Sidebar Sidebar
  • 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 29 problem a really weird one help

fnkycoldmedina

fnkycoldmedina

Well-Known Member
May 18, 2005
2,266
98
58
pa
ok i have this old school screamin 29 a few years now .the only thing i changed recently was the addition of a new style roger beep.everything else pretty much the same .here comes the problem....i go to hook the radio to my base antenna,was gonna try some mics on it .funny thing is when i connect the base coax the channel display goes dead and the radio wont key.unhook the coax hookup a dummy load and she works good again.so i,m looking farther into this now and it seems the coax ground is doing this ...touch the center lead all is ok ..screw the coax on and out goes the display.not the meter light just the channel display and it wont key.so i keep checking,i check the coax lines ...nope all good ,so i look over at my linear...a mobile linear on the same power supply i,m using to power the 29......ok lets bypass that ....radio straight to the antenna...works fine ...hmmmm...this is tricky now .......so when the amp is inline hooking the coax to the 29 kills the display and wont let it key.run it right to the antenna coax and whalla ,works fine.so i,m thinking some more ....i been at this awhile now..got my vom in hand .so i get to thinking ....somethings grounding out in the amp making this 29 do that?but my base amp works fine with it ?that has its own built in supply......so i start doing checks on the coax with my vom .....i am checking grounds ...with one lead on power supply ground i use the other lead on the coax ends .....the outer shield should make the buzzer go off ...and it did ......but so did the center conductor....this should not be .this is because its running thru the amp ...hhhmmm......amp works fine with the base using the same coax .........then i remember i got another 29 ...so i go get that 29 .....i hook it to the same coax and have it going thru the amp as well ...no problems here ......so this one runs fine ,the base runs fine .....but if i hook my best 29 to the same coax my channel light goes out and it wont key ???any ideas?
i went back n forth with this for awhile i even checked the radio to make sure the rf lead wasnt grounding out on the radio ....it wasnt...i checked the power lead to ground in the radio no buzzer ...the radio ground to chassis is ok and all.i can honestly say i have never run across this befor .and i,m stumped .it must be the radio ?the base works fine thru the amp ...the other 29 worked fine .......this started happening (well i just found out today but)after i installed this new roger beep ..could it be that?i just dont see how?it works like it should.the beep breaks into the key line like any other then has 12v power and ground and audio lead ...just not seeing this ?help.......i can check anything you like
 

Something you did is touching chassis ground that shouldn't be. You are concentrating on the amp and how it affects the setup. You don't need to. Do this little test. Run the radio straight to the antenna like you said. It works now right? Now, take a jumper and run the ground from the power supply to the chassis of the radio. Poof. Power out. You have a hot radio case. Betcha you can measure voltage between power supply ground and the radio case. Check your roger beep install.

The reason that the amp is causing you trouble is because the chassis of the amp is grounded directly to the ground of the power supply. The radio chassis is capacitively isolated (no DC path) from the negative power lead. As soon as you hook up the amp, there is a DC path from radio chassis to power supply ground.

I believe this also tells you something about your antenna ground. It doesn't mean good or bad, just tells you something... LOL!

Mopar
 
well the only new addition was the roger beep.i will start by unhooking the chassis ground from that .but it is grounded where everything else is on the same can.got about 3 things grounded there.oh wait i know something else i can check .ok....you got me thinking now ...what you said makes sense to me now .sometimes ya just need a nudge.i may have struggled with this for awhile but i think you have really helped.thanks.....oh man..if its what i think it could be the whole face gotta come off .this is one custom 29 ...lol...i,ll have to put a picture up...lol...
 
i would leave the face on for now.
also, you are considering too many variables at the same time.
eliminate certain possibilities by testing, and then move on.

just opinions here, forget the amp, and the power supply.
confirm them as working by trying a different radio.
channel display ok? radio keys?

ok, amp and power supply are not the problem.

now, apply power to radio (the "problem" radio), no antenna hooked up.
radio on.
using voltmeter, set to measure resistance, touch one lead to one of the metal tuning cans, and the other to the chassis of the radio. (the metal case)
is there a short?
yes? you have grounding issues as these should be isolated from eachother.
no?
good, you've eliminated one possibility.

now, set the voltmeter for DC voltage, touch the black lead to one of the tuning cans, and the red lead to the metal chassis.
do you read any voltage?
yes?
you have somehow connected voltage to the metal chassis.
no ?
good, another possibiliy gone.

now, you say you have "all three" grounds going to the same can.
this might be causing a ground loop.
try moving these ground wires to different points on the PC board's ground trace.

all of these possibilities are leading me to believe that the roger beep install is the culprit, and you have connected either voltage or grounds or both to the wrong place.

i can tell you that, most likely, nothing on that roger beep should be connected to chassis ground, so if it is bolted somehow to the chassis, look very closely at the areas it is secured at for bolts or nuts touching a PC board trace.

best of luck,
loosecannon
 
the chassis is hot like mopar said...i did a check between power supply ground and the chassis and read voltage from 7.3 to 8.6 ..up n down it was going .this is bad ....too late to probe around anymore tonight.i can say this though...i installed 3 of these beeps ..one in every radio i have and installed the same way in every radio.unless a part went bad on the beep?whats the possibilities that a radio component is going ?let you know more friday its late .thanks for the help ..it is helping ,thats the good thing.

could the protection diode be going?its huge on the back of the power plug...
 
fnkycoldmedina said:
screw the coax on and out goes the display.not the meter light just the channel display and it wont key
Click to expand...

This quote is incredibly revealing. Everything you needed to know was right there. This is telling you that your problem is something you have hooked to the 8v circuit in the radio. Figure out what that is, and figure out what is touching the case. You don't have a ground problem, you have a hot wire or component shorted to the case. It isn't a factory component on the radio. It is something that was installed later. I would bet money on it. There is always the chance that I would lose, but I feel it would be a safe bet.

What is installed in the radio that needs 8v rather than 12v? Answer that and you will be well on your way to finding the gremlin.

Mopar
 
Hey Joe ........I know the feeling bro :( I hope you find that bug !! Who knows dude ? maybe taking out that Rodger beep might fix things ? Reminds me of a squeal I was having once on a 29 , God Damned thing drove me nuts trying to figure it out !! turned out to be a hair line fracture in a solder joint that went from the board (from parts ) to the chassie, heated it up and that was that .....Why ? hell if I know ! just fixed it :) Dude ? you ever been in a radio shop ? I've been in a few and then some , you ever go into the shop where the techipooh is doing his thing ? or maybe even farther back into the shop ? You bet'cha !! did you see all those piles of radios stacked to the ceiling ? Guess what 90% of all those babies were :x :oops: :cry: :evil: :twisted: :? :( you betcha !!
Good luck to you Jose , I sure hope you can weed it out.
 
found the problem and the radio is repaired.but not after i ripped it all down n took nearly everything out of it ..lol....turns out it was my keyup birdie button,it was grounding out on the chassis,i repaired that and put it all back together now and its working fine .i am borrowing power (8 volts or so)from the channel display obviously.i should have thought of that since it was 8 volts or so i was seeing on the chassis.brain fart...lol.....thanks guys you really were helpfull.hate to lose this old school philippines made 29..its a screamer

if you guys want to see my custom 29 go to the pic section i,m gonna post a pic ...
 
yah sometimes it sucks ass to have to dig and dig to find a problem but what are the other options other than sending it out to be repaired. Glad you got it figured out
 

dxChat
Help Users
  • No one is chatting at the moment.


      Top Bottom
      Back