How do i replace timer mechanism in a malibu ml300rtw transformer with a photo cell

SOURCE: I have a Malibu ML300RTW transformer with a 50 ft.

Usually 50 - 75 ft. is as far as you want to go, especially with 16 gauge wire. What happens is the lights get dimmer as the length of the run increases. If you don't mind the lights getting noticeably dimmer, go ahead and add the other 4 lights. Using a heavier (12 gauge) wire will help a lot. You'll want the 12 ga. to start at the transformer, preferably running the whole length. Also, using a smaller watt bulb will help.

If you have a 120 volt outlet near the approximate middle of the new (total 150 ft.) run, you could relocate the transformer to that location and have two 75 ft. runs in 2 directions.

Testimonial: "Thank you for your response. Your advise was exactly what I needed to know."

SOURCE: Malibu Model: ML200TW Transformer Automatic timer not working

You can buy cheap plug-in timer, set power pack to override, and plug power pack into cheap timer.

I wrote power pack troubleshoot checklist:
http://www.fixya.com/support/r8391011-power_pack_not_working

SOURCE: timer stops at on and off trips and doesn't turn on or off

The clock motor has gotten weak and now it is overheating and will not recover.
There are no replacement clock motors.
To buy more time with old power pack, purchase cheap plug-in timer, set power pack to override, and then plug power pack into cheap timer.

For more information on power packs showing replacement trippers, useful links and tips
http://www.fixya.com/support/r8391011-power_pack_not_working

SOURCE: What timer motor fits in Intermatic ML121RT Power Pack ?

The PB912M10 timer clock motor is not sold separately. Aug 2010.
If power pack transformer still works: buy cheap plug-in timer, set power pack to override, and plug power pack into cheap timer.
With receipt, power pack might still be under warranty.
Open following link for power pack tips and resources:
http://waterheatertimer.org/Malibu-power-pack-stopped-working.html

SOURCE: how easy is it to

Question is not 100% clear.
Add a comment any time, and that sends email, and I will respond with more information if needed.

1) Are you trying to repair a transformer?
I do not know of any repair for the transformer, or available parts sold to repair transformer, although the product does have a warranty, and Intermatic might have a repair protocol for the transformer.
http://www.intermatic.com/customer%20service.aspx

2) Are you replacing existing transformer with a new transformer?
The transformer plugs directly into outdoor outlet, and wires are connected to A and B terminals.
Open following link to read how to set up and use transformer in troubleshoot-format:
http://waterheatertimer.org/Malibu-power-pack-stopped-working.html