Question

[Slim304]

Got a 29 lx that was hooked up to 24 volt melted the black wire on the plug on the back of the radio checked all the protection diodes and they're fine now the radio won't turn off could it shorted out the volume squelch knob or is there somewhere else I need to check

 

[Cable Guy]

Id definitely check the volume knob on-off switch pot. Maybe the contacts were fused.

 

[Slim304]

Replaced the volume squelch knob and still no change

 

[BC Coyote]

Oof! 24 volts ! If you had it hooked up to that much power for more than a second or two there could be multiple fried components, especially if the protection diode didn't pop.
Does it still transmit and receive ?

 

[brandon7861]

Oof! 24 volts ! If you had it hooked up to that much power for more than a second or two there could be multiple fried components, especially if the protection diode didn't pop.

If the protection diode didn't pop, it wouldn't have let 24v past the power jack. Its when that diode does pop and stops shorting out that power can get past it and cause further damage.

The diode saves the radio, the fuse saves the diode. No more missing/foil/pen spring fuses!

Replaced the volume squelch knob and still no change

Whenever you have a catastrophic failure and think you have it sorted out, it is always best to run the radio on a current-limited supply and slowly bring up the voltage to make sure nothing else is shorted.

According to the 29LX LE schematic (only sch I have), the switch is the only way for power to enter the rest of the radio. If replacing that didn't help, I wonder if there is still something shorting inside causing the new switch to weld together.

Since it is now turning on and staying on, I assume you either found a good protection diode soldered across the inside of the power plug or you missed it and it is indeed an open and 24v did get past it. You said "protection diodes", plural, so I assume you checked the diodes on the board and may have missed the actual protection diode on the power jack (if that is where they put it, and it often is). What is the current draw while it sits there powered on?

The power switch isn't in the volume pot itself, it merely rides the same shaft. Did you swap the entire Vol/Sq/SW assembly (or at least disconnect the wires from the switching section and check it's functioning with a continuity meter)? Those are not fun to find. You must have either had a donor radio sitting there (hopefully that one was ok, did you check) or figured out where to order an equivalent assembly to have replaced it already. I don't mean to question you like this, but a good switch will not let power into the radio. There is no other way for voltage to get past it to turn on the radio. So either it is not really replaced, the replacement was bad, or it fused again and you should have a large current draw and parts quickly warming up inside.

 

[Cable Guy]

I thought it is a reverse protection diode, not an over-volt diode. I would think the diode has no impact unless it is only rated for 13.8 volts and blew shorted. I can't imagine what is left. That switch should turn it off if it's good. Id leave the switch disconnected and try it. If it doesn't come on, but does if you short the leads, it's definitely the switch unless, UNLESS that switch is digital and some ic that controls the on, off function has blown shorted.

 

[Slim304]

Oof! 24 volts ! If you had it hooked up to that much power for more than a second or two there could be multiple fried components, especially if the protection diode didn't pop.
Does it still transmit and receive ?

Yes still works fine goes off when you turn off the power supply and it was given to me like that

 

[brandon7861]

I thought it is a reverse protection diode, not an over-volt diode. I would think the diode has no impact unless it is only rated for 13.8 volts and blew shorted.

I agree, its revere protection only. 24v wouldn't hurt the diode if the polarity was correct. But to melt a power lead over blowing a trace or something inside, I assumed it was connected backwards and the diode took the hit. Sorry for the confusion. I have never seen a power wire melt any other way

My mistake for making that assumption.

Edit: are there numbers on that diode, maybe we can look up the reverse voltage rating

 

[Cable Guy]

We must sometimes make assumptions when doing this. Doesn't always pan out like we thought. I don't know about that model, and didn't have the schematics so I assumed it was close to a 29ltd as far as the power input and referenced that.

 

[Shadetree Mechanic]

Just quickly looking at the schematic, the power switch is the only way the radio can get power. This has me stumped, there is nothing that could short out to supply power. I am interested in what the solution ends up being, but to me, if it works, it works.

 

[nomadradio]

Ah, this radio doesn't have a NOAA weather radio channel does it? Pretty sure it stays active when the power switch is off, and can power up the radio when an alert tone comes through.

Not sure where you would find a diagram to this feature if this radio has it.

73

 

[Handy Andy]

How did you fix the "black wire"?

This may just be a re-assembly problem.

The power switch is in the front, but there are traces that go back to where all that protection is. If you soldered two pads together by accident - this result would make sense; you just inadvertently bypassed the switch - now every time you plug it in - it's on.

There is one wire that POWERS the radio - rises up from the rear power "choke" (if you have the discrete component version of the LX unit) - shown as W20 - while W21 is the RETURN from the main switch back to the board over at the "inner corner" of the Audio "Reactance" transformer - which is the larger of the two "chokes" back there.

IF you soldered the wires wrong - or neglected to test the wires at the harness (that loom of wires bundled together) to make sure that this "short" did not melt two wires together - this has to be rewired correctly.

 

[Captain Kilowatt]

If the protection diode didn't pop, it wouldn't have let 24v past the power jack. Its when that diode does pop and stops shorting out that power can get past it and cause further damage.

The diode saves the radio, the fuse saves the diode. No more missing/foil/pen spring fuses!

I think you should look at that again. The protection diode is simply for polarity protection and has nothing to do with over voltage protection. You could supply the radio with 50 volts and that 50 volts will still get into the radio everywhere. If the polarity was reversed, that diode SHOULD cause the fuse to blow, assuming a fuse was used. If polarity was correct, then that over voltage can indeed do considerable damage to the radio.

 

[brandon7861]

I think you should look at that again. The protection diode is simply for polarity protection and has nothing to do with over voltage protection. You could supply the radio with 50 volts and that 50 volts will still get into the radio everywhere. If the polarity was reversed, that diode SHOULD cause the fuse to blow, assuming a fuse was used. If polarity was correct, then that over voltage can indeed do considerable damage to the radio.

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