Problem with Neumann lathe - turntable suddenly stops

[GeorgeZ]

Hello,

I'm looking for any help with the following problem:

Our VMS-82 lathe suddenly stopped cutting. After some checking we have found that fuses and both transistors BUZ11 (T8 and T11) on "Turntable motor power amplifier" PME 82 card were burnt out. We can replace them and continue with cutting but something suspicious remains. The defect is hidden somewhere because the problem returns after some time (approx. once per two month).

Any help, hints, tips, tricks or servicing experience appreciated. Thanks a lot.

[andybee]

yes, I see, your machine is ready for the trash.
I pick it up tomorrow, let´s say, 10o´clock?
so, you don´t need to pay for the waste



try to contact flo from vinylium, I think, he is VERY experienced
in the lathe motor of the VMS-82

maybe the transistors have a heating problem? I know, that
there is a white liquid between transistor and cooling plate,
to move the heat faster away from the transistor. normally,
this liquid is dry after 10-15 years, and is better to be replaced,
there a grey pads available now.
maybe this is a solution.

good luck!

[cohearent]

Just gotta jump in here...
The "liquid" used for transistors is white silicone compound, sold as "heat sink grease". It is more of a paste than a liquid. Although it does harden a bit with time it shouldn't ever need replacing. It is mostly used in conjunction with mica insulators to allow the heat to conduct from the device, through the mica, to the aluminum heat sink. DON"T EVER USE THE GRAY SILICONE PADS!!! They are JUNK!!! I tried to use them in a Class A amp and it blew up in a day. I then measured the heat sink temperature at the sink and it was HALF of what it should be. I don't know how these things got foisted on an unsuspecting world, but don't be fooled. Maybe in a less critical application they "work", but can't be ideal. You can't beat the old school method here.

[mossboss]

Hello,

I'm looking for any help with the following problem:

Our VMS-82 lathe suddenly stopped cutting. After some checking we have found that fuses and both transistors BUZ11 (T8 and T11) on "Turntable motor power amplifier" PME 82 card were burnt out. We can replace them and continue with cutting but something suspicious remains. The defect is hidden somewhere because the problem returns after some time (approx. once per two month).

Any help, hints, tips, tricks or servicing experience appreciated. Thanks a lot.

This does not appear to be a heat related Issue It seems that they are going into oscilation and cook themshelves
If I where you I would check to see if there is something starting up that dirties the line that supplies power to the system
We had similar issues here Large motor starting up about 10 miles down the road screwed up our lathe
You may be able to find it may be not It matters not as you may not be able to do anything about it anyway
Get your self a UPS from an old computer instalation and place it between the power supply and the lathe
Get one that has a bank of sealed lead batteries in it Like 220 V ac input down to 12 volts DC into the batteries 12 V DC out of the batteries back to 220 V ac Dont worry if the do not hold charge You dont need it as a back up
But I can assure you Nothing like Spurious voltage spikes noise or 2nd third or further harmonics or 50 HZ hum will get past it and it is most likely the best investment you ever do
Having a cutting lathe near an industrial area as you have and I do you will get these issues which are hard to pin down as well as find the cause
I would start with it
I am assuming that this is not an issue realated to the lathe electronics as if you had a problem they will not do it every second month
They are dead or alive immediately if there was an internal problem which by your description here it does not appear to be the case
If you have loose wires somewhere along the supply line It may do it as well but knowing you I take the view that you checked this more than once discounting it
To me this points to something injecting crap on the power supply of the building the power supply does not catch it as it cannot so cooked output trannies
Happened to us here
Hope this helps
Cheers

[markrob]

Just gotta jump in here...
The "liquid" used for transistors is white silicone compound, sold as "heat sink grease". It is more of a paste than a liquid. Although it does harden a bit with time it shouldn't ever need replacing. It is mostly used in conjunction with mica insulators to allow the heat to conduct from the device, through the mica, to the aluminum heat sink. DON"T EVER USE THE GRAY SILICONE PADS!!! They are JUNK!!! I tried to use them in a Class A amp and it blew up in a day. I then measured the heat sink temperature at the sink and it was HALF of what it should be. I don't know how these things got foisted on an unsuspecting world, but don't be fooled. Maybe in a less critical application they "work", but can't be ideal. You can't beat the old school method here.

Hi Kevin,

I wouldn't call them "junk" or claim that they were foisted on an ususpecting world. Their failure to work was due to mis-application. It is true that they are generally not as good of a heat conductor as mica and grease, but if they are designed into an application taking this into account, they are fine. Also note that there are several different variations of these pads that have different thermal resistances. The tradeoff is generally breakdown voltage vs. thermal resistance. One issue with the mica grease combination is that if the grease is not applied properly, the results can be worse than the Silpad method. A very common error is the application of too much grease to the device and sink. In a production environment, we try to stay away from using grease because its messy, labor intensive, prone to mis-application, and does not work well with the vapor degreaser we use to clean our boards. As far as being ideal, mic and grease are far from it. There are other methods that are better. No method is the best.

One failure mode I've found is that after many years, the clamping force holding the power device to the sink declines due to many heating and cooling cycles. If it gets too bad, you will have a heat related failure no matter what you use as the interface from device to sink.

Mark

[GeorgeZ]

Thanks for your hints...

Our technicians bypassed some circuits in that card last Spring and everything has been working since that time, except the bypassed motor brake, but the brake is not crucial for us. They will try to replace some ICs and fix the problem when the next maintenance cycle will be achieved.