Thanks John.  Much appreciated.
I've passed it on to my friend with the QMD.
👍

You can easily check for proper encoder operation, both for slippage and proper direction, by going Test>Encoders on the Nexus.
From the manual:

The following information applies only to German equatorial mounts:
Northern hemisphere:
a) Position the telescope on top and pointing
roughly at the celestial pole (Polaris)
b) Go to Test->Encoders
c) Move the telescope in RA only towards the
west - if the RA /AZ value on the screen is increasing then the encoder steps’ sign for the RA encoder is correct; if not then take a note of that
d) Now move the telescope in declination only towards the west - the Dec/Alt value should be increasing - take a note if it is not (assuming the first alignment star will be east of the meridian)
e) if the encoder value during c) or d) was not changing as described then go to Settings- >Telescope->Adjust and change the sign in front of the corresponding encoder steps to opposite
f) if the first alignment star is in the west then the logic for the step d) is opposite (i.e. the value should be decreasing)

John

Thanks John, Spikey.
I contacted AP and the ENC10-900 can indeed be ordered still, for a price of course.  And my friend said he's sure the encoders on his are not slipping, so something is up.  I'll let him troubleshoot it with Serge.
Too bad about not being able to upgrade the 900QMD otherwise.  Oh well.  
Cheers!
Christopher

You can upgrade the encoders.  I recently had the encoders for a 400QMD mount upgraded by AP from the 4000 tic to 10000 tic.  Contact AP about this.

You cannot upgrade a QMD mount to GTO.

My concern for your friend is that the encoders might not be the problem.  The QMD mounts have an encoder wheel which sometimes slips on the encoder shaft, giving inconsistent results.  If you search the web for “QMD encoder tutorial”, you can find an old CN thread discussing this.

Hi,

I also upgraded from an Argo to Nexus and it was plug and play so you might still experience an issue
even after upgrading to 10K US digital encoders.
I would contact Serge at Astro Devices to help walk you through this problem. His customer service is like
AP's, second to none. Even though he is located in Australia he will call you directly back after emailing/calling him.

Contact:
Contact (astrodevices.com)

Good Luck,
John

I have a friend who has and is still happily using his 900QMD mount.  Recently he tried connecting my Nexus DSC to its 4000 tick encoders, but the Nexus didn't like it and did weird things, loosing its way.  His encoders work fine with his old (Argo Navis?) DSC unit.  Can one still upgrade the encoders on the 900QMD to the 10,000 tick encoders or better?  I think they were AP part ENC10-900 (from an old page). 
And a long shot, but can one upgrade a 900QMD to GTO with new motors and controller?  Heh.  That would help me justify upgrading my Mach1 from CP3 to CP4 as I could give him the CP3.  :)