Macbeth TR-527 Densitometer Restoration (Repair, Schematics, Operation)

Hi all,

To those interested, I am linking my work on repairing and restoring a Macbeth TR-527 Reflection and Transmission Densitometer. Work involved reverse engineering the light path and full electrical circuit, figuring out the electrical operation and replacing or repairing defective elements, both electrical and optical.

The article will probably interest electrical engineers most, because it goes in depth on how the TR-527/TR-524 works electrically, but I have also discussed the theory of operation in broader terms. They can still be had relatively cheaply from the usual places and are mostly repairable.

Reverse-engineering and repairing a Macbeth TR-527 Transmission and Reflection Densitometer

I am also still working on a series of articles detailing how I designed and built a homemade densitometer, but I have so far only covered the introduction to the theory.

I hope this can help someone trying to bring these lovely instruments back to life

Nice, I have two of these. One I bought like 15 years ago off ebay and the seller didn't bother to protect anything, it was literally in a box with nothing else. When it arrived it was a wreck; dented, the bulb smashed, and the plastic disc broken in three places. The spare bulb was good and it powered up and wasn't out of calibration, lol. Got my money back and kept it.

Great work!

I was lucky, I got two densitometers at the same time and they came with complete documentation, including a multipage service manual with schematics and parts diagrams.

I have been able to keep both working fine over the years.

I like the Tobias/X-rite construction for its simplicity. That's what I emulated for my homemade design:


It is photodiode-based with a custom logarithmic amplifier. The whole thing probably weighs 1/10th of the Macbeth, with all its cast metal parts...Shipping from the States costs me as much as the ebay bid haha.
It is a good instrument, though, the reflection densitometry head is a masterpiece of machining. That was the whole reason I got it in the first place - was trying to copy the reflection geometry, but I was too curious to let the rest of it go. I made a CAD model of it for those interested.

Hey, that's interesting; thanks for the good read! I'm hoping to come back to this. I'd love to give that a thorough read.

Very nice sensitometer!

I never got into reflection densitometery and still avoid it. Problem with prints, where blacks are important is that the high density is hard to read even with the best calibrated equipment. Good thing about testing film, low density is easy to read and easy to make a tester to read it.

Fantastic work. Congratulations !
I have a Macbeth TR1224. A later generation, no PM. Color filters are kaput, but I use it only for BW.

bernard_L said:

Fantastic work. Congratulations !
I have a Macbeth TR1224. A later generation, no PM. Color filters are kaput, but I use it only for BW.

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Thanks! How do you know the filters have degraded? I have an old eastern German specrophotometer on my dining table that I’m trying to computerize, but it is a slow ordeal, so I don’t have a way yet of knowing.
Meanwhile, I’m also collecting all the schematics and supplemental documents into a pdf for easier access (without all the verbose descriptions). I’ll post it here soon.
It’s also possible I have made mistakes here and there, but I’m confident they are minor.

Nikola Dulgiarov said:

How do you know the filters have degraded?

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Some years ago:

Now the B channel readings are completely wrong in one of A or M status (don't remember which just now) so I just use the other one, whichever. I don't know how the "V" value is derived from RGB measurements.
Maybe I'll try to install some of these; probably not compliant with A/M standards; don't care.
Also: custom wavelength, more expensive:

What I like with the TR1224 is the output of values via serial port; no more typing.

Interesting threads, Bernard! I have purchased some glass filters from abroad also, I have found the quality to be in line or better than the price (vs. Edmund, Thorlabs, etc). I was also gifted some ORWO gelatin filters, they look pristine, but maybe they have also aged in time...

I made a small add-on for the TR-527, plugs in the back and then into a USB to send readings over serial port with 18-bit resolution (which is overkill, but what I had available). I had the idea to also send the filter position like the original Macbeth upgrade, but I kept it simpler in the end.

Also, here are the compiled schematics and photos in full resolution (large file!).