Johnson 70 hp not charging

"My 1979 70 hp Johnson is runn

"My 1979 70 hp Johnson is running well now except there is a problem with charging. The console guage indicates around 12 volts, my volt meter measurement on the battery read 12.75 with engine off, then 12.57 with engine running. There is not change in the console guage if I increase the RPMs in neutral. The only other odd thing going on now is that the tach reads 10 rpms with key off, then with key on it drops to zero. With engine running it is operating normally. My question is this, how can I test to see what exactly is causing the problem with charging? Is the tach issue with key off an indication of another problem?
Thanks!"

"u need to test,the rectifier.

"u need to test,the rectifier...very common,fault,easy,cheap fix...look up in the search,on here,joe reeves tests..."

"Thanks. By the way, is this

"Thanks. By the way, is this test done with battery disconnected or connected?"

"I just put a universal full w

"I just put a universal full wave 40 amp rectifier from radio shack on mine a couple of months ago.
It was about 15 bucks and is twice as strong as the original.
Works perfect!
By the way, number one rule to any one working on a motor----
Dissconnect the battery first!"