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Heidenhain scale repair

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Dave Halford31/08/2019 12:05:46
799 forum posts
8 photos

Hi all,

I have an ancient Hiedenhain DRO 721 without memory.

One of the scales had the bulb holder board missing, I have replaced this with a Lilly pad style 5v LED.

This works well is one direction, but stops counting negative after a short distance. Swapping the scale inputs over proves it's the scale and not the box.

Has anyone an answer please (apart from buy a new scale smiley)



Stueeee31/08/2019 13:20:22
52 forum posts

I have a Heidenhain 731 DRO on one of my lathes. I have been told that the filament bulb used in the scales for these is quite special; not sure whether any LED would behave in the same way as a filament arrangement.

When I had an issue with one of my scales, after doing the usual stuff re cable connections etc. it had to go back for repair to Heidenhain, this was 12 or so years ago, and they were far more interested in selling me a (massively expensive) complete new DRO setup, rather than mending mine, although they did (expensively again) mend the scale with a two week turn round.

elanman31/08/2019 13:50:01
31 forum posts
3 photos

Be very careful who you deal with, I got well turned over when I sent one of my scales to an Heidenhain agent for repair. They had said on the phone that it would be repairable then when it came to fixing it they just wanted to sell me a new scale. Make sure you get a quote before you get too deep in.

The good news is that EMS-i sell compatible scales with the round plug on. Not super cheap but about 25% the cost that the Heidenhain agents were offering. More important was the fact that EMS-i were really helpfull to deal with.

In this day and age you may well be able to get a whole new setup for no more than the cost of repair of the scale. And if you only have a simple DRO at the moment you will get a lot more features.

I don't seem to be able to paste a link but you can google EMS-i





Edited By elanman on 31/08/2019 13:53:08

Edited By elanman on 31/08/2019 13:55:43

mgnbuk31/08/2019 13:53:19
777 forum posts
61 photos

Heidenhain used to use a 12v lamp running on 5v in read heads and encoders - they were just a dim glow, but lasted thousands of hours (20,000 hours was quoted IIRC). When a lamp failed, the read head or encoder was replaced. IIRC the lamp positon was adjusted at the factory to position it correctly behing the read head grating to optimise the signal output. As far as I can remember it was only the very high line count rotary encoders intended for direct mounting on rotary tables that had the lamp changed when they failed, but it required the encoder to be sent back to Heidenhain for repair - as these encoders cost many thousands of pounds, a repair of several hundred was cost effective. Scales were able to be repaired relatively cheaply by the end user by replacing the read head.

Have you tried cleaning the the scale grating if you only have a localised problem ? I have done this on contaminated scales that read intermittently by removing the head assembly & the sealing lips, then carefully cleaning the grating with tissues and IPA. Go easy. don't apply a lot of pressure or use sharp tools and beware the sharp edges of the glass ! With the sealing lips out you can examine the glass grating for contamination & obvious damage - the grating should look like a contiuous grey band down the length of the glass strip. Any scratches or discontinuities here would scrap the scale.

Your scale must be old to have a lamp, as Heidenhain went to Leds many years ago - maybe IR Leds, as I don't recall seeing the dim glow when looking down the length of a scale with an end cap removed on later scales. Be aware that Heidenhain counters were usually sine wave input, not square wave. Early units were 11 micro amp output, current items are 1v peak-to-peak . Various signal converters are available - I have used 5v TTL square wave to 1v peak-to-peak converters on retrofits.

Given Heidenhain repair & new part prices, it would probably be cheaper to replace the entire setup with a new Chinese counter & scales and sell on the working parts of the Heidenhain setup to recoup some of the cost. Not knocking Heidenhain in any way here - they are a very professional producer of high quality industrial equipment and their service is second to non, but their pricing reflects that and isn't exactly "home workshop friendly".

You could give them a ring on 01444 247711 and ask the Service Department for advice - they don't bite.

Nigel B

Dave Halford31/08/2019 14:33:40
799 forum posts
8 photos

Very old = 1970 something.

What happens is you can wind the table the full travel, or any part of, for more than an inch and it works normally in one direction only. reversing more than an inch causes the decrease in count to stop. This happens anywhere along the scale.

I have swapped read heads and the glass scale works normally in both directions.

The LED is quite bright, I tried a piece of paper to reduce the brightness, but this prevented counting in either direction so diffuse light is not the answer.

elanman31/08/2019 15:59:59
31 forum posts
3 photos


Yes I have also found the Heidenhain service dept very helpful, but only on DRO display issues. Both times I've had scale problems I've told to use the agents. This was a good few years ago so things could be different now.



mgnbuk01/09/2019 13:17:41
777 forum posts
61 photos


I have not had to use a Heidenhain agent, though I don't doubt what you say from recent experience - I just bought heads direct from Heidenhain to exchange myself. I have dealt with Heidenhain for a long time (since 1983) through my previous employment (a CNC retrofit company) & have just continued to do so where I am currently (maintenance for a machine shop). The last scale I bought was several years ago to replace one that had been damaged on a Cincinnati machining centre - the machine head had dropped during transport & punched the read head through the bottom of the scale, damaging the glass in the process - about £1500 + Vat for an 1100-ish long TTL output scale with distance coded reference marks IIRC.


Sounds like your scale itself is OK. It is a long time since I had the conversation, but I have a recollection that the reason that the lamp was not user replaceable was that precise focussing was required. Being a good Yorkshireman, I had enquired why a £300 (at the time ! ) replacement head was required because of a failed £1 lamp. As the 11 micro amp scale signal is the un-amplified output from the photo cells, the lamp position had to be set precisely on a test rig to get the correct signal output - it would have cost more to send a head back to have the lamp changed & reset than a new replacement head mass produced at the factory cost. From an industrial point of view, I can get a machine up & running more cost effectively by fitting a new head if one fails - OK £500 or so isn't cheap, but it doesn't take long to cost a lot more than that if a machine is out of production.

I don't know enough about the components used in the heads, but could it be that your Led is too bright, too diffuse, or the wrong colour temperature ? My recollection of the originals with the lamp is that they were a very dim yellowish light - as you would expect from an incandescent lamp running at less than half it's rated voltage. Could you mask the Led to replicate the illuminated area produced by your good head with the lamp ? And / or dim the Led with a series resistor ?

The last time I had a DRO related failure (the counter failed on an Anilam readout on a Harrison lathe) I didn't bother getting a price to have the counter repaired or replaced, just bought a complete system from Machine DRO at around £400 including Vat and delivery. A bit more fitting time to swap both scales & mount the new counter, but the new installation was warranted & has proven reliable.

Nigel B

Machine-DRO02/09/2019 10:47:00
22 forum posts
25 photos

Good morning,

Sorry to hear you are having issues with your Heidenhain DRO system

We can supply an adaptor with the round 9 pin connector, that will convert signal from the M-DRO optical or magnetic encoders to be compatible with the Heidenhain consoles. Please note this is not compatible with all Heidenhain encoders so we would need some further details from you.

If you wish to discuss further please contact us on 01992 455921 or

All the best,

Machine-DRO team

Dave Halford02/06/2020 11:54:09
799 forum posts
8 photos

Finally shortened 4" and remounted the scale more solidly.

Scale works both ways properly now so very touchy on running parallel in both vertical and horizontal plane.

mgnbuk02/06/2020 12:29:14
777 forum posts
61 photos

very touchy on running parallel in both vertical and horizontal plane.

IIRC the mounting tolerance for parallelism with the axis movement was 0.1mm over the length of the extrusion in both directions - we used to generally get to better than 0.05mm, but the ultimate limiting factor was the straightness of the extrusion. The read head to extrusion gap was 1mm, usually set automatically by the plastic transit clamps that held the read head to the extrusion for shipping & which were removed after installation. The read head was was mounted such that the front face was level with the extrusion. The original Heidenhain manuals for the scales can be found online & give all the required mounting information.

Nigel B.

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