Go Back   WorldwideDX Amateur Radio Forums > Citizens Band Related > General CB Services Discussion


Amateur Callsign Lookup
Enter Callsign:

Stock Cobra 2000GTL "air noise" on TX

Reply
 
LinkBack Thread Tools Display Modes
  #1  
Old 09-25-2007, 06:10 PM
Member
iTrader: (0)
 
Join Date: Aug 2006
Posts: 49
Crazy_D is on a distinguished road
Default Stock Cobra 2000GTL "air noise" on TX


Got a stock Cobra 2000gtl. When deadkeying, (and the mic gain all the way down, or at any setting for that matter) I get a air noise or whooshing noise that makes me sound like I am in the mobile or something. I tried 3 different mics (and with the Mic Gain all the way off), and concluded that the problem is either in the input mic preamp circuitry, balance modulator area, or something into the AF audio chip area.

Also, accompaning this whoosh noise (kinda sounds like light white noise).... the deadkey wiggles up and down alittle, (it is set at about 4watts). Also, I see a small movement on the mod meter when the mic gain is all the way down.

My thoughts is either:

A) Bad cap somewhere (probably a disc cap)
B) something needs to be realigned or such
C) Maybe audio chip needs replacing or some other transistor


Any thought please let me know, and I will be willing to try to fix this nuisance.

Thanks


Reply With Quote
  #2  
Old 09-26-2007, 06:25 PM
Senior Member
iTrader: (0)
 
Join Date: Jul 2005
Location: Northern Alberta Canada
Posts: 363
bushwacker is on a distinguished road
Default

Hows the power supply?
Got any voltage fluctuations?
60 cycle hum?

If I were to guess, without any more background info,... I would say the problem could be in the power supply.

You have what kinda sounds like a 60 cycle humm coming into the radio. That and you also have deadkey fluctuations which could again be related to fluctuations in voltage from the power supply.

If you have another external power supply around that you know is good... power up the mainboard/radio with that and see if you have any noise.

Rule out the power supply first so at least you know you are not dealing with a bad power supply . It might not be the power supply... but it definatly could be so you have to rule it out.

Take care

60 cycle humm with voltage fluctuations is common for older power supplies
__________________
Bushwacker-- Bush Station 588... VA3

Bench/ Communication Tech
Tridon Communications

Reply With Quote
  #3  
Old 09-26-2007, 09:40 PM
Member
iTrader: (0)
 
Join Date: Aug 2006
Posts: 49
Crazy_D is on a distinguished road
Default

Hi,

I checked the internal power supply, and also, I connected the 2000gtl to a Astron VS-70m and powered the radio by that and the noise was still there.

I can rule out a problem with the internal power supply.

Thanks!

Reply With Quote
  #4  
Old 09-28-2007, 03:12 PM
Senior Member
iTrader: (0)
 
Join Date: Jul 2005
Location: Northern Alberta Canada
Posts: 363
bushwacker is on a distinguished road
Default

OK cool..... now rule out the audio IC and the mic

Get the radio to TX without a mic and check if its still there.

Remove the power to the audio IC and check the same.
__________________
Bushwacker-- Bush Station 588... VA3

Bench/ Communication Tech
Tridon Communications

Reply With Quote
  #5  
Old 09-28-2007, 06:18 PM
Member
iTrader: (0)
 
Join Date: Aug 2006
Posts: 49
Crazy_D is on a distinguished road
Default

Hi,

I have the radio powered off the Astron VS-70M for testing purposes.

I now key the radio with a 5pin mic plug jumpered to make it key when I stick it in the mic plug.

I still have the noise in this configuation.

Reply With Quote
  #6  
Old 09-29-2007, 11:37 AM
Senior Member
iTrader: (0)
 
Join Date: Jul 2005
Location: Northern Alberta Canada
Posts: 363
bushwacker is on a distinguished road
Default

OK.... you will have 1 pin on the audio ic that is the audio in... .... unsolder that leg of the audio IC... key the mic and see if you have any noise... if you do the noise is coming in through AFTER the audio IC

Do you have this noise when recieving in AM? or is it just transmit audio?

If its just transmit...check the input and output of the audio IC.
If you can recieve good without this noise, I would think the problem is bettween the mic input and audio ic OR audio ic and rf amp.

I assume you checked for cold solder joints and loose connections... check them all in the audio input /output of that IC.

I wish I could hear what it sounds like.

Oh well... its hard to help people over the net.... my advice is just work through it in stages.... once you find what area or stage the noise is coming from you can concentrate on it.
__________________
Bushwacker-- Bush Station 588... VA3

Bench/ Communication Tech
Tridon Communications

Reply With Quote
  #7  
Old 09-29-2007, 12:28 PM
Member
iTrader: (0)
 
Join Date: Aug 2006
Posts: 49
Crazy_D is on a distinguished road
Default

Hi,

I will try this tonight. (lifting pin1 of audio IC chip to turn it off)

This noise is ONLY on AM transmit only. SSB is clean. Also RX is clean, so this noise has got to be in the audio chip area, as looking at the schematics, the TA7222 chip is only for AM audio, and the SSB audio is diverted into the AN612 balanced moduator chip via TR21 (2sc945) AM audio switch.

I know what you are wanting me to do, is turning off the audio chip circuit by removing the 13.8v going into it. Once thats turned off, and if the noise goes away, then we know its either the Audio chip or associated circuitry. If this "air" noise still persists then we would need to look towards the back end of the radio. But since it only happens in AM mode, and the "mode switch" is before the final output stage, it MUST be in the AM audio chip area.... because if the noise was also on SSB, then we could rule in the output section....

Will let you know more later tonight.....

I think we are onto something here....

Reply With Quote
  #8  
Old 09-29-2007, 12:41 PM
TonyV225's Avatar
W9WDX Amateur Radio Club Member
iTrader: (8)
 
Join Date: Apr 2005
Location: Wisconsin
Posts: 5,722
TonyV225 is on a distinguished road
Default

If not the audio IC if it not that look at the Capacitors ithat general area I would say a leaky Cap in the audio section.
__________________
""""was not thinking of a cber sat with his makita screwdriver winding round the presets untill his meter said s9+60db from a guy on a handheld 15 miles away""""

Reply With Quote
Reply

Bookmarks
Worldwide Radio Forum



Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are On



All times are GMT -7. The time now is 08:57 AM.


Powered by vBulletin®
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
Search Engine Optimization by vBSEO 3.6.0
Worldwide Radio Forum