A postcript to this. Turns out I didn't kill the mixer but, as part of a plan to keep the analyser going while the original went away for repair, I bought a cheap mixer with blown low band.
The diode was shorted - a common result reported by others. The dead diode came off easily with a flick of a scalpel blade. I installed a Skyworks SMS7630-079LF as replacement (cathode to ground), fixing it in place with dabs of silver conductive paint (Electrolube SCP). This dries to form a reasonably secure connection, I planned to add a bit of epoxy for permanence.
It works! I haven't tested exhaustively as I realised the mixer wasn't the problem (see my other post re SYTF), but feeding signals directly to the low band input, the frequency response looks flat 10MHz - 1.8GHz and I've no reason to think it won't be fine over the full band 0 range.
Cost - ?2 for 5x diodes and ?10 for the SCP (both from Farnell). Got to be worth a try if you have a dead mixer and limited funds. Also, it doesn't mean you can't get a 'proper' repair later, the dried paint washes off with acetone to leave a clean surface for bonding.
If you do try it, be careful with the amount of paint; it can wick under the body of the diode and short it out.
Steve
