I have agreed to look at, and possibly repair, a sprinkler system that
has been in the ground for at least 10 years. It has also had many
hands twiddling with it over those years. (One of those problems where
you never know what you'll run into.)

The controller has 8 zones with 1 to 4 sprinkler heads per zone.
The heads are a mix of partial circle, full circle, and rotary.
Input pipe to the valves appears to be 3/4" PVC.
The sprinkler heads have 1/2" threaded risers.
Not yet sure what size pipe comes from the street.
The controller is properly operating all 8 valves.
Most all heads are Rainbird, or equivalent.
I don't know how well it worked when new vs now.

The zones with up to 2 low volume heads appear to work ok. Any more
than 2 heads greatly reduce the amount of water coming out of all the
heads. The zone with 4 heads is not much more than 1-2' spray radius
per head. The field has had a few faucets added to the supply line
and the loss of volume/pressure when a zone is activated is very
apparent at the faucet also.

My first impression is either loss of pressure or loss of volume.
Anything else?
Assuming that the whole field uses 3/4" PVC, would that provide enough
volume for 4 heads at any one time? Would there be any noticeable
improvement using 1" for supply?

I want to explore all reasonable possibilities before I start tearing
things up. All opinions are welcomed.

Red