bad Newall dro facinoma

Here is a puzzlement. Just got in Newall sapphire Dro and spherosyn scales. Seller stated counter was bad. He was right. Here are some findings related to failure.

the older Spherosyn reader heads require 12 vdc and 5 vdc power. The 12 vdc regulator ic, and ac transformer are rock steady. 11.92 vdc available at plug for encoder.
When one encoder is plugged in, into either x or y dedicated receptacle the display will work for up to 60 seconds then drop out with either a 0 on the lcd strip or sometimes blank. When a volt meter is plugged into the 5 vdc female hot pin on the unused connector, a curious pattern is documented.
+ 4.75 vdc for 10 seconds
+ 500 mvdc for 10 seconds
- 4.25 vdc for 10 seconds.

this cycle is as regular as a pendulum.
one 7812 regulator ic is present and rock stable. two 7805 5 vdc regulator chips are present and output stable.
however, measuring output at two diodes shows wild fluctuation but never negative voltage. and checking capacitor shell on several adjacent to regulator chips shows lots of fluctuation but never negative.

it would seem either a capacitor or diode-- or both is at fault. but the swinging polarity is something new to me.

any thoughts?

pics

https://picasaweb.google.com/pulsenpal/FailedNewall#


jh

The first suspect on an older piece of gear is always the electrolytic capacitors.

Buy replacements from Mouser at www.mouser.com . They have a huge selection.
You can always use a higher voltage, and moderately higher capacitance, if you can't find an exact replacement.
Be careful of the dimensions, including lead spacing if they're radial style mounted on the PC board.

- Leigh

Newall still has some parts for those DRO's.
There phone number is (614) 771-0213
It sounds like the power supply is bad
There is not a lot that can go wrong with those newall dro's.

success---- sort of

definitely had one bad electrolytic cap-- 1000 mf 16 vdc. replaced with Radio Shack.
Also replaced 7812 and 7805 voltage regulators. This seems to have resolved most of power issues to Spherosyn reader heads. I unsoldered all power electrolytic caps-tested and then added back one by one.. 5 vdc and 12 vdc input to reader heads stable and display for most part stable. What appears to be ground issue with blister membrane input will be investigated. Overall, an almost usable DRO.

did you know you can use screwdrivers in place of Newall ball tubes?

here is proof----

https://picasaweb.google.com/pulsenpal/FailedNewall#5705119373334648562

jh

For the price of the caps, just replace them all.. Sometimes they test OK but don't work right in circuit...

No goofy zener diodes, or such in circuit???

abarnsley

very perceptive, abarnsley. There is a critical zener diode 6.2 vdc at about one amp. Radio Shack did not have the proper replacement but this will be also replaced. The swing polarity seems to fit one of the failure modes of zener diodes, from what I read. Two separate 5 vdc power supplies are present. one 5 vdc is fed by ac step down transformer and it is dedicated to the bulk of logic and control elements. at least I think. the second 5 vdc is dedicated to logic elements in the
spherosyn reader head. this 5 vdc element is parasitic, and is dependent upon rectified voltage from the 12 vdc regulator chip which in turn is supplied ac voltage from step down line power. and the 12 vdc output is used only for the coil inductor in the reader head. again, this is what I believe to be . no manufacturer of dro's provides this level of proprietary info. my first
spherosyn unit -new in 1980--had all reader head elements on display. wires ran everywhere and it was easy to blueprint supplied voltages, etc. but any coolant making its way past the poorly designed cover played havoc with these components. the current reader heads on my bench --build year 1992, are fully epoxy potted. no ability to troubleshoot anything in readerhead. but unlikely to suffer coolant injury.

what are your thoughts on this?

at least now the display is usable, an apparent logic ground issue remains but I have to read the programming manual and apply.

jh

I have a Newall, a millpack D7, if'n my memory is right. Worked well for me for many years. Then the axis display showed signal fail. Had to replace the power supply. Fixed problem. Too short of a time later started to have more problems with display and I think the axis lead connectors might have been the problem. They seem to have too much weight and needed to have more support for the leads to keep from straining the receptical ports.
Changed over to CNC retro and DRO is waiting for my attention to prove it's still useble. May use on a lathe if it is salvageable.

Krutch

No power supply engineer, but if the zener is controlling voltage on that PS sub circuit... replace it... (along with caps) With the cost of the low voltage/low current parts, vs shop time.. shotgun em out of there... ( I would have gone for the zener first, before the 78xx's.. But you have to have the zener on hand...)

Ground issues of all types, normal on machines with coolant and oil around...

We do have a well stocked electronics store up here in AK.....

Not everyone has that luxury...

I did stay at a Holiday Inn last night... LOL..

de WL7WW