DJ-C5 photos below
On both radios, first remove both the top and bottom covers (3 screws on each side).
On the front the screws are hidden below the plastic keyboard cover. The black marker diamond indicators at the top center and lower left and right show their location beneath the cover.
On the back the screws are clearly visible.
Remove the built-in telescoping antenna.
Route RG-174 thru the antenna hole and along the front left edge of the PCB. Strip and separate about 1/2" of coax shield and center conductor.
Remove the housing U-bracket nearest the Li battery connector from the radio (pull the plastic frame out a bit), tin the inside front corner of the bracket and sweat the coax shield end to it, being careful to keep solder out of the tapped holes. Also keep the sweat joint as low-profile as possible to avoid interference with PCB components.
Then put the U-bracket back in place and terminate the center conductor as shown in the pix for the DJ-C1 or per 2 above.
If you can find a coax connector that mates to the one on the DJ-C5 PCB, then the coax connection could be less tedious. But it's on the opposite side of the PCB from the hole where the original antenna came out. And the MMCX connector I got from Mouser didn't mate.
Finally, use a small Ty-Rap to strain-relieve the coax just inside the hole where it exits.
Pretty much the same process works for the DJ-C5, except:
1. There's no 3-pin header to access the 50-ohm point. Instead, there's a mini coax (MMCX?) surface mount coax connector on the back side of the PCB. The disconnect from the matching network is a solder bridge between that connector and the edge of the PCB. Use solder wick to completely remove that bridge - the gap between the pads is VERY narrow - maybe 10-15 mils!
2. There's no convenient hole to bring the center conductor thru from the front side of the PCB to the back -- so find the front-side trace that goes to the coax connector, clear off some solder mask and tack the coax center conductor to that trace.
alinco_djc5_IMG_077.jpg - Closeup
In 0377, look for the two solder pads with a gap between them NNW of the little coax connector. The solder bridge between those pads connects the 50-ohm antenna I/O port to the matching network for the built-in whip that gets removed - it serves the same purpose as the jumper on the 3-pin header on the DJ-C1.
IMG_0373 shows the termination of the mini coax to the 50-ohm point on the front side of the board. Like the 2m mod, the coax shield is sweated to the inside of the mounting bracket. IMG_0374 is a wider view of the front side showing the coax routing thru the hole in the top vacated by the whip mount.