Cobra 138XLR key up ping/pop

[357]

Anyone know how to get rid of this?
My radio has a ping when I key up and with the mic gain off it gets worse like a quick squeal.
I seen someone say something about the PA circuit that does this.

The PA circuit has been rewired. This modification removes the annoying "pop" or "ping" sound transmitted during ssb key-up. So far I have worked on seven of these radios, all exhibit the same problem.

http://www.chicagopartsnetwork.com/...a-138XLR-am-ssb-40-ch-variable-power-alc.ASPX

 

[357]

The ping can be heard when I key up, its worse when I turn the deskmic down to zero.
Here is what I'm talking about.

 

[Robb]

Feedback with Talkback?
Check to see if it has a resistor running from the PA jack to the ext speaker jack on the trace side of the board?
If it does; then cut one leg and see if it stops feeding back . . .

EDIT:
I don't know the 858 pll boards very well at all. unit_399 knows them very well. He might chime in here.

 

[222DBFL]

There is some definite feedback!! Like Robb stated check for a talkback mod. If not, try a different mic. Could be the mic itself needs to be cleaned and also add a snap on ferrite choke at the base of the mic on the cord. One that fits tight as possible. The RS small ones work well for most mics.

 

[ExitThirteen]

It's a capacitor going bad. I can't remember which specific cap it was, as I have always just recapped the whole radio and it goes away. But I DO know that it is a capacitor issue. Keep thinking I had it written down in my notes, but so far, I haven't been able to locate anything on this issue yet. If I find something in my notes I will post it here.


~Cheers~

 

[Robb]

Since it is an old radio, he should recap it anyway. But worth it, as it is the 858 pll chassis. A keeper!

 

[357]

Thanks for the replies.
There is no talkback. There is another "pop" that I hear on all 858 boards when running a power mic.
The first word you say you will hear a "pop" sound.
I wish I knew who it was on Chicagopartsnetwork so he could tell me what he did.
I have a wack of caps coming and a cap/esr tester so I think I'll be caping 3 radios...at least the tantalum's and audio caps in the 138 but total recap on the Centurion and 135xlr.

Not really looking forward to it.
I also ordered a variable solder iron cause my 30w iron sucks and burns up the board

 

[Harley3315]

ever figure this out I have one doing the exact same thing.

 

[Shockwave]

The only way this can happen is if the microphone switch connects audio before the transmit relay closes. That's why different microphones with different switches behave differently here. One of the best ways to handle these types of switching transients in relay controlled radios is to wire the microphone to connect the speaker ground in receive as is done with all electronic switching radios. Figure out which side of the speaker is ground and connect it though an unused mic pin to the black receive control wire in the mic. I've been doing this on Tram D-201's for years to remove 100% of that loud speaker pop when keying or unkeying the mic.

PS: This doesn't exclude the possibility that something could be delaying the internal relay from keying instantly. Check to make sure its reaction time is correct when you key the mic. If there is a known capacitor issue, that could be contributing to the problem.

 

[Harley3315]

99% sure I changed every electrolytic cap in the radio but might give that a go it does it regardless of which microphone I use.

 

[Shockwave]

If the feedback is present after you hear the internal relay click, this modification may not help you with that. If the feedback is only present just before the relay clicks, this will solve your problem.

 

[Harley3315]

Well no I did add talk back but regardless of if it's on or off the problem persists it's not so much a pop more like a free key up ping . None the less kinda annoying. But it last a couple seconds and goes away. I've done a very extensive amount to this radio in hopes my rx would be cleaner. This town has horrible noise from factory florescent lights maybe other things.

 

[357]

I think it was just aggressive limiting or something.
I cant remember if it went away or not, sorry.

 

[Shockwave]

99% sure I changed every electrolytic cap in the radio but might give that a go it does it regardless of which microphone I use.

If just one got put in backwards somehow, this could be causing the problem too. This is easier to do than you may think when replacing many at one time. It's always a good idea to confirm the polarity of the part you are removing first. I've seen the silkscreen on the PC board tell a fib more than once here.

 

[Harley3315]

I'll look it over again wish I could find a top and bottom view of the board like a Sam's photo fax.