I've been following the recent thread about using high-side injection crystals in the GE Receivers. I've tried that on both VHF and UHF MASTR II, MVP, EXEC II, etc., and that always worked out well.
I have several 2-Meter and UHF Motorola MICOR Repeaters locally, and they've been in operation for so many years that I never gave a thought about ordering high-side injection crystals for any of the receivers. They always seemed to work very well just sending them in to ICM and having them rebuilt for the new frequencies using the standard crystal formulas. Some of the Repeaters I inherited with the channel elements already re-crystalled, and none of the receivers' elements had been ordered with high-side injection. I'm just getting ready to send in another pair of UHF MICOR elements to ICM, and am wondering if I will gain anything by requesting that the Receiver element be recrystalled with a high-side injection crystal? Someone recently mentioned that I'd need to reverse the diodes in the discriminator if I do that, so that the AFC will work properly. In a VHF MICOR Receiver, there are just two diodes that need to be reversed, when moving a high-split Receiver down to the 136-150 MHz range. Looking at the UHF MICOR Receiver schematic, I see that there are four diodes. Which ones get reversed, if you're changing the injection to high-side? The two "output" diodes, or all four? The Motorola service manual part numbers for the diodes (in both the VHF and UHF Receivers) are listed as P/N # 48D84616A01 - "Diode, Planar hot carrier". I know these diodes are rather small and quite fragile. They're difficult to unsolder and then resolder after reversing polarity without breaking them, as careful as I've tried to be. I did that on several of the 2-Meter MICOR Receivers that I completely rebuilt with 136-150 MHz factory parts. Sometimes the diodes broke, sometimes not. Someone on the list recently mentioned this diode fragility problem, and recommended that the user plan for replacement as part of the Receiver frequency change project, rather than trying to reuse the original diodes. If there's any advantage at all to going with high-side injection on this latest UHF MICOR Receiver, I'll go ahead and order some new diodes from Motorola, just to be ready for any that I break. But how many, and which ones get reversed? LJ
