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  #1  
Old 26th November 2009, 01:00 PM
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Default Enlarger timer issues

Hereís a request for a sanity check

I use an Omega Superchromega DII dichroic enlarger, with the wonderfully-named Chromegatrol combined power supply and timer. The timer part has become increasingly erratic. Not only is it inaccurate, itís unpredictably innacurate, so I canít simply work out a correction factor for exposure times. The logical solution seems to be to add a spare Paterson digital timer that I have into the circuit to give me some semblance of accurate exposure time. I plan to do this by putting the timer between the mains socket and the power supply, leaving the power supply on Ďfocusí mode so that when the Paterson timer decides to let power through, the enlarger lamp will come straight on..

This seems a sensible approach, but Iím a little worried about only having the cooling fan in the enlarger head running when the timer is on. The fan normally runs continuously when the psu is switched on and independently of whether the lamp is on or not. Is there an exposure time beyond which I shouldnít go to avoid overheating problems?

Any thoughts/words of widom gratefully received.
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Old 26th November 2009, 01:36 PM
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If the enlarger has been designed to have its cooling system running all the time I think you would be lucky to get away with doing otherwise. I suspect there will be a high bulb failure rate (are they expensive bulbs?) but worse of all the excessive heat build up could do damage to other components and wires within the head. Even if you do some experiments to establish a 'safe' bulb light time you can bet that one day you will forget and leave the bulb on for longer with disastrous effect :-(

Is having it serviced / repaired totally out of the question?

Neil.
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Old 26th November 2009, 01:48 PM
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I agree with Neil on the overheating issue. I'm not familiar with the Chromegatrol although I do get asked from time to time how to use one of our timers with that enlarger type. Switching the mains would work, but you may run into problems with reliability because the power supply wasn't designed to be switched on and off frequently, and there may also be a time delay between applying power and the lamp coming on.

I believe the enlarger uses a standard 24v lamp, but I'm not sure about the fan - do you know whether that's 24v or mains voltage? If the latter it would be fairly straightforward to rig up a separate mains supply to it.

Otherwise, your best bet is to keep the Chromegatrol power supply and as you suggest switch it to "focus", and get hold of a power relay with a mains voltage coil. Wire the relay contacts so as to switch the 24v to the lamp, and connect the relay coil to your Paterson timer. That way you'll keep all the functions of the Chromegatrol intact apart from the dodgy timer, and you won't cause any problems with its reliability or overheating. And of course you can replace the Paterson timer with a StopClock quite easily .

Hope that helps!
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Old 26th November 2009, 01:49 PM
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I'm not familier with this enlarger. What is the rating of the lamp? It could be a simple solution to change the transformer, and use your Paterson timer with that.
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Old 26th November 2009, 01:49 PM
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Quote:
Originally Posted by B&W Neil View Post
Even if you do some experiments to establish a 'safe' bulb light time you can bet that one day you will forget and leave the bulb on for longer with disastrous effect :-(

Is having it serviced / repaired totally out of the question?

Neil.
Thanks Neil. Disastrous effects are not high on my list of priorities.

Repair is not out of the question, provided I can find someone local to look at it. It is very heavy and posting it off somewhere will be prohibitively expensive.
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Old 26th November 2009, 02:00 PM
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Richard's ideas sound good to me - especially if gives you the option to splice in a StopClock. Once you have one of those you will never look back ;-)

Neil.
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Old 26th November 2009, 02:03 PM
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Is this the item? eBay link.
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Old 26th November 2009, 02:04 PM
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Is your Chromegatrol the one with decade switches to set the time? If so, a squirt of switch cleaner might just sort it. Problems with timers like that are usually wear or dirt in the switches or potentiometers that set the time. If you can keep the power supply you'll benefit from the voltage stabiliser therein.
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Old 26th November 2009, 03:07 PM
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Crikey, thanks for all the helpful comments and thoughts. For reference, this is the whole beast and this is the psu. (both ebay links)

The enlarger bulb is 250W, 24V if that is any help. The lead from the head that plugs into the back of the psu has a total of 7 pins (6 square/rectangular and 1 round) and I'm not keen on messing with that. The mains cable is hardwired into the back panel of the psu.

Dave/Richard - Given that I'm not particularly comfortable playing around with wiring and things electrical, Iíll have to leave that to the professionals. On the subject of switch cleaner, do I just apply if from the outside, or will I have to take the front panel off and give the inner face a squirt as well? Sorry for the naÔve question but this is all new territory for me.

Richard - funnily enough, I would like to get a stop-clock at some point, but the question of connecting it up has always baffled me. Is this likely to be feasible?

I guess this gives me an excuse to concentrate on lith printing for the time being, as the lack of timer precision is going to be less critical than on things like split-grade printing.

Thanks again.
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Old 26th November 2009, 03:40 PM
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Quote:
Originally Posted by PaulG View Post
On the subject of switch cleaner, do I just apply if from the outside, or will I have to take the front panel off and give the inner face a squirt as well? Sorry for the naÔve question but this is all new territory for me.
You would need access to the switches themselves so that would require at least that the cover is removed. But if you've a friendly electrical shop or electrician nearby I'm sure they'd be able to do that.

Quote:
Richard - funnily enough, I would like to get a stop-clock at some point, but the question of connecting it up has always baffled me. Is this likely to be feasible?
To use a StopClock (or other timer) you'd really need either to use a relay method I suggested earlier (which would require surgery on that multiway plug), or replace the Chromegatrol completely with another transformer and wire the fan up so it runs continuously. Either way it's not an easy task I'm afraid. The only power supply / timer combo that's easy to work with is the DeVere Transtab because it is designed to work with an external timer - either a DeVere one or somebody else's - and we can supply a suitable cable.
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