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Board # EPT01481 3 Z
PLL 8719
Cobra 148 GTL-F

Problem No audio on TX, Good Dead Key no swing PA is just static
Replaced Audio Chip - No change


Picked up a 148 GTL-F, bought a new bezel and faceplate, worked great so only did the channel mod and talkback. About two weeks went by with no problems, then no TX audio, just a dead key. RX Audio ok...Checked PA mode, just a consistant Static hiss when keyed.... I replaced the audio chip, no change. Voltages on the audio chip are(starting from the back of the radio):

1 )13.1
2 )6.95
3 )0.0
4 )0.0
5 )1.16
6 )1.16
7 )0.0
8 ) 6.6
9 )13.8

Thanks again for any direction!
73's

Just noticed I can get a swing above dead key by creating a squeal by touching the foil side of the board by the audio chip during key up, but no audio from the mic. Checked the mic gain switch, it's ok...

The only Sch I have is for the EPT014811Z, but I am pretty sure it is the same as the EPT014813Z.

Try lifting the collector of TR24 and the emitter of TR27 and one end of C232, to see if the audio returns.
Also, check C99, C101, C104, and C108.
You could also try injecting a 1 KHz tone in to Pin 4 of the IC6 audio IC.

Lifted points as you asked, and pulled cap 232 leg, No change...

When connected:
TR24
E- 0.0/0.0 rx/tx
C- 0.0/0.0
B- .70/0.4

TR27

E- 0.5/.5
C-3.0/3.0 (when E lifted it is 7.0 RX/TX)
B-1.10/1.09 emits a hum when tested in TX with DVOM

I'll try the other caps now, and get back, Thanks for the direction Rick!

Have you have checked the voltage regulator (IC4)?
What voltages do you have around TR22 and TR23 in TX?
Do you have a means of injecting a 1 KHz tone?

IC4 starting at the back of the radio:

1 - 8.2
2 - 13.8
3 - 8.2
4 - 0.0
5 - 13.8 - on tx goes to 0.**
6 - 8.2 - on tx drops to 0.17
7 - 0.03
8 - 0.11 - on tx goes to 8.2

Checking other voltages now...

Damn just noticed I listed TR22 as TR27

I'll go back and lift leg of TR27 and give you the results. The voltages above that say TR27 are actually TR22....
My Bad eyesight, heh heh

OK correctly lifted legs and cap as suggested, no difference.

Reinstalled parts:


TR27

E 2.92/2.92
C 0.0/0.0
B 2.92/2.92 ( all are the same RX/TX)

TR24
E- 0.0/0.0 rx/tx
C- 0.0/0.0 rx/tx
B- .70/0.4 rx/tx

TR22

E- 0.5/.5
C-3.0/3.0 (when E lifted it is 7.0 RX/TX)
B-1.10/1.09 emits a hum when tested in TX with DVOM


TR21 0.0/2.98/0.0 same on RX/TX

Sorry - can't add tone either....

It looks like you have the wrong voltages around TR21.

In AM and RX mode you should have Base 0.35 vdc Emitter 0.0 vdc Collector 3.05 vdc all voltages are ± 5%.

In AM and TX mode you should have Base 2.60 vdc Emitter 0.0 vdc Collector 1.95 vdc all voltages are ± 5%.

It looks like the Base is not being turn on, check the voltage at the junction with R112 and R113 it should be about 6 to 6.8 vdc in AM TX mode, if not check TR31 and diodes D36, D37, and D38, on the Collector side of TR31 you should find 8.2 vdc in TX mode.

Thanks for all the help Rick, I found the needle.... (in the haystack)

R 113 open, replaced and static was gone, but still just a deak key, no audio...

I did a headphone for audio on pin 6 like you said, no audio... followed it all the way back to the 5 pin mic board... The small black diode or choke between mic and radio circuit was open, qick swap and everything is back happy. What diode or choke should be there? I used a temp just to check... It is listed as as L404 on the mic circuit board.

Where is the diode connecting too and from?
Are you sure, it is a diode and not a choke?

I believe it is a choke, The board is marked L404

I thought it would be a choke, it will be a 5.6uH.

Thanks for the choke value, I replaced the choke, buttoned it all back up works great again!

It's great to have the help you give, even when it turns out to be a simple fix after looking everywhere, I wouldn't have found for a long time without your direction.

Now I owe you another pint...... LOL!