[Doug]’s newly-installed Yaesu FT-891 mobile transceiver failed to power up despite a careful installation, and it turns out to have ultimately been caused by a reversed cable. There’s a happy ending, however. Since the only real casualties were a blown resettable fuse and a badly-burned resistor that damaged the PCB, [Doug] was able to effect a repair. Things could have been worse, but they also could have been better. Damage could have been prevented entirely with some better design, which [Doug] explains during his analysis of what went wrong.
The destroyed SMT resistor and pads were easily replaced with a through-hole version, thanks to the schematics.
The main problem was that the generic RJ12 cable that [Doug] used to connect radio components had its connections reversed. This would not be a problem if it was used to connect a landline telephone to the wall, but it was a big problem when used to connect the radio components together. According to the radio schematics, the two center wires carry +13 V and GND, which meant that a reversed cable delivered power with reversed polarity; never an optimal outcome.
Once the reversed power arrived at the other end, [Doug] discovered something else. Diodes whose job would be to protect against reverse polarity were marked DO NOT INSTALL, probably to shave a few cents off the bill of materials. As a result, the full 13 V was soaked up by a 1/8 W surface mount resistor which smoldered and burned until a fuse eventually blew, but not before the resistor and pads were destroyed. Thankfully, things cleaned up well and after replacing the necessary parts and swapping for a correct cable, things powered up normally and the mobile radi ..
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