'80 cb750f not charging battery

[sch071]

Just started digging into this problem on my cb750. Right now I am charging battery that has been drained. Before I do some tests when engine is running I cleaned as many connection as possible, did some TLC on fuse box that does not look bad at all but I did sand all connections anyway and checked fuses.
Checked that 5 wire plug with a 3 yellow wires that come of stator ( multimeter set on 200 ohms) and I have readings like 00.5 between all of them (I assume this number is OK since readings should be .41 to .51). Checked rotor and got same 00.5 readings and I think darn thing is fried (should be 4-6 ohms, readings should be 04.0 to 06.0 when multimeter is set to 200 ohms). I know this issue has been answered over and over but any input by someone that already have experience with this matter is more than welcome.
P.S. I will fix darn thing eventually and I will ride this bad boy first chance I have, until then,  well I have to clean all this snow outside.

[Re-run]

Have you checked the battery while the bike is running? If the battery is old, it may be just not able to hold much charge anymore. Once the batt is charged, get it on the bike and start it. Rev to over 3000 rpm and check the voltage. You should be hitting around 14 but should be under 15 and above 13.

[sch071]

Yes, that is my plan as soon as battery is fully charged, what scares me is multimeter readings of the rotor.

[LukeM]

Somewhere in the Repair section are some tips on how to check the charging system.
Some things to consider:
When the engine is idling, there's not enough voltage/current coming from the system to charge the battery.  You need to have the engine above 2500-3000 rpm before you'll see it.
Typically, when things are working well, you'll see 14 - 14.5 volts at the battery terminals when the engine is running at that rpm.
If memory serves, the resistance between the 2 rings on the rotor should be between 3 and 6 ohms. Less than that, and there's probably a short in the rotor, rendering it useless.
Get the FSM from the home page of this forum, turn to chapter 16, and go through the checks on the regulator, rectifier, alternator and battery.
One more thing: check the 3 yellow wires coming from the alternator, going to the regulator/rectifier.  They should be nice and clean, with no burn marks. This is a common problem with this era of Japanese bikes.  You can bypass the connector and solder the wires directly if the molex connector looks burnt or warped.  Happened to me on my Shadow, and soldering the wires together fixed that problem.

Take it step by step, don't get frustrated, and yell for help if you get stuck.  We'll help if we can.
Luke M

[sch071]

I just had a chance to fire up bike ( after I fully charged battery) and after engine warm up (  charger connected all the time), removed charger and measured voltage @ 3000 rpm. 12.7V, @ 4000rpm 12.9V, @ 5000rpm 13V. Removed negative lead @ idle, bike was running rough, and as soon as I tried to rev engine, just stopped. Since rotor ohms already did not mach what suppose to be, I am about to start looking for one good used rotor. Anyone??
Guys, thanks for your support and advice, one of the reasons I did get this bike is support on this site.