My Duplamat 300w transformer decided it was done after 3 years. Since I couldn't buy another one in the USA, I decided to make my own. Special thanks go out to several members of the Aquatic Plants Digest and the authors of the Krib article on DIY cable heating . This is a pretty big transformer at 24V 12A. All the normal disclaimers apply. Don't try this without the required skills.
Parts list:
From Allied Electronics
227-2010 24V 12A transformer, Allied brand
528-0158 Panel mount banana jacks (2)
626-5112 Hospital grade 14 gauge power cord (a little excessive)
740-5195 Fuse 5mm x 20mm 250V 4A
798-0019 Panel mount fuse holder
806-3015 6" x 6" x 5" Aluminum instrument case (vented)
Total delivered price for above list was $117
From local Lowes:
#10 x 1" machine screws (4)
#10 washers (5)
#10 lock washers (4)
#10 nuts (5)
3/16" spade terminals
1 rubber grommet
Heat shrink tubing
The articles referenced above, and the wiring diagram that comes with the transformer, will tell you all you need to know. Be sure to fuse the black primary power wire, ground the chassis, ground your tank water, and use a GFCI outlet. This is some serious power, take it seriously.
Power cord, fuses, red panel mount banana jacks, fuses, black panel mount fuse holder, transformer (11.5 pounds worth:), case, rubber feet, screws
Inside from back transformer bolted in. All wiring terminated in insulated 3/16" spade terminals soldered in except for ground (on chassis to transformer mount bolt) and wire bug for white primary lead.
Completed unit from front with old Duplamat 300w as reference. The case is a bit bigger than it needs to be, but should vent well.
vented backside of case
That's it! Total cost of roughly $125. The Dupla unit cost me $271 in 1998. The best part is that the transformer itself is a $45 item. Over the long term, this will result in a significant reduction in maintenance costs.