Welcome to Phoenix Swamppirate. Not familiar with the amp myself but I'm pretty sure someone here will have some input. Haven't seen anyone stump the PL gurus here yet. Sit back, set a spell...someone will be along soon.
They were a mess... A HOT MESS!!! I took them on trade for a preamp build. I still need to do the face plates and power switches. but, all in all, I think a great trade. If they don't hold a candle to the WOPL's, they are history
I'm sorry for the slow reply. I am recovering from some eye surgery I had yesterday.
The Phase Linear DRS900 was first introduced in January 1982 and by December 1982 Phase Linear shut down its Lynnwood Washington plant. Jensen (which had acquired Phase Linear) moved production to Laredo Texas where production continued until 1984.
Needless to say, Dean N. (Phase Linear's last service department manager at the Lynnwood Washington plant) didn't have much time in 1982 to write service manuals as the number of employees in the service department got smaller. Therefore the service manual for the DRS900 was limited to just (8) pages; a cover page, (2) pages of schematics, (2) pages of parts layout, (2) page parts list and a (1) page of "amplifier test procedure".
It should be noted that the DRS900, DRS250 and DRS400 work on a multiple power supply (in that they had an additional secondary voltage coming of the power supply transformer). The DRS900 low power output is a minimum of 150 watts per channel RMS into 8 ohms. "But when your musical reproduction requires extra effort, the DRS900 automatically switches to a higher power supply that provides 4.8dB additional headroom."
It has been my experience that when the DRS900 switches to the higher voltage rail that the amp is capable of delivering 420 watts per channel into 8 ohms both channels driven.
Typical repair issues with the DRS900 include the following; the power supply (which is difficult to get into), blown output transistors, and defective Darlington transistor(s), etc. NOTE: There are some higher rated Darlington's that Dean suggested to me at one time to use as replacements that are far more reliable than the ones Phase Linear used at the time. I can't remember off the top of my head what the part numbers are but they are buried somewhere in all of my boxes.
By the way, the DRS900 LED meter movement is in my opinion better that the series 2 meter movement in that it is far more accurate and linear.
NOTE: The DRS250 and DRS400 both used the same power supply transformer and had a higher than normal failure rate in comparison to other Phase Linear amplifiers.
Unfortunately, at this time I have a dozen 700 series amplifiers as well as a waiting list of other customers for me to take on any additional repairs. I also have (2) DRS900's of my own that I brought with me from Seattle to Connecticut that I need to work on. If you aren’t in a big hurry I may have some time next year to work on yours.