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Message |
    
Perrym1962
New member
Username: Perrym1962
Post Number: 1
Registered: 11-2002
| | Posted on Monday, August 28, 2006 - 6:43 pm: | 
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I have a fairly new cobra 29 ltd which was use in rock quarry. I transmits perfectly and works well with the exception of receive. Will not receive at all. I assume it is because of all the close range talking. Perry in Ky. |
Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 2011
Registered: 9-2004
| | Posted on Monday, August 28, 2006 - 11:29 pm: |
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here's a quick test that you can do to determine whether the problem is in the radio or just a broken mic wire.
make sure the radio is off and the power is disconnected.
take the mic off the radio.
using a voltmeter set to read continuity (most digital VOM's these days have a setting where it beeps when there is continuity. if not, set it for resistance, and look for the needle to move all the way over)
take the bottom cover off the radio, leaving the speaker wires attatched to the speaker, or hooking up a known good external speaker.
touch one lead of the voltmeter to the metal part of any one of the "tuning cans" in the radio. they are all over the place, and are shiny metal boxes with a tuning screw in the top. DO NOT turn any of the screws!
touch the other lead to one of the pins on the mic jack on the radio.
check each pin until you hear the beep or see the needle move over.
that is your ground pin.
now get a short piece of wire about 12" long. strip both ends just a bit.
turn the radio on.
touch one end of the wire to the ground pin on the mic jack, and touch the other end to each of the other pins.
one should make the radio switch to transmit, and you will see the needle on the radio's meter swing over.
one might cause a loud sqeal that may scare you.
the other should cause the radio to receive.
if the volume is up a bit, you should be able to hear static coming from the speaker.
if the other pins did what they should, but none made the receiver work, first make sure its not the speaker by using an external speaker, and also check the wires going to the stock speaker. then i would suspect the front end transistors inside the radio.
this is definitely a job for a good tech.
not a golden screwdriver buddy.
if, however, you did manage to get some sound out of the speaker; congrats! you just have a broken wire in the mic cord. this is VERY common.
most of the time its on the plug end, but sometimes its at the mic end.
on the plug end, cut the plug off about 2" back from the plug, and take the cover off of the mic plug.
make a diagram noting what color wire goes to what pin.
strip the newly cut end of the mic cord, and carefully strip all the wires inside about 3/16".
unsolder the piece of wire on the mic plug, and solder on the newly cut end of the mic cord.
use a very small ammount of solder flux on the mic plug to ease soldering. (like enough to cover the tip of a toothpick)
replace the cover on the plug, and try it on the radio.
if all went well, you should now have receive!
if not, but the jumper trick worked, its time for a new mic.
good luck, and feel free to post any questions.
matt |
Perrym1962
New member
Username: Perrym1962
Post Number: 2
Registered: 11-2002
| | Posted on Tuesday, August 29, 2006 - 5:05 pm: |
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I guess it's going to have to go to the tech. It's not the mic. Even if I have to spend some on it, it will be ok because it was gave to me. Except for some dust it looks almost new. I thank you for your response. Perry. |
Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 2015
Registered: 9-2004
| | Posted on Tuesday, August 29, 2006 - 6:16 pm: |
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oh well, we tried.
did you do the tests to confirm that it wasnt the mic? or did you just try another known good mic on the radio?
make sure you at least plug in an external speaker before you go the tech route.
good luck,
matt |
Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 2016
Registered: 9-2004
| | Posted on Tuesday, August 29, 2006 - 6:27 pm: |
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one thing i was just thinking of.
you say that the radio will transmit normally, but do you know for sure whether there is any modulation or not?
you cant get a radio check if you cant hear the other person talking.
here's what im getting at.
if the audio chip is bad, then the radio would still key on transmit, but with no audio. (no modulation)
it would also kill the receive audio, but the needle would still move around with signals.
so, if you have a modulation meter, and see that there is audio in transmit, or the needle on the radio's S-meter moves a bit when you talk into the mic, then its probably not the audio chip.
however, if there is no modulation on transmit, and no audio on receive, but you can see the meter moving around when signals are received, then i would strongly suspect a bad audio chip.
post youir results, and if it looks like it is a bad audio chip (also very common) then i'll show you how to replace it.
shouldnt cost more than $10 for the chip, and just carefully unsoldering and resoldering the legs of the chips.
let me know what you find out,
matt |
Perrym1962
New member
Username: Perrym1962
Post Number: 3
Registered: 11-2002
| | Posted on Tuesday, August 29, 2006 - 10:26 pm: |
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I tried 4 different mics. It has audio through the headphones on my dosy meter. I replaced tr-23, tr-11 and 12 just for giggles. I tried an external speaker and also transmitted with another radio nearby and the needle won't so much as wiggle. Also I had a friend to listen to me while I transmitted and he said it sounded good. There is no meter movement on receive period. Thanks again for your post.. |
Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 2019
Registered: 9-2004
| | Posted on Tuesday, August 29, 2006 - 11:25 pm: |
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so, no meter movement on receive.
are you saying that the radio's meter doesnt move when you transmit also?
if so, that is confusing.
everything else is pointing towards the front end transistors. usually they are FET's.
are these the ones you replaced?
looking at the top left of the schematic, the first transistor you come to should be labled "RF AMP". you might try replacing this one too.
good luck,
matt |
Perrym1962
New member
Username: Perrym1962
Post Number: 4
Registered: 11-2002
| | Posted on Wednesday, August 30, 2006 - 9:48 pm: |
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The meter works on fine on transmit.. |
Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 2031
Registered: 9-2004
| | Posted on Thursday, August 31, 2006 - 10:05 pm: |
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ok perry, i was mistaken when i said that the front end transistor(s) might be FET's, its the mixer section where they usually use the FET's.
anyway, i did some reading today, and it seems that there are two diodes right before the front end transistor, which is TR7. if one or both of these diodes short from a severe overload situation, it will cause the symptoms you describe.
these diodes are D1, and D2, and they are 1N914's.
you should be able to find them at radio shack even!
they may have blown TR7 which is a 2SC1674, but either way you want to replace this transistor.
you see, the 2SC1674 is kind of a noisy device and there is a lower noise replacement that will increase the signal to noise ratio of your receiver by up to 6db!
replace it with a 2SC2999 or an NTE107.
they shouldnt be to hard to find.
so, hopefully, for two diodes and a transistor, you can have a kick ##$$ radio!
good luck, and let me know what happens,
matt |
Perrym1962
New member
Username: Perrym1962
Post Number: 5
Registered: 11-2002
| | Posted on Thursday, August 31, 2006 - 10:51 pm: |
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I appreciate the responces from you. I will try it this weekend. Thanks again, Perry in Ky. |
Perrym1962
New member
Username: Perrym1962
Post Number: 6
Registered: 11-2002
| | Posted on Friday, September 01, 2006 - 6:09 pm: |
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Replaced D1 and D2 as well as TR-7 still nothing. May be ready for a tech or the landfill. I have 14 other radios, so it isn't so bad. |
Kid_vicious
Senior Member
Username: Kid_vicious
Post Number: 2037
Registered: 9-2004
| | Posted on Friday, September 01, 2006 - 9:36 pm: |
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well, im sorry i couldnt be of more help.
best of luck to you with that radio.
make a post if and when you get it fixed, so others searching the site can benefit from what we've discussed.
later,
matt |
Perrym1962
New member
Username: Perrym1962
Post Number: 7
Registered: 11-2002
| | Posted on Saturday, September 02, 2006 - 12:43 am: |
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p.s. I also changed IC4 |
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