VMS80 pitch computer becoming less efficient throughout the day

[pizzahotline]

Hey Trolls, I have this issue with a Neumann VMS80 lathe regarding it's ability to calculate how much disc space will be taken up.

When the lathe is started in the morning it works great, it will do a dry run and accurately calculate the percentage amount a side will take up. The problem is, towards the end of the day this percentage will be about 3-5% greater than the results in the morning. My only thought is that this is a heat related issue where the components of the pitch computer drop in efficiency as they get warmer throughout the day.

Has anyone else heard of this issue, if so, do you know any solution?

Thanks

[dmills]

That has almost got to be something in the analogue doings (which are fairly extensive according to my copy of the diagrams), could be deeply annoying to find, even with a can of freezer spray.
Binary search might be your friend, find a likely lad signal somewhere half way thru the doings, attach multimeter/scope/data logger or whatever is appropriate, do your two test runs and see if it changes. Once you have something that does, you can start divvying the thing up and testing each way.

IIRC there is a **lot** of 4000 series cmos in there, so take static precautions not all of it has well protected IO pins, and some of that sand will be hard to find today.

[Joe90]

Hello pizzahotline, I'm not sure if you have resolved this issue as your post is a most one year old.

Although i'm new to cutting discs, i do have a number of decades experience in both electronics and machine tool.
Your assumption that this issue is related to heat is correct. In my experience of restoring vintage audio equipment, motor drives and control systems is,.. you need to start by replacing all of the capacitors.

Capacitors are made of perishable materials and, if they are old (25 years plus), heat will causes their values will drift over time. Therefore they will need to be replaced.
New capacitors are not expensive but the bulk of your expenditure will be in labour. It is a job one can DIY but you will need both good soldering and good de-soldering skills.
When choosing replacement capacitors, you can go higher in operating voltage.... but not in capacitance. They will also be rated for operating temperature. Get the highest values that are practicable for the situation. If in doubt, consult a professional. Most audio electronics engineers will be able to re-cap any circuit board you give them, so you may want to remove the boards yourself and have them worked on at a local audio repair shop.

Also, if i re-cap a unit, i don't replace all the capacitors in one hit. I start by doing the power supply first, then re-assemble everything and try using the unit again. If no improvement but the unit still functions,.. tackle another of the printed circuit boards in the unit or.... if the unit contains only one large PCB, divide that board into smaller areas and do each area at a time. Each time you complete an area of re-capping, re-assemble and test the unit is working correctly.

It may be tempting to just blitz the thing and replace all of the caps in one hit however, i advise against this. If you do, and you make a mistake
i.e. miss identify a capacitor and replace it with one out of range by a factor of 10 because you missed a decimal point on a tiny silk screened component from the 1970s..... or..... you put one in the wring way around... or lift or brake a trace with poor soldering technique,... you will have to go back and find that error and re-work it.
So, small steps are easier to rectify. You don't want to be in a position where you know you made a mistake... but have no idea where to begin to look because you did so much work in one hit.

I hope my words help and remember, there is no reason why kit from the 70s can't be recapped to give another 40 years of reliable service. Good luck,