• You can now help support WorldwideDX when you shop on Amazon at no additional cost to you! Simply follow this Shop on Amazon link first and a portion of any purchase is sent to WorldwideDX to help with site costs.

COBRA 25 LTD INCOMING AUDIO PROBLEM ?

Switch Kit

Well-Known Member
Apr 6, 2005
3,584
108
63
Radio transmits and receives just fine ,no receiving audio coming from the speaker or ex.speaker jack ? I can see it is receiving by the radios meter but just no incoming audio ? This one is really baffling me ? Tried a few different mic's and the same results.(I put head phones in the ex.speaker jack to hear if I could pick up anything on receive and I did hear a very very faint signal ,as if the radio was trying to cough it up but no cigar) Anyboby have any idea on what I might be able to look at on this one ? It's a 25 LTD (not classic) ...I had a PC-122 once that had this same problem and it turned out to be the two biggest caps in the radio had to be changed and low and behold the incoming audio came back on that one ...IM wondering if this might be the same case on this 25 LTD ? ....I thought I might ask some of you guys out here that know a lot more then I do on this one before I statred to change parts around. Thank you and always appreciate any help given. Switch Kit (and the radio came this way which I would guess to say may not help much ?)
 

Switch Kit

Well-Known Member
Apr 6, 2005
3,584
108
63
Audio chip ? OK ...but as I said ,the radio transmits on the meters and shows plenty of audio received out ,theres just no receive audio coming in ,the radio transmits just fine . Unless what you are saying is that the in coming audio can be tied into the audio chip one way or the other ? ...is this what your saying here ? thanks
 

NorthStar

Administrator
Mar 21, 2005
1,387
66
58
Yep I seen more than one occasiona TX and the RX not come out of the speaker and it being the chip. But it could be something else.
 

Switch Kit

Well-Known Member
Apr 6, 2005
3,584
108
63
Wow ! I really think I can even confuse myself sometimes )-: .....the radio does transmit good clean audio ...good watts and forward swing ......the radio does show receive by it's meter and as I said ......I could here a little scratch of audio there with head phones on going through the ex.speaker jack .....I will change the audio chip all the same and hopefully that will be problem ? if not ? ....it will be back to the drawing board .....just for grinnys I did change out the 3 biggested caps and there was still no change .......HMM ......transistor ? resistor ? diode maybe ? thanks NS Switch Kit
 

Switch Kit

Well-Known Member
Apr 6, 2005
3,584
108
63
Well ,I changed the audio chip )-: notta change . This 1983 Cobra LTD is so clean !! Haven't a clue were to start next ?
 

Switch Kit

Well-Known Member
Apr 6, 2005
3,584
108
63
thank you Freecell .....I did change both of those along side the big transformer (had the same problem in a pc-122 once and changing those fixed the problem .not this time) the thought did come cross my mind there Bub on the volume control .......thanks for lodging it in there again ,I'll give it a try .thanks Bub........NEWS AT 11 ?
 

Switch Kit

Well-Known Member
Apr 6, 2005
3,584
108
63
New's at 11 is here !!! )-: changed on/off volume control ,no change . Hmmm ? lets see here .....
1. changed two big caps next to transformer
2.changed audio chip
3.changed IC2 for ginnies (no change) weather or not it had anything to do with it ? Parts isn't a problem here for me.
So what about it fellows ? any other ideas I might be able to look at on this one that anybody might know ? .......I do have one other question ? ...What is the little driver/transistor looking thing that is mounted to the chassie right next to the on/off switch in this radio ? thanks again for all your's help. SK
 

Switch Kit

Well-Known Member
Apr 6, 2005
3,584
108
63
I tried a few mic's Nomad ......no change ....that of course was the first thing that I did to this radio . I suppose I can just keep on changing part after part but just wish I knew where a good starting place might be ? What is that transistor /driver/voltage regulator on the side of the chassie right next to the on/off switch ? .....Hmm ? well it's got to be a resistor/transistor or cap .....the only question to that would be which one ? If this radio wasn't in as nice of condition that it's in I would have just gave up on it by now ,but much thanks to all for the suggestions thus far. SK
 

nomadradio

Analog Retentive
Apr 3, 2005
5,810
8,719
573
Louisville, KY
www.nomadradio.com
Hi SK,

I'm pleased to hear you tried the mike first. Just didn't catch any mention of that in the thread.

A great many breakdowns fall into the 'routine' category. Statistically, the same small list of things tend to break the most. From one brand/model to another, the list will change, but the same principle tends to apply.

Troubleshooting one of those with suggestions from forum posts can be surprisingly easy. Just takes somebody with experience on that radio, asking the right questions, mostly.

Then you have that "one in twenty" or a hundred. A failure that just doesn't happen over and over in dozens of that model. An "oddball" problem. This is when a basic tool like an oscilloscope pays its way. Simply tracing your audio signal, from the detector, to the volume control, past the squelch circuit and then into the audio power chip is pretty fast and efficient that way. It's been long enough since I had to do that without a 'scope that I've forgotten how. Reminds me of using a dime for a screwdriver. Awkward and error-prone.

That being said, there's one more "low-tech" test I haven't suggested. All that it does is narrow down where to look.

Think about the loud "HUM" you get from a guitar amplifier if you pull the plug out of the guitar with the volume turned up. Touch the tip of the guitar plug with a finger, and you get the same loud hum.

Turn on the radio, and turn the volume full up. Take a screwdriver with a thin tip, touch it to the clockwise lug of the volume control. Lay one finger along the screwdriver shaft. It works better if you don't have your other hand in contact with the radio chassis.

You should get an "AC" hum out the speaker, fairly loud. Now do the same to the volume control's center lug. You should hear the exact same sound and level. If not, the volume control is shot. Bridging the center lug to the Clockwise lug with the screwdriver tip will get you FULL volume channel chatter if that's all that is wrong.

If you can get NOTHING from this test, the problem is downstream from the volume control.

If you get a loud hum, and the volume control is okay, the problem is upstream from the volume control.

The "shotgun" approach has two main drawbacks. Statistically speaking, you'll probaby replace a bunch of parts that are not bad before "pin the tail on the donkey" gets you back your receive audio. And every time a part gets replaced, the odds that some small accident, like a solder bridge will occur. This is what's called "muddying the water", adding new defects to the old one. Seen it a lot.

Many kinds of radio breakdown don't call for all the 'high-tech' tools and toys to track down. Some of them do.

73
 
  • Like
Reactions: Shadetree Mechanic

Switch Kit

Well-Known Member
Apr 6, 2005
3,584
108
63
well thank you very much Nomad for your time and know how ,You surely have a way with words (-: I like that "Low Tech" approach you mentioned.Soldering isn't a problem for me with this stuff ,I did that much for 12 years for a living ,as far as a scope goes ,I don't own one and if I did ,I more then likely would need some books or some one to tech me how to use one .The on/off volume switch is good just like the first one I took out ,but you taught me how not to have to put myself through that change again in the future (at least with these type radios) ,thanks. I would conceder this one a "oddball" problem as you put it and the "shotgun" approach is all that I have really going for me here.You said it would either be upstream and down on this, there after finding out that the volume control was in the good ..weather up or down ,I honestly wouldn't know where to begin in those aspects ? so I guess like radios in the past ,I just keep on changing parts if it continues to mean that much to me ? ....at least you yourself Nomad can understand what your doing ,where in for me ,it's more so poking and hoping and getting lucky from time to time ....I've always thought of myself at being fairly good at these 25s 66s and 427s ...or should I say this perticular board that seems like it's been around for a very long time.....and as you said ,the "oddball" ones can truly be a test for somebody like me from time to time ....so thanks to some pretty good books out there , internet and sites like these over the last few years I've been able to learn a few things along the way and gotten some pretty good advise from others like yourself ...back to the drawing board. thanks "THE INTERNET TECH" (-:
 

dxChat
Help Users
  • No one is chatting at the moment.
  • @ BJ radionut:
    There will be NO FT8 robots running unattended at Bouvet,:ROFLMAO:
  • @ BJ radionut:
    10M BAND.....>28400 SSB .... Always give your full callsign. Listening is the key so be sure we call you before replying. 3Y0J will always listen up (using split) so it will be useless to call us on our transmission frequency.
  • @ BJ radionut:
    I like these guys already. :cool: Sounds to me like ROBO callers in ANY Mode will be ignored!!!!(y)
  • @ BJ radionut:
    3Y0J: Update January 29, 2023: Ken estimates Marama should arrive at Bouvet on Monday evening just before sunset. He says he anticipates a good weather window for at least two days then. The team will attempt to move the equipment and supplies onto the island using the zodiac then. They have two zodiacs. They are hoping to accomplish it in those two days. :cool: