Cobra 148GTL keyup AM, SSB with no Audio

[PETERpH]

I need some help: I have a Cobra 148 GTL old radio; it was working great until, I increase the Voltage from the power supply by accident then a boom noise came from the radio. (WHEN OFF) I was able to find the diodes that burn out in the radio and replace them, it is back ON, I could not hear anything trace the voltage and find out that, I burn the TA7222 Audio Amplifier I was able to replaced then the RX is back. However as soon as I press the microphone, red light come-ON watts on AM are being reflected on the watt meter. But I have no audio (modulation) SSB I can hear a crack on external speaker as soon as I press the mic but I have no audio. I have check most of the transistors PNP and some NPN with no luck finding the problem. I have no signs of a dielectric capacitor expansion. PA doesn't transmit a voice either. I do not have a frequency counter to check PIN 3 for 10420MHz on the IC MB8719. However I do have an oscilloscope with little of knowledge on CB's I do see check points on the schematics but I am not sure what to look for. Any help will be appreciated.


Thank you.

Peter.

 

[Dmans]

PeterpH,
Search the attachment below. It is the 148 schematics from a Sam's manual. it will show plenty of voltages on the schematic to help you.

Good Luck!

73
David

 

[PETERpH]

PeterpH,
Search the attachment below. It is the 148 schematics from a Sam's manual. it will show plenty of voltages on the schematic to help you.

Good Luck!

73
David

Thank you, I will look into later on today. I am assuming I have to adjust the TX once the TA7222 Audio Amplifier was replace? I don't have a Frequency counter I will be calculating the Frequency respond from the check points.

Thanks again

PETERpH.

 

[Dmans]

I would check all voltages to and from the Audio Amplifier (as a starter) compared to the Sam's manual. Also check for solder bridges around the TA7222 .Brighter minds may chime in with more info.

73
David

 

[Robb]

Is the carrier high (8-10 watts) in AM TX?
If so; then check the AM regulator for failure.

 

[nomadradio]

Check C18. When it fails as a short, the mike amplifier shuts down. Simply unsoldering one leg from the foil trace will let you see if you get mike audio with it unhooked. We use a 330uf or 470uf cap to replace it. The original 47uf value proves to be too small when the carrier is turned down, causing a feedback squeal in the AM transmit audio.

73

 

[PETERpH]

Thank you for all of your replies, I have trace the voltage and mic signals. However, 99% of the Voltage are high in average of 1v 1.8v, I have order a kit of dielectric capacitor to replace them all. I found a short on a TR-37 XMT and a high voltage on a TR-38 XMT almost 3 time more of what the specs are (TR-28 is the AM driver). However, I thing I will look deeply once I replace all the Cap's to verify TR-38 "XMT" is no over powering the bias.

BTW all the voltage in and out from the IC6 AF POWER AMP are good including IC 4, IC 3. Are (ok) I am not happy with a high voltage in some areas, I hope ove I change the Cap's the voltage will be more stable.

I will post more information once a replaced the parts and do more testing.

Thanks again.

PETERpH.

 

[Dmans]

Good job. Sounds like you are headed in the right direction. Keep us posted.

73
David

 

[PETERpH]

Good Evening.

The radio is back on and I HAVE AUDIO, I am still waiting for the Cap's to be replace. Once I replace the TR-38 I have audio modulation.

I have being checking all frequencies and I notice that Channel (1) is on 26.965 MHz then I switch to Channel (40) the frequency is on 27.325 I am missing 8 channels According to the channel chart is channel (32) and Channel 40 it should be 27.405MHz

I have check all the TP points as the alignment required and I did find 9.56v on TP-10 when I try to regulate the voltage from L 21 nothing happened

**** I start checking the channel frequency from channel 1-4 everything is good once I switch to channel 5 the frequency change to channel (2) 26.975 channel 6 is like in channel (3), channel 7 goes to a "26.995" this frequency is NOT on the (40) channels. Channel (8) also switch like is in channel 2 same is channel (5) once I switch the channel (9) is the same is channel (3) etc check the chart below.

Do you guys believed that L 21 has to do with this?

Channels

1.- 26965 11.- 27.005 21.- 27.135
2.- 26975 12.- 27.105 22.- 27.145
3.-26.985 13.- 27.115 23.- 27.255 *******????
4.-27.005 14.- 27.125 24.- 27.155
15.- 27.135
5.- 26.975 16.- 27.155 25.- 27.165
6.- 26.985 17.- 27.165 26.- 27.265*******????
7.- 26.995 ***?? 27.- 27.275
8.- 26.975 18.- 27.135 28.- 27.285
9.- 26.985 19.- 27.145 29.- 27.295
10.- 26.995 20.- 27.165 30.- 27.305

31.- 27.315
32.- 27.325

33.- 27.295
34.- 27.305
35.- 27.315
36.- 27.335

37.- 27.295
38.- 27.305
39.- 27.315
40.- 27.325

 

[PETERpH]

Update from last night.

I was able to recover all the channels from 1-40 they are all correct under the frequency chart. However, it does not change the PTT from Channel 13 to channel 2 no audio....

I am little frustrated I really don't know if is word spending so much time in a radio I had for over 15 year.

 

[nomadradio]

When Alice goes down the rabbit hole she finds herself in more trouble than she bargained for.

One reason that a tech who uncovers multiple faults hoping for a 'simple' repair uses the expression "Down the Rabbit Hole".

What the effort is worth is entirely up to you.

The combination of excess voltage and decades-old electrolytic capacitors could turn into a list of parts that cause trouble as they break down one by one.

Could.

73

 

[Handy Andy]

Update from last night.

I was able to recover all the channels from 1-40 they are all correct under the frequency chart. However, it does not change the PTT from Channel 13 to channel 2 no audio....

I am little frustrated I really don't know if is word spending so much time in a radio I had for over 15 year.

When you have this condition...STOP - before you can damage the PLL from excessive voltage on the input pins.

Got a Truth chart?

Something that looks like this - I've got two, but I want to point out something between both...

For the 2816 PLL

​

For the MB8719...



Might want to investigate that and compare it to the true logic levels you're encountering.

It sounds more like you have a bad channel switch.

Or a wrong one - review the above carefully. I've seen someone try to take a Cobra 29 channel selector and put it in - in place of the OEM one on a 148 - they don't work - at least, you shouldn't..

There is also the condition of shifted pins off by one - condition - seems it rolls a bank of frequencies all the time until the logic pinout of the BCD changes to the next set of pins - kind of like messing with a 8 pin Switch header, to set program address in the older days of booting and IMSAI / IBM 360 machine coding days.

OR the wiring from the switch to the board, isn't right, shifted over or crossed - in any case slowdown you are encountering a problem that makes the radio stop accepting input PTT or otherwise, because the radio thinks it's out of lock so it won't transmit. You have got a receive to something, but it's not going to let you transmit until whatever condition that is causing this programming problem is cleared.

 

[Dmans]

I have a Cobra 148 GTL old radio; it was working great until

Andy,
Sounds like Peterph may have owned this unit for some time.

I might would begin replacement of all electrolytics and a full alignment.

73
David

 

[Handy Andy]

Andy,
Sounds like Peterph may have owned this unit for some time.

I might would begin replacement of all electrolytics and a full alignment.

73
David

Yes, I caught that, but one thing I did see - is something I've had show up at the shop before...

Wrong channel selector used...

Programed pins fall into place, but that "4-th detent" - tells me he will have a lot of issues with frequencies - the PLL not keying up, but the pins seem to work - scenario.

Not too many notice that the 29's 2816 or the '66's SM5126 - may use the same selector because they use the same ROM code - but the MB8719, because you use the OTHER SIDE of pins, they are mirrored.

The 8719 has Hex (8 pin) addressing, but the 5126/2816 use OCTAL or Base 6 (6 pin) between both - so the operator only needs the 6 but there's the "detent" used on the shaft to "set" the address to load - so his channel selector doesn't have enough pins to make this functional...or at least the Binary of it - note the Pin 16 of the 8719 versus Pin 1 on 2816/5126 - those pin trains are 01011 01011 - the 2816 is 01010 - the repeating sequence is what I'm seeing...

I'll let this thread go back to you - I know he's working with a lot of age-related problems - but the frequency should shift as normal - it isn't - so I'm suggesting the Channel - if he can program those pins MANUALLY - and get the Channel to work - the PLL is ok, the Selector is not the one programmed to work with the 8719 - even though the pins do seem to make it functional.

 

[PETERpH]

Update. I received the caps and replaced them, voltage still a little high, but stable. All the channels that were not working, now they do.
I can see modulation on the Oscilloscope BUT I can't hear anything ONLY white noise. (Soft white noise). I unplug the antena to the dummy load and is the same white noise.
I have check from the TA72222 to TR-43 AGC AMP CI-183, CI-187, TR-19 SSB DET AM AF AMP. I removed all of these ceramic capacitor to check them of the board IC-72, IC-75, IC-71, TR-13, TR-12... everything looks good.

I purchased a Tektronix CFC250 frequency counter for this project and other testing equipment. I recheck everything from what I check last week with the Oscilloscope, everything looks to be normal. At this point I am looking forward to fix it and continue to learning and understanding more of electronics....


Does any have an idea of what could be?