Tobias Densitometer Tune Up

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ic-racer

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I was doing some year-end cleaning and got out my spare densitometer for a checkup. Seems to work OK but the button did not work all the time.

Thanks to Mouser's extensive database of switches, I was able to get an exact replacement.
Densitometer Switch 1.jpg
 
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ic-racer

ic-racer

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Quick check of the innards. There was some oxidation on the blue multiconnectors on my other identical unit, so I DeOxed all the connectors here. The filter cap still works, but I'll have to check with HAL 9000 if we should change it or let it fail first. Last time I changed a filter capacitor based on his recommendation, the old capacitor was fine and had no evidence it would fail. HAL 9000 got all bent out of shape after that and made a big fuss...

Densitometer Switch 2.jpg
 
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Check the calibration...checks out OK!

Densitometer Switch 3.jpg
 
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Bill Burk

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I was doing some year-end cleaning and got out my spare densitometer for a checkup. Seems to work OK but the button did not work all the time.

Thanks to Mouser's extensive database of switches, I was able to get an exact replacement.
Haa I have half a dozen of those pushbuttons that I picked up at a garage sale a while ago. If I knew you needed one I would've mailed it to you for nothing.
 

MTGseattle

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Bump for an old thread?

I just acquired one of these thanks to @Jon Shiu
I opened the table up tonight just to see what was inside the thing. There were no table light bulbs installed. I question the need for them when just checking negative densities, but I found this odd.
Also, it looks like the source lamp had a nickel sized drop of epoxy or something securing it to the inner base of the machine. (Half on the inner slope of the reflector, and half down on the case) Is that normal? It seems like this would make changing the source lamp a pain.

There's a small strip of some type of clear tape on top of the table running across the aperture. I'm guessing that should also not be there, and was maybe placed there for shipping? I'm pretty sure Jon got it that way based on his pictures.

Lastly, should I contact Tobias and see if I can get a calibration target or is there a way to make a Stouffer wedge work in that capacity?

Anyway, I'm happy. It seems to work, and it was quite a bit cheaper than anything else I had been watching. I'm just not wanting to dive into film testing without at least checking the calibration values.
 

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The lamp is certified to NIST standards, so the expoxy blob is there to exactly orient the bulb filament. It's just like the Macbeth bulbs; they are dead nuts calibrated and not meant to be changed the normal way. You buy the entire mounting bracket and bulb; that is why they are so expensive.

However, since you probably don't require NIST standards in your work (not a put down, only intra Lab work would), you could break the epoxy and replace the bulb with a like bulb and eyeball installing a new bulb.

This ties in with the calibration strip. You can use a standard Souffer wedge with known densities as your calibration wedge; There's nothing magical about their brand target. Just have someone with a good sensitometer read it and give you the values OR purchase a calibrated wedge and use those; plenty close enough.

You can then use those values and that wedge to recalibrate the unit when you change the bulb to get it to as close a match as possible.

Everything is relative; you just need consistency within your own processes.

The tape across the aperture was put there just so it wouldn't fall out during shipment. There are various sizes you can obtain from 1mm to about 3mm, if I recall correctly. You should probably remove it to keep it from ageing and yellowing, but in the end it probably wouldn't really hurt as it would be essentially "zeroed-out" when you calibrate the unit.
 

MTGseattle

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That's interesting regarding the bulb since one can do all sorts of messing around with the little mirror inside the unit. Although as I type I realize that having only a single part being user adjustable probably reduces some headaches.

What's your take on the 2 bulbs that serve "Table lamp" function? They're small candelabra base (I forget the nomenclature) style. I guess they afford one a bit of 'light table" on the unit for looking at the part of the negative you wish to read.
 
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Bump for an old thread?

I just acquired one of these thanks to @Jon Shiu
I opened the table up tonight just to see what was inside the thing. There were no table light bulbs installed. I question the need for them when just checking negative densities, but I found this odd.
Good idea you opened it to check it out. With any used darkroom equipment, it should be given a check over.

The table lamp is 7W C7 candelabra base night-light lamp.

Also, it looks like the source lamp had a nickel sized drop of epoxy or something securing it to the inner base of the machine. (Half on the inner slope of the reflector, and half down on the case) Is that normal? It seems like this would make changing the source lamp a pain.

The main lamp is 7P6V.

The main lamp should be firmly secured in the socket with two screws on the lamp pins. There is no mention of gluing the lamp in the service manual. Maybe the screws are stripped?
The procedure for replacing and aligning the lamp and mirror is below.

There's a small strip of some type of clear tape on top of the table running across the aperture. I'm guessing that should also not be there, and was maybe placed there for shipping? I'm pretty sure Jon got it that way based on his pictures.
The aperture plate is not held in place other than gravity. Likely it was taped in place for transport and the tape should be reomved. There were two different size aperture plates available. 2mm and 3mm. Under the aperture there should be a blue filter.
Lastly, should I contact Tobias and see if I can get a calibration target or is there a way to make a Stouffer wedge work in that capacity?

You can get a calibrated step wedge from Stouffer. Us the step that is closest to 2.0 density for the calibration. Remember to calibrate and do all the readings with emulsion side UP.

There should be very little drift if the power supply is good. Even with a perfectly functioning unit, Tobias recommends double checking Zero in between every few readings. Mine is pretty good, I can get all the way through a strip, writing down the values as I go, and it still zeros out the same when I'm done.

Screen Shot 2024-01-10 at 10.58.53 PM.png
DSC_0036.JPG
Screen Shot 2024-01-10 at 10.46.13 PM.png
 
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MTGseattle

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Thank you @ic-racer . I did get the manual with mine, and a little pack with the alternate aperture and 2 hex keys. I really appreciate the last photo you added.
I'll get one of mine posted tonight.
 

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Good idea you opened it to check it out. With any used darkroom equipment, it should be given a check over.

The table lamp is 7W C7 candelabra base night-light lamp.



The main lamp is 7P6V.

The main lamp should be firmly secured in the socket with two screws on the lamp pins. There is no mention of gluing the lamp in the service manual. Maybe the screws are stripped?
The procedure for replacing and aligning the lamp and mirror is below.


The aperture plate is not held in place other than gravity. Likely it was taped in place for transport and the tape should be reomved. There were two different size aperture plates available. 2mm and 3mm. Under the aperture there should be a blue filter.


You can get a calibrated step wedge from Stouffer. Us the step that is closest to 2.0 density for the calibration. Remember to calibrate and do all the readings with emulsion side UP.

Good information from ic-racer and Kino. Some time ago I contacted Tobias for a calibration slip - I was a bit disappointed at the price: $150, not including shipping. I bought the much more useful, calibrated 4x5 Stouffer step wedge instead. Shipped, it ran me about 60% of the Tobias strip.
 

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One of the table lights in my Tobias TBX-U has failed. The Tobias replacement bulb is available - for ~$85!! It's a miniature bayonet or single-contact-base bulb, twist-lock rather than the screw-in bulb 7P6V noted by ic-racer. Any ideas for an off-the-shelf replacement?

tobias_table_bulb.jpg
 
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ic-racer

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Some options here. Do you know the voltage?

 

tom williams

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Some options here. Do you know the voltage?

That's a good lead ic-racer. McMaster-Carr doesn't occur to me when I think electronics parts. They have some near matches that I think I can make work, the best match being a 12vdc LED rather than the OEM-style incandescent. But I think I can shield the sensor source light so that the LED light doesn't enter into the detector reading- that may even be overcautious. Or chance a 13vdc bulb in the 12vdc circuit.
 

MTGseattle

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I did not get a separate service manual. The booklet I have does have parts list and calibration instructions.

I sourced a Stouffer T5100C 5-step 2x4 inch sensitivity guide. the steps are listed as .05, .94, 1.92, 2.90 and 4.00 respectively. I think this will only help me test/calibrate the "white" filter setting.
 
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I did not get a separate service manual. The booklet I have does have parts list and calibration instructions.

I sourced a Stouffer T5100C 5-step 2x4 inch sensitivity guide. the steps are listed as .05, .94, 1.92, 2.90 and 4.00 respectively. I think this will only help me test/calibrate the "white" filter setting.

See the values of the color-calibrated step wedge in post #3 above. If your step is anything like ones I have used in the past, it may be pretty close to neutral density. In which case the color densities will be similar to the white density.
 

tom williams

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Some options here. Do you know the voltage?

I measured 11.8V at the table light fixtures. McMaster-Carr offered no exact matches to that voltage and the physical dimensions of the original bulbs. I bought McMaster-Carr number 9527T301 bulbs, rated at 13V 4.3W .33A, manufactured by CEC Industries LTD. Trade number 1816. The 4.3W choice was a not-too-bad guess: the M-C bulbs are a tad brighter than the originals.
.
The original bulb (the glass part) diameter is 0.39" and they're 1.12" long. The CEC bulbs are .41" in bulb diameter and 1.19" long. The bases are identical (Bayonet base, single contact, bottom indexed miniature light bulbs). Ten pieces, $17 shipped. I'm calling it a decent alternative to Tobias' $85 per bulb. A cursory test didn't show a discrepancy between the surviving original bulb and the brighter bulb - I'll make a stringent test once I get a fresh step wedge from Stouffer.
 

MTGseattle

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@tom williams A few years back I had a devil of a time sourcing a bulb that had parallel pins like that. I must have using the wrong terms or something.
 
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I usually order a bunch of spares of all incandescent specialty lamps these days.

bulbs.jpeg
 
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