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Zeta Tonearm VTF Drift

I believe that the nephew of the original GB Tools owner/designer was on some forums as he was discussing the Zeta and Mission Mechanic arm and others, see here: https://www.audio-forums.com/thread...mechanic-tonearm-and-now-rega-rb-1000.100492/

It may be that the person will see this post and even try to help out by trying to get the tonearm serviced for you unless you try to contact Moth or something, there may be someone on PFM that can service it even. I will try to ask around for you if you want, I am sure it may not be a complicated service.
 
Thanks for the link and offer of help. I just sent an email to Moth Group and will see if they have any options available.
 
I did some more investigation over the weekend. My previous assumption of drift was incorrect, based on low sampling frequency. I did some experiments with two sets of scales and checking with cal weight to eliminate other factors. The downforce actually varies every time I move the arm across to the scales. I now understand why getting the VTF in range was such a PITA. I did notice some consistent behaviour: Allowing the cartridge to drop as low as possible before weighing increased tracking weight and the lifting the cartridge to it's highest possible position reduced tracking weight. My guess is that there is a lump of hardened oil/grease that is pushed one way or the other. I was seeing a difference of 0.3g to 0.4g between lightest and heaviest! I also tried WD40 per Jamie123's suggestion, but there was no observable change.

I am now waiting for Moth to reply and will decide next steps based on that.
 
So an update, but not the end of the story yet. Before sending the arm to Moth, Mike Harris had me wash out the bearings with IPA (which I did ~ 5 times) and apply sewing machine oil. I saw some slight improvement in repeatability but it was still more variation than the Kleos could handle. Mike has now had the arm for a while and cannot find anything wrong with it, with very repeatable down force on Cartridge Man scales - in fact he went as far as to say that it was the best adjusted and smoothest Zeta he has seen! My thought is that the IPA/oil and the vibration of transatlantic shipping freed up some gunk that was causing the issue. The arm is now heading back to me, where I will be able to re-test.
 
The arm is back and Mike is right, VTF is now repeatable. So it seems the solution for sticky Zetas is washing with IPA, lubricating with sewing machine oil, time and (maybe) a bit of vibration.
 
Time, the great healer. You really need to flush them out, not just add ipa. The travel probably helped a lot.
 
So one year on I discovered the real cause of the issue that explains all the observed effects. One of the vertical bearing lock screws was not fully tightened so the bearing tension was being adjusted by the vertical movement of the arm - lifting the head shell increased the bearing pressure - lowering VTF, lowering the the head shell had the opposite effect. It also explains why a long air trip seemed to improve the situation. With the bearing pressure adjusted (using the paper test) and locked I am now getting consistent VTF within +/- 0.002g.
 
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