Help! My enlarger won't turn on!

A 1 amp fuse would be less likely to blow than a .63 Amp fuse. The number is the number of amperes the fuse can stand before it fails. If the fuse is higher than what's called for, it might be because someone has substituted a larger one because the correct one was failing repeatedly (possibly indicating some fault in the circuit it is protecting), or else it's an oversight.

Clarimort said:

I just noticed: The fuse that was inside the transformer is a T1A (so 1 Ampere) but the sticker on the transormer says it needs a T0.63A.
Could that be the reason why the fuse blew? I'm gonna get one with 0.63 Ampere tomorrow.

I also checked my Kaiser Timer and noticed that it sais "2,5 AF" next to the fuse to I am guessing it should be a fast one with 2,5 Ampere. But inside is a slow one with 2,5 Ampere. Is that bad? Should I change it?

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Chosing the right one, T or F is of importance for proper working!

Chosing T instead of F may ruin your device.
The other way round your fuse may blow prematurely, not ruining anything than itself, but the same time setting your device out of order without reason.

That T1A was used instead of a T0.63 rather reduced the chance of blowing.
The likely reason for blowing I already gave earlier.

As you are lacking basic understanding of electrics, just replace a fuse by the exact type stated on the device or its manual. And do not try some shortcut.

The fact that a T1 amp, rather than the specified T0.63 amp, fuse was used mostly suggests to me that someone found T1A fuses in a shop before they could dig up a 630ma one...

You "Can", with very big quotes around it, make use of a fuse that has a higher capacity rating [and slower trip speed] and have a device run reliably. However doing so will give up protection rating to the equipment, and possibly yourself.

Using a lower rated fuse, or one designed to trip faster than specified, is a good way to spend a lot on fuses...

Clarimort said:

I have to be the dumb one:
What is it?

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Control board from Durst AF closed loop enlarger (2501/2506/Optromat models use this common one).
Feel the beauty of the "Working methodically from the wall to the bulb ..." advice.

I went to college with a kid that grew up on a very small farm. They didn't have a lot of cash money, heated with wood stoves etc. They had 25A fuses where 15A was called for. It was old "knob and post" wiring with asbestos covered wire. Each individual conductor ran through a ceramic tube or stood off from the structure on a ceramic post insulator. I always imagined the wiring was like an electric toaster. . Very old house 19th century.
Miracle they didn't burn the place down. And of course nothing was grounded, (earthed).

Martin Rickards said:

Another point, probably unrelated, is not to handle those Xenophot bulbs bare-handed. The grease on your fingers can reduce their life. Always try to wear cotton/latex gloves when changing them.

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I use a clean sock as a mitten.