Cobra 148 GTL problem

[eldorado303]

Hello guys. I have recently bought a non working
148 gtl. I would like to know some opinions on what
might be the problem(s) with it. I have no RX, no
band hiss whatsoever. Also no TX, the light does not
change to red when the ptt is pressed. The PA does
not seem to work also. I have measured voltages on
both MB3756 voltage regulator, and the PLL.
Voltages are as followed: MB3756
Pin 1-11.8V, Pin 2-13.8V, Pin 3-8.3V, Pin 4-0,0V
Pin 5-13.8V, Pin 6-8.3V, Pin 7-NC,
Pin8 (w/PTT pressed)-0,0V
PLL Voltages I did not right down, but most of the pins
were around 2,3V, and some up, some down,
but they do not check ok with the values on the
schematic. Some of the MB3756 voltages are not
ok, as one can see on the schematic.
Any opinions on what might be causing this
no RX/TX problem?

Thanks in advance,

João G.

 

[9C1Driver]

First you need to decide if it's even worth fixing. If it's an old school model then yes. If it's a Soundcrapper or new model then dump it on e-bay as fix/repair.

 

[Robb]

If it has a mb3756 in it, it is old and worth fixing.
Restoring as many 148's as I can get from Hamfests - lol.

Here is where I would start:

1) Check pin 9 on the 8719/pll chip; it should be ~8v all the time. If you aren't seeing 8v; then the regulator has failed or there is a shorted electrolytic cap(s) on that 8v rail. Old radio; then old, dried out caps are a commonplace issue.

2) BTW - Replace all 10v caps in the radio using a 16v or 25v with the same capacitance value, as a rule.

3) Check to see if the Cobra Service Bulletin #1226 was done to it, which adds a 18v zener diode near the voltage regulator. That diode may be faulty or probably not even present. I do that SB mod to all 148/142 and all Uniden chassis that have the mb3756 regulator. This doesn't fix anything, except keeping that regulator from failing. That part is becoming more hard to find, so it would be wise to keep it working.

4) Then check the VCO voltage as the Service Manual describes.
http://www.cbtricks.com/radios/cobra/148gtl/graphics/cobra_148_gtl_sm_pg48_pg63.pdf

 

[Radio Tech]

I agree with Robb. Worth fixing.
And as he said check for the 8 volts. Since the radio sounds as if it is pretty much dead at the moment, must be either missing voltage or a bad solder connection. If there is no 8 volts again as Rob said it could be the regulator. However there are 2 more components in that circuit that can take it out. C84 and C95. Either of the two will take the 8 volt down. But that should make IC4 run very hot.

Once 8 volts is restored you should see things start to come alive.

 

[eldorado303]

Hi guys. I was just checking the stuff you told me.
The PLL pin 9 is not at 8v its at 11.2v. Also I've
checked C84 and C95 ( not with my multimeter
because it doesnt have capacimeter ), but visually
they seem ok, the are not swollen or open.
( of course I know it doesnt mean they are ok, but
since I dont have the capacimeter its all I could do. )
IC4 is not hot, just normal. The #1226 service bulletin
hasnt been done on it, but it has D44 there. ( I dont have
the proper diodes to change it now )
Anymore ideas on this are welcome. I might change IC4,
because Pin 1 is at 11.2v instead of 8v, and the last pin
doesnt show 8v when TX. Also I want to ask if you guys have
any idea on why the PLL voltages are not correct, they are around
2v, instead of being 4,5v and so on. Could it be the MB3756
the sole cause of this?

Thanks,

João

 

[Robb]

If your multimeter is accurate; then 11.2v is too much voltage.
Do you have another regulator to replace it?

http://www.cbtricks.com/radios/cobra/148gtl/graphics/cobra_148gtl_sch_redraw.pdf

 

[eldorado303]

Thanks Robb. Yes, the multimeter is accurate.
I dont have any mb3756 here, but I guess I'll buy one
soon and give it a try. Other thing, how can I know if
the PLL is ok? I have the CH numbers on the display,
but since no RX and TX, is there any way I can know?

 

[Robb]

Hope your pll didn't get fried by too much voltage.
Certainly wouldn't do any more testing until the 8v issue has been dealt with - first.

 

[ExitThirteen]

If you have 11.2V at pin 9 of the PLL, most likely it has taken out the PLL too. It doesn't always happen, but I am willing to bet that the PLL is bad as well. Replace the MB3756 regulator first, and if you get 8V at pin 1 when you're done, and 8V at pin 9 of the PLL and you still do not get TX/RX, the PLL has gone fault and will need to be replaced.


Hope this helps!


~Cheers~

 

[loosecannon]

i agree with Xit13, but would like to add a couple of things to try.

first, when you replace the MB3756 regulator, leave pin 1 unsoldered from the board and sticking straight out.

power up the radio and measure the voltage on pin 1. not the PC board trace, but the pin itself. you should measure right about 8 volts.

my guess is that you will see that 8 volts there, but if you dont, you have a problem in the power supply circuit around that MB3756.

if you see the 8 volts there, you can solder pin 1 to the PC board, BUT!!!
do not turn the radio on yet.

first, unsolder pin 9 of the PLL and use your continuity meter to check that it is not making any connection to the PC board. (use solder wick for this)

after you've confirmed that pin 9 of the PLL (MB8719) is not connected to anything, then you can turn the radio on, and check for the 8 volts again on the PC board trace that connects to pin 1 of the MB3756.

if you see anything other than 8 volts on that line, stop right here and figure out which part on that line is causing the erroneous readings.

the reason for doing it this way is so that you dont blow a good PLL chip.

if you do see 8 volts on that line, then you can solder pin 9 back in place, and make another check to see that its still 8 volts.
if it isnt, then your PLL is definitely bad and needs to be replaced.

(a little note on that- MB8719 chips are extremely hard to find these days, and many of the ads on ebay from china are different versions like the MMB8719 or the RCI8719, which are not the same, so be careful)

as for testing the caps, you can do a quick test with your continuity meter to see if its shorted, but you have to remove the cap from the chassis first.
you wont see any difference in the looks of good caps vs. bad ones.

good luck,
LC

 

[eldorado303]

Thanks for the help guys. I've bought a MB3756,
and am just waiting for it to arrive through the mail.
I will do those tests like you told me. And I'll keep you
up to date.

Thanks,

João G.

 

[9C1Driver]

Thanks for the help guys. I've bought a MB3756,
and am just waiting for it to arrive through the mail.
I will do those tests like you told me. And I'll keep you
up to date.

Thanks,

João G.

That's great that you have one worth saving. The guys that have posted tech help here are some of the best the forum has. Looks like you will have er fixed up in no time.

 

[eldorado303]

Hi guys. I've received the MB3756 today and I did like you told me,
I desoldered pin 9 at the PLL, and tested both pin 1 on the
voltage regulator and the pin 9. Pin 1 on the MB3756 tested 8.3V,
and I also got 8.3V at pin 9 on the PLL. I then soldered pin 9 of
the PLL back and measured voltage. Also 8.3V. But most of the
PLL voltages are all messed up, ie:
pin 1- 3,15V instead of 3,4V ( this one is not very bad )
pin 2- 1,93V instead of 4,28V, pin 3- 0,03V, pin 4- 3,15V
instead of 3,45V, pin 5- 3,15V instead of 3,45V, pin 6- 0,03V
instead of 8V, and so on. Pin 9 is correct though, at 8,3V.
I still have no TX, I mean the light doesnt change from green to red
when pressing the PTT.

Maybe the PLL is out?

Any ideas?

Thanks,

João G.

 

[ExitThirteen]

It could very well be that the PLL has failed and will need to be replaced, based on the voltage readings you gave before you replaced the MB3756. You may have to locate a new PLL which is not the easiest thing to do, since it has been discontinued for some time. It may cost you $15-25 USD to get one, but still would be worth it for that radio.

You could also try adjusting L19 (VCO) slightly to see if TX/RX comes up. Try this first before replacing the PLL.


~Cheers~

 

[Robb]

Use a voltmeter/DMM and test the VCO voltage by attaching the + test lead to the top of R207:



If L19 will not adjust; then it is a fair bet that the pll is fried.
It is still worth fixing - IMO.