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Cobra 140 GTL Audio Issues (Hiss/Static)

...locate C28 by TR15 - remove the cap or just lift a leg. See if it quiets the RX side.

It would be prudent to check the RF gain pot for voltage across it too Just make sure RF gain is sending a voltage back to the board

Andy,

C28 was located closer to TR13 than TR15. Lifting either leg of C28 caused the receive audio to drop out, leaving only a static hiss which is unaffected by squelch. Reconnecting either leg of C28 brings the receive audio back. (I've been running my Signal Generator with a 1k hz tone on 27.185MHz for a good reference signal)

With C28 lifted, the static still follows the speaker jack (i.e. Static is heard from the External Speaker Jack when in CB Mode, and heard from PA Speaker jack when in PA Mode.

TR32 is present. I have not pulled it to check for shorts though. That is my next step.

RF Gain Voltages:
Center Pin of Pot: 0.97v Low / 2.78v High
Outer Pin of Pot (Used): 6.5v Low / 2.78v High
Unused Pin of Pot: 0.97v Low / 2.78v High
 
Andy,

C28 was located closer to TR13 than TR15. Lifting either leg of C28 caused the receive audio to drop out, leaving only a static hiss which is unaffected by squelch. Reconnecting either leg of C28 brings the receive audio back. (I've been running my Signal Generator with a 1k hz tone on 27.185MHz for a good reference signal)

With C28 lifted, the static still follows the speaker jack (i.e. Static is heard from the External Speaker Jack when in CB Mode, and heard from PA Speaker jack when in PA Mode.

TR32 is present. I have not pulled it to check for shorts though. That is my next step.

RF Gain Voltages:
Center Pin of Pot: 0.97v Low / 2.78v High
Outer Pin of Pot (Used): 6.5v Low / 2.78v High
Unused Pin of Pot: 0.97v Low / 2.78v High


Ok! Nice work. We can rule out RF gain for now...

The steps taken - resulted in knowing the RX was not ADDING to the noise, then focus on the Audio Amp chip area.

The process of "Breaking connection" to any portion of the receive, Mic or PA amps - you have isolated it to the Audio Amp itself. So if you need to, go ahead a do the same for the Mic and PA sections - to be sure.

Then once we know NONE of this noise is arriving to the Audio Amp section - focus on that section as being the culprit.

Look for these kinds of caps - I call them Mylars, Green Chiclets, but "PC Crowd" calls then Polystyrenes...
GreenChiclets.jpg
So if you have any of those in your radio, you already may want to replace these with standard Disc Caps - the beige or brown types - use the same values. These "styrene's" have been known to have an affinity for moisture and although a much higher breakdown voltage than their Disc counterparts - you don't have those kinds of hazardous voltages running thru those areas, or thru the radio even. (Sorry for the Preposition) So they are CHEAPER than their Disc counterparts and work well for their package size and work well in audio bandpass filters - but when the fail - they fail - miserably.

When you do some more research, you'll see that Uniden and other radio makers, may use these but in very rare instances - because of a need to fill that they couldn't drop in another more reliable type to meet the specs. But as with experience, you'll appreciate certain brands of radios sticking to quality more than cheaper easy to fix parts that when it comes to set it and forget it - makes you wonder why these Styrenes ever existed.

So again, we will be looking for those Green Chiclets and also into a possible recapping of Electrolytic caps (Grey ones rated 10V are problematic too - they just "kill" the circuit by shorting out when they've dried out.
 
Ok! Nice work. We can rule out RF gain for now...

So again, we will be looking for those Green Chiclets and also into a possible recapping of Electrolytic caps (Grey ones rated 10V are problematic too - they just "kill" the circuit by shorting out when they've dried out.

All electrolytic caps were replaced in the initial troubleshooting phase. I'll start checking out the Mylar caps.

Thanks for everything you've done so far.
 
I know its poor taste to bring up an old thread. However, I have a tram d300 with a pc-385 board so its internally identical and I also have the EXACT same problem. I have recapped the radio all tantulums and electrolytics. I still have the exact same problem as discussed in this thread. I was wondering if the OP ever resolved the issue or if any experts would like to give it another go? Im open to all suggestions.
 
When you turn up the squelch does the noise still exist?
  • It may serve you better, to know that as these radios age, many "noisy" problems and "quirks" in performance of the Receiver show up so you may find yourself replacing a lot of components - this is normal - you'll know why as you work thru the rebuild/repair process..
  • In nearly every case, Age of the radios' more sensitive filtering parts, like the electrolytic caps and many of the Mylar (Green Chiclet) caps are failing - drying out and or losing internal cohesiveness - its' simply "old" and they're showing their age.
The Tips earlier - after going thru this thread, are, to me, workable but you need to follow an order of steps that after I've done this - helps me locate the problem "parts" faster.

Start at the Audio Chip itself. Isolate its' input from the rest of the board - this way, if that noise still persists - you know it's not generated by something further up the Receiver chain, or the Mic amp sections.

You do that by removing C110 0.1uF or 104M/K cap - from the receiver Pre-amp and C82 0.047uF (473) - Mic Amp / AF Switch (this cap number may differ - but it comes from TR27 part of the Mic Amp chain going to the AN612)

Power up the radio and let it warm up - listen for that noise - be patient at this step, for the radio is not injecting noise, hiss or any sort of background audio into the amp to help it warm up faster - this takes time to bring the conditions back that give you the noise.

If you can, and are EXTREMELY careful - reheat soldering joints CAREFULLY - DONT CAUSE ANY SHORTS - it's is better to wait ...

But - if pressed for time - just apply small amounts of heat - while it is powered, you'll near hum and static - it's normal - this helps you in locating the faulty parts.
  • In the reheating process you may locate the poor soldering area and the condition goes away, you may have found the "barely able to make contact" solder pad - but then too, reheating these parts can also recondition them into working temporarily - so you will have to do this several times - letting it cool down after each attempt.
  • Pay attention to the detail of where the noises encountered during this reheat process are the strongest
    • - in the Audio amp chip itself has many feedback and Equalization pins so they all can add to this problem in one fashion or another.
The next process, is to reattach the Receiver Preamp - reinstall C110 that 0.1uF cap (or whatever value it had) BUT REMOVE C108 from TR36 - this takes away the Volume control but you want to know if this pre-amp is ADDING to the noise... Repeat the above like you did for the Audio Amp but for the Receiver Preamp...leave the Audio Amp alone - it may be fine but then too - you may find it "acting up" again, then between these two sections is the problem.

If not, as in it's all too quiet...you'll have to proceed - by removing C108 of the Pre-amp and now reconnect the Mic amp AF Switch C82 that 0.047uF (473)

This may sound tedious - and I agree, but it is the only way you can locate isolate and rebuild the noisy sections of the radio.

Nine times out of ten, this noise is related to just the Audio amp and its' support - replacing parts with fresh is not the easiest job but this set of steps helps me locate the noise sources faster.

Now, to cut to chase...
Locate the limiter - has it been removed? You might want to put it back in - if only for your receiver. In this radio the Limiter ENGAGES in Receive (RX) mode thru D41 - which shunts any noise in the mic cord to ground before it's amplified - the Mic amp itself is powered all the time.

For your benefit, if "defeating the limiter" is necessary, keep the limiter in place, and locate R103 - 1 Meg (105 or Brown - Black - GREEN ) . Replace it with a 560 ohm resistor (Green - Blue - Brown). Locate R125 -1.5K ( Brown - Green - Red ) and replace it with a 3.3K ( Orange - Orange - Red ) or LARGER value​

When you switch to TX mode, the D41 stops conducting and the Limiter opens up - this also occurs for the PA preamp too - thru D40 - if they are disabled - noise can be picked up in the mic amp and sent into the Audio amp skewing this test - so it's why I recommended the removal of these two Caps C110, C82 listed above - to isolate the Amp and rule out any potential problems where the biggest source of noise AMPLIFICATION comes from - the Audio Amp itself.

You can apply this to any radio to help isolate the sections from themselves by removing their connection capacitors that keep the DC Bias (if any) back in their respective places. The Audio amp itself can be the main culprit and isolating the other sections helps you determine this.
 
Last edited:
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When you turn up the squelch does the noise still exist?
  • It may serve you better, to know that as these radios age, many "noisy" problems and "quirks" in performance of the Receiver show up so you may find yourself replacing a lot of components - this is normal - you'll know why as you work thru the rebuild/repair process..
  • In nearly every case, Age of the radios' more sensitive filtering parts, like the electrolytic caps and many of the Mylar (Green Chiclet) caps are failing - drying out and or losing internal cohesiveness - its' simply "old" and they're showing their age.
The Tips earlier - after going thru this thread, are, to me, workable but you need to follow an order of steps that after I've done this - helps me locate the problem "parts" faster.

Start at the Audio Chip itself. Isolate its' input from the rest of the board - this way, if that noise still persists - you know it's not generated by something further up the Receiver chain, or the Mic amp sections.

You do that by removing C110 0.1uF or 104M/K cap - from the receiver Pre-amp and C82 0.047uF (473) - Mic Amp / AF Switch (this cap number may differ - but it comes from TR27 part of the Mic Amp chain going to the AN612)

Power up the radio and let it warm up - listen for that noise - be patient at this step, for the radio is not injecting noise, hiss or any sort of background audio into the amp to help it warm up faster - this takes time to bring the conditions back that give you the noise.

If you can, and are EXTREMELY careful - reheat soldering joints CAREFULLY - DONT CAUSE ANY SHORTS - it's is better to wait ...

But - if pressed for time - just apply small amounts of heat - while it is powered, you'll near hum and static - it's normal - this helps you in locating the faulty parts.
  • In the reheating process you may locate the poor soldering area and the condition goes away, you may have found the "barely able to make contact" solder pad - but then too, reheating these parts can also recondition them into working temporarily - so you will have to do this several times - letting it cool down after each attempt.
  • Pay attention to the detail of where the noises encountered during this reheat process are the strongest
    • - in the Audio amp chip itself has many feedback and Equalization pins so they all can add to this problem in one fashion or another.
The next process, is to reattach the Receiver Preamp - reinstall C110 that 0.1uF cap (or whatever value it had) BUT REMOVE C108 from TR36 - this takes away the Volume control but you want to know if this pre-amp is ADDING to the noise... Repeat the above like you did for the Audio Amp but for the Receiver Preamp...leave the Audio Amp alone - it may be fine but then too - you may find it "acting up" again, then between these two sections is the problem.

If not, as in it's all too quiet...you'll have to proceed - by removing C108 of the Pre-amp and now reconnect the Mic amp AF Switch C82 that 0.047uF (473)

This may sound tedious - and I agree, but it is the only way you can locate isolate and rebuild the noisy sections of the radio.

Nine times out of ten, this noise is related to just the Audio amp and its' support - replacing parts with fresh is not the easiest job but this set of steps helps me locate the noise sources faster.

Now, to cut to chase...
Locate the limiter - has it been removed? You might want to put it back in - if only for your receiver. In this radio the Limiter ENGAGES in Receive (RX) mode thru D41 - which shunts any noise in the mic cord to ground before it's amplified - the Mic amp itself is powered all the time.

For your benefit, if "defeating the limiter" is necessary, keep the limiter in place, and locate R103 - 1 Meg (105 or Brown - Black - GREEN ) . Replace it with a 560 ohm resistor (Green - Blue - Brown). Locate R125 -1.5K ( Brown - Green - Red ) and replace it with a 3.3K ( Orange - Orange - Red ) or LARGER value​

When you switch to TX mode, the D41 stops conducting and the Limiter opens up - this also occurs for the PA preamp too - thru D40 - if they are disabled - noise can be picked up in the mic amp and sent into the Audio amp skewing this test - so it's why I recommended the removal of these two Caps C110, C82 listed above - to isolate the Amp and rule out any potential problems where the biggest source of noise AMPLIFICATION comes from - the Audio Amp itself.

You can apply this to any radio to help isolate the sections from themselves by removing their connection capacitors that keep the DC Bias (if any) back in their respective places. The Audio amp itself can be the main culprit and isolating the other sections helps you determine this.
Yes. The noise exists even after incoming RF is squelched out. Noise exists on PA as well. I have done everything the OP did except I actually have recapped all electrolytics and I am waiting on a couple more tantulums from digikey. It is definitely an audio issue not a radio issue. I will read through your reply thoroughly and report back. Thank you.

ALSO, if you are the same "handy andy" as on cbtricks with all the uniden pc122xl info, I appreciate it, because my neighbor is working on a trc465 that is identical internally but currently only puts out 4 watts on ssb as well as AM. Your info is helpful in understanding how that radio works. His power supply failed and overloaded the radio and his ssb dropped from 12 watts to 4 ever since. But thats for another thread I guess. Thanks a bunch!
 
Hang in there...we're pulling for you!
57283094e3c74_redgreenpullingforyouallinthistogether.gif.228243d7738824e145eb3078a220cde0.gif
 
Ok, had to give you guys a break...some humor (or humus)

When you "squelch out" and the noise never changes, then you still have to pull those two caps C110 and C82...
  • IF you find C110 and C82 doesn't stop the noise, then the problem is at the Audio Amp.
  • When the noise is gone, and you reconnect, try C110 first...your receive.
If the limiter has been disabled, you might want to put that transistor back in operation. You'll need it for RX to stay quiet "listening". Thanks to foreign manufacturing putting in a lot of bells and whistles in products, it encroaches in a lot of todays electronics even in the shack, the products' own timers, power supplies and keypress oscillators can produce a PLETHORA of noise.

These newer CFL, LED and even Monitors and Tablets and DSL - emit so much RFI noise, it's easy for them to be interfering with 27MHz band as well as spurs in others.

I didn't want to have you sitting in the dark -literally- because - it's the only way to keep down the noise.

Otherwise, the noise picked up in the mic amp adds to this problem and deafens your ears. Seriously, do what you can to quiet the speaker - I'll show you how to rework sections to help you and keep your neighbors happy at the same time. See above for starters...

But let's look at the Nosie level, in the radio - first...

Remember to:
  • Swap - Reconnect - ONE section at a time
  • Be PATIENT - Allow time for symptom to reappear.
  • If Problem is not in that reconnected section - proceed...
  • MIC amp section -
    • There are two parts, Mic Amp for TX and PA amp (No TX)
    • Noise present in MIC Amp?
      • - BE SURE TO CHECK PA SEPARATELY
      • Place in PA mode - Remove C88 - 0.047uF (473)
      • Does Condition Disappear? - PA amp faulty -investigate R95, R96, R97 and C86
There is also another problem - dealing with AGE of the Radio...​
  • You already know Electrolytic's need to be replaced
  • Mylar's (Green Chiclets) are also suspect
  • Power supply FEED - Both Battery and 120VAC line
    • A TSB (#1226) was issued to PROTECT MB3756 (IC-5)
    • This Diode modification keeps the radio from excessively high Input Voltage Supply spikes from damaging IC-5 - MB3756 and also to prevent REVERSED Polarity problems in Power Up and Down conditions as Downstream Caps charge and discharge can place excessive loading on IC-5's Regulation.
CobraTSB1226.jpg

I happen to have a lot of Respect for the 140-148 Series line from Cobra - as well as Unidens effort with the Grant, Jackson, and Washington. I love working on these - both radio lines deserve a lot of credit for the abuses they have taken from owners wanting more out of their equipment, then realizing - too late - they could have bought Equipment more suitable for their needs than to take it out on these radios.

I can only help you get this corrected - the process is tedious, but rewarding once you've solved the problem.
 

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I have just finished fixing a similar problem, on an 8719 Washington, slowly rising noise in RX with volume off.
I replaced C109 to fix the issue. Strange thing is, the schematic shows it as a 0.1 tantalum. Mine had a 22uf tantalum. I replaced it with a 22uf electro and it’s fine now.









QUOTE="Handy Andy, post: 719536, member: 52309"]Ok, had to give you guys a break...some humor (or humus)

When you "squelch out" and the noise never changes, then you still have to pull those two caps C110 and C82...
  • IF you find C110 and C82 doesn't stop the noise, then the problem is at the Audio Amp.
  • When the noise is gone, and you reconnect, try C110 first...your receive.
If the limiter has been disabled, you might want to put that transistor back in operation. You'll need it for RX to stay quiet "listening". Thanks to foreign manufacturing putting in a lot of bells and whistles in products, it encroaches in a lot of todays electronics even in the shack, the products' own timers, power supplies and keypress oscillators can produce a PLETHORA of noise.

These newer CFL, LED and even Monitors and Tablets and DSL - emit so much RFI noise, it's easy for them to be interfering with 27MHz band as well as spurs in others.

I didn't want to have you sitting in the dark -literally- because - it's the only way to keep down the noise.

Otherwise, the noise picked up in the mic amp adds to this problem and deafens your ears. Seriously, do what you can to quiet the speaker - I'll show you how to rework sections to help you and keep your neighbors happy at the same time. See above for starters...

But let's look at the Nosie level, in the radio - first...

Remember to:
  • Swap - Reconnect - ONE section at a time
  • Be PATIENT - Allow time for symptom to reappear.
  • If Problem is not in that reconnected section - proceed...
  • MIC amp section -
    • There are two parts, Mic Amp for TX and PA amp (No TX)
    • Noise present in MIC Amp?
      • - BE SURE TO CHECK PA SEPARATELY
      • Place in PA mode - Remove C88 - 0.047uF (473)
      • Does Condition Disappear? - PA amp faulty -investigate R95, R96, R97 and C86
There is also another problem - dealing with AGE of the Radio...​
  • You already know Electrolytic's need to be replaced
  • Mylar's (Green Chiclets) are also suspect
  • Power supply FEED - Both Battery and 120VAC line
    • A TSB (#1226) was issued to PROTECT MB3756 (IC-5)
    • This Diode modification keeps the radio from excessively high Input Voltage Supply spikes from damaging IC-5 - MB3756 and also to prevent REVERSED Polarity problems in Power Up and Down conditions as Downstream Caps charge and discharge can place excessive loading on IC-5's Regulation.
View attachment 36407

I happen to have a lot of Respect for the 140-148 Series line from Cobra - as well as Unidens effort with the Grant, Jackson, and Washington. I love working on these - both radio lines deserve a lot of credit for the abuses they have taken from owners wanting more out of their equipment, then realizing - too late - they could have bought Equipment more suitable for their needs than to take it out on these radios.

I can only help you get this corrected - the process is tedious, but rewarding once you've solved the problem.
[/QUOTE]
 
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I know its been a long time but I am finally back on this radio.
The current problem is that I cannot find c109 or c110. Can anyone point me in the general location?
 
Hmm. Curious, what is your board number?

The PC-385 - has gone "plaid" with many a different variant in both parts layout and traces...
upload_2020-12-1_8-56-5.png
 

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