Have been working on two Revox B790s for a friend. #2 (I think might be older than the other, and has different tonearm parts) plays for several seconds and then something tells it to retract. I have tried everything; I mean I've spent 12 hrs trying to make tiny changes that would help, after spending many hours just cleaning switches and the unit itself. Weight was a big problem, Audio Technica 140LC cart is 4.3g and just offsetting that has been frustrating.
Height was a problem with #2 as this arm sometimes wants to touch the plastic cover on the bottom rail to attain correct tracking force and vertical height so I removed it and secured the wires inside with tiny amounts of epoxy so nothing touches. One unit plays fine, and tracks at a decent force. For that unit (#1), I added a couple grams of lead weight to the rear of the tonearm and neatly secured it. I have even tried adjusting the IR lamp that sprays light on two sensors in the headshell, to make sure they both get light through the vertical shutter. I have centered the lamp to the shutter using an IR camera I have.
The other unit (1) will adapt ok with the screw-on lead weight in the kit so that I get my tracking force. needle height is at 20-21mm as manual says. Needle touches inner ridge on mat as required. Have tried adjusting the solenoid height up or down and made sure the rails are smooth. It (2) will play, track and return, but won't remain in play position. The solenoid doesn't seem to be letting it go, something is telling it to release.
At one point or another I had this issue with the first unit but everything fell into place and careful balancing has allowed it to work ok. Nothing I do with the second older unit seems to help. Both have the same cartridge. I am ready to give up on it but if someone has an answer I'll sure try it if I have not done so already. I am wondering if some component beyond my control has gone out of spec.