CLASSIC V NG
FlightDetent,(taking care with the gap so as not to confuse).
Firstly,
The B737 Classic powered with a Garret APU(i can`t remember if it is the same when fitted with a APS200 Sunstrand APU) , APU GEN OFF BUS may illuminate after APU start but nil freq or volts show on the AC meters due to the way it was previously shut down.
If during the previous shutdown the APU Generator was triped off line with no other AC Generator connected to the bus`s the excitation field in the Generator collapses(as there is no AC power on the ships main Bus`s) this usually happens during shutdown and securing of the aircraft once terminated at night.
During the next start(normallyfirst flight of the day pre-flight) the APU will start normally,the APU GEN OFF BUS light will come on normally as the APU is above 95% and is ready to load.
Selecting APU on the AC meters panel will not provide an indication as the generator field requires "flashing" ie it recieves a field excitation from the APU GCU(Generator Control Unit) where by allowing it to both indicate and be connected to the bus`s through the AGCU.
This is not a fault with the aircraft but a design problem!
Depending on your companys procedure for APU normal shutdown you may never see this problem or see it regularly.
Secondly,
quote;
Second attempt, all O.K. CPS higher than usual, but within range, on bus, O.K., one, two, three, .... KABOOM - darkness all over.
You mentioned that the B737 CLassic that not long after you connected the APU to the Bus`s the other day that you ended up in darkness,i suspect that you have a totally different problem.
If, when this happened the APU also did an AutoShut Down(interpretation of your description of event) it is most likley that the APU TachGenerator loss of signal occured ,as will cause the M280 Apu Control Unit to Autoshutdown the APU 10 seconds after it reaches operating speed (ie ready to load withthe APU GEN OFF BUS light illuninated)with nil indications.
Thirdly,
Indication of Standby Bus off?
For the B737NG the Standby PWR OFF light would illuminate and depending on the failure a TR UNIT light and/or ELEC lights depending upon whether you are in flight or on the ground.
For the B737 Classic you get the Standby PWR Off light as long as the battery is switch on.
Hope this info helps