South-east of Old Olney is an Enclave camp where you can test the controller. All you have to do is unlock the deathclaw cage and let it kill the Enclave soldiers. You can also sneak up the nearby collapsed bridge and get within range of the deathclaw and watch it attack the Enclave soldiers. Although a lot of the time you never get the 'completed' notification for this (Optional) task and the marker stays on radar until you enter the Old Olney sewer.
Once you get to Olney, bring up the local map on your Pip-Boy and look for the entrance to the sewer. Once you get into the sewer, find the ladder which leads to Old Olney Underground. Arriving there, you are greeted by the ghouls Wint and Kidd who are the last survivors of a group of ghouls which tried to establish a settlement here - a plan the deathclaws apparently did not approve of. The ghouls leave after your conversation, deducing that it's safe to leave the way you came.
From here on, the way to Olney Powerworks is pretty linear. Navigating the deathclaw-infested Old Olney Underground, you will find Wint's and Kidd's dead comrades, like Badger, Connelly, Dunbar, Carl and Sanders, and eventually reach a door to Old Olney S. Wilson Building. In the building you encounter a couple of Enclave soldiers and a door on the second level (behind two Enclave soldiers) that leads to Olney Powerworks.
The old power plant is guarded by sentry bots which can be deactivated from a couple of security terminals. You can take the short way by going through a door on your left after you enter the area, and then hacking the very hard terminal there, or take the long way by veering to the right. Either way, you will eventually reach the room with the tesla coil, which is inside the bright electrical chamber. Removing the coil while the power is on will cause a large amount of damage. To acquire the tesla coil without power damage, jump down into the Tesla Coil chamber, enter the adjacent room, and use the hard terminal to shut off the power. Then go back into the chamber and remove the tesla coil from its base.
Another way to sustain less damage is to jump into the Tesla Coil chamber and activate the three emergency switches inside the chamber. You still receive a shock, but it is not as harmful. If skill level prevents access to the shutdown switch, throwing a few pulse grenades into the chamber will knock out the power and allow you to access the three switches.
Once done, a nearby ladder takes you out to a street north of Old Olney. Here you are likely greeted by a couple of deathclaws, and upon defeating them you can fast travel back to the Citadel. Hand the coil over to Tristan and receive your next assignment, Who Dares Wins.
While attempting the optional quest to use deathclaws to your advantage, make sure you tell your followers to wait; otherwise they might kill the deathclaw, thereby blowing the option.
The "help" provided by the deathclaw control scrambler is questionable as the only Enclave-controlled deathclaw encountered during this quest is located in the Enclave camp outside Old Olney and cannot leave the radius of the Enclave transmitter without dying. One more controllable deathclaw will be encountered within the Mobile Base Crawler.
The manhole the ladder connects to is initially exit only, so forget about short cutting in that way. You can, however, use it to return to Olney Powerworks later on.
There are two sets of around 10 alien power cells (considerably more if the PC has the Scrounger perk [40~50]) in ammunition boxes located in the nooks above the water fountains, next to the twin turret guns, at the end of the long hallway full of computers. The water fountains have little "flying saucers" on top of them as clues, like miniature versions of the full sized ones found in playgrounds.
The room containing the remote research terminal to disengage the tesla coil also contains three sets (x12) of alien power cells: one on a stove near the back of the room, one on a table to the left of that stove, and the third on a table next to the terminal itself.
If you acquire the scrambler and travel to your Megaton home, an Enclave deathclaw may appear at your house, follow you outside, and begin attacking Megaton settlers.