Strange Surges : VMS70

[BongoJim]

Hi Guys,

Frank from The Carvery here. I wonder if you can help. I am experiencing some strange issues on my VMS70. Essentially I am getting an apparently random lift of the stylus from the lacquer. At first I thought it was perhaps a phase issue as the audio I was cutting was shall we say 'experimental'. It then happened with a mono track. My deduction was that it was coming from an electrical surge of some sort. When I cut a silent groove I don't get it. When it does happen it is never in the same place twice, although often in the same track...

To give you an idea of the frequency of the problem six out of twelve sides have been affected and only ever on masters, not dubs (typically).

Things I have tried;

Replacing the vacuum system
Platter stability
Cutting Mono
Altering groove depth
Lubricating all mechanisms
checking vacuum pipes for snagging


Any ideas welcome...

thanks

frank

[EmAtChapterV]

Is it just on one channel, or both simultaneously (180 degrees out of phase)? I've seen it happen in one channel where IIRC something went wrong in the feedback section and/or power supply of the cutting amplifier that made the drive section start to kick out semi-random spikes of DC current. (The rail voltage sagged and it started to clip, I think?) So I'd give the amp a once-over.

[jjgolden]

Hi Frank, Here's some questions to help us help you:

While Cutting 100%Mono, does your depth meter show any movement?
Do you have a zuma or non-Neumann pitch system?
Does it exhibit the same results with both 33 and 45 programer?
Is your 14" disc completely flat while the suction is on and plater is rotating.(no debris on the outer area of the platter)

JJG

[BongoJim]

Both channels. It happens even if the track is mono.

[markrob]

Hi,

I'm not a pro, so take what I say with a big grain of salt.

I wonder if you have some kind if infrasonic issue in your signal chain ahead of your drive amps. I assume when you cut a silent groove, you still had the drive amps on. Was the full signal chain operational at this time? How long (in terms of a rotation) was the lift off? That would give you an idea of the length of the disturbance (1.8 seconds per rev at 33RPM). Does your metering show any sort of infrasonic pulse (as was suggested) in either channel. If a low frequency pulse appears on only one channel, the stylus will move at a 45 degree angle and could cause lift off. Could the problem be in the depth control signal path? I seem to recall that these systems use a DC bias on the drive coils to control depth.

Mark

[BongoJim]

Hi JJ,

I did recently notice quite a lot of 'pumping' movement in the dept gauge, which I thought might be to do with my suction unit having worn down and the brushes sparking causing surges. I opened it up an indeed there were some small sparks (which I now think could be acetate dust). I then replace the vacuum with an old unit but still got the problem.

14 inch lacquer is nice and flat, no ridges, also there is no increased depth before the loss of contact and the recorded signal does not seems to be altered, phase wise.

So far I have only tried it at 33. Pitch system is standard Neumann.

I just cut a side with the pitch system in but no audio and got no variation in depth. then again, yesterday I managed to cut a couple of 18min sides with no problem...

It really is very intermittent.

[BongoJim]

Hi Mark

Lift of is generally around 2 inches in length. the groove then goes back to normal. I was just about to do another cut with the servo from the head mech removed to see if that helps

[jjgolden]

With the lathe turned on and no audio feeding it, your depth meter should read 30mA.
If you move that pot above or below, do you see any erratic movement on the depth meter?

I would have look at both the depth potentiometer and the black knurled lead in depth pot below it.
They could be corroded and in need of some exorcising and/or deoxit for better contact.

JJG

[BongoJim]

I normally have it set around 40ma, not sure that makes a huge difference though right? when I move the pot even with the head up the needle movement is not smooth. It kind of jumps from one depth to another. once set however it seems pretty fixed. Will try some deoxit on them too.

One question; out of the two cables that come out of the back of the head mechanism, one of them goes to the pitch control, the other to the overhead. What does the overhead one do?

[jjgolden]

With this info I'd suspect that pot Is has some corrosion or is worn and not making good contact. The movement of the needle should be very smooth.

I believe 30mA is the optimum null point with regards to calibration between LPI/Depth and the programers.
If the dexoit does not do the trick, I'd move it down to 30 and set your depth with the screw on the back of the suspension box for each cut.
Once you've set it at 30, there's no real practical reason to touch it again. I've never known it as a creative tool or for setting static depth on a per cut basis.

Regarding the wires at the back of the head mechanism (suspension box?) You may check with a manual if you have one.
I don't want to say the wrong thing here.


JJG

[opcode66]

Are you sure your dash pot is correct. It is there to slow upward movement. If too loose then it might allow the head to jump up too far.

[concretecowboy71]

My guess is that if you are getting movement on your gague it is electronic inside the panel on the lathe.

I was getting random lift outs for a while and the figured out I had a sticky spot on my microscope track and when I wheeled it over while the lathe was cutting, the vibration would cause a lift out.

[BongoJim]

So I cleaned the pot. Needle movement is now smooth, although when I turn the lathe on the meter reads 70ma and then slowly comes back to 30ma...

[BongoJim]

@opcode66

I think the dash pot is fine. I did top it up with oil but to be honest this is only the second time I have had anything go wrong with this lathe in five years. My guess is that this has to be an electrical problem as a physical one would have more consistency in its error.

[Greg Reierson]

I would also suggest going through all of the cables and card connections in the entire system. If it had gotten to the point where the pot was dirty there's a good chance you have other connections that need attention. Clean contacts will solve / prevent many problems.

[BongoJim]

Thanks Greg. Will try again in the morning and report.

thanks

[gold]

If the depth meter is acting funny that's the place to start. 99% of the time it's a connection. First I'd try cutting at a different depth. As has been suggested it could be a bad spot on the pot. Next I'd clean and re seat the depth panel connector. If your head suspension has a leaf switch and not a microswitch I'd clean and exercise that. Then work backwards towards the AS cards that supply the depth current. Re seat those. Possibly re cap them is all else fails.

[BongoJim]

thanks for that.

Yes I cleaned the pot, which is now smooth as well as the connector, unfortunately the problem persists. I wonder if it is not an amp issue, where the amps are producing too much vertical signal? I am going to try and take the connector from the depth panel out of the equation and see if the signal is still producing vertical spikes in the head.

[jjgolden]

-Did you clean the "Lead In Depth pot" as well?

-Another place to look may be the neumann pitch computer cards.
Carefully remove them, inspect the connections for any corrosion, then reseat them.

-Also, Check it with another 33 programmer if you have one, or check it will your 45 programmer.
All of these things have an influence on Vertical movement.
Eliminate all of the relatively simple things first before diving into the deep end.

-Another thing to check is to unplug the preview feed to your system and do some test cuts with 100% mono program and see what happens.
In other words, you want to be sure there's no intermittent RF or buzz etc. being sent from your preview audio channels to the lathe.

JJG

[gold]

-Another place to look may be the neumann pitch computer cards.

That's what I meant by the AS cards. There are two types. I'd swap like to like and see if the problem follows the card.