PC Engine GT - Batteries drain after LCD mod

[2huwman]

I sent my GT off for an LCD mod a few years ago. When I got it back I noticed that the batteries would drain overnight (with the GT switched off of course). I mainly use it with an AC adapter and no batteries, so it wasn't a big deal, and it had been such a hassle getting it back from the person who modded it that I didn't really want to send it off again.

Anyway, his advice was to just take the batteries out, but he eventually said that it was probably happening because of the voltage regulator that he had fitted, and it might be fixable if the ground wire went to the off/on switch instead of the voltage reg.

That didn't make a lot of sense to me as my knowledge of these things is pretty minimal, but I've taken to photos of the inside of the GT in the hopes that someone can advise a way to fix this. I've attached one photo and here's a link to more high res ones:

https://photos.app.goo.gl/Fb2BawyuWAzGxhom9

I guess it's the large black wire he's talking about going to the screen

Thanks very much for any help or advice!

[MasterofBiscuits]

He's connected the power wire for the screen mod and the positive wire from the battery compartment to the positive leg of the DC adapter input, in the part of the circuit before the power switch. So even when the switch is off, the board will continue to draw power from the batteries. Does the screen even turn off? I'm assuming he's done that because the screen mod needs more than 5v, but there are other places you can tap unregulated power from.

Edit: this also means he's bypassing the switching of the DC adapter to cut battery power when running on DC input, so the batteries will be receiving power too - not good.

If you remove the red wire that runs from the screen mod to the point on the GT where the red wire from the battery connects, and run it to the small top leg of the voltage regulator (Q502 - immediately below the R508 text), I think that should fix it since this part of the circuit comes after the power switch.

Before you do this it might be advisable to post some pics of the other side of the mainboard, since I can't see a ground wire from the battery compartment, so maybe there's something else funky going on underneath.

[2huwman]

Thank you, that makes sense. I took the photos after I removed the battery ground wire in preparation of doing your lithium battery mod. I realised I needed to fix this battery draining problem before I went any further with the lipo battery! The battery ground wire was going up to the connector socket just below tp509 before I unplugged and removed it.

I'll get some photos of the other side of the PCB so you can see if there's anything else going on. I can't remember if the screen did actually turn off - it might have stayed on and I didn't notice as it was blank.

[2huwman]

I've attached a photo of the back of the PCB and here are some higher res shots:

https://photos.app.goo.gl/SMedkyj4JWMYeaPu6

Can you see any issues here? Thanks again.

[MasterofBiscuits]

I've attached a photo of the back of the PCB and here are some higher res shots:

https://photos.app.goo.gl/SMedkyj4JWMYeaPu6

Can you see any issues here? Thanks again.

Can't see any surprises there, looks ok.

Before you do the battery mod, I would check to see if the screen can run off 5v. It looks like it currently runs off the 9v from the batteries, and once you do the mod you will only have 5v. You could get round it by adding another boost converter, but it would cost you some battery life.

[2huwman]

Will do - thanks very much.

[Keith Courage]

You can wire the screen to the 7V on the power switch instead of the batteries. The center poll on the switch turns the power on and off.

[2huwman]

Thanks guys - I've tested the screen and it won't turn on with 5v, so I'll need to use a 7.4v battery. Will either of the changes you've suggested be OK with this?

[MasterofBiscuits]

Thanks guys - I've tested the screen and it won't turn on with 5v, so I'll need to use a 7.4v battery. Will either of the changes you've suggested be OK with this?

Depends how you wire up the 7.4v battery - you would need to do it differently to my lithium mod guide. The easiest way would be to wire it to the battery input terminal and run it basically as stock. In that case you could wire it how Keith or I suggested.

If you did wire it like this you wouldn't be able to use the DC jack for charging unless you disconnect it from the circuit somehow (cutting traces etc). Probably easier just to add in another charging method like a USB socket. The adafruit board would not work, as it's designed for one 3.7v cell, but you can use a simple 2 cell charging board.

[2huwman]

Thanks again - battery drain problem is now fixed! I've fitted a 7.4v 3000mAh battery and got about 4 hours 15 minutes of play time with my Bonk 2 Hu Card!  I've also found a 5000mAh battery that should fit (case has been modded) so hopefully will get over 6 hours.

I've fed the balance charging cable through the hole on the right side of the unit above the TV tuner connector (had to file down the inner part of the shell to get the wires through) and I use a USB 7.4v USB lipo charger to keep it simple.

[MasterofBiscuits]

Glad to hear it's all working now

[2huwman]

Sorry - one more question: Is it OK to power the GT with the AC adapter in the original dc port, while having this new lipo battery connected? Or could it end up accidentally (and unsafely) applying current to the lipo battery?

[MasterofBiscuits]

Sorry - one more question: Is it OK to power the GT with the AC adapter in the original dc port, while having this new lipo battery connected? Or could it end up accidentally (and unsafely) applying current to the lipo battery?

How have you wired it up? If you just have the lipo attached to the original battery input it should be fine as there is a physical switch in the DC socket that cuts the battery circuit when you plug the AC adapter in.

To be sure I would disconnect battery, plug in AC adapter and test voltage on the battery connector - if there is none, you are good to go.