Alternator or regulator trouble?

Good Day members..My question regards a problem with either my regulator or alternator.

Car is a 72 Polara ex-CHP 440 with the large case alternator (presumeably the 100 amp). The car will be running fine and Ammeter shows above half (charging)..then randomly the lights all grow dim and the Ammeter shows below half (not charging)...at random again everything goes back to normal?
Once the guage when below half I pulled the positive cable..the car quit..I was always under assumtion that it would continue to run from the alternator power?? Is this correct and the alternator is bad?

Is this a symptom of a bad voltage regulator or an alternator diode getting ready to quit??

Any help would be great!

Could be something simple such as a brush worn out. Also, check the connector at the regulator. They were known to get loose and not make connection.

Quote:

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Is this a symptom of a bad voltage regulator or an alternator diode getting ready to quit??

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not the reg & I wouldn't think a diode could act like that. Loose connection in the field circuit or loose connection inside the alt. I'd clean the bulkhead and all field terminals & if still acting up I'd take the alt in & have them spin it up & see if it will stop charging while on the tester