What breaks on the EF Johnson 5100 and how to fix it

Here are the things that break in the 5100. I’ve handled several dozen 5100s (900, VHF, and UHF) and these are the most common ones.

• The UI board (the one with the display, key contacts, and connectors) sometimes goes south. There’s really nothing you can do about that. Causes issues like “appears to stay keyed constantly” and random errors without error codes. Toss it and replace.
• The ribbon connectors themselves (between the knobs, buttons, accessory conector <=> UI board) sometimes get dirty. Causes issues with programming, knob functionality, or button functionality. Clean and re-seat connectors with isopropyl alcohol or a good T-Shirt.
• The big plastic connector between the “RF Deck” and the “UI board” sometimes doesn’t make good contact. You will see “DSP Fail” or some other type of failure on boot. These can be sporadic. It can splinter pieces of plastic off (just remove those bits and discard), get dirty, or just not seat correctly. Clean that off, or swap RF Decks with another radio and by chance the two will mesh and your errors will disappear.
• The RF deck itself can go on the fritz. Bad final PAs lead to a few 1/10,000s of a watt of power output. One once gave me a horrible buzz. To fix, toss the RF deck and get a new one.

If you come across anything not listed here, feel free to comment. Also, my repair methods are to gain as many usable units out of a box of 5100s, not to make a single unit work at all cost.