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No sound from Gamegear after recap, its not the sound board
Post Posted: Fri Apr 07, 2023 8:12 am
Hello there, I've been working on a Gamegear that was gifted to me by a friend, it was her system from her youth but she knows I collect such things so it came my way.

I didn't work so first job was a a badly needed recap during which I also found a broken trace on the power board. All the done and it sprung to life but no sound, just noise from the speaker if I turn the volume all the way up.

I've tested the sound board and speaker in another gamegear and its fine. Tested my power board in the other gamegear just in case it was a missing voltage but no its fine.

Looking at the schematics I tested IC4 (the amplifier on the main PCB) and discovered that I can see what I think is sound input on pins 3 and 5 but there is no output other than a noisy looking signal of around 3v, which is what I assume I'm hearing on speakers.

I found a spare LM2904 which I believe should be compatible for the UPC385 on the gamegear so swapped that out but it made no difference, the behaviour is identical.

Has anyone come across this sort of failure before and would be able to offer some advice as to what may have went wrong? I'm confused in so much as its effecting both channels, if it was one of the passives that had failed you'd expect it to only effect 1 channel, I think it unlikely that multiple have failed effecting both channels but if its not the sound board, not the amp and not the main asic then what else can it be?
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Post Posted: Fri Apr 07, 2023 10:48 am
As you say, if it's affecting both channels it seems unlikely to be a passive near the amp on the main board since both channels are isolated.

Common factors then are supply rails / Reference voltages into IC4 - you have the +5V analog supply, ground and also a +2.5V DC bias - have you checked all those at the pins?

Where are you measuring the output from IC4 and how? There are two test points (TP8 and TP9 on the schematics I'm looking at) which are after the AC coupled caps C64 and C67 so those should read no DC bias. Assuming you have a scope, can you see a signal on either of those?

Otherwise, what's the deal with the headphone jack, is that part of the soundboard assembly that you said worked in another GG or is it fixed to the shell?
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Post Posted: Fri Apr 07, 2023 1:27 pm
I'm in work just now so can check again later but from memory testing last night the +5v analogue supply was a little low, about 4.8v and the 2.5v was high coming in over 3v.

The output I was just measuring on pins 1 and 7 but I did check at the two test points and it looks identical, just noise at the same 3+v level.

The headphone jack is part of the sound board. Testing that sound board in another gamegear its fine (as I said above).

Looking at the schematic again am I correct in saying the analogue 5v and the 2.5v are generated from the main ASIC? I wonder if there's a failure on one of the caps C7 - C10 causing the 5v to drop a bit but the 2.5v to run high.
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Post Posted: Fri Apr 07, 2023 1:49 pm
CRG(Glen) wrote
Looking at the schematic again am I correct in saying the analogue 5v and the 2.5v are generated from the main ASIC? I wonder if there's a failure on one of the caps C7 - C10 causing the 5v to drop a bit but the 2.5v to run high.

No, the analog 5v is just derived from the main supply, Via inductor L3 and +2.5 is a simple voltage divider (R25, R27)

I'd say a read of 4.8v is nothing to worry about (you would normally expect lower than nominal from a circuit drawing current) and I doubt a read of 3v is causing your problem although I guess it could cause some clipping at high volumes. The GG isn't famous for its high def audio.

That you're reading what sounds effectively like a DC voltage coming out of the op amp with same bias as going in suggests either the input isn't actually driving an audio signal (are you in a position to test the input signal?) or else the device isn't working.

Do check the voltages right at the pins though when you get home - you want to rule out broken tracks around that device, and you mentioned you tried replacing it so perhaps it's not the device itself.
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Post Posted: Fri Apr 07, 2023 9:40 pm
So yes it looks as if there is some sort of voltage problem.

Comparing my good GG to the bad with no game in, both using the same power and sound boards. I also swapped back to the original UPC358 as in all likelihood its probably fine.

Good GG IC6 (its a twin ASIC model)
Pin 3 - 2.51v
Pin 2 - 2.57v
Pin 1 - 2.57v

Bad GG IC4 (this is a single ASIC model)
Pin 3 - 4.77v
Pin 2 - 3.7v
Pin 1 - 3.7v

Above values are the same on pins 5,6,7.

Analogue 5V is at 4.86v. Inductor L3 is good, as are C42 and R26 and R27. Most of the small passives test within spec. A few of the 470k resistors between 2.5v and the ASIC which read only 440/450k. Probably not far enough out to cause this problem though and I suppose is within 5-10%. I've tested all vias in the area and they all seem fine. There plenty of corrosion on the power and sound boards but the main board wasn't that bad. There was something odd looking on the side of the ASIC facing IC4 so thats been reflowed also. I suppose there might be something broken under here but removing it would be a pain.

I'm not sure why the voltages are so different between the boards. Does the sound system operate different between the 2 versions of the GG?

How could I go about testing the input signal?
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Post Posted: Fri Apr 07, 2023 10:30 pm
CRG(Glen) wrote
How could I go about testing the input signal?

Well, an oscilloscope is the obvious weapon of choice, but I'm guessing you don't have one as you're measuring DC voltages -presumably with a multimeter?

I think you might be onto something though with those measurements at pin 3 of IC 4. I'd say that ought to be averaging out at the bias point of +2.5V as per your working 2 ASIC unit.

The question now is why. One option is that the ASIC is faulty, and it outputting a steady DC voltage on both channels. This is possible but I'd say moderately unlikely. I would have thought that a fault would likely be isolated to a single channel within the ASIC.
The other thing is going back to that +2.5V supply, maybe there's a problem there. Can you check R27 - with the power off, is there continuity between one side of it and ground, and between the other side of it and R26 (or R25 - can't tell from this blurry schematic)? Then, with the power on, what is the voltage at the end not connected to ground?
EDIT: looks like there's a test point M2 there as well, might make it easier.

Since you mentioned dodgy looking things on the ASIC worth also checking for shorts between pins 86 and 87 of the ASIC, between each of those pins and neighbours and with ground and VCC. Also I'd check continuity between pin 86 of ASIC and pin 3 of IC4 and between pin 87 of ASIC and pin 5 of IC4 if you haven't already.
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Post Posted: Sat Apr 08, 2023 2:43 pm
If I probe those inputs on my scope it does look like an audio signal.

See attached image Pin3 IC4. Comparing to the other gamegear pin 3 of IC6 the 2 signals looks almost the same apart from the level. I hope this shows that the ASIC is fine and is producing sound the fault must be within those voltage levels.

R27 and R26 are both 10k resistors and test fine (one just over, one just under 10k). There is connectivity to ground where it should be, with power on the other side of it. Voltage on the side not to ground is 4.8.
Pin 3 IC4.png (20.9 KB)
Pin 3 IC4.png
pin 3 IC6.png (21.07 KB)
pin 3 IC6.png

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Post Posted: Sat Apr 08, 2023 3:02 pm
Its fixed. Reflowed the resistors and capacitor creating that 2.5v and audio is back. All voltage levels now test the same as my other GG.

Thanks for the help Willbritton. I'm so happy to get this going.

I think what I'll do is remove those resistors and properly clean up the area. Must still be a bit crusty from the previous leaky cap.
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Post Posted: Sat Apr 08, 2023 3:25 pm
Oh that's really great CRG, so glad you got it working!

I guess there must have been a short between +5V and +2.5V, as you say could have been some detritus from leaky caps.
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