Cobra 138XLR Info On Reversed Power Wire Repairs

[Captain Kilowatt]

They must be making them different today (cheaper). I remember years ago seeing many 3 amp diodes in the DC input and they would withstand the short burst of forward current allowing the fuse to blow first providing the proper fuse was installed. 1N4001's IIRC. They can take up to 30 amps peak current surge and even at 8 amps they will last a full second which is long enough to take out a fuse in most cases.

 

[kopcicle]

Anecdotal :

General Electric 3-5xx AM and SSB radios shared the same 3 prong plug with Cobra/Dynascan/Uniden/President et all. There was just one difference, The General Electric was wired backwards! So every GE that came in "backed up" was rewired to comply with the "standard" and every three prong that came through the door was verified and immediately , if backwards, destroyed.

Nearly all of these diodes could be identified by a pale blue/gray cathode band. All failed shorted, full continuity. Some had succumbed to the "if a 1.5A fuse failed, put a 3, 4, 10A fuse in its place. Toasted input chokes, melted power sockets, vaporized traces. Curiously enough the PTBM-xxxx 02A Cybernet AM chassis were nearly impervious to "backup" damage. Just replace the fried DC parts and move on. The SSB radios weren't so lucky. The Cobra/Dynascan/Uniden/President radios that were subjected to the GE plug didn't fair so well either. If much of anything got past the diode , and it usually did, you would see final, driver, regulator (MB3756 in the 8719/34 boards), PNP modulator, audio and PLL smoked as well as a couple of the transformers. It wasn't unusual to see capacitor confetti.

My favorite is the 3-5804D that launched a filter cap that ricocheted off 3 walls, the floor and the ceiling before coming to rest inches from where it started. My tears came when an early Grant 858 was plugged into a GE power cord after hours of repair from a lightening strike.

Like others here the fuse, power cord and internal wiring were checked "at the door" in front of the customer. Still it amazed me the number of customers that blamed me for reverse polarity failures due to the reverse power plug. Even after a complete explanation "Power plugs are all the same , see, it fits" it was still my fault. In a few cases, even if I had never seen the radio before it was my fault.

Just another reason why I don't operate a tech bench anymore.

 

[OldTech03]

I had these issues many times with truck drivers. I finally just started putting bridge circuits in the radios of those that made a habit of hooking them up backward's the worst of the bunch was the slip-seaters changing trucks every day sometimes twice a day. And yes before you respond it did drop the output a tad but you never had to worry again about reverse hookup and most of the slip-seaters didn't use the radio for maximum power mostly for talking close range and really it only drops output a half watt in most cases so it's just an all-around better setup and they loved not having to worry about what wire went on wich terminal.