cobra 140 GTL problems

[zman]

I inherited a few CB mobiles from my cousin that were his dads.
One is an old Cobra 140GTL. It will not key up or receive anything. Basically it is dead. I know the PLL 8719 is ok as i removed it and replaced with a junker PLL from a scrapped rig then swapped the PLL into a rig i know works.. So thats out of the way. Im thinking the audio chip might be bad. Anyonr have any clues to help me out? Id like to get it going is it was always a talking radio when i used to chat with him on the air back in the 1990s..

[Tombstone (R.I.P.)]

Yeah, the audio chip will give you the problems that you describe. You can use a small audio amplifier and probe the chip with the radio on. If you get receive through the amp then audio chip is probably the culprit. Connect the ground of the amp's input to the radio's circuit board ground and use the hot lead of the amp to do the probing. I have a small disk capacitor in line of my audio probe in case I hit voltage. It's easier to just run your finger across the chip's leads. if you get a hum then the chip is ok. Just a hint, it could be something else, but that's where I'd start.

Tombstone

[zman]

Would the audio chip prevent the radio from keying up?

[Tombstone (R.I.P.)]

I believe it will. I had a 142 here awhile back and a bad audio chip made it not key up. Did you check the chip? There may be other issues though.

Tombstone

[Sandbagger]

Jan 24, 2010 1:41:50 GMT -5 zman said:

Would the audio chip prevent the radio from keying up?

No it wouldn't.

What you need to do is to see what exactly the radio does and doesn't do. Does it light up? Does it show receive signals? Do you hear a small pop or click in the speaker when you turn the squelch control up and down? Does the TX indicator turn red when you key it, or does it stay green? If you put an external speaker into the PA jack and activate PA mode, does it work when you talk into it? Have you tried a different microphone? If the mic wiring is broken, you may not have TX, and/or no receive audio.

If you know the mic is good, PA works, but the TX/RX indicator stays green, a likely cause is the PLL being unlocked. When this happens, it inhibits the transmitter from operating, and with an inoperative PLL circuit, there will be no receive either. I know you mentioned that you eliminated the PLL chip, but there is far more to the PLL circuit than the chip itself. Unfortunately the PLL is an extremely difficult circuit to troubleshoot remotely.

On the other hand, you could just have a bad MB3756 regulator. Check for 8V on pin 1 all the time, pin 6 on receive, and pin 8 on transmit. Pin 5 is your TX switch, and should be pulled toward ground when you hit the mic switch.

[zman]

I have tried the regulator and it seems ok. I swapped it for the regulator in my madison. So that is ok as well as the 8719 PLL. The PA function will not work. Basically the radio turns on and the channel indicator lights up as well as the meter but thats it...

[Sandbagger]

Jan 26, 2010 16:35:45 GMT -5 zman said:

I have tried the regulator and it seems ok. I swapped it for the regulator in my madison. So that is ok as well as the 8719 PLL. The PA function will not work. Basically the radio turns on and the channel indicator lights up as well as the meter but thats it...

Well, a bad audio chip will certainly cause the PA to not work. But the fact that you have no receive or transmit as well, leads me to believe there's some other common failure. If the radio used to work, but came out of storage like it is now, the most common things to fail are those pesky 10V electrolytic capacitors, so there may be a short across the 8V line (either the RX or the common) which is making the radio appear dead.

[zman]

Well its the audio chip after all. I installed the one from my madison (LOL my guinea pig radio!) and it transmits and receives but it wont transmit audio on AM (works great on SSB) and i dont have receiver audio. However the numbers on the chip are not like those on the chip from the madison. MB3758 from the 140 has the numbers 7813 and 616 but the MB3758 from the madison has 7801 and 613. So now im on the look out for a audiochip from a junker 140 or a 142GTL.

[Sandbagger]

Jan 27, 2010 0:35:01 GMT -5 zman said:

Well its the audio chip after all. I installed the one from my madison (LOL my guinea pig radio!) and it transmits and receives but it wont transmit audio on AM (works great on SSB) and i dont have receiver audio. However the numbers on the chip are not like those on the chip from the madison. MB3758 from the 140 has the numbers 7813 and 616 but the MB3758 from the madison has 7801 and 613. So now im on the look out for a audiochip from a junker 140 or a 142GTL.

Glad to see you are making progress, although there appears to still be a few issues there. I'm surprised you didn't notice S-meter deflection before, nor were you able to transmit a dead carrier. When the audio chip goes, it typically prevents audio, but transmit and receive should still work (although you won't hear it). So I'm a bit confused there. In fact, your symptoms now (TX on SSB, no audio on AM, and no receive audio) would be the classic symptoms of a BAD audio chip. Are you sure this chip is good? I'm also confused about the chip's number. Are you sure you changed the audio chip and not the regulator/switch? The regulator is an MB3756. According to the schematic, the audio chip should be a TA7222. I would also replace C118 and C114, near the audio chip as they are notoriously falure-prone 10V caps. If C118 opens, you'll have no audio even if the chip is good.

Do not worry about the discrepency in the 70xx numbers on the audio chip. Those are the date code for when the chip was manufactured. The first 2 digits are the year, and the 2nd two are the week. So a 7801 chip was made the first week in 1978. The other 3 digit number might be a lot number, I'm not sure.

[zman]

Your right. I did replace the regulator. The S meter does show deflection as shows a signal on CH 19. i just dont have an receiver audio or transmit audio on AM.

[zman]

The audio chip is good because i swapped the one from the madison with the one from the 140. So I know the audio chip and the 8719 PLL are all ok. I installed the regulator from the madison in the 140 and now the radio half works so i know the regulator in the 140 is bad. Now i need to find out why the radio has no receive audio or trasnmit audio on AM?

[zman]

radio shack has the caps but they are 35V.

[Sandbagger]

Jan 27, 2010 17:38:35 GMT -5 zman said:

radio shack has the caps but they are 35V.

Going higher in voltage rating is fine (just don't go lower). Those early 80's vintage Uniden-made radios seemed to have a common problem with the small 10V caps. They tend to either open, or nearly short, which can create havoc in all sorts of places, and they can be hard to track down without good test equipment to check out the radio.

[Sandbagger]

Jan 27, 2010 15:59:22 GMT -5 zman said:

The audio chip is good because i swapped the one from the madison with the one from the 140. So I know the audio chip and the 8719 PLL are all ok. I installed the regulator from the madison in the 140 and now the radio half works so i know the regulator in the 140 is bad. Now i need to find out why the radio has no receive audio or trasnmit audio on AM?

Ok, so the regulator was the cause of the radio being nearly "dead". That makes perfect sense. But now it appears to have the symptoms usually associated with having a bad audio chip. But you've replaced the chip with a known good one (I assume) and the problem remains. So there must be an open cap or some other component blocking or preventing the chip from amplifying the signal. At this point, it would help greatly if you had an oscilloscope and/or an audio signal generator to trace where the audio is going (or not). Shotgunning parts without tracing the circuit can become tedious exercise and potentially expensive. Although in a 30 year old radio, it wouldn't be such a bad thing to replace all the electrolytic caps, but there can be quite a few in a solid state rig.

[zman]

Well i just ordered a new regulator for the rig instead of using my madison as a guinea pig. The regulator from the 140GTL was installed in the madison and it didnt work. So i definetly know that part is faulty.
i installed new caps to replace the older 10 volt caps. So i just have toi wait until i get the new regulator. I dont own any test gear at all besides a watt meter and a multi meter.

[Sandbagger]

Jan 28, 2010 15:05:06 GMT -5 zman said:

Well i just ordered a new regulator for the rig instead of using my madison as a guinea pig. The regulator from the 140GTL was installed in the madison and it didnt work. So i definetly know that part is faulty.
i installed new caps to replace the older 10 volt caps. So i just have toi wait until i get the new regulator. I dont own any test gear at all besides a watt meter and a multi meter.

Well, tracking down problems in a radio is best accomplished using test equipment. Many people can repair common failure mode problems by shotgunning the most likely to fail parts and it'll work in most cases. But once the usual stuff is gone through and the problem remains, it will become much less intuitive which parts to change next.

You don't have test equipment, but there are a few things you can do to help narrow the problem down. You are missing audio now. So you can test the chip's function to some degree by simply touching the pins on the chip and nearly driver transistors with your finger or while holding a metal probe. You should hear a 60 hz hum, buzz or some sort of clicks or other noise coming out of the speaker if the amp is working. And by working back through the audio chain from the chip, you should be able to find the stage which isn't passing the audio. Of course, if the audio amp circuit is the problem, you won't hear anything.

[zman]

Ok, i finally got a new regulator installed and the receiver came back alive but i still dont have any receiver audio or audio output on am but it barks on SSB. Im still plugging away! Thanks for the help guys!!!

[Hawk "615"]

Have you replaced all the 10v elecrolitic casps?

[2600]

Should take a particular look at C92 when the receiver audio alone goes down. That one's rated at either 6 Volts or 10 Volts. Pretty prone to short and kill all receiver audio.

73