148GTL-DX - how to install talkback?

[Cutlass327]

I have the MK2 version, with the PB010AB board. It doesn't have a PA jack, but the CW key jack instead.

I've tried across speaker ground to chassis ground, RX to ground, ext speaker ground to chassis, and no luck.

I plan to put it on a switch, as I only want to use it to monitor setting mic gain and such and then turn it off. IS it possible to to this?

 

[999]

Cut D80 and switch it. Straight to the right of IC1.

 

[999]

 

[NZ8N]

I have never been a proponent of "cutting" anything, there is always a better, neater way to do it.

Ground the trace under the red circle, this disables the Mute circuit on the speaker amplifier (TA7222) when in transmit, this is the same thing that TR39 does in CW mode. This line can be switched too.

Add a resistor (blue lines) between Pin 1 of the Mic amp op-amp output, to Pin 4 of the TA7222, between 1 Meg (lower volume) to 100 K (higher volume). This works in FM and SSB modes too.

 

[Cutlass327]

Thanks guys!

I have searched for weeks for this info, and also how to unlock the clarifier... Between info that doesnt work or for the Usual 148GTL, I have had no luck...

 

[Cutlass327]

View attachment 37158

I have never been a proponent of "cutting" anything, there is always a better, neater way to do it.

Ground the trace under the red circle, this disables the Mute circuit on the speaker amplifier (TA7222) when in transmit, this is the same thing that TR39 does in CW mode. This line can be switched too.

Add a resistor (blue lines) between Pin 1 of the Mic amp op-amp output, to Pin 4 of the TA7222, between 1 Meg (lower volume) to 100 K (higher volume). This works in FM and SSB modes too.

So you ground that one trace to "activate" the talkback and then use the resistor with it to control volume?

 

[NZ8N]

Exactly correct. It is that simple. And, the Galaxy and other radio boards patterned after this original 148GTL-DX design work the exact same way.
The reason the speaker ground resistor trick does not work is because the speaker is already grounded, not through the Microphone like a conventional radio is.

Now, about that Clarifier, remove D84 in front of VR6 (feeds +8v TX), and find the high side of the Fine Clarifier pot, not sure if it is the Blue, Orange or Red wire , I think maybe Red, follow it down to the main board, it should have +8v in RX only, move it to a +8v always trace. It might be the Blue wire, not sure.



http://www.cbtricks.com/radios/cobra/148gtl_dx/graphics/cobra_148_gtl_dx_sm_pg35_36.pdf

I haven't seen a Cobra 148TL-DX PB-010 radio in a long time, so my memory might be a little rusty.