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Hi Jim
I have a 169 T337D as well. I recently rebuilt the front alternator, and due to the fact that the field circuit was shorted out also replaced both stock regulators with the Zeftronics variety. I also replaced the battery with a Concord platinum series as the Gill I had was marginal at best capacity wise. Finally I also bought a battery minder and have been keeping my new battery connected to this when not flying. After doing this I have also seen the voltage jumping up and down as you describe, and the OV light flashing on and off. I also have a VI meter so I can see the actual voltage, not just the light and it seems to be similar to what you are seeing of 28-30V. Finally I also see the alternator lights flickering indicating the alternators are offline, but I see current so I know they are not. All this appears to settle down and stops completely after about 10 minutes of flying. I believe the reason for this is that the Zeftronics regulators regulate to 27.5V. This is close enough to the battery voltage for a "just off the charger" battery that with the forward drop of 0.7V across the alternator isolation diodes the system doesn't charge the battery all the time, leading to the alternator offline flicker indication. I think the regulator doesn't like the fact that the battery is so close to the regulation voltage and this causes the voltage to jump around as it seeks to get charging going. After a few minutes of running current off the battery for part of the time, the battery voltage drops enough that current flows steadily into the battery but doesn't increase the battery voltage to the point where this won't happen. This leads to the end of the flickering and OV behavior.
You don't mention if you did anything besides replace the regulators, or if the problem is persistent or just around at start up. Anyway thought Id pass this along as its similar to what you are seeing.
Ben
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