Cobra 140 GTL Audio Issues (Hiss/Static)

[wcsd106]

Hi All,

I recently picked up a Cobra 140 GTL (Uniden PC-385 board) and was in the process of bench testing it. When I first started testing it, I thought the squelch was malfunctioning, but I'm beginning to think it is something in the audio chain instead. Here is what's happening:

When the radio is on, you can hear a white noise similar to static coming through the speaker. This white noise is present whether the squelch is turned up or not and does vary with the volume control.

For example, if I inject a 1000hz tone using my RF signal generator into the radio, and monitor the audio output on my oscilloscope, I can see the 1000hz sine wave. The amplitude of the wave is varied by the volume control as would be expected. If I turn the squelch clockwise until the signal generator tone is muted, I still hear the hissing sound similar to static.

The hissing sound remains and can be controlled by the volume control.

Any ideas?

Thanks.

 

[Robb]

You using a switch mode power supply for the radio?
Florescent lights?
Tried it on a battery yet?
etc . . .

 

[psycho]

The 140 GTL looks identical to the 146 GTL.

 

[nomadradio]

Nope. It's the mobile version of the Cobra 142GTl. Same as Uniden Washington and some others.

I would start by unsoldering pin 4 of the TA7222P audio power chip.

If the noise goes away, It's gotta be a fault between the volume control and that pin.

And if it won't, that points to the TA7222 or maybe one of the electrolytic caps attached to it like C112, C113 or C114..

Not a common fault. The TA7222P was an incredibly reliable chip after the first few years' production. But this radio was one of the first to use it. An early-production chip can misbehave like this.


73

 

[midnight special]

pc 385,,,,, have you made sure all the screws holding board to chassis is tight? make sure all the caps soldered from board to chassis is good,,,,

 

[sonoma]

if it was mine I would recap the radio since it is so old 1st and then try to repair it if a recap does not fix the problem. more than likely it has a bunch of caps leaking or shorted in the radio.
as Nomad stated you need to at least change out the caps at the audio chip and I would do all the 10 volt caps at 1st if you do not want to replace all of them on the 1st go round.

 

[Handy Andy]

It's the...
"...You gotta' wear earplugs, when you know the squelch should be on..." part that tells me the "Static" is from caps thru the audio line - that dried out.

You did not say you recently got this radio recapped, this effect (swoosh as well as squeals amongst others) is a sure sign the caps along and thru the audio path - from the detector, throughout and into the 1st audio amp (TR13), squelch (TR15 into TR14) - volume amp buffer (TR36) and "out of lock" mute are all in the chain. when you adjust squelch and the squeal - hiss - crackle - roar - doesn't go away, that is a strong clue that the noise is from leakage - for various parts that are supposed to allow audio to pass, but keep the individual stages of the amplifiers and audio signal controllers - isolated form each other a re-cap is nearly a necessity due to the age of the device and all parts related. (THX! Sonoma - as I researched this - your post popped up)

There are also other factors, of mods, proper wiring and Mic...

The 140 GTL I know, uses a 5-pin cord and NO! Pins 2 and 4 are not shorted together. Not a good idea to try using a power mike or any other type of mic design on the radio unless you trust the radio is properly wired for the mic, and the mic itself is also properly wired FOR that radio. Use Stock Mic wiring - open the handset up - why? Much of the noise may be from improper wiring of the Ground for Speaker, Internal and External Speaker and PA - which can be an enigma in itself.



I'll try to summarize it, are you using the correct mic wired for the radio? Can you verify that by using more than one mic known to be good? A Cobra 140 can use a Cobra 148GTL as well as a Uniden Grant XL 5-pin mic - they all wire up, the same way for 5- pin so if you have a spare -try the others out to make sure the stock mike or whatever you use can work on another radio, e-g. - swap that radios mic to your new radio and that new radios mic back to the one on the bench to verify. IF both work - and you get speaker - onto the next step...

Test your PA CB switching - a lot of times a switch contact can open up from lack of use or just having a bad day and then you're left wondering what happened - It was working fine a minute ago! type of moment...Those contacts can be from the speaker jacks, they short to internal wiring to reroute if not used, onto the CB PA switch and it has at least two "poles" for rerouting Audio as well as switching control (Turns off TX but turns on and routes audio thru PA speaker) and Mic amp (TR29 to TR28) swaps to PA amp (TR30)

Here's a PDF - it's attached, a schematic of the Cobra 140 - hope it helps!

 

[wcsd106]

Nope. It's the mobile version of the Cobra 142GTl. Same as Uniden Washington and some others.

I would start by unsoldering pin 4 of the TA7222P audio power chip.

If the noise goes away, It's gotta be a fault between the volume control and that pin.

And if it won't, that points to the TA7222 or maybe one of the electrolytic caps attached to it like C112, C113 or C114..

Not a common fault. The TA7222P was an incredibly reliable chip after the first few years' production. But this radio was one of the first to use it. An early-production chip can misbehave like this.


73

Thanks, everyone for the replies. Nomad, the noise doesn't go away completely when I lift pin 4 of the audio IC, but it does get much quieter. I can still hear noise in the speaker, though.

I had already begun to suspect capacitors, especially since this is one of the cobras with the 10v electrolytics all through it. I've started replacing some of them and was planning on doing an entire recapping. Looks like that may be a good plan of action to begin with anyway.

I'll keep this updated as it moves along.

 

[357magnum]

Thanks for all the info all ! My old girl ( 140 gtl ) hasn't been touched since the uppers were put in over 35+ years ago ! Now I have things to look at or might just find someone here to send it to ! Sold all my bench equipment a few years back after my seizure . Some things are just too hard for me to do now . Mine just started the hissing looks like it needs a well deserved tune-up & cap job as it has served me well over the years & still use it as my mobile .73 & God Bless , Leo

 

[Handy Andy]

When you mentioned Pin 4 and the noise not going completely away, there are NFB and EQ sections the TA7222AP uses for control and equalization - if these caps are bad - the loop that they are in simply adds to the noise you hear.

Good idea to start out by replacing those Electrolytics and secondary ones to replaces are the DISC and Styrene types that can add a level of fun to your day. These styrenes as well as disc can absorb moisture and you hear their failure from it - heating it with a soldering iron will make it show up real quick.



The Critical Affects Performance is exactly as it says.

Can add to the noise and is a low voltage Tantalum. Replace with correct value if you can, this cap affects the tone and performance of the audio this radio will have over the air as well as in it's receiver. Again part of a Negative Feedback (NFB) filtering and EQ offset for the input to the amp and can add a squealing tone and/or unstable operation if you use wrong values.

 

[wcsd106]

Well, I made a list of every electrolytic cap in the radio and placed an order for the ones I didn't already have today.

It'll probably be Monday or Tuesday before it gets here.

 

[wcsd106]

Parts arrived and I've started recapping the radio. I had several NOS TA7222AP audio ICs, so I've replaced the audio IC for good measure as well. I'll keep the old one around.

I've got a few more caps to do (about 6) but I powered it up on the bench today and right now the hiss is still there.

The electrolytics I've got left to replace are the ones near the final/drivers and the 2 near TR13.

I did try switching between CB and PA. When I switch to PA, and plug an external speaker into the PA Jack, It's almost as if it just swaps the speaker jacks. I still hear static through the PA speaker as if the radio is receiving, and when I turn the squelch up, that static dies and the constant noise continues. Correct me if I'm wrong, but I don't believe the receiver noise should be coming out of the PA jack.

Microphone is the correct mic and is wired correctly. I have tested and verified that.

Going to finish these capacitors tomorrow.

Anyone have a transistor / IC voltage chart for this radio? I'd kill for one right about now. LOL.

 

[red97k1500]

Not really what you asked for, but it might help?

 

[Handy Andy]

When the noise occurs in PA, then the region that is affected is from Mic to PA or CB Mic amps - it uses two of them, and you "select" which pre-amp thru the PA/CB switch.

So if audio "hiss" is still in there - and SQUELCH is NOT removing it - then the problem is the Mic amps that use the same line for RX as RX - so to help you in ISOLATING the noise even better...locate C28 by TR15 - remove the cap or just lift a leg. See if it quiets the RX side.

IF not, reinstall that cap and locate TR32 - did they remove it? If so - reinstall that one. That - if not installed, will leave the Mic line open for noise to gather up in the Mic amp or even PA amp and inject into the Audio Amp chip line that it shares with C110 (from RX audio amp) and C82 (AM AF amp) it uses the RX power thru a diode D41. It turns on the TR32 to silence the Mic lines.

It would be prudent to check the RF gain pot for voltage across it too - for if your RX is goofy as you say you get hiss in RX - there could be a problem with the RX power feed thru the radios RX strip and RF gain controls.

Just make sure RF gain is sending a voltage back to the board - check terminals on pot to ground back at one of the shield cans for power flowing to the pot and back to the main PCB.

 

[wcsd106]

Andy,

Thanks for the information. I'm going to hit the bench again this evening and try to narrow it down. If I have a moment, I'll take a video of what's going on with the radio. I know sometimes that will help especially in a case of, "It's making a noise."