Cobra 148GTL weak RX audio in all modes

euhara · Post by **euhara** » Thursday 27th Nov 2008, 13:57

Hello to the forum,
Guess I will look for more help and post my problem, I try to help myself much as I can until I get lost as where to look next.

I have a older Cobra 148GTL with low volume in receive mode.

Board number PC-412 is all I can see
PLL Chip is MB8719

When I got this radio it was dead, and I got it going and aligned and tuned it up two of the mike wires (yellow and white) were crossed so It had no SSB output and just a DK in AM mode so I fixed those and everything works fine now except even at full volume the RX audio is very weak. Perhaps the crossed wires caused some of the current fault.

The PA volume is very loud when volume is turned up, and raises and lowers with the volume control. Squelch works normally near as I can tell.

Checked voltage at IC4 Voltage reg. MB3756 and it is:

Pin 1 RX AM MODE 8.30 TX 8.39 SSB All pins basically same as AM
2 14.38 13.93
3 8.31 8.40
4
5 14.38 164.5mV
6 8.31 .360v
7 nc nc
8 .550 8.41

IC6 AF Amp AM RX MODE TX
1 13.76v 13.10v
2 7.28v 6.98v
3 ----- -----
4 ----- -----
5 1.209v 1.290v
6 1.200v 1.296v
7 30.4mV 121.9mV
8 34.2mV 124.3mV
9 7.04v 6.86v
10 14.56v 14.09v
SSB mode volts basically same as AM mode

Hope I did this right so someone can possibly see the fault.

Thanks, Jim

http://www.youtube.com/user/TheDefpom · Post by **The Defpom** » Thursday 27th Nov 2008, 14:15

have you tried an external speaker, to rule out a problem with the internal one.

I will look into this further shortly

http://www.youtube.com/user/TheDefpom · Post by **The Defpom** » Thursday 27th Nov 2008, 14:35

OK, firstly I would check that the wire leading from pin 3 of the mic socket is heading to the rear of the radio and connects to the ext. speaker socket, to confirm that the RX wire is correct, as if that is incorrect it will kill RX audio.

After that I would start by assuming that there is a component fault somewhere, this was mentioned elseware in the forum fairly recently, but I will put it here too...

Replace C186 it can fail, causing the RX audio to be partially shunted to ground and dropping the voltage on TR45 before teh audio makes it to the audio amp.

If that does not work, check in order: TR45, C72, TR19, C73, C74, TR12, TR13, any of which could cause the same symptom.

euhara · Post by **euhara** » Thursday 27th Nov 2008, 14:53

Thanks Scott,
I have the audio connected to an external speaker, easier to work on that way. Since the PA speaker is so loud i assume everything must be connected. Am I wrong?
So I am thinking it must be some faulty component and will check all you suggest.

regards, Jim

http://www.youtube.com/user/TheDefpom · Post by **The Defpom** » Thursday 27th Nov 2008, 15:03

The RX speaker feed comes via a different source than the PA speaker feed, that I why you need to confirm that the RX wire from the mic plug does indeed go to the external speaker socket.

When in PA mode, the PA socket gets its earth directly from the PCB, but in CB mode the earth comes from the RX pin on the mic socket, if there is an incorrect connection there, it could mean that there is not a true earth for the CB speaker (internal and external), which would effect its audio level or ability to produce any sound at all.

euhara · Post by **euhara** » Thursday 27th Nov 2008, 15:51

OK thanks Scott
for the fast replies, Replaced C182 and no change. The wire is connected from mike pin 3 to speaker wire in board at rear of radio it is a green wire. But can't find TR45. Is it possible the board is marked wrong? As it should be by C182 but the board is numbered TR43 there although the number on this transistor is C458.

TR19 voltage is right on but I get a slight tone from the speaker when I probe the base pin. Waiting for your answer before going further.

Thanks, Jim

http://www.youtube.com/user/TheDefpom · Post by **The Defpom** » Thursday 27th Nov 2008, 16:03

C186 not C182

After having a second look at my diagram, it could be TR43 not TR45, the 3/5 is hard to read due to a mark.

http://www.youtube.com/user/TheDefpom · Post by **The Defpom** » Thursday 27th Nov 2008, 16:11

I am 99% sure it should be TR43, not 45, I compared the diagram to one from a very similar chassis, and it was 43.. I will have to fix up the numbering!

euhara · Post by **euhara** » Thursday 27th Nov 2008, 16:17

Sorry Scott,
it is C186 next to TR43 which I replaced. Can you furnish the voltages for TR43 I can't find a reference to that one, also what type is Tr43?? On Rick's schematic which he redrew the Transistor number next to C186 is in fact TR45 and is a 2SC458. Board must be wrong I think.

Thanks, Jim

euhara · Post by **euhara** » Thursday 27th Nov 2008, 16:39

OK Scott,

By the way thanks for having such a good Forum. It is the most active helpful one I can find.
I just discovered that the external socket must not be grounded properly as when I physically ground it the audio comes up full. So there must still be a wire wrong in the mike.
Can you straighten me out on that as I have no idea what brand mike it is but is an amplified hand held.

Thanks, Jim

http://www.youtube.com/user/TheDefpom · Post by **The Defpom** » Thursday 27th Nov 2008, 16:56

Lets try this without a mic, to give us a definite cause, be it the radio or mic.

Unplug the mic, now with a bit of wire or some other metal object short between pin 3 and pin 4, this should make the ext.+ int. connection earth properly, and would normally turn on the audio, HOWEVER, you had an issue with some wires there, so the wiring on the rear of the socket may be incorrect also.

euhara · Post by **euhara** » Thursday 27th Nov 2008, 17:09

OK Scott,
You are correct the audio works fine by shorting those two pins and this radio has been golden screwdrivered and wires chopped up but I have fixed all those except at the mike internal plug (the one in the radio.) Can you help check those out or maybe be sure I have all correct in mike plug? Don't know whether it would help for me to look at a mike wiring chart as I have done that.

Thanks Jim

http://www.youtube.com/user/TheDefpom · Post by **The Defpom** » Thursday 27th Nov 2008, 18:29

It would be best to sort out the internal wires, otherwise it could cause problems later in its lfe for people trying to wire mics for it.

Lucky for you, once some time ago I made some notes...

Pin - Function - Colour
1 - Audio - Pink
2 - Audio Earth - Coil Inductor
3 - RX - Green
4 - Earth - Black
5 - TX - RED

euhara · Post by **euhara** » Thursday 27th Nov 2008, 19:09

Many thanks Scott,
Success!!

Got the Mike wires on right and thanks to you it is finished and working fine in all modes.

It is a fine looking Radio and will now look good on my shelf for a spare.

Cheers,