LNB TR shows a side view of the replacement transistor.
LNB diode shows a side view of where the diode goes.
LNB switch is a schematic of the transistor switch area in particular. It shows why the switch fails.
A transistor usually has a relatively high collector to emitter voltage tolerance. Called the breakdown voltage or (BV ceo) And they usually have a relatively high base to collector breakdown voltage called (BV cbo). That is the to connection which normal have a large voltage across them. But the emitter to base breakdown voltage called (BV ebo) is usually 2 to 6 volts for most transistors.
Under normal conditions the (in black) the emitter to base voltage is around 0.7V But if you have reverse polarity coming in. The voltage across the E to B junction goes up to the incoming voltage that is because the collector is shorted to ground by the reverse protection diode in the voltage regulator of the particular LNB. If the incoming voltage is more than -2 to -6 volts, then the transistor is going to break down and start conducting in reverse. Even with 250 or so milliamp of stray current and the switching transistors will be on their way to toasty-vill. And when the magic smoke comes out, you can’t put it back in.
It only takes 10 to 20volts of stray AC voltage to do the damage. Low enough that you would not feel it if you were holding the lnb/dish in one hand and the coax end in the other. But when you are connecting the coax (or when a lose connection forms during normal service), the transistors will go poof.
The components and regulators can stand large forward spikes (30 to 60V) without any problem. Stray forward polarity tolerance is high enough that running across anything higher in the filed is unlikely. The problem comes when the input goes negative. 10 to 20 volts is well within the normal stray voltages you would find in the field environment. That is why they need to put a diode in the input of the switch to sink the any stray negative current surges, and spikes that comes with lose connection and making or breaking coax connections during assembly of the system.
I can tell that they anticipated that it was a possible problem because they put a diode in the base circuit of the transistor switches. That prevents the bias resistors from conducting reverse current and turning the transistors on when biased in reverse, but they overlooked a critical problem. The emitter to base junction can not stand that high of voltages. So the bias resistors don’t need to conduct base current to cause the transistor to start conducting. It will do it on it’s own when the E to B voltage gets above 2 to 6 V.
That is why I have a big arrow pointing to the diode with the statement ”Diode won’t save it”
So to the design team. Nice try, but no cigar.
I have two LNB’s that I have repaired from the same damage. One took out one transistor (the one show) and the other took out both transistors. After adding the diode in both units I have had no more problems.