Renix no charging issue

This morning I discovered I have a charging problem and have spent the whole day so far trying to diagnose. I have searched and didn't find anything that I haven't already addressed. hopefully someone finds something I missed.

Here is all the relevant data:
1990 XJ limited, ac and abs removed, auto trans
I previously had a Cb radio which I changed out for a UHF radio last sunday and went wheeling. For the first part of the day the uhf radio worked fine but later in the day I could hear the others but they said I was really broken and couldn't understand me. The UHF radio is wired in where the CB was. There was a lot of rain and mud on that trip but other than the radio the jeep ran fine. We got home in the dark, I drove about 30kms on the highway with headlights on, no indication of electrical probs.
Jeep sat until today cranked a bit slow and I noticed the volt meter needle wasn't where it usually is but it did start. drove into town (just a few kms) and no change - I was not getting any charging.
checked output voltage right at the alt and only got batt voltage, and dropping - my alt was not putting out any juice.
I tested as per the FSM and all the tests were good (windings, stator, etc), brushes had lots of length, slip rings were solid and clean. I cannot find anything wrong with this alt.
As per the FSM, with key ON, I have 4 v at one of the wires on the plug, and batt voltage at the other.
Grounds checked and ok.
I don't know where to go from here, other than to buy a new alt just to have a 'known' good one.
Ignition switch?

IIRC, you should have battery voltage at two wires in the plug with the key on. I'd look for the resistance in the 4v wire-- and it will probably be the one that goes through the ignition switch.

I think that the alternator on your year has the voltage regulator built inside. My '88 alt died after some heavy water crossing. I just replaced the alt and things started going again.

Suggest you remove the alt and take to a Pep Boys or similar retail outlet. They often can bench test these and they do it free. I'd bet on the alt as your problem so it's likely you will be taking it off eventually. If the alt tests OK then your problems get really deep.

And a simple test just to be sure that the gauges are telling the truth is to turn on headlights then start motor. If headlights get no brighter when motor is going then it's a good indication that there is no charging.

well, I bit the bullet and bought a rebuilt alt from Can Tire - Champion Brand, 100 amps for $162.

Installed and working fine.

I'm guessing it was the voltage regulator (internal). The FSM does not give any procedures for checking it but there is one comment something like "if all the tests check out fine, then suspect the voltage regulator"

I have never had an alt fail like that on me - it has always been something I could diagnose and repair/replace - brushes, diodes, etc.

After I got it running I was on the UHF radio and noticed the dash voltage meter noticeably dips when transmitting. That is some powerful radio - we have tested them out to about 8kms and they are very clear. Maybe my voltage reg was on the way out and having the blower motor, taurus fan and using that radio just pushed it over the edge.

It's to bad we can't have radios like that in the US.