Strange problem with PHD

[Pomerol]

I have been using PHD very successfully for about 2 years with no significant problems.

I have noticed over the last few weeks a new problem.Calibration in W,E and N goes normally but during S the star doesn't return to the central crossed lines until calibration is finished and when it starts "Guiding" it jumps back to the central position.

Guiding normally continues for a few minutes and then it starts to make S corrections and the Dec line on the graph starts to track away from the correct position such that the photograph being taken is unusable.If I leave it running eventually it will make one big correction back to the central correct position. It may then run correctly for several minutes before the cycle starts again.

I haven't changed any equipment, PA and balance have been checked and are good.I have tried with several stars and their focus position but am not able to find/correct what is happening.

Clearly something is going wrong on S callibration......any input most welcome.

Ian S.

[adamsp123]

Check the ST4 cable isn't corroded, this is what I had a year or so ago with similar symptoms

[Photosbykev]

Could this be mechanical backlash in the N-S axis? i.e PHD is driving the mount towards the South but the slop in the gearbox means the scope isn't moving

[Pomerol]

Thanks for the replies.....have checked the ST4 cable, looks OK.

Had also sent an e-mail to Craig Stark (he wrote PHD) about the problem, he is very clear that the problem is backlash in Dec so will have to persue fixing that.

Many thanks

Ian S.