don't know if a leaky cap would hesitate to discharge, so I think Andy is right about the input cap.

 

[KK4JW]

"Back to the bench" this evening.. Pun intended.

I'll go probing some caps and see what I come up with. Again, my most heartfelt thanks to you guys for the help with this!