Tracking down inconsistent exposure

What type of enlarger head are you using? Cold light heads are notorious for light output variation due to head temperature. I don't have any direct experience with color heads or condenser heads, but like you I'd suspect voltage fluctuations. Trying to pinpoint these types of problems can be problematic, but your best bet is to pick one variable at a time and eliminate. Good luck.

 

What about developer temperature, that can vary, I use a dish warme. My main enlargers all have colour heads so voltage stabilised power transformers, that does make a difference.

Ian

 

I used to have a Jobo timer. It was electronic and at first everything was fine. From some point on, I had inconsistent print density. I'd make my test strips and the final print would be considerably different in some cases. Eventually, things got worse and it would turn on the lamp for a randomish amount of time. I never really found out what exactly the problem was, but I suspect it was the selector(s) switches. IIRC, they select different resistors and vary the time this way. Perhaps the contacts were problematic, who knows, I really hated the damned thing.

Anyway, see if you can try another timer.

 

I knew an old man who had a very old cold light head; maybe an adapted Graflex type? Anyway, he modified a large, electromagnetically fired packard shutter to sit between it and the lens bellows. He kept the cold light on constantly and had the timer wired to the shutter. Claimed he had very repeatable results...

 

Power voltage variation?

 

I'm also thinking voltage variation as a possibility. I've known photographers who use voltage regulators to avoid that.

 

Agreed... voltage stabilisation is a must! My enlargers plus main safelight are connected but not other safelights which I actually see brighten then dim whilst on - an intermittent flickering. However, I’m not sure that these fluctuations can cause an appreciable and visible print density variation as measured in stops or part thereof. Knowing the electrical supply does cause voltage variation I think stabilisation is a wise precaution as it eliminates a variable in the quest for consistency.

 

I had my exposures off by about stop (I guess) between making test strip print and the corresponding print within about 3 minutes . Measured variation is ususaly between 220-235V. First cause is the electrical heating in my darkroom, the second is the house heating. There is also variation in the distribution net. What I do now is turning off both heatings which fortunately I can do from my darkroom, so i do it only when working with enlarger. I know I will need the stabiliser.

 

My Besseler 45 condenser enlarger came equipped with a voltage regulator, so voltage changes must be a common problem for manufacturer to include as a given.

 

0ne way to accurately test your enlarger timer is to connect a Graylab 300 timer to the enlarger timer’s output socket. Preset the Graylab to a longer time than wanted. Then, when you trigger the timer it will run the Graylab for the duration and record the exposure time delivered by the enlarger timer as elapsed time on the Graylab.

Repeat this several times to learn if the enlarger timer is accurate or not and whether the delivered times are consistent or not. I did this to confirm that some capacitor-controlled timers were failing and needed their electrolytic capacitors replaced.

 

Mine transformer is from manufacturer and is marked as stabilised, but its output also varies ...

 

I´ve hat a similar problem twice; once the halogen bulb was just before spent, the second time the stabilizer of a stabilized power source was about to start smoking.

Furthermore, I make sure the system has run for about a minute before measuring with the Heiland Splitgrade, I have experienced readings being off when measuring immediately after starting a printing session. But that really is the first minute only.

 

Your transformer may be the problem. If it isn't stabilizing, it might be de-stabilizing.