[time-nuts] Thunderbolt question
csteinmetz at yandex.com
Sat Aug 5 16:07:07 EDT 2017
> I’d say it would be an MMIC amp similar to this device [Avago MGA-87563]
If a chip similar to the Avago part Arthur referenced is what is
installed, which seems plausible, the 0.749v on the RF input (Pin 3) is
a fault and is caused by an external source of voltage (3.417v) imposed
on the RF output (Pin 6) through the internal feedback resistor to Pin
3, attenuated by the gate resistor.
Avago says this particular chip needs to have 0vDC at Pins 3 and 6, so
if the connected parts would impose any DC voltage on those pins,
external blocking capacitors must be used on Pins 3 and 6. You might
check to see if there are blocking caps (at least at Pin 6), and if they
are good. (Alternatively, the internal output capacitor from Pin 6 back
to the output FET source may be bad.)
Of course, don't expect a bad external cap to be the only other problem
-- if it is bad, the 6-pin amp may well be bad, as well as whatever is
connected to the other side of the cap.
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