H7140 PDP11 PSU

Correct. was either Well, it's not totally shorted, then, which is the common failure mode of chopper transistors. Does your meter have a 'diode check' range? If so, use that, and check the base-emitter and base-collector junctions with the probes both ways round. This will at least tell you if the junctions are good. You really need to remove the transistor from the PCB to test it, though. But perhaps before that we should check a few more things. The chopper circuit is driven by E2 (p88), a good ol' 555 timer wired as an astable. For this to work, it needs to be gettign power. This power comes from one of two places : When the supply is running, it comes from T1 on the PSU motherboard, via pins 7 and H of the connector and diode D7 (p88). At start-up it comes from the mid-point of the mains smoothing capacitors via pins 2 and B and Q6 (p88). The base of Q6 is driven from the network R2 and D2 (p82). You should check the power resistors on the motherboard, actually, R1, R2 and R3 (p82). Now obviously the first can't be doing anything since the supply is not running. But the second source should be there. You should measure the supply voltage at pin 8 of E2 _with respect to pin 1 of E2_. That is, the black meter probe is connected to pin 1 of E2, the red one to pin 8 of the same IC. Be warned that this circuitry is directly connected to the mains, and will have _lethal_ voltages on it with repect to ground. So connect the wires, make sure they can't touch anything, then plug the machine in and see what voltage you get. Unplug the mains and wait for things to discharge before touching anything. -tony -------------------------------------------------------------------- mail2web - Check your email from the web at http://link.mail2web.com/mail2web