TurboExpress not working with batteries (works okay with power adapter)

[nyandeyanen]

Hi everyone,

Although I'm new to this forum, I've been reading through it quite a bit recently and registered just for this. I have a TurboExpress which I purchased recently and recapped, did some invasive repair to the controller board at times with 30 gauge wire (it was a mess, one of the transistors fell right off!), replaced the interface ribbon and got the system working. Two stuck pixels on the stock display, but otherwise it was working great. Even the TV tuner worked perfectly with AV input using a stereo RCA to 1/8" cable. Then I found an LCDDRV board on eBay and decided to try installing it for fun with an extra 3.5" LCD I had kicking around. I've installed enough McWill and BennVenn displays in Game Gears and Atari Lynx II systems, what's the worst that could happen?

Well, the LCDDRV board came up with a blurry display on initial installation whenever all 3 R/G/B wires were attached, so I ended up probing all over the board with a multimeter, trying different wire types and looking for any form of ground leaks or solder bridges. I figured out which lines leading to the controller board to bridge in order to disable the automatic screen dimmer/color killer in case that was related, and disabled the backlight circuit entirely by cleaning off a solder bridge. I tried anything I could think of to find where what I thought was interference. Somewhere along the way the battery power circuit stopped working. I did resolve the LCDDRV issue in the end (sticking a 75ohm resistor on each of the R, G, and B wires and putting one between the csync wire and the ground of the sync stripper IC has the new display working great, it seems the TE was overdriving the LCDDRV board's inputs) but now it's not so much a portable as a US region version of my CoreGrafx tied to the wall with a 3.5" screen and no CD game support.

I've checked the caps several times and went through the circuit looking for any problems. Everything is working fine except the batteries. Any idea of which specific parts I should be looking at? Perhaps a coil or a transistor? I'm happy to send pictures of the board if you think it will help, though I don't see any particular visual indicator of a blown part. Just some creative soldering in places due to leaky electrolytics damaging pads.

[Dylanward]

Hello,
Here’s a list to do first before adjusting pots.
1. Clean system
2. Put new capacitors on the system all of them.
3. Check for traces that could be damaged and disconnected.
4. Check battery posts and wires.
5. Under neath the grey mask on the board there’s usually a failed component.

[nyandeyanen]

Hello,

Thank you for the response! I've already taken care of items 1 through 4, in fact I re-soldered the wires to the battery posts just in case the joint wasn't solid and checked continuity to the board with batteries installed. I see the voltage on the power connector's pins at the board and traced out the voltage to a few components, but I'm not certain where the step down is taking place for the batteries. I'm reading up on buck converters now,

I have not replaced the coils/inductors under the grey blobs yet, though I did clear out the two small blobs to verify continuity. They seemed to be OK as far as I could tell with my simple multimeter. If those are the likely suspects I can replace them all. Since the device works fine with an AC adapter and only the batteries fail to power the system, I wondered if that might imply a specific component but thank you for the hints.

[Keith Courage]

There is a bad trace near the Power Jack/Battery wire area. Scrape off the gray chunky stuff there and look for a bad connection. Be careful when scraping off the gray stuff. There are some components under there.

[nyandeyanen]

Thank you! I'll check. I've already removed the gray chunky stuff to trace the power leaving the batteries. I'm guessing this reinforces the possibility that those coils are involved, or at least a trace leading to them.

[Dylanward]

That is correct. Your on the right path. Either trace or component.

[MasterofBiscuits]

This happens all the time on Game Gears that I work on - the DC Jack has a physical leaf switch inside it that disables the battery circuit when you plug the ac adapter into it. This can often fail due to oxidisation or the switch simply wears out over time. The TE/ PCE GT has the same type of switch - I would test if the switch is actually working using a multimeter.

[nyandeyanen]

Thanks to everyone for the input! It turns out that the L501 inductor coil (and one of its pads) was the cause. No continuity, though I thought I'd checked it previously. Sticking the pad back to where it's supposed to be and replacing the coil with a new one has it working perfectly. Now I have to either get the LCDDRV working with the TV tuner (there are RGB and Csync lines, probably need a switch though) or just replace it with a controller port and it'll be done.