5120 - fuse blew when plugging in adapter

[desertrout]

Aside from what I understand is expected behaviour using one of the cheap 6V/5Ah SLA batteries and the grey 5140 adapter (I know I know), my 5120 has been working well over the past few months, using it on a semi-daily basis, on battery alone as well as with the adapter.

Today, when I went to plug in the adapter shortly after I had turned on the machine, it shut down.  I didn't fully plug in the adapter - just touched the port. A quick check revealed I had blown the 7A fuse at F1.

I'm no EE - a relative idiot, to be frank. What could potentially have caused this to happen?

 

[techknight]

you probably shorted it out at the DC jack, which means its time to take a look at that 9Z30 MOSFET right next to it and verify it isnt shorted. 

 

[desertrout]

Thanks. What would be the symptoms / risks if Q1 (the 9Z30) was shorted? Aside from testing it out of circuit. I have replacement here if need be, mostly curious.

Also, what would have caused it to short out at the jack in the first place?? Something I'd like to avoid in the future...

 

[techknight]

The transistor was probably already bad. It is designed to regulate the incoming current flow from the AC Adapter to charge the battery, and maintain the system at the same time. As mentioned many times before, its a really bizarre power circuit. Quite predominantly the Achilles heel of the portable design. 

 

[desertrout]

Huh. That's a lot of responsibility for one component. I'll take it out and check it later today.

 

[desertrout]

So, I removed Q1, tested ok. Replaced it with a new one anyway. Fires up, went to plug it in and it shuts down. Fired up again off battery fine, so the fuse is ok, but.... WTF? Is this just an issue with my adapter then? What is going on?

 

[LaPorta]

What is the output of the adapter?

 

[techknight]

your adapter could have shorted internally. 

 

[desertrout]

What is the output of the adapter?

7.7V, supposed to be 2A but I haven't tested that. I've resolved to move to a 1.5A max 7.5V adapter before using it again.

 

[LaPorta]

Yeah, I'd see if the output was over-amped. No idea why that would happen, but anything is possible with old stuff.

 

[desertrout]

your adapter could have shorted internally. 

I assume that would be verified by continuity between the + and - of the plug? If so, there isn't any.

 

[LaPorta]

Well not necessarily, there could be a short inside the circuit inside that is just leading to a bleeding of current without it being reduced to its proper amperage. See what it is with it plugged in.

 

[desertrout]

Yeah, I'd see if the output was over-amped. No idea why that would happen, but anything is possible with old stuff.

It's a recapped 5140, so unless I bunged that up or tweaked the pot too high (somehow)... I guess maybe?

 

[desertrout]

Well not necessarily, there could be a short inside the circuit inside that is just leading to a bleeding of current without it being reduced to its proper amperage. See what it is with it plugged in.

If there's continuity? I checked plugged and unplugged, appears to be nothing. 

 

[LaPorta]

Check the amp output.

 

[desertrout]

Ah. Will do.

 

[desertrout]

Classic. Just need to use my eyes. I'm going to go ahead and guess that the chipped insulation on the plug tip has something to do with it... (still going to check amp output... something I want to do on all my psu's / adapters):

 

[techknight]

Yikes! Yea, thatll do it.