I have a cobra 148 gtl Taiwan model. This radio isn't doing anything except turning on. No rx/tx/PA. When I key up the tx/rx light on front doesn't switch. Also nothing on the channel display. It's just blank. Where would you guys start on troubleshooting this? Thanks
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Need help with cobra 148
- Thread starter Mk121
- Start date
First off, the ANL, ANL/NB, & PA switches are often hacked to use as switch for additional channels.
Did you just get this radio recently?
Won't receive or transmit?
If the TX/RX LED light won't change from green to red when you key the mic, that usually is caused by the PLL circuit being 'out of lock'.
A number of things can be responsible. I like to think of the frequency synthesis portion of a radio as a bicycle chain. If just one link breaks; then the bicycle cannot be pedalled, so it goes no where. Many parts to this 'bicycle chain' in the radio. PLL is an acronym for 'Phase Locked Loop' - a chain locked together in a loop. PLL voltage too high/low, VCO voltage too high/low, a dead diode in the VCO or PLL circuit, PLL damaged, crystal gone bad, or as simple as an open circuit due to a poor solder joints or a wire going to the additional channel switch has broken off. There are more; those are the more common ones.
A DVM and a frequency counter are essential to work on it.
That, and a bit of patience along with the service manual . . .
From CB Tricks:
"...The MC145106 is a phase–locked loop (PLL) frequency synthesizer constructed in CMOS on a single monolithic structure. This synthesizer finds applications in such areas as CB and FM transceivers. The device contains an oscillator/amplifier, a 210 or 211 divider chain for the oscillator signal, a programmable divider chain for the input signal, and a phase detector. The MC145106 has circuitry for a 10.24 MHz oscillator or may operate with an external signal. The circuit provides a 5.12 MHz output signal, which can be used for frequency tripling. A 29 programmable divider divides the input signal frequency for channel selection. The inputs to the programmable divider are standard ground–to–supply binary signals. Pull–down resistors on these inputs normally set these inputs to ground enabling these programmable inputs to be controlled from a mechanical switch or electronic circuitry. The phase detector may control a VCO and yields a high level signal when input frequency is low, and a low level signal when input frequency is high. An out–of–lock signal is provided from the on–chip lock detector with a “0” level for the out–of–lock condition."
Even though the Cobra 148 doesn't have a 145106 PLL, it also has this same feature. A freq counter will tell you this anyway, as it won't be able to see just on freq; it will be jumping around all over the place.
Did you just get this radio recently?
Won't receive or transmit?
If the TX/RX LED light won't change from green to red when you key the mic, that usually is caused by the PLL circuit being 'out of lock'.
A number of things can be responsible. I like to think of the frequency synthesis portion of a radio as a bicycle chain. If just one link breaks; then the bicycle cannot be pedalled, so it goes no where. Many parts to this 'bicycle chain' in the radio. PLL is an acronym for 'Phase Locked Loop' - a chain locked together in a loop. PLL voltage too high/low, VCO voltage too high/low, a dead diode in the VCO or PLL circuit, PLL damaged, crystal gone bad, or as simple as an open circuit due to a poor solder joints or a wire going to the additional channel switch has broken off. There are more; those are the more common ones.
A DVM and a frequency counter are essential to work on it.
That, and a bit of patience along with the service manual . . .
From CB Tricks:
"...The MC145106 is a phase–locked loop (PLL) frequency synthesizer constructed in CMOS on a single monolithic structure. This synthesizer finds applications in such areas as CB and FM transceivers. The device contains an oscillator/amplifier, a 210 or 211 divider chain for the oscillator signal, a programmable divider chain for the input signal, and a phase detector. The MC145106 has circuitry for a 10.24 MHz oscillator or may operate with an external signal. The circuit provides a 5.12 MHz output signal, which can be used for frequency tripling. A 29 programmable divider divides the input signal frequency for channel selection. The inputs to the programmable divider are standard ground–to–supply binary signals. Pull–down resistors on these inputs normally set these inputs to ground enabling these programmable inputs to be controlled from a mechanical switch or electronic circuitry. The phase detector may control a VCO and yields a high level signal when input frequency is low, and a low level signal when input frequency is high. An out–of–lock signal is provided from the on–chip lock detector with a “0” level for the out–of–lock condition."
Even though the Cobra 148 doesn't have a 145106 PLL, it also has this same feature. A freq counter will tell you this anyway, as it won't be able to see just on freq; it will be jumping around all over the place.
Last edited:
he said no channel selector light up either. sounds like he might have lost the 8 volt rail on the radio since the channel selector does not light up. he needs to check for 8 volts at the band switch for LSB/USB/AM voltage. does the meter even move at all . does the PA function work.
Robb gave you a good starting point and this will also need to be checked.
Robb gave you a good starting point and this will also need to be checked.
If the 8V on the pll is steady; then the regulator should be OK.
Did you test the pll with a freq counter?
Check other specs on the pll/xtal/vco voltage as per the service manual?
Did you test the pll with a freq counter?
Check other specs on the pll/xtal/vco voltage as per the service manual?
The 8v is on the regulator against the side of the chassis. The pll has no voltage on any pins. I do have a frequency counter but no I didn't use it.
Check the schematic and trace the voltage line from the voltage reg to the pll.
http://www.cbtricks.com/radios/cobra/148gtl/graphics/cobra_148gtl_sch_redraw.pdf
Use the service manual and test the vco voltage.
http://www.cbtricks.com/radios/cobra/148gtl/graphics/cobra_148_gtl_sm_pg48_pg63.pdf
If your radio did not have the service bulletin change to the voltage reg; then you may want to do it. If your regulator is toast; then you can still get them on ebay and they are genuine. All of the regulators I bought from China worked 100%.
http://www.cbtricks.com/radios/cobra/140gtl/graphics/cobra_140gtl_142gtl_148gtl_tb1226.pdf
http://www.cbtricks.com/radios/cobra/148gtl/graphics/cobra_148gtl_sch_redraw.pdf
Use the service manual and test the vco voltage.
http://www.cbtricks.com/radios/cobra/148gtl/graphics/cobra_148_gtl_sm_pg48_pg63.pdf
If your radio did not have the service bulletin change to the voltage reg; then you may want to do it. If your regulator is toast; then you can still get them on ebay and they are genuine. All of the regulators I bought from China worked 100%.
http://www.cbtricks.com/radios/cobra/140gtl/graphics/cobra_140gtl_142gtl_148gtl_tb1226.pdf
I tested tp9 for voltage got 17 mv. I'm not really sure how to fully read the schematic. Do I follow the line that says always 8v over to the pll? If so what would be the first test point as an example? Also I put the frequency counter on all the pins of the pll and got nothing. Thanks for helping a newbie out.
most likely the MB3756 voltage regulator is bad.
that, or one or more of the associated electrolytic caps has gone bad.
they are the ones that look like tiny beer cans wrapped in plastic.
You said that this was a Taiwan radio which means that it was made in the late 70's or early 80's. this means that the electrolytic capacitors are either bad, or are going to go bad one by one over the next few years.
The reason i mention this is because unless these are all replaced, you can run around in circles trying to find a problem, only to find out that during your troubleshooting, another part has gone bad. the recommendation is to replace all of them at once, then align the radio. you can find kits on ebay for this.
for now, you probably just want to see if you can make the radio work again,
so, i will qualify my next statements with the fact that you may end up chasing your tail due to one or more shorted capacitors in the radio.
most of the time when a problem like the one you describe comes up, its the voltage regulator itself that has gone bad.
get your DC voltmeter and put the negative lead to PC board ground. (any of the metal tuning cans is good for this)
now put the positive probe to each pin of the MB3756, starting at the pin closest to the back of the radio. this is pin 1.
here are the voltages you should find:
pin 1 should be 8 volts all the time. both RX and TX.
pin 2 should be 13.8 volts all the time. this is the input voltage going into the radio.
pin 3 should be 8 volts all the time. this is just a reference voltage.
pin 4 is ground, and should show 0 volts.
pin 5 is the RX / TX switching pin. it controls the voltages on pins 6 and 8. you will read different voltages on this pin in RX than you will in TX.
pin 6 is 8 volts, receive only. it will drop to 0 when you key the mic.
pin 8 is 8 volts, transmit only, it will drop to 0 when you unkey the mic.
chances are pretty good that one of these voltages is not there, or way off what it should be.
if you find this to be the case, unsolder that pin from the PC board and very carefully bend it up so that it is no longer making contact with anything.
now measure that pin again. is the voltage now correct?
if it is, you have a bad part on the trace on the PC board. follow it wherever it leads looking for a part that is off value. start with the electrolytic caps. you will need to unsolder them from the board in order to test them.
put your voltmeter to read resistance, and put one lead on each leg of the cap.
it may show 0 ohms for a second or two, but should read like an open connection.
if you find one that reads as a direct short, replace it.
really, you should replace any that you pull from the board with new ones just to get a jump on the re-capping.
if you find that the voltage on the pin is still wrong even after removing it from the board, then the regulator itself is bad and needs to be replaced.
the caveat is that there may be a bad component on the board that caused the regulator to fry, and the new one will fry too.
this is where troubleshooting skills come in handy, and i don't have the time or strength to go into all of that in a thread.
you might want to buy two of them just in case if you are planning to try replacing it.
this is a great radio, and is very much worth spending a few bucks on.
IMO the taiwan cobras are the best out there. they are just old and need some TLC.
hope this helps,
LC
that, or one or more of the associated electrolytic caps has gone bad.
they are the ones that look like tiny beer cans wrapped in plastic.
You said that this was a Taiwan radio which means that it was made in the late 70's or early 80's. this means that the electrolytic capacitors are either bad, or are going to go bad one by one over the next few years.
The reason i mention this is because unless these are all replaced, you can run around in circles trying to find a problem, only to find out that during your troubleshooting, another part has gone bad. the recommendation is to replace all of them at once, then align the radio. you can find kits on ebay for this.
for now, you probably just want to see if you can make the radio work again,
so, i will qualify my next statements with the fact that you may end up chasing your tail due to one or more shorted capacitors in the radio.
most of the time when a problem like the one you describe comes up, its the voltage regulator itself that has gone bad.
get your DC voltmeter and put the negative lead to PC board ground. (any of the metal tuning cans is good for this)
now put the positive probe to each pin of the MB3756, starting at the pin closest to the back of the radio. this is pin 1.
here are the voltages you should find:
pin 1 should be 8 volts all the time. both RX and TX.
pin 2 should be 13.8 volts all the time. this is the input voltage going into the radio.
pin 3 should be 8 volts all the time. this is just a reference voltage.
pin 4 is ground, and should show 0 volts.
pin 5 is the RX / TX switching pin. it controls the voltages on pins 6 and 8. you will read different voltages on this pin in RX than you will in TX.
pin 6 is 8 volts, receive only. it will drop to 0 when you key the mic.
pin 8 is 8 volts, transmit only, it will drop to 0 when you unkey the mic.
chances are pretty good that one of these voltages is not there, or way off what it should be.
if you find this to be the case, unsolder that pin from the PC board and very carefully bend it up so that it is no longer making contact with anything.
now measure that pin again. is the voltage now correct?
if it is, you have a bad part on the trace on the PC board. follow it wherever it leads looking for a part that is off value. start with the electrolytic caps. you will need to unsolder them from the board in order to test them.
put your voltmeter to read resistance, and put one lead on each leg of the cap.
it may show 0 ohms for a second or two, but should read like an open connection.
if you find one that reads as a direct short, replace it.
really, you should replace any that you pull from the board with new ones just to get a jump on the re-capping.
if you find that the voltage on the pin is still wrong even after removing it from the board, then the regulator itself is bad and needs to be replaced.
the caveat is that there may be a bad component on the board that caused the regulator to fry, and the new one will fry too.
this is where troubleshooting skills come in handy, and i don't have the time or strength to go into all of that in a thread.
you might want to buy two of them just in case if you are planning to try replacing it.
this is a great radio, and is very much worth spending a few bucks on.
IMO the taiwan cobras are the best out there. they are just old and need some TLC.
hope this helps,
LC
If you look at Service bulletin #1226, you will see that it will correct the voltage supply before the regulator fails. If the regulator has already failed; then I would do #1226 and install a new regulator.
All of the older Cobra radios that use that chip that I owned, I have done this simple and low cost procedure to them and never had any failures after that point.
You will need a 18v/1/4w zener diode and then perform the upgrade. Later versions of these radios already include this change. The newer Cobras don't even use the mb3756, so this does not apply.
You will need to trace this small portion of circuitry down and see if the upgrade is already in there, or if it still needs to be done.
http://www.cbtricks.com/radios/cobra/140gtl/graphics/cobra_140gtl_142gtl_148gtl_tb1226.pdf
All of the older Cobra radios that use that chip that I owned, I have done this simple and low cost procedure to them and never had any failures after that point.
You will need a 18v/1/4w zener diode and then perform the upgrade. Later versions of these radios already include this change. The newer Cobras don't even use the mb3756, so this does not apply.
You will need to trace this small portion of circuitry down and see if the upgrade is already in there, or if it still needs to be done.
http://www.cbtricks.com/radios/cobra/140gtl/graphics/cobra_140gtl_142gtl_148gtl_tb1226.pdf
Well I did like you said with checking the pins. Pin 1 had no voltage. Unsoldered the leg for pin 1 and rechecked it. Sure enough no its showing 8v. So you think I should get one of those recap kits or try chasing down the exact part that went bad? Thanks
hey MK121,
Sorry about leaving you in the lurch, i just saw this post and somehow missed the message that you had posted back.
so, this is at least promising, as the voltage regulator may just be working right.
maybe...
so we know right now that there are one or more components on the trace that comes out of pin 1 of the regulator that are shorted to ground.
in the most basic sense, you need to unsolder and lift the leg of any component connected to this trace, and see if operation is restored.
use the "rick jackson" schematic on the cbtricks site for a reference so we are on the same page.
with the radio off, and DC power disconnected, re-solder pin 1 of the reg. to the PC board.
if you look at the schematic and follow the line that comes out of pin 1 of IC4 (MB3756), you will find little dots that indicate a part is connected to that "line".
this would be the end of the part to lift from the board and check for operation.
WARNING! without experience doing this, there is always the chance that you could let the magic smoke out of the radio by doing these tests.
so, when you have removed a particular component lead, and you want to turn the radio back on in order to test it, you should do it quickly.
for example, have your voltmeter negative lead already connected to PC board ground with an alligator clip or something so that you don't have to physically put it there with the radio on.
do all your work with the radio off, and with DC power disconnected.
each time you remove a component lead, have the voltmeter on DC volts and be ready to put the positive lead on to the pin 1 trace and see if you have restored the 8 volts.
if not, turn power off immediately and re-solder the part you removed.
then move on to the next one, doing the same thing.
the trick is to not leave the radio on for very long while some component is disconnected.
my first guesses would be any electrolytic capacitor connected to that line.
in this order, these are the first four parts i would check.
C84, C18, C109, then C95.
after that, start looking at other capacitors and resistors.
use your eyes and a magnifying glass to try and see if any of the parts on that line look burnt or discolored. this would be a telltale sign of a bad part, but you never know what caused that part to go bad, and you may find that a new part blows as soon as it is connected.
i highly recommend the re-cap kit, regardless of what you find, as it's only a matter of time before other caps start going bad.
you could just replace them all and hope for the best, but you will learn more if you try to find the problem first.
it would be a good idea to have the re-cap kit on hand when you start trouble shooting, so that as you remove electrolytic caps, you can replace them with new ones.
don't go crazy replacing all the caps without checking the radio.
you want to know if you put one in backwards or something, so turn the radio on and check it as you go.
one more thing.
what i have laid out here is not necessarily the way a tech would go about finding your problem if he had it at a shop with proper test equipment.
this is just a way for you to try doing it yourself, so i wanted to put in the disclaimer that you could see smoke coming out of the radio, and i can't cover every possible outcome in an internet thread.
i am merely using an educated guess as to what you should try, and there are others here much smarter than I who may come up with other ideas.
that said, good luck with it, and post back anything you find along the way.
LC
Sorry about leaving you in the lurch, i just saw this post and somehow missed the message that you had posted back.
so, this is at least promising, as the voltage regulator may just be working right.
maybe...
so we know right now that there are one or more components on the trace that comes out of pin 1 of the regulator that are shorted to ground.
in the most basic sense, you need to unsolder and lift the leg of any component connected to this trace, and see if operation is restored.
use the "rick jackson" schematic on the cbtricks site for a reference so we are on the same page.
with the radio off, and DC power disconnected, re-solder pin 1 of the reg. to the PC board.
if you look at the schematic and follow the line that comes out of pin 1 of IC4 (MB3756), you will find little dots that indicate a part is connected to that "line".
this would be the end of the part to lift from the board and check for operation.
WARNING! without experience doing this, there is always the chance that you could let the magic smoke out of the radio by doing these tests.
so, when you have removed a particular component lead, and you want to turn the radio back on in order to test it, you should do it quickly.
for example, have your voltmeter negative lead already connected to PC board ground with an alligator clip or something so that you don't have to physically put it there with the radio on.
do all your work with the radio off, and with DC power disconnected.
each time you remove a component lead, have the voltmeter on DC volts and be ready to put the positive lead on to the pin 1 trace and see if you have restored the 8 volts.
if not, turn power off immediately and re-solder the part you removed.
then move on to the next one, doing the same thing.
the trick is to not leave the radio on for very long while some component is disconnected.
my first guesses would be any electrolytic capacitor connected to that line.
in this order, these are the first four parts i would check.
C84, C18, C109, then C95.
after that, start looking at other capacitors and resistors.
use your eyes and a magnifying glass to try and see if any of the parts on that line look burnt or discolored. this would be a telltale sign of a bad part, but you never know what caused that part to go bad, and you may find that a new part blows as soon as it is connected.
i highly recommend the re-cap kit, regardless of what you find, as it's only a matter of time before other caps start going bad.
you could just replace them all and hope for the best, but you will learn more if you try to find the problem first.
it would be a good idea to have the re-cap kit on hand when you start trouble shooting, so that as you remove electrolytic caps, you can replace them with new ones.
don't go crazy replacing all the caps without checking the radio.
you want to know if you put one in backwards or something, so turn the radio on and check it as you go.
one more thing.
what i have laid out here is not necessarily the way a tech would go about finding your problem if he had it at a shop with proper test equipment.
this is just a way for you to try doing it yourself, so i wanted to put in the disclaimer that you could see smoke coming out of the radio, and i can't cover every possible outcome in an internet thread.
i am merely using an educated guess as to what you should try, and there are others here much smarter than I who may come up with other ideas.
that said, good luck with it, and post back anything you find along the way.
LC
LC,
That's helpful information. Thanks for taking the time to explain everything. I have the recap kit ordered. Hopefully it will be here for the weekend. I downloaded the schematic you were talking about and will use it like you said. Also I will give the caps you mentioned a look first. Hopefully no magic smoke will appear.
That's helpful information. Thanks for taking the time to explain everything. I have the recap kit ordered. Hopefully it will be here for the weekend. I downloaded the schematic you were talking about and will use it like you said. Also I will give the caps you mentioned a look first. Hopefully no magic smoke will appear.