check them wires!
Wow, round gage 76C+. That's a rare bird! You're part of a small group that knows what a DDR is...
Like most things on helicopters, the issue of displaying N1 in the 76C+ isn't a simple as it appears. In fact, the N1 you are looking at is a "corrected" N1, and not necessarily the exact N1 speed the engine is turning. It's due to the fact that Turbomeca rates their engines primarily on N1 speed and the precise rated (takeoff, 2min, 30sec) n1 speed is different for different ambient conditions (alt, temp).
So the kind Sikorsky engineeers, realizing you really didn't want to pull out a chart each time you set takeoff power, came up with a scheme for a "biased" or indicated N1. This allows you to memorize only one number for each rating. The mathematical correction is made in the DECU and sent (via ARINC) to the indicator. As a backup, the N1 indicator itself is also capable of calculating "Biased N1". It receives a raw N1 signal from the alternator, and oat from the ship's OAT. So what you're actually doing when conducting this test is confirming the gage calculated N1 is within tolerance to the DECU calculated value.
So with all that I STILL haven't answered your question! I don't have an exact answer for you right now, but will find the indicator spec and let you know. In the meantime, check all the inputs to the indicator for proper connections. Seems to me it's missing a necessary input to do its own calculation.
R. Cal