No Tx audio on cobra 148 GTL

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mike262
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No Tx audio on cobra 148 GTL

Post by mike262 » Tuesday 29th Dec 2009, 6:15

Radio Make and Model: Cobra 148 GTL
PCB Number: PC 412 AE
PLL Number:
Test gear available:
Symptoms: No audio on Tx

What is working: Everything Else

What has been tried:

Did it suddenly happen ?:
Anything else that may be of help?:

Hi,

I have an 1990 Philippines made Cobra 148 GTL PC-412 AE

when I got the radio it had the channel mod done. I put the radio back to stock. After putting back TR24 (it was removed from the radio) I lost the audio on tx.

I have a good carrier, but no modulation. With TR24 removed I have modulation. The PA works. I have audio on SSB, but it only puts out about .6 watts.

Without TR24 everything works great. With TR24 no modulation in AM and only .6 watts output in SSB.

I checked IC4 and all voltages are good. I got the following readings on the below transistors:

TR24 AM mode only

RX TX

E C B E C B
.01 .01 .73 .09 .01 .89

TR26 AM only

4.02 6.0 .62 1.75 .12 .61

TR27 AM only

3.06 0 3.2 2.25 0 2.15

TR25 AM only

6.64 .73 6.25 2.26 .90 1.53

TR23 AM only

1.10 4.26 1.76 .11 2.21 .67

Bias voltages are correct. Its on frequency. VCO voltage is good.

The only other odd thing is I have barely any movement in the meter on Rx. If the signal is close it swings the meter, otherwise it barely moves. The meter moves normally in all other modes. I have also adjusted VR1 with almost no change.
The radio receives well, even weak signals. I have much more movement with my other 148's. I have also tried two other replacement meters.

Its a great sounding radio as long as TR24 is removed.

Thanks for any help, I'm stumped.....

Mike

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The Defpom
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Re: No Tx audio on cobra 148 GTL

Post by The Defpom » Thursday 31st Dec 2009, 0:50

This symptom sounds familiar, it sounds like the TR24 is loading down the DC supply feeding the audio circuit, so its killing it, you are replacing TR24 with a 2SC945 ?

I will have to look up this in my notes, I may be able to direct you to a likely part at fault.

As to your meter fault, try replacing the VR, sometimes they crack, if it is still a problem let me know.

mike262
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Re: No Tx audio on cobra 148 GTL

Post by mike262 » Thursday 31st Dec 2009, 4:20

Ok,

I'm using a 2sc945. I tried more than one just in case. I'll check the VR.

Thanks,

Mike

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Re: No Tx audio on cobra 148 GTL

Post by The Defpom » Friday 1st Jan 2010, 1:29

Try lifting one end of R128 and see if that restores normal TX audio with TR24 in place, if it does then the fault is (or around) TR26 or its feed from the AM TX power control.

Some of your voltage readings are a bit odd, please recheck TR25 and TR26 readings.

I found my notes, it was on a sister chassis, the fault was the equivalent part of C109, so it could be worth bearing it in mind as something to replace.

To find out if the audio chain is losing power in TX (RX actually shunts some power to shut off audio via D59), check the voltage at both sides of R111 in TX, you should only see a small drop across it, if there is a large voltage drop (more than about 1 or 2V) then I would be looking at replacing capacitors.


For your meter fault, if the VR1 is OK, then try replacing C33, and check D12, D13, D14, D66, D67 and lastly TR48 (which is the AM S meter driver).

mike262
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Re: No Tx audio on cobra 148 GTL

Post by mike262 » Friday 1st Jan 2010, 7:41

Okay,

When I lift one end of R128 I get audio back. I also checked the voltage across R111 while transmitting. With R128 in place I get 1.72 in RX and 6.19 in TX. If one leg of R128 is lifted I get 1.72 in RX and 1.73 in TX.

I checked the voltages again for TR25 and TR26.

TR26 (RX)(AM)

E4.0 C6.02 B.64

TR26 (TX)(AM)

E1.76 C.12 B.61

TR25 (RX)(AM)

E6.26 C.75 B6.67

TR25 (TX)(AM)

E1.56 C.90 B2.31

I replaced TR26 C109 C73 C19 and C18 - No change.

Thanks again,

Mike

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Re: No Tx audio on cobra 148 GTL

Post by The Defpom » Monday 4th Jan 2010, 12:11

With R128 in place, try lifting one end of R161 (this is the SSB ALC feed), I just want to ensure that it is not staying turned on in AM.

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Re: No Tx audio on cobra 148 GTL

Post by mike262 » Sunday 10th Jan 2010, 14:44

Sorry for the late reply;

When I went to lift one end of R161 I saw my other TR26 sitting on the bench. I never checked it. When I swapped it out everything came back. Looks like TR26 was the problem.

as for the meter in RX, I have to check c33, everything else checked out.

Thanks again,

Mike

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Re: No Tx audio on cobra 148 GTL

Post by The Defpom » Sunday 10th Jan 2010, 21:03

Great, at least it was TR26.

Now just to sort the meter...

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Re: No Tx audio on cobra 148 GTL

Post by mike262 » Friday 12th Feb 2010, 14:35

No luck with the meter. When I hook up a pre-amp the meter works as normal. Without the pre-amp no swing on low to med signals. Also, If I increase the volume to listen to low signals, when a strong signal comes in I get blasted from the speaker.

Mike

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