Thread: Heidenhain DRO VRZ 753 B dont react when moving axes

HEIDENHAIN O.M.A. Controls

didn't cut and paste right

this gibberish is telling yo it won't count if the memofix is in the red position


MEMOFIX switch has two
Plaque frontale a .deux positions: positions marked RED and GREEN. <








R







OUGE et VERT. a) RED: Counter Stops when
a) ROUGE: ARRET COMPTEUR en traversing over reference
Passant au-dessus de la mark
marque de reference b) GREEN: Counter resumes counting
b) VERT: DEPART COMPTEUR en when reference mark is
Passant une nouvelle fois traversed over once again
au-dessus de Ia marque de The reference mark indicator indicates
reference counter STOP. 8
Lorsque Ie voyant de I’affichage de Ia When the MEMOFIX switch is on RED,
marque de reference reste allum6, the counter remains stopped when
Ie compteur est a I’arr&. En Position traversing over the reference mark.
(( ROUGEs, Ie compteur reste arr&k, In this condition the counter tan be
meme en Passant une nouvelle fois preset for the next starting Operation.
au-dessus de Ia marque de reference. In switch Position GREEN counting is
Dans cetta Position, on pewt prk- resumed when the reference mark has
selectionner une nouvelle Valeur pour been traversed over once again and
Ia prochaine operation. the reference mark indicator will be
Dans Ia Position « VERT», Ie compteur extinguished. The counter continues
se remet a compter en Passant counting even whentraversing ov

Originally Posted by Johan.A

We found broken components that we missed when we first looked .

Yup, power-supply components .

The two broken diodes are part of a full-wave rectifier (or bridge rectifier). From the anode/cathode bands one can see that these two diodes provide the positive voltage, and the capacitors are presumably the filters for that. Thus it appears that the DRO has a separate low-power supply for the light bulbs on the scales. An interesting question is, why are both diodes broken? This happened because too much current was drawn from them, then they cracked/broke from heat. Two things may have happened: (1) someone shorted out the wires which should feed the light bulbs in the scales or (2) one of the filter capacitors shorted. Since you said one of the capacitors tested bad, the second explanation seems likely. A short in the wiring to the scales might also be responsible, but this would take a good fraction of a minute (my guesstimate) to crack the diodes.

Well done, please post another note after you have swapped these parts, I bet it will work fine then. If you want to be safe, check that there are at least tens of ohms of resistance between pins 3 and 4 on the scales, to rule out any short circuits within the cables, connectors, or read head.

Hello all

I have helped Johan fixing this DRO. I can confirm that the bad capacitor was shorted. I have replaced all the capacitors and all four diodes now and the 5V supply is back. It is not tested in the machine yet but I know that Johan is very eager to do that.

I was a little worried that the voltage regulator LM309 was broken too but apparently it is not.

My friend did a exelent job with the DRO

X and Z axis work as it should but not Y

The bulb work . I get some reaction when I move Y axis

I can get a - and sometimes 0.01 .

Can it be dirt on the scale or reading head ?

Johan

I cleaned the scale . Still dosent work

Originally Posted by Meconer

I can confirm that the bad capacitor was shorted. I have replaced all the capacitors and all four diodes now and the 5V supply is back. It is not tested in the machine yet but I know that Johan is very eager to do that. I was a little worried that the voltage regulator LM309 was broken too but apparently it is not.

That was very nice of you to fix it for him.

On mine one of the 3-pin regulators was cooked, I think it was a 7812 in a TO-3 can. But if the power supply rails are all OK, then I'm sure the DRO will be fine. If you want to improve it further, add some more heat sink grease under the components on the heat sink. After 40 or 50 years the original grease is probably not working so well.

Originally Posted by Johan.A

I cleaned the scale . Still doesn't work

First step: swap the connectors for X and Y at the DRO, to identify if the problem is with the DRO Y channel or with the Y scale.

I tried that before cleaning

OK, so you have isolated the problem to the Y-axis scale. Next step requires a multimeter (cost 15 Euro) set to diode test mode.

(1) With the diode test function, check that pins 1 and 2 show about 0.6 volts with EITHER orientation of the red/black leads, meaning first connect red to 1 and black to 2 and then connect red to 2 and black to 1. This checks the photodiodes for the first quadrature.

(2) With the diode test function, check that pins 5 and 6 show about 0.6 volts with EITHER orientation of the red/black leads. This checks the photodiodes for the second quadrature.

(3) With the diode test function, check that pins 7 and 8 show about 0.6 volts with EITHER orientation of the red/black leads. This checks the photodiodes for the reference mark.

What you are seeing (flickering but no counting) is consistent with one of the diodes being open-circuit in one of the quadratures. If so, you will see that in steps (1) or (2).

Thanks Ballen . I guess I have to wait for my friend to do the measuring .

This morning when I started up the machine the DRO worked on all axis.


Then when i tried it again after about 30 minutes the Y-axis dident work again

I can se on the display that it does react . It flicker when i mowe the axis and I can get a 0.01 .
But it cant read properly.

I have only cleaned the glass scale with a cotton swab .
Maybe its dirt on the reading head ?

Now in the afternoon when I put on the machine , the Y axis works again

Originally Posted by Johan.A

Now in the afternoon when I put on the machine , the Y axis works again

Temperature change in the shop? My guess is that the filament on the lamp on that scale is getting ready to fail.....Open at times then makes contact...
If the scale stops working check for continuity on the lamp power leads. Pins 3 & 4 on the connector going to the scale.
Cheers Ross

What Ross has suggested makes complete sense.

An alternative explanation is that one of the photocells has an intermittent connection. The four photocells which measure the two quadratures are attached to a small circuit board, which is located behind the glass scale. The back of the photocells are soldered to the circuit board, and the front of the photocells have very fine wires that connect them to the circuit board. If one of those fine wires is broken or damaged it can lead to the behavior that you are observing. It might also be something simpler: an intermittent connection inside the connector, inside the head, or where the wiring enters the head or the connector.

If you have a multimeter then you should test for this. First connect a voltmeter or continuity meter to the lamp pins, and then wiggle the cabling, where it goes into the head and into the connector, to see if you can interrupt this connection. Then repeat for the four quadrature photocells.

At the end of the day, if you can't locate the source of the intermittent connection, you should take the read head out of the scale, inspect it with a loupe or a low power microscope, and clean it again. Be careful about the fine wires described above. If possible use pure isopropyl alcohol for cleaning.

If a solution is not easily accomplished,and you are lucky, you might find a read head on e-bay for your series scales....Have done so in the past for read heads that were too damaged to repair.
Also be aware that the incandescent lamps for those heads (if so fitted, could be LED) are in short supply....may no longer be available or limited to stocks held by repair facilities.
Replacement of the lamp (if its the problem) is a job that would require calibration equipment or at the least an O-Scope to set the lamp output....Likely a job for a professional Heidenhain repair shop.

Cheers Ross

Originally Posted by Johan.A

Now in the afternoon when I put on the machine , the Y axis works again

Did you figure out what was causing the intermittent failure?

Hi

No I havent turned it on anymore .

There have been alot of other things to do in my garage workshop to get it in working order
taken down cabinets and carpented and painted a wall where the machine should stand

I did get a late answer from Heidenhain today . He have marked components wich breaks sometimes

Johan

Hi Johan,

Thanks for the photo. Would it be possible for you to post a larger version? Try reducing the file size on your PC BEFORE you upload it to PM, that might prevent it from being compressed to the size of a postage stamp.

Originally Posted by ballen

Hi Johan,

Thanks for the photo. Would it be possible for you to post a larger version? Try reducing the file size on your PC BEFORE you upload it to PM, that might prevent it from being compressed to the size of a postage stamp.

It dident work out . Now its two small pics . I e-mailed you the picture

Originally Posted by Johan.A

It dident work out . Now its two small pics . I e-mailed you the picture

Maybe Bruce will post a larger image...
Cheers Ross

That was strange . Now I resend it for you .