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Revox B 790 tonearm popping up, what am I doing wrong?

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Revox B 790 tonearm popping up, what am I doing wrong?

Postby mrow2 » 31 Mar 2011 18:08

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.
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Re: Revox B 790 tonearm popping up, what am I doing wrong?

Postby jlbruyelle » 31 Mar 2011 23:13

Hi mrow2, I had a similar problem with an early B790 arm too: sometimes it worked well, but sometimes it would go back up as soon as it touched the vinyl. I traced the problem to a (normal) safety behaviour of the arm control to an abnormal condition in which the arm went sideways as it went down. It seemed to be related to the screw-in lead counterweight that was off-axis, and I managed to fix it (mostly), but this particular arm was so finicky that I finally gave up on it as soon as I found an used B791 arm on the bay, which took its place on the TT and works great. I kept the old arm for parts.

If your arm is able to play a few seconds before retracting, it might be a servo problem: track following does not work, and when the limit is reached, the solenoid is deactivated in order to prevent the stylus from egressing the groove. Checking the signals on the arm control PCB according to the service manual should help you sort out the problem. Also, note that in those old decks the electrolytic caps and (especially) trimmers do not always age well and can very easily cause failures, so I would consider changing them if you have the equipment, and the time, to recalibrate the thing.

Adding weight on the rear end of the arm is not necessarily a good idea, as it tends to cause such stability problems in my experience. OTOH, Revox sometimes installed a brass weight in the front (visible through a hole), to adapt very light cartridges. If yours has this "counter-counterweight" installed, you might try to remove it (it is normally glued in place).

My other B790, with the "normal" arm (snap-in, not screw-in, counterweight), never had such problems.
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