TRC-458 common problems:
1) C179 2.2uf 16v tantalum cap. Replace with a 2.2uf 25v electrolytic. This cap is often found failed or failing. Replace.
2) Ribbon cable from board to channel selector/LED channel display. At the board end of this cable where it slides into the receptacle is often dirty and corrosion is present. Clean off the cable end with a ScotchBrite pad or fine steel wool until all corrosion is removed. Must use care so as not to ruin this delicate and hard to replace cable. Clean the socket with very fine sand paper (500 or 600 grit) - lightly sanded - and douse with electrical contact cleaner and blow it out with compressed air.
Make a wedge of a thin piece of flat scrap plastic (about ~.030" thick) with the same length of the inside of that socket. Slide the cable into the socket and then gently wedge that piece of plastic behind the cable into the socket in order to force the cable to make contact. Have seen this issue with the four or five 458's that I've worked on and is the only fix that is simple, easy, and effective.
If this cable doesn't have proper contact to its socket; you will have nothing out of the radio. Receive and transmit will not work. Common problem.
3) Clean the relay contacts by carefully removing the relay cover and spring and careful not losing the spring when you remove it. Use a clean paper Q-tip with the cotton end removed with electrical contact cleaner between the six contacts on the relay body. Do NOT use a file OR sandpaper, as these contacts have a special surface on them that must be protected. Clean the floating contacts with electrical cleaner and a clean paper card - like a white matchbook cover. Reassemble. If this is damaged, it is hard - but not impossible - to replace.
4) Check with CB Tricks to see if your radio has the Service Bulletins issues dealt with. The Cobra 139XLR is practically the same radio and many of the Service Bulletins won't be necessary because the 458 had them dealt with already. But I'd check to be sure.
Just finished rebuilding a TRC-458; pictures and story forthcoming . . . film at 11 . . .