I did a hack to a Sony watchman and now it’s continuously drawing .018 amps

[sideburn]

Hey guys, I’m hoping you can help me sort out what I did to cause this old portable tv to be continuously drawing 18ma now even when the power switch is off.

During my poking around and wiring things up I did create a short and noticed what looks to be a mosfet or a voltage regulator on the pcb was getting extremely hot. It would burn my fingers. Finally once I isolated the short circuit that I had created everything worked still like normal so I continued on with the project.

Everything works fine now but I notice that the batteries were draining even when the power switch is off. It turns out that it is pulling 18 milliamps continuously when the power is off. Could this be that mosfet / voltage regulator that was getting hot to the touch?

Here’s a link to a video I made of the hack I did:

Thanks for any help if you’ve got any ideas what could be the cause or even a workaround fix to the problem. Like a low current cutoff or something.

The part that looks like a voltage regulator or mosfet is green flat with 3 leads coming off of it. I could pull it and see if it’s a known part and not some proprietary Sony labeled part.

Thanks again,

-Tavis

 

[dave9]

As far as seeing that flat green regulator or 'fet, it could be handy to have a link to exactly where it is in the video or a standard high res picture as the video looks a bit dark to me (tho' could just be my monitor settings).

Yes you can damage an IC and cause it to have leakage current, though it is the only thing we'll be fixated on since the rest of what you did is unknown, but for that matter a transistor (I mean some other one) can fail and start leaking too. It wouldn't have to be the one that got hot though it seems a more likely candidate since it did.

Time to get the markings off of it, look for a schematic, see if there's any silkscreening on the PCB to help ID it. Transistors are often marked with Q(n), meaning the letter Q and a number but in that tight space I could understand no markings.

Then there's grabbing a multimeter and seeing what you can read, voltage measurements relative to ground on the 3 legs of the IC. Typically a regulator, even a variable one set to a voltage, would be set to a round number though with very low voltage consumers like memory or a processor, could be a single digit volts plus a decimal place.

Anyway IF that component is leaking it seems more likely a transistor because presumably there would be voltage at it all the time, with power off for it to leak and if it were a voltage regulator then it would always output, while not necessarily if a transistor that isn't damaged.

So, you might expect voltage on one pin with it working correctly (or maybe not, just throwing an idea out there) but shouldn't have voltage on either of the other two pins when it's turned off, with certain exceptions like devices that have a soft-off mode and sample for a logic switch or infrared or RF remote control signal to turn on, or a timer circuit, whatever. I plead ignorance of the capabilities of a retro watchman, never owned nor took one apart.

Anyway if you can ID it and have the capability to harmlessly desolder it and solder in a new one, of course that's the best solution. If not, and if the watchmen-gaming-gadget functions as it should, 18 mA extra current doesn't seem like much except when it should be off and saving the battery so I would add a switch to disconnect the battery or just take the battery out (again I haven't had hands on one so don't know how much of a hassle that would be).

 

[sideburn]

Thanks for the info. Maybe I’ll take it apart again (for the 1000th time) and take some pics and get that part number. My only real concern about the drain is if I leave nimh rechargeable in it and forget about it I guess it would ruin then after they drain all the way. I’d hate to put a switch on it. I like keeping it originanonnthe outside. I thought maybe I could put a low current cutoff on it somehow though.

 

[dave9]

It could start reverse charging the weakest NiMH cell(s) in a series once they reach 0.0V first, so yes it would be best not to let it drain down to nothing.

As far as a low current cutoff, how would it know it was drawing low current unless it was drawing current instead of being cut off? If you don't want a switch sticking out then there might be some way to rig up a capacitive touch switch, if there's spare room in the chassis for it, but that too may draw some current.

A switch could be tiny and recessed if you find a way to mount it, could have a tiny hole nobody will ever see that you stick a paperclip in or something.

Also consider that this (probably) transistor may be in the process of dying still. Some damage takes a while to get to its worst state. It may then be necessary to replace it.

 

[sideburn]

Hmm good point Ideas. Yes I was wondering the same thing that it could be in the process of dying. So he’s to try to find the bad transistor. Hmm.