It works in all but the earliest PL400s.  The earliest PL400s produced had the power supply capacitors mounted directly to the left side of the amplifier chassis using 2 round capacitor brackets.  The rectifier bridge was also mounted to the chassis side in this amplifier vintage.  The White Oak Audio Capacitor upgrade WILL NOT install in amps of this vintage.  The kit WILL install in any PL400 that has the transformer mounted with the aluminum die cast bracket that incorporates both the capacitor cradle and bridge rectifier mounting.  This includes the Phase Linear 400 Series II although due care must be taken in the series II amplifiers because the front panel LED logic boards in this series of amplifiers leaves very little room.

It works in all but the earliest PL400s (but is not applicable for the PL400 Series II).  The earliest PL400s produced used 2 fuse type incandescent bulbs above the upper edge of each of the VU meters instead of the 4 grain of wheat style incandescents that installed into holes in the lower edge of the later Dixson VU meters. Generally speaking, if your amp has a Phase Linear PL18 light board in it, the White Oak Kit will work in your amp. The White Oak Audio PL400 LED Light Board Upgrade Kit WILL NOT install in amps using the fuse style lamps above the meters.  The kit WILL install in any PL400 that has the PL18 PC board.  The PL400 LED Light Board Upgrade kit is NOT APPLICABLE for the PL400 Series II.

It is easy to test your VU meters.  Using a 1.5V AA cell battery and a 390 ohm resistor, attach the negative side of the battery to the minus terminal of the meter.  Attach the positive side of the 1.5V battery to one side of the 390 ohm resistor.  Attach the other side of the 390 ohm resistor to the plus terminal of the meter.  If the meter is good, the needle should move to approximately the -4 dB meter position on the meter scale.

Generally speaking, White Oak Audio Design is NOT in the business of installing upgrades in Phase Linear amplifiers. White Oak Audio Design's business is designing and marketing high quality upgrade products for installation by DIYers or by a qualified service professional.
We do work with and rely on qualified upgrade and repair professionals to install our upgrade products. These professionals are well versed in White Oak Audio products and are master upgrade technicians that perform quality repairs and restorations.