VMS70/SX74/VG74B Overload when lowering head

[I.S.D]

Hi Everyone,

I've got a new issue with my rig today and I'm stumped.....

Cutting on a Neumann VMS70/SX 74/VG 74 B

When I lower my cutting head, the right channel on the amp rack drops into Over Load

I recently lost my auto lift, after cleaning the contacts on the leaf spring in the head assembly it returned. I may have been a little rough on cleaning the contacts because I lost heating current to my stylus, I found a blown fuse on a card (NG 66 - H), once replaced the current retuned. But i noticed my right channel groove was a little odd looking, then I noticed on my amp rack the right channel was cutting with way more current. I check my desk, all outputs are equal. I was stumped, so I disconnected all inputs to the lathe, removed the cutting head and visually inspected, no issues. Once I reinstalled the cutting head and attempted to test cut an unmodified groove my issue started to occur, the right channel would drop into Over Load every time i tried to lower the cutting head.

Any ideas?

Thank you!

[Greg Reierson]

I had this exact problem a few months ago. In my case it was the wiring harness between the suspension box and the head. The strain relief had broken on the head side and many of the wires were frayed. Once one of them finally failed the circuit breaker saw an open circuit and popped the breaker. Took quite a bit of digging to figure it out...

Remove that wiring harness and test the continuity. While you're in there you should clean the pins and sockets. Mine were very dirty.

Here's a link to that diagram: https://we.tl/t-2TVHUmf1kR

I'd put a dummy load on the suspension box and test the circuit breaker before offering up the head coils for testing purposes.

[Greg Reierson]

Just re-read your post. You also may have bent a leaf when you were cleaning it. Those things are notoriously touchy. Check there as well.

[I.S.D]

I've spent a lot of time looking at and working the leaf switch, it appears to be functioning properly...

I also checked the contacts on the connector cable you mentioned, everything contact is passing

I talked to Chis Muth and he thinks it may be the right feedback coil in the cutting head, he suggested i talk to Roberto at cutterheadrepair.com but also mentioned he wasn't sure who could repair it for me.

Any idea how I'd test the right feedback coil?

[Stowvinyl]

speak to Flo at floka.com he can repair your head.

[gold]

Any idea how I'd test the right feedback coil?

If you use a dummy load and you get the right feedback signal then that points to the head.

[Phinster]

Check continuity of rt chan feedback contacts L and P on the head

[gold]

Check continuity of rt chan feedback contacts L and P on the head

That will tell you is it's an open coil but an open coil isn't the only way it can break. Maybe the most common though. A dummy load is a better test because it eliminates the head.

[montalbano]

Hi, I've got this issue several times.
If SAL goes in overload, when you lower the head and open the circuit, and you have no modulation, this most likely happens because there is no signal continuity as the SAL expects to be.
So what you have to do is 1st of all measure with a tester the Ohms in the DRIVE and FEEDBACK signals. Check the pins: Head/head connector/connector on the back of the VMS - and finally the SAL itself. This will help you to see where exactly the problem is in the signal chain. In the worst case, the head is cooked, but in many other cases it is a very simple thing.
E.G. In my case it was a simple loose welding in the head connector. Invisible to see at a first sight, because there was the cap covering all the wirings.
Good luck!
Phil