0ld style Uniden Grant. Keys when powered up?

[Low_Boy]

What if D44 was shorted? Would that pull down pin 5 to ground?

That is something I have not checked and will when I get home.
Thanks

 

[loosecannon]

I believe you have the pins reversed is the way you posted your voltage readings.
Seeing that you labeled pin two as no connection and pin 7 with 13.8 volts on it is a dead giveaway.
LC

 

[ExitThirteen]

LC is correct, and with that being said, pin 3 does have the correct voltage on it. Thank you for the correction, LC.


Check D58 and D11, they are located near VR3, the squelch pot. Make sure they have not gone short. Let us know how you get on.


~Cheers~

 

[Low_Boy]

I feel very embarrassed. Eating then heading home and will post findings.
Thanks

 

[loosecannon]

I feel very embarrassed. Eating then heading home and will post findings.
Thanks

i get turned around all the time when flipping PC boards over and over.
LC

 

[Low_Boy]

D44, D58 and D11 all test good.

 

[loosecannon]

low boy,

now that you have the correct pinout of the MB3756, please go back and re-try the tests i laid out for you.

remember that when you thought you had unsoldered pins 6 and 8, you actually unsoldered pins 1 and 3.

I really think you have a bad voltage regulator, or something on the pin 8 PC board trace that is shorted to PC board ground.

the way to tell which it is, is by unsoldering the pin and checking the voltage on it.

for example, if you unsolder pin 8 from the PC board, turn the radio on, and find that you have 8 volts on pin 8 in TX mode, and then you put your meter in continuity mode and then check the trace itself (pin 8 still un-connected) against PC board ground, you would know that some component on that trace that goes to ground has shorted.
LC

 

[Low_Boy]

Getting ready for bed but I will check this out tomorrow and post the findings.
Thank you.

 

[Low_Boy]

well that's a completely different story!
saying that the mic TX pin is shorted to the chassis is completely different from saying that it is shorted to PC board means something very different.

of course it is shorted to PC board ground, because the radio is in TX mode.

put the mic jack and squelch knob back in place, and move your attention back to the MB3756 voltage regulator.

i know you have checked it before, but just for fun, lets do it again.

radio on, no mic connected, radio is in TX mode right?

measure the voltages on pins 1-8 of the MB3756 and post them here.

you should measure 8 volts DC on pin 6 if the radio is in receive mode.

you should measure 8 volts DC on pin 8 if the radio is in transmit mode.

pin 5 is the pin that switches that voltage from pin 6 to pin 8.

my guess is that you will find 8 volts on pin 8, seeing as how the radio is stuck in TX mode.

if that's the case, unsolder pin 6 and pin 8 from the PC board and bend them up so that they are not making contact with the board.

now check the voltages again.

you should see 8 volts on pin 6 and when you key the mic, you should see that voltage move to pin 8.

please post the results of your experiment.
LC

Okay I pulled the real pin 6 and 8 from the board. Pain-8 showing 8 volts.pin 6 showing nothing then I key the mic pin 8 still shows 8volts pin 6 still showing nothing. I then tried another regulator same problem. I then tested for continuity between the pulled pins and the trace they came off of and saw nothing. Then tested from tab to ground on each. And pin to ground on each and saw nothing.

 

[loosecannon]

sounds like your problem lies on the trace of pin 5, which is the RX/TX switching pin.

look at the schematic, at the components that are connected to the pin 5 trace, and make determinations based on metering the components either in place or out of circuit.

basically it's C133, R162, TR35, or the wiring on the CB/PA switch.

i know you said that you already replaced TR35, but don't take it for granted.

remember that when you key the mic, with the radio in CB mode, it grounds the emitter of TR35.
check to see if this pin is grounded when the radio should be in receive mode.

if it is, pull TR35 out and check the trace that connects the emitter to the TX pin of the mic jack, through the CB/PA switch. if that trace is still grounded, that points to your switch wiring.
if it's not, it points to TR35.
LC

 

[Low_Boy]

D35 With radio off emitter and collector show continuity with chassis ground NOT board ground. With radio fired up emitter, collector and base show continuity with chassis ground and board ground.

 

[loosecannon]

EDIT!
check the wire that goes from your CB/PA switch to the mic jack. i think you have it connected to the wrong side of C407.
refer to this schematic:
http://www.cbtricks.com/radios/cobra/148gtl/graphics/cobra_148gtl_sch_a_size.pdf





is that with the transistor in place or just the empty traces?


LC

 

[loosecannon]

if this is the case, there is a chance that your new TR35 is smoked also.

fix the problem, and then replace TR35.
LC

 

[Low_Boy]

EDIT!
check the wire that goes from your CB/PA switch to the mic jack. i think you have it connected to the wrong side of C407.
refer to this schematic:
http://www.cbtricks.com/radios/cobra/148gtl/graphics/cobra_148gtl_sch_a_size.pdf





is that with the transistor in place or just the empty traces?


LC

With transistor in place.

 

[Low_Boy]

Just found one more thing. With the radio pluged in with power on or off there is continuity between board and chassis ground. With radio unplugged no continuity. With tr35 in or out.

One more thing TX light is not on. Remove the antenna and the TX light comes on. I have been doing all my testing with the radio on a dummy load because the radio transmits when powered on so I never noticed this until juat now.

This is not a shorted disc cap on the power plug. They test good and I also removed them for a test. Same thing. I can not find a bad bypas cap.