Sega Game Gear VA0 shuts off after Cap replacement

I've recently got my hands on a lot of 9 Game Gears, recapped all of them, switched some lcd's and replaced 1x CPU and 2 SRAMs. Those 7 units work perfectly now, but I'm having issues with the other 2 units.

Both of them do work, but sometimes they're are shutting down immediately after powering on. I can combat this behavior by turning the brightness up/down a bit, most of the time they'll start up and run smoothly for hours. First I thought the 20k pot was the issue, I've even replaced one with a 10k resistor and cleaned the other one to see if this solves it. It didn't.

I was looking at the schematics for the 2 Asic VA0 and measured the resistors connected to the pot to see if maybe one has gone out of spec, they're all ok. There are also 2 transistors in this section, Q5 and Q6, but since the fault is sporadic, it's very hard to track down.

After reading up on it, the 1-sec-shutdown seems to be a more or less common problem with GG's even after cap replacement, has anybody figured out where this is coming from? It looks like the watchdog is kicking in and shutting the units down, but what exactly and why I can't figure out.

 

Are the caps you used that control the brightness the exact same rating? Ive frequently have had the same issue and have only had varrying results

 

Double check Cap C60

One leg goes to ground, the other to pin 32 on the cpu

 

I've used the same cap kits for all 9 units, because all of them are 2-Asic-VA0 boards. The capacity matches the old ones 1:1, the voltage is higher rated, but that shouldn't make any problems. Also the soldering is pretty solid, double checked for bridges and traces, everything seems alright. I don't think it's cap-related in this case, or at least not the new ones.

C60 is only going to the single Asic on VA1 revisions, on my VA0 it's the first Asic (IC1) of the two. No shorts.

I do remember reading somewhere that the Game Gear runs a self check at start up and looks especially for over- and undervoltage at this stage, if something is missing or out of tolerance, the watchdog shuts the unit down. It would be helpful to know what exactly the process is.

 

@X5900

I only know from some basic testing on a VA1 model. So I'll explain what I know based on the VA1 (Or what I think I know)

VONF/VRES detects and compares this voltage to VREF. Once its out of spec shutdown starts. You can control the shutdown behavior by adjusting R29 and R30. I didn't make any notes when I was testing my battery charge circuit.

But I think this might point you in the right direction.

 

How much higher are the voltage ratings? I have had issues where the voltage was too high. but if its only about 10v more then thats likely not it.

if you happen for some reason to have the old cap you should check what capacitance it was reading out and try to match that to see what happens.

 

That's a good hint, since both are very close to the leaked caps, maybe there's still some conductive residue left under some of the smd packages in this area, which could - in theory - also sporadically change the level of resistance. I think I'll take it out and clean it again, see what happens.

 

I once had that problem on my GG and it was a broken/corroded trace that I did not see. I had it mended and the thing worked perfectly afterwards.

You're not supposed to need to tweak anything on it if you used the same values as the original capacitors on the replacements.

The watchdog is supposed to power the unit off before it fully deplete the batteries (so they don't risk leaking). The only thing it checks is the voltage.