Possible alternator issue on my 75, stops charging when warm..
 

Ok, here are the symptoms. Charge light is on when car is first started cold, takes a blip for it to go out, then my volt gauge goes from 12 to 14. I understand that's kinda normal on some 911's, takes a blip to get the rectifier to start current flow in the alt or something..

Anyhow, it's always been fine, replaced the battery with a new Optima this year and it's been normal for several months, but now out of nowhere after 30 mins of driving my voltage drops to 12 and never recovers. The charge light never comes on in the dash, but my gauge stays at 12 no matter the revs. Turn on lights and it drops to 11.5.

Testing on the battery when running in this condition shows 11.8, turn car off and battery recovers to 12.2. A little low, but it's been running on battery, so that makes sense.

I trickle charge it to be safe, battery back at 12.7, and car starts and charges fine at 14v, then 30 mins later problem is back. Ideas??

Since it works to start with, First two things that come to my mind are intermittent electrical connection or alternator brushes. Things expand when they warm up. I’d clean up the connections and if the alternator is old maybe time for a rebuild with new brushes and bearings.

A bad alternator can ruin your new battery. However, pulling the alternator and cleaning the grounds, etc. all the way under the car (alt to engine; engine to body; etc.) is a very good idea. Do not trust any of your ground wires - they can hide a ton of corrosion under the outer vinyl covering, so have a very close look or consider age and replacement.

Plus, you get to polish your engine fan! What could be better than that?

Still have the original style mechanical remote voltage regulator beside the CDI?
Sounds like it may be heating up and failing.
A lot easier and likely cheaper to swap that out with the appropriate electronic version than pulling the alternator.

Thanks all. I'll take a look tonight for that regulator and check all the ground straps.

My dated 74 alternator finally failed after near on 50 years of service. Brushes like mentioned above. Could’ve jut replaced them but thee was a lot of wear on the shaft so I replaced alt.

Are you using a voltmeter at the battery to test your voltage? If not, I would confirm readings with a good meter.

Another possibility is if you're using a voltage meters that plug into the cigarette lighter to check charging voltage it could be defective (link below). My '77 has the same symptoms as you describe and I use one of these meters. if I remove the cig voltage meter and then reinstall it, it shows charging voltage and all good. This reminds me to buy a better one.

https://www.ebay.com/itm/182161792450?hash=item2a69b05dc2:g:8KIAAOSwvg9XVg2 4

I had a related issue where the diode solder had gotten old and would move when I pulled on the diode lead. It wasn’t immediately visible but would stop charging when the car heated up and the solder expanded. I removed the old solder on all the diodes and redid them with fresh solder. This solved the problem.

Another time a brush was sticking in the holder. Both times the problem was intermittent. Also check that one of the brush springs is not broken although if it was I would expect the problem not to be intermittent. I would gently pull and confirm the diodes are secure and the brushes move freely.