cobra 148 gtldx tx audio problem

[chris307]

first some details about the radio

pb-010aa taiwan

on fm the transmitted audio is very low and same with all modes
on ssb i can hear my self on my 847 (close prox to the 148) but the 148 shows no deflection on s-meter when talking but when the roger bleep comes in i get full delflection 30+ and the bleep can be heard 30 miles away loud but no audio can be heard due to the limited audio to drive the ouput stage

checked the voltage regs and they are fine
radio put out full tx power on fm and will put out full tx power on ssb when the bleep comes in

people can hear me on fm but they nead to turn up their volume flat out and then its only just

removed tr24 to check it aint gone bad and shorting the audio to ground and this made no difference

checked the mic and this is working fine

any thoughts

thanks for looking

[Brian Morris]

Hi,

If you can hear the roger bleep then this means pretty much all of the audio chain is working. Check the mic, the mic socket pcb (check caps and chokes), check the mic gain pot and it's associated wires, and check that R224 is still 10k. There's not really anything else it can be.

Regards,

Brian.

[chris307]

mic gain pot was shorted to ground so got the audio back

ssb audio was realy bad so checked the driver and final lol

had a 2078 as a driver and a a 2166 as a final

changed to 2166 driver and 1969 final and set the bias and the audio is great now

need a replacement for a 2sa1012 maplin dont stock the tip24 any alternitives from maplin?

[Brian Morris]

Maplin don't stock much in the way of components these days, but a TIP2955 should do the job, but not quite the same case, so you might have to do some frigging around to get it in.

Regards,

Brian.

[Brian Morris]

Or, get a TIP42 from Bardwells, cheap as chips!

http://tinyurl.com/6j4gc7

Brian.

[chris307]

replaced the 2sa1012 with a tip42c and the fm is back fine
thanks for the link

fm audio is fine but ssb is realy bad

sounds realy bubbly and distorted like its getting badly overdriven
if i turn the mic gain down almost off or take off the test board out i can moniter the audio on anouther set and its ok as soon as i turn up the mic gain it goes bubbly wobbly non linear sounding exactly like the power lead is too long and thin a mistake i made years ago any thoughts?

[Exit_Thirteen]

Provided you reinstalled the modulation limiter (TR24), I'd check the MB3756 IC chip next to the audio chip... I've had many Cobra 148's and other MB8719 PLL boards that were real gargly/warbly on SSB only to find out that the MB3756 voltage regulator IC was bad.

If TR24 is installed, I'd check that regulator IC next.

Hope this helps!

-Rick (Triple 7)

(edit) For the life of me, since I haven't worked on a DX model of the 148 in ages, I can't remember if it has that MB3756 voltage regulator IC chip in it or not...if it does not have that IC in there, look for a 3 pin regulator instead, could be labeled LM7808 (8 volt regulator) or A473 (2SA473). Sorry if this confuses you, but the point being that I'd look into the voltage regulator, that can cause the gargle/warble on SSB.

[Rick]

Exit_Thirteen

The Cobra 148GTL-DX is not anything like a Coabr 148GTL you have in the USA. The Cobra 148GTL-DX chassis is known as the PC879 and PB010 and was designed by Uniden back in the 70s and was the forerunner to most of the modern generation of DX radios today.

[Rick]

As for bad SSB TX audio, check the chassis screws also check the final and driver bias, C150 can cause some odd problems in this area.

[Brian Morris]

Have you set up the bias for the driver and the final in accordance with the service manual? Check R263 connected the base of the final, sometimes this burns out and goes open circuit.

Brian.