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No Mic Audio Cobra 148 GTL

HavaV10

HavaV10

Active Member
May 24, 2023
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Lake Havasu City, Arizona
Picked up this Taiwan made Cobra 148 GTL the other day. Very clean inside and hasn't been chopped up. No extra channels either. It has a issue, no audio going through when the mic is keyed up. Also the Green/Red led does not turn red when the mic is keyed up. Also no audio coming out of the PA jack. Everything else works perfect. There is good 4 watts output on AM. Maybe someone could tell me a component to check. Maybe TR23 and TR24? Both should be C945 but in loc TR23 there is a C458 in there. In the photo I'm keying up the mic on SSB no red led TX light. Thanks for any help.
 

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Isn't the c458 an equivalent for the c945? Not saying it's not the problem, but that might be the reason someone put it there.
 
Next question is whether the PLL is unlocked or not. If so, this inhibits both the red transmit LED and any transmit power.

Do you hear the channel that's selected? If so, the receiver is working and the PLL both. If the PLL is not locked onto the correct receiver frequency, this will lock out the transmit side.

If the PLL is locked, make sure pin 8 of the MB3756 chip has 8 Volts DC on it when you key the mike. If not, the problem is with this chip or one of its inputs. If pin 8 is coming alive properly when you key up, it's time to sniff for a weak low-level transmit signal.

A spare radio with a coax jumper makes a RF 'sniffer'. Back the sleeve of the jumper's far end over the plug body. The center pin is now a half-inch long 'sniffing' antenna. See if the second radio hears anything when the mike is keyed.

If so, the radio is producing a transmit signal and losing it downstream from the transmit mixer circuit.

If you can't even hear a weak signal waving the sniffing antenna around the S042P transmit mixer chip, this points to trouble upstream from that part. This chip almost never goes, bad. Two signals feed into it. If nothing is coming out of it, that means one or both of the inputs is missing.

73
 
nomadradio said:
Next question is whether the PLL is unlocked or not. If so, this inhibits both the red transmit LED and any transmit power.

Do you hear the channel that's selected? If so, the receiver is working and the PLL both. If the PLL is not locked onto the correct receiver frequency, this will lock out the transmit side.

If the PLL is locked, make sure pin 8 of the MB3756 chip has 8 Volts DC on it when you key the mike. If not, the problem is with this chip or one of its inputs. If pin 8 is coming alive properly when you key up, it's time to sniff for a weak low-level transmit signal.

A spare radio with a coax jumper makes a RF 'sniffer'. Back the sleeve of the jumper's far end over the plug body. The center pin is now a half-inch long 'sniffing' antenna. See if the second radio hears anything when the mike is keyed.

If so, the radio is producing a transmit signal and losing it downstream from the transmit mixer circuit.

If you can't even hear a weak signal waving the sniffing antenna around the S042P transmit mixer chip, this points to trouble upstream from that part. This chip almost never goes, bad. Two signals feed into it. If nothing is coming out of it, that means one or both of the inputs is missing.

73
Click to expand...
thanks, yes receives fine on the channel selected. I changed out TR22 and TR23 with no change. There is barely I mean barely audio coming through the mic. Had another radio next to it on the same channel. Super super weak audio I can hear my voice barely but doesn't move the S/RF meter at all and the red TX light has yet to light up when I'm keyed. I will check pin 8 on that IC4 Thank you for your time.
 
Pin 4 is ground. Should read zero. Makes it look like the transmit-side light is bad.

Time to check C18. Should have around 6 Volts DC on the positive side. It's rated at 10 Volts. That makes it the highest statistical risk among old electrolytic caps. When it shorts, all power is lost to the mike amp.


73
 
nomadradio said:
Pin 4 is ground. Should read zero. Makes it look like the transmit-side light is bad.

Time to check C18. Should have around 6 Volts DC on the positive side. It's rated at 10 Volts. That makes it the highest statistical risk among old electrolytic caps. When it shorts, all power is lost to the mike amp.


73
Click to expand...
changed out C18 still no transmit audio. Also changed out all the 10V and 16V caps still the same. Might just change them all out. Think I'm going to replace the CB/PA switch next.
 
nomadradio said:
Pin 4 is ground. Should read zero. Makes it look like the transmit-side light is bad.

Time to check C18. Should have around 6 Volts DC on the positive side. It's rated at 10 Volts. That makes it the highest statistical risk among old electrolytic caps. When it shorts, all power is lost to the mike amp.


73
Click to expand...
I checked voltage at C18 and yes there is 6V positive on both receive and transmit.
 

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