Batboard

So, does anyone know exactly what blows up when you apply reverse polarity voltage to a GTX portable? How screwed am I? No, this is not the one I'm selling on eBay... but I sure do wish I'd quit messing with them when they were all working!

Jeff

OUCH ! How much current was able to be supplied before you noticed it?

There's a fuse on the circuit board where the battery and programming terminals are located. If you're lucky, maybe that's the only thing that went.

If that wasn't in the circuit at the time, the next "saviour" is CR405, a surface-mount diode located at the top outside edge of the RF circuit board near the volume control. I can't tell which side of the board, but if you're holding the radio like you're using it, it's on the surface facing the front of the radio, to the right of the volume control switch. That probably makes more sense once you have the RF board out of the chassis. The vol, channel, and sp/mk jacks are along the top edge, from right to left, as you look at the board X-ray view.

That diode is across the B+ line prior to the power switch. If that didn't short out and save things, and you then powered it up, I'm afraid that all sorts of things may have gotten zapped: regulators, ICs, etc. How much smoke got let out? Did you hear any "rice crispies" sounds (snap, crackle, pop)?

Bob M.

It had whatever current the battery pack could supply. No smoke or sizzle, but also no operation after. I gotta start drinking less coffee!

Is that fuse an axial lead picofuse? Do you have the rating?

Jeff

The schematic says 4A. The parts list just says FUSE with a Motorola part number. You'd have to pull the back off and see what it looks like mechanically, but a Picofuse is highly likely.

Bob M.

That may be the one we bought some of for a friend's Yaesu a while back. I may be able to get one by walking around the corner. It's good to have friends.

Thanks, Bob.

Jeff

If it is a Picofuse, they're easy to come by. If it's some kind of surface-mount fuse, then I don't know. I did check the part - it's NOT the same as used in a MaxTrac (1A). GTX portables probably draw about 1.3 Amps so the 4A rating is really to protect against internal shorts from lighting the battery on fire.

Next time, use a bench supply so you don't get the polarity reversed. Better yet, don't let a "next time" happen !

I hope that's all that went wrong. After you open the radio, turn the power switch off and check the B+ line to ground with an ohm-meter to make sure the protection diode didn't short out. Then check for proper forward and reverse continuity. Finally check it with the power switch turned on. You don't want your new fuse to go "poof" right away.

Bob M.

It is a 4 amp surface mount but there is loads of room and you could use a pico fuse in its palce if you wantrd to.
I hate the GTX series anyway and have boxes of scrapped ones.

kcbooboo wrote:Next time, use a bench supply so you don't get the polarity reversed. Better yet, don't let a "next time" happen !

I'm not sure that's good advice. The only thing that would have been different with a bench supply is that the current might have been limited sufficiently to protect the fuse. Oy!

Jeff

I was thinking more along the lines that you'd have color-coded wires or binding posts that had been used successfully before, rather than trying to hook something up to a battery where polarity is often not obvious.

Also, depending on the supply, you might be able to set the current limiting to something around 2 amps. Yes, you'd protect the fuse, but the fact that there was no output voltage and maybe a big spark would have been an indication of something seriously wrong.

I guess it depends on your setup and available equipment.

I found a nice GP300 programming adapter that I use with my GTX portables. It fits between the radio and the battery. Pull the battery, attach the adapter to the radio, and now slip the battery back onto the adapter. It has a DB25 cable to plug into a RIB, and it also has a cable for hooking up to an external power supply of up to 7.5 volts. An internal SCR crowbar hopefully prevents any damage to the radio. So far I've only used the radio's battery on the back of the adapter, but it sure is nice being able to fully operate the radio (for adjusting power and deviation). And it makes programming a bit easier. Best of all, unless you use the DC cable and an external power supply, there's no way you can let the magic smoke out of the radio.

Bob M.

I was working assembly line style on multiple radios, and did it right about 9 out of 10 times. It's that one moment of inattention that will kill you.

That adapter is surely the way to go, but I'm too cheap (er, broke) to spring for the ones I've seen.

'JK

Whew! I lucked out on this one, guys. It was just the fuse. Even the diode that blew the fuse is still good. Yippee!!

Jeff