Have a Tristar 120 that gives no TX/RX indication on low channels. Works fine on regular and upper ch. Swapped with another Tristar 120 and it works fine. No issue with mic, ant. or wireing. Could it be the ch switch?

only the CB band to TX on.. you may RX on any frequency.
ch.1-ch.40 am and SSB if it had it.. are the FCC allowed channels. yes there are people on those other bands. user beware.

is it a total no TX. or just not getting out very far.?
could have a broken mic wire. or the CB / PA switch may be in a wierd spot try moving it.. are you getting the Correct RX frequency on the channels.. ie are you getting channel 20 on acutal ch 20. and if you switch to ch. 6 are you getting ch 6? if you could explain more..

MAKE SURE R-F GAIN IS TURNED UP FULLY AND ANTENNA IS NOT SHORTED

without more detailed info, quick suggestion - thermal fault, if all TX audio works when in this fault condition, it may be fair to assume audio IC is ok and check electrolytic capacitors and dry joints.

I'll bet that the A.W.I. is out of adjustment. give it a little twitch of a turn and see if the red light goes out.

May be bad antenna match on that end. Check SWR at CH 1 and at CH 40. If quite different readings you will need to adjust the SWR and maybe the antenna itself.

to get this fixed is going to cost you more than the radio is worth. Your decision.

But thats how a walkie talkie works, it stays in Receive Rx mode UNTIL you press the PTT button,, then it Transmits Tx, and when released it goes back to Rx, receive?

Check xtal X1 (7.3450 MHZ). It's in the am osc. I had exactly the same problem. I live in a highly corrosive coastline area

Same problem for me also blocking on the tape amateur 144/148 MHz not of emission nor of green reception led does not ignite malgres the modifs. Have a solution thank you