Hi guys, i've searched on this topic but haven't found any threads with this specific problem so here it goes:
Radio Make: Cobra
Radio Model: 148 GTL (taiwan)
PCB number: PC-412AD
PLL number: MB8719 (8334 C06)
IC number: MB3756 (8345 M67)
What doesn't work: No TX (light comes on) no RX (light comes on) no PA.
What does work: Everything else works. Lights come on and i believe the signal meter moves on RX (but no audio)
What you have tried so far: checked the microphone and it's OK. inside visual inspection everything seems to be solid with no loose wires or components.
I picked this radio up from a garage sale in a lot. It was in the box with all its hardware and accessories and it looks like it has never been used. Someone mentioned that the audio chip might be bad but im not sure how i can go about testing it out or checking any other components in the audio circuit. Anyhow, hope someone can help, this is a great radio and i would hate for it to go to waste, any help will be greatly appreciated!
New Cobra 148 GTL no TX/RX/PA
New Cobra 148 GTL no TX/RX/PA
Last edited by Shadow on Wednesday 3rd Sep 2008, 8:31, edited 2 times in total.
Re: Cobra 148 GTL no TX/RX/PA (Lights on no one home).
It sounds like the radio has been connect up the wrong way round, so it would be best to check the reverse polarity diode D55 at the rear of the radio where the power cables come in, you may find that it is open circuit in which case replace it with a 1N4003.
It is most likely that the audio chip (IC6 TA7222AP) has gone south.
Also, check the voltage around the RX/TX switch regulator (IC4 MB3756) you can find the voltage for this IC on my Sch diagram, which is on CBTricks http://www.cbtricks.com/radios/cobra/14 ... redraw.pdf
It is most likely that the audio chip (IC6 TA7222AP) has gone south.
Also, check the voltage around the RX/TX switch regulator (IC4 MB3756) you can find the voltage for this IC on my Sch diagram, which is on CBTricks http://www.cbtricks.com/radios/cobra/14 ... redraw.pdf
Rick.
Thanks for the details! I checked D55 and it looks fine. resistance in one direction, open in the other.
I'm not sure where the audio chip is. im assuming it's one of the chips that are screwed onto the side of the case but there are 2 or 3. I've tried to read the schematic but i don't think I'm at that skill level yet. Im also having problems locating the rx/tx switch regulator.
Thanks very much for the help so far
I'm not sure where the audio chip is. im assuming it's one of the chips that are screwed onto the side of the case but there are 2 or 3. I've tried to read the schematic but i don't think I'm at that skill level yet. Im also having problems locating the rx/tx switch regulator.
Thanks very much for the help so far
Re: Cobra 148 GTL no TX/RX/PA (Lights on no one
It would be best to let us have the PCB number.
What is the part numbers on the IC bolted to the chassis.
Without the above info it will be hard to help you as you can not read schematic.
What is the part numbers on the IC bolted to the chassis.
Without the above info it will be hard to help you as you can not read schematic.
Rick.
Re: Cobra 148 GTL no TX/RX/PA (Lights on no one
Hi Rick, i appreciate the help. Sorry for the delay but I got laid off last week and things have been kinda hectic.Rick wrote:It would be best to let us have the PCB number.
What is the part numbers on the IC bolted to the chassis.
I updated my first post with all the info. The number on the PCB is PC-412AD
PLL number: MB8719 (8334 C06)
IC number: MB3756 (8345 M67)
Let me know if there's any more info that you need, thanks for being patient with me.
-
ramblingman
- Technical Helper
- Posts: 331
- Joined: Sunday 21st Jan 2007, 16:35
- Location: mississippi
The MB-3756 Is the Regulator i.c.....And the audio chip
should be a TA-7222AP,it should be the largest i.c., that
is mounted on the right side rail on your radio,Knobs facing you.
You will not be able to see the part number,until you remove the
screw and pull the chip back..As the part numbers are facing the
rail.It will be a 10-pin i.c.,,It might be a TA-7217AP..Also before
i went to all the trouble of changing those..Make sure you check the
simple stuff first.Check the P.A. switch,It could be bad..Also check
for poor solder connections around the pll area..Use a non-metallic
scratching tool,and bump and lightly scratch around the pll area,and
make sure its not a bad connection.Then once youve checked that.
Start taking should voltage readings at the audio i.c,,and the regulator
i.c. and the pll chip.And lets us know your readings.
should be a TA-7222AP,it should be the largest i.c., that
is mounted on the right side rail on your radio,Knobs facing you.
You will not be able to see the part number,until you remove the
screw and pull the chip back..As the part numbers are facing the
rail.It will be a 10-pin i.c.,,It might be a TA-7217AP..Also before
i went to all the trouble of changing those..Make sure you check the
simple stuff first.Check the P.A. switch,It could be bad..Also check
for poor solder connections around the pll area..Use a non-metallic
scratching tool,and bump and lightly scratch around the pll area,and
make sure its not a bad connection.Then once youve checked that.
Start taking should voltage readings at the audio i.c,,and the regulator
i.c. and the pll chip.And lets us know your readings.
Re: Cobra 148 GTL no TX/RX/PA (Lights on no one home).
Look at SIL chips with the chip ID front of you the pin number go from lift to right.
The Audio and Voltage reg in you radio are of the type SIL that stands for Single In Line.
For more information check out
http://en.wikibooks.org/wiki/Practical_ ... le/Pinouts
The Audio and Voltage reg in you radio are of the type SIL that stands for Single In Line.
For more information check out
http://en.wikibooks.org/wiki/Practical_ ... le/Pinouts
Rick.
Ill bet the house on this one. Don't mess with the audio chip, instead, replace all the electrolytic capacitors in the radio and it will come back to life. That radio is going on 30 years old and the caps are dried out. Cobra used cheap caps in the Taiwan model. I will dig up a list of their locations and a suitable replacement for you this weekend. You can get replacements at radio shack
Buck
Buck
Thanks Buck, i've actually heard about this more than once, you are the 3rd person who has told me it's the caps. I gotta find a parts list to see the values unless you have one handy.Buck wrote:Ill bet the house on this one. Don't mess with the audio chip, instead, replace all the electrolytic capacitors in the radio and it will come back to life. That radio is going on 30 years old and the caps are dried out. Cobra used cheap caps in the Taiwan model. I will dig up a list of their locations and a suitable replacement for you this weekend. You can get replacements at radio shack
Buck
The part you're looking for is C95, a 47 UF (usually) electrolytic rated at 10 Volts. The 10-Volt (and 6-Volt) rated capacitors tend to fail first, around the 20-year mark. More often than not, this occurs after a long visit to the shelf. Within a week or two of being put back into service, one of these 10-Volt caps will short internally. You can replace it with Loss of all transmit is just one of the common symptoms caused by one shorted tiny filter capacitor. Another one will kill all receiver audio, and another will kill all mike audio, but still show you a dead carrier on AM transmit.
Anything from 33uf to 220 uf at 25V will do the job.
Buck
Anything from 33uf to 220 uf at 25V will do the job.
Buck
I will dig up the locations of the other electrolytic caps that should be replaced also. I have them in my notes somewhere. Also note this help comes from good ol 2600 from over at CBTricks. I can't take credit for this one but I can speak from experience. I'm 99% positive this will solve your problem.
