Can I convert stereo to mono for a phono input

I would think that the read-out of a recording meter is very unreliable as to assessing channel balance.  First, the needle is bouncing around.  Second, you have no way of knowing how the material was encoded into stereo, meaning the fault may be in the recording.  Further, what do you know about the state of calibration of your meters?  IMO, if you don't hear the problem, why drive yourself nuts over a few dancing meter needles?  Try feeding each meter separately with a single signal; you don't even have to know for sure the voltage or db, so long as it is the same signal fed alternately to each channel.  This way you might determine if the meters are equally sensitive.

If the cartridge is fairly new, you could make a case with the dealer to replace it, if all else fails.  A decent cartridge should have inherent channel balance within less than 2db, but I don't know what bible that is written into.

By the way, as to your response to my earlier diatribe, getting the azimuth "right" is not going to cure your channel imbalance, if indeed there is one.  That was sort of my point.

Lew.  The fact that the meters register differently when I switch the leads to the phone a preamp shows that it is not the meters. As for the signal, it is a test tone on a analog test record, I would think it is good, and I noticed this with multiple  recordings sources  during the process of recording.  I am away from home, but I need to look more closely at the meters to see what degree of resolution they have.

This can not be done.

Peter, Sorry for being so pedantic and possibly condescending.  I am sure you know what's up much better than I do, since you've got the stuff in front of you.  I speak as someone who endured for several years an evident channel imbalance (L>R) in my main listening room.  I never was able to figure it out.  Then one day I closed both halves of a large double door that connects the listening room (our living room) to the adjacent dining room.  Lo and behold, the perceived (by everyone) L channel bias went away.  Sometimes it's the room.  But in your case, you say you DON'T hear a problem; you're worried about what the meters tell you.  A certain old Borscht Belt comedian would say, "don't look at the meters".

It remains a little odd to me that Phonograph cartridges which are inherently balanced devices, don't have XLR outputs.

Some do (well, some **arms** do anyway). Most tone arms have a balanced connection somewhere (even a lowly BSR or Garrard); its just not got the XLR connector.