Zone VI Compensating Development Timer Temperature Sensor

I've owned one of these timers since Fred first released it and mine has the original C-clamp with thermistor (black) temperature sensor. At some point later on, this sensor was replaced with a metal sheath. I believe my temp sensor may have grown "tired" because the timer is losing about 5 - 6 secs every 30 secs, based on accurately measured water temp of 68F. Does anyone know if a replacement temp sensor is available that works with this timer? My foot switch is, also, acting wonky but I wouldn't think this piece too hard to find a replacement for.

Thank you for any help provided.

Try gently cleaning the sensor spot on the clamp. They build up grime over time, which makes them slower to react. Also make sure all your wiring contacts going into the timer box itself are clean and not corroded. I don't use their footswitch at all, but a waterproofed in-line one. If I did go back to a footswitch, it would be a completely rubberized one intended for damp environments. McMaster Carr probably still sells those.

I would go to Richard Ritter directly, he should know, even if the design was done by Paul Horowitz and he may still be at Harvard faculty (I contacted him some years ago, getting contact through Harvards website) and he graciously responded on the Enlarging Timer.

Foot switch is easy to find new, start on Amazon.

Hassasin said:

I would go to Richard Ritter directly, he should know, even if the design was done by Paul Horowitz and he may still be at Harvard faculty (I contacted him some years ago, getting contact through Harvards website) and he graciously responded on the Enlarging Timer.

Foot switch is easy to find new, start on Amazon.

Click to expand...

I've already contacted Richard and his suggestion was to watch for one on these forums or eBay. I'd really rather get mine working properly as I don't think it's the timer itself gone bad.

Alan9940 said:

I've already contacted Richard and his suggestion was to watch for one on these forums or eBay. I'd really rather get mine working properly as I don't think it's the timer itself gone bad.

Click to expand...

Then contact Paul Horowitz.

Hassasin said:

Then contact Paul Horowitz.

Click to expand...

I will try Paul. Thanks.

Alan9940 said:

I will try Paul. Thanks.

Click to expand...

Let me just add, that when I contacted him some years back, I just used what Harvard had for contact and sent him a completely unsolicited email. He responded within a few days, was not long, and gave details for the light sensor Zone VI used in the cold light heads for compensating feature.

I am also surprised a bit Richard Ritter blew you off like that. But maybe Zone VI is just an old story for him that possibly draws ire more than sympathy now.

Would you please post any information you receive from Paul for the edification of others? This sort of info should be preserved if possible.

Steve Goldstein said:

Would you please post any information you receive from Paul for the edification of others? This sort of info should be preserved if possible.

Click to expand...

Will do.

Not all of these were sold by Zone VI. When Calumet bought out Z VI, they sold some too, but dropped the ball on the warranty or repairs, because the actual subcontracted manufacturer of these dried up too. That was a long time ago. I'm glad mine is still working, given a bit of maintenance attention from time to time.

What I failed to mention is that if contacts get dirty, sometime the unit forgets what compensation setting it's supposed to be on, and just recalls the previous one. Remove the cable, inspect it, and re-insert it.

FWIW, I contacted Paul and he responded back in <1 hour! There was quite a bit of "technical speak" in his reply outlining possible issues with the electronics of the timer that flew well above my head, but he is willing to take a look at it. According to him, it can be fixed.

Alan9940 said:

There was quite a bit of "technical speak" in his reply outlining possible issues with the electronics of the timer that flew well above my head

Click to expand...

Maybe if you could share it, it might prove very useful to others.

@Alan9940 Good to know he is still as he was back in the day, clearly a man of integrity But yes, that technical part would be good to see.

koraks said:

Maybe if you could share it, it might prove very useful to others.

Click to expand...

Sure...

These were his guesses without actually looking at my unit:

1. It could be a bad thermistor, though they are extremely robust. He added here that if developer was leaking into the sensor assembly, it would lower the resistance, which is what NTC thermistors do when hotter so the sign of the effect is backwards. (no idea what this means?)

2. Drift of the timing capacitor - unlikely, because it's a film capacitor (I'd be nuts to use an electrolytic.)

3. Drift of the timing resistor - this is more likely, especially since I used (I think) carbon-comp resistors, not metal-film, but I'd have to look at the BOM (bill of materials) or the unit itself to see.

4. Drift of the calibration trimmer - this is also possible.

He further added that to disambiguate these possibilities what he'd do in his lab is to use a fixed resistor (I think 10.0k, but again I'd have to look at the schematics) and set the oscillator to whatever the nominal frequency should be (i.e. to generate an accurate 1 sec cadence.)

As I said, most of this "electronic speak" is above my head but those with this knowledge may find it useful info.

Alan9940 said:

I've owned one of these timers since Fred first released it and mine has the original C-clamp with thermistor (black) temperature sensor. At some point later on, this sensor was replaced with a metal sheath. I believe my temp sensor may have grown "tired" because the timer is losing about 5 - 6 secs every 30 secs, based on accurately measured water temp of 68F. Does anyone know if a replacement temp sensor is available that works with this timer? My foot switch is, also, acting wonky but I wouldn't think this piece too hard to find a replacement for.

Thank you for any help provided.

Click to expand...

You probably can take the footswitch apart for cleaning. Most of them are held together with a shaft that has c-clips on both ends. One carefully removes the c-clip on one end, and - as you pull the shaft out, hold the switch surface in place and gently release it (so the spring underneath doesn't go flying. Then spray the electrical contacts with DeOxit and reassemble.

I have one of these timers as well but - knowing it would die some day - I designed and built my own replacement. If you're handy with electronics, this is an easy project to build. If you're not, find a motivated youngster

Thanks for posting info regarding the timer you designed. I scanned through the attachment and my eyes started glazing over.

Alan9940 said:

Thanks for posting info regarding the timer you designed. I scanned through the attachment and my eyes started glazing over.

Click to expand...

If you're not an electronics nerd, try to find one. This isn't actually all that tough to build. The hardest part was fashioning a case for the thing. It works really great and I use it all the time.

Great and durable unit, I had and used one heavily for years, to the point where it needed a tune up. I too had the black resin type sensor, and the 'switch' has an O-ring, with two or one small magnet. Mine needed just a refresh of the O-ring and the magnetic properties with the correct pole of a strong magnet. I switched from a foot pedal to large, positive click large switch that I mounted to the wall. (All that vaporized in the Lahaina fires).

As for the drift, 5-6 sec over a min of development at 68F is likely correct, and you're just seeing the proper precision. I would actually calibrate your process to the Zone VI unit and trust it. I used mine enough on prints to where I would trust it with my medium and large format in deep tanks.

That 5-6 sec may be from the thermometer used. Many photographic thermometers can be off, not saying yours is. I used to use a Kodak Process one that was most accurate and precise, enough I could calibrate other ones, usually Paterson's color ones. I used to buy those in batches of 5, test and select the two best, and return the others. Most were 'close enough' but I'd chose the two that most closely matched my Kodak one.