I use a transformer to supply 12vac to a photographic enlarger. A fault has occurred and the output voltage is now only 2-3 vac. There are adjustable potentiometers on the circuit board but adjusting these does not achieve a high enough voltage to the transformer.

The output from the voltage regulator PCB to the primary winding of the transformer is only 30-60 volts not the 240v required to produce 12vac at the transformer output. Can anyone guide me on what the fault may be such that I can repair the pcb, or alternatively give me a circuit diagram for a modern equivalent that I could build to replace the old pcb with. I've attached a couple of photos. Thank you!

 

Looks like an ordinary mains dimmer, you could remove the pcb, and connect the transformer Primary directly to the mains 230V, and then measure the secondary side.

 

Looks like an ordinary mains dimmer, you could remove the pcb, and connect the transformer Primary directly to the mains 230V, and then measure the secondary side.

Hi Dave, thanks for your post. I thought that too, but when I tried it it blew the fuse in the transformer, but it is only a 0.63A rated fuse. I thought that the starting current might have blown it - although I did not have the load connected. I'll try it again with a higher rated fuse and see what happens. I'd really like to have the voltage stabilization circuit running if I can as its will ensure that the light intensity from the enlarger is more consistent i.e. not dependent upon mains voltage. The original equipment manufacturer sold 2 versions of the transformer, one with voltage stabilisation and the other without - I don't really know if the variation in mains voltage that occurs today is a problem at all.

 

Hi Dave, thanks for your post. I thought that too, but when I tried it it blew the fuse in the transformer, but it is only a 0.63A rated fuse. I thought that the starting current might have blown it - although I did not have the load connected. I'll try it again with a higher rated fuse and see what happens. I'd really like to have the voltage stabilization circuit running if I can as its will ensure that the light intensity from the enlarger is more consistent i.e. not dependent upon mains voltage. The original equipment manufacturer sold 2 versions of the transformer, one with voltage stabilisation and the other without - I don't really know if the variation in mains voltage that occurs today is a problem at all.

Ok then measure the transformer Primary on ohms to see if it's shorted out, if it's ok try it directly on the the mains, and if it's working fine, get an ordinary light dimmer and put it inside the box,.

The original one has probably blown the Triac .

 

Ok then measure the transformer Primary on ohms to see if it's shorted out, if it's ok try it directly on the the mains, and if it's working fine, get an ordinary light dimmer and put it inside the box,.

The original one has probably blown the Triac .

Hi Again, many thanks. I know the transformer is fine as I've already tested primary and secondary windings and it functions, in that it drops the 68v on the input to 3.4v at the output. I'll get a dimmer and see what happens. Thanks again. Regards Andy