XY Transistors Shorting?
by Bob Roberts
The main reason for the xsistors blowing is poor connections from their physical location on the frame to their destination on the chassis. The harnesses to each pair should be eyeballed & even checked for continuity as often times a pin will be connected, but by the insulation only & not the wire. The ends that are soldered to the sockets should be examined closely, as well. You should be sure the xsistor mounting screws are tightened properly since they are what connects the xsistor collector to the circuit. If your header connectors are the red MTA type be aware that they have been a known source of bad connections that have plagued the industry since their inception. Another problem with those MTAs, & other connectors, is a feature that many think they require... the ramped version. How the ramped version works is that the ramp & the counterpart ramp on the header lock together when plugged up. Seems like a winner... the connector can't work it's way loose & come off... but over time this action rings out the solder joints on the header causing intermittency & in turn short out the output xsistors. The headers need to be inspected & resoldered if necessary... which is almost always!
The short check list:
Over the years I've heard hundreds of ops, hobbyist & even techs say they got bad xsistors from this place or that place because they shorted right out immediately & maybe even did more visible damage. I've had techs say they changed out xsistors with huge overkill xsistors & they solved their problem! NO they didn't & the bad rap for the 2N3716/2N3792 escalated. The problem was in lack of due diligence. Putting in a pair of xsistors that could take a couple hundred volt spike & keep on ticking only masked the problem that may be exacerbated down the line.
The 2N3716/2N3792 pair are well equipped to handle the job in any of the XY monitors, but like any other components they need to have solid, stable connections to the rest of the circuit.