Help me with my sanity check - GBA component check and diagram reading - Page 1
Forum section: electronics / beginners

author: Black Phoenix
author_rank: Super Contributor
timestamp: April 24, 2021, 02:10:29 am
content: Heck, this thing is making me question a lot of things. I'm trying to help someone repair a Gameboy Advance that have a Custom Winding SMD Transformer broken (T1):Easier said that done since this part it looks like it is custom made and all the search I've done online regarding the printing on it return into nothing. It also helps that a lot of different markings are available for the same part:I could desolder it and measure his inductance but I don't have a LCR meter available. So I though of trying to know the rate of the transformer and try to get a compatible one. So using the schematics available online I try to do some measurements:So basically on T1:Pin 1 and 2 are connected together;Pin 3 and 4 are connected together;Pin 5 and 7 are NC;Pin 6 goes to a pair of Caps in parallel;Pin 8 is GND.So with a lead on a GND pad I made some measurements and:Pin 1 - 2.23V;Pin 3 - 2.33V;Pin 6 - 0.01VNow my sanity check to see if I understood the circuit perfectly:Pin 3 and Pin 1 check since they are in the same side of the transformer, the primary side, so their voltage being .10V apart is normal;Pin 6 is strange since the DA (Digital Analogue) needs more than .01V. Look that we are talking of .01V above GND that is 0V. But now that I check It should be 0V since the DA have a pair of Caps in Parallel, that are powered by Pin 1/Pin 2, going through a Zenner Diode (D2). So the measurement is right. But then what is the VOut on Pin 2 of the Voltage regulator U4?It gets 2.23V via Q4 on pin 4 of U4 that is connected to Pin 2 to drive it, plus 3V directly from the battery on Pin 1 of U4, and should output, by thepage 2 of the PDFto the Audio Amp 3V via Pin 2 of U4.Am I right on my assumption or I done a big mistake on the measurements/reading the schematic? What kinda compatible part is available as replacement without needing to follow the route of getting a donor board and remove the component from it?

author: theHWcave
author_rank: Regular Contributor
timestamp: April 24, 2021, 09:11:30 am
content: Did you measure DC or AC voltages?Which position was the power switch? (not having a Game Boy and no idea how that switch would be used - it looks strange). If it would be in the most right position, T1-pin3 should be the battery voltage.What brings you to the conclusion that T1 is broken in the first place? It looks quite alright on the pictures?What are the voltages at VDD3, VDD5, VDD-15 and VDD13 , and VSHD (shutdown?)

author: Black Phoenix
author_rank: Super Contributor
timestamp: April 24, 2021, 10:22:35 am
content: Quote from: theHWcave on April 24, 2021, 09:11:30 amDC voltage;The Power switch have 2 positions, On and OFF. Pin 1 is OFF and Pin 4 is On, Pin 2 and Pin 3 are Commons, connected between them that closes the circuit when the power switch is on the ON position.Exactly, T1-3 is the voltage of the battery.Quote from: theHWcave on April 24, 2021, 09:11:30 amThe images is not from the PCB in question, is just for a view of the part in question. The PCB in question was missing the component, so it doesn't power up.My measurements on a know working board was to try to get more info of what is the output voltages when the input of 3v is supplied, to get a similar "rate" on a compatible part. When I mean rate (probably not really the right word for it) is an example of:Input 1V, Output .2V so it is a 1:5, were the input voltage is divided by 5 and you have the output. Or Input 1V, Output .5V, so a 1:2 if you understand what I'm saying.Quote from: theHWcave on April 24, 2021, 09:11:30 amIs it really needed to know that to find what kind of SMD transformer model could be compatible? That voltages are provided by U4 (Voltage Regulator) that is working without problem (I used a donor board and put a know working SMD transformer from a donor board and it is working) and the voltages output by U4 don't go into any of the Pins of the SMD transformer.My objective is really get to know what is that T1 in terms of part no or compatible component from any manufacturer, since is a lot common for this SMD transformer crack his ferrite core if the PCB or even the GBA is dropped on the floor, specially when most of them have more than 15 years after being manufactured. 40% of the boards I pickup used have this component cracked or missing parts or even totally shredded.

author: BrokenYugo
author_rank: Super Contributor
timestamp: April 24, 2021, 03:38:12 pm
content: Push come to shove you could probably just unwind one to get the winding data, find a close enough bare core and wind your own. With winding data it may also be possible to calculate things close enough to find an off the shelf solution.This is probably running at higher frequency than a multimeter is gonna measure properly. An oscilloscope would be a more suitable tool for probing right at the transformer.

author: Black Phoenix
author_rank: Super Contributor
timestamp: April 24, 2021, 04:01:52 pm
content: Quote from: BrokenYugo on April 24, 2021, 03:38:12 pmThat was what I was afraid it would be the cause, since I don't own an oscilloscope.Quote from: BrokenYugo on April 24, 2021, 03:38:12 pmI may try that since I have some cracked ones but working that I would not mind using.

author: theHWcave
author_rank: Regular Contributor
timestamp: April 24, 2021, 07:04:35 pm
content: Oh ok, I did not get that T1 was missing. That is of course a big "hint".I agree with the other comments, your best bet is to unwind one, counting the turns and orientation and then use that data on a similar core to create a replica

author: Black Phoenix
author_rank: Super Contributor
timestamp: April 25, 2021, 01:26:53 am
content: Quote from: theHWcave on April 24, 2021, 07:04:35 pmIt was my fault also for providing stock photos instead of the PCB in question and not explaining myself in a better way.

author: amyk
author_rank: Super Contributor
timestamp: April 25, 2021, 02:17:33 am
content: I am struggling to understand the schematic and why this part needs to be a transformer and not a regular inductor. The VDD3 part (3.3V?) is a straightforward boost converter, apart from some slightly perplexing things like why REG_EXT is always grounded, but the 5V part makes no sense; the output will always be shorted to ground through DA2.

author: Black Phoenix
author_rank: Super Contributor
timestamp: April 25, 2021, 11:06:23 am
content: Quote from: amyk on April 25, 2021, 02:17:33 amAsk Nintendo, that is the schematic directly from a leaked Service Center Manual. By design it have a boost regulator built in, most of the GBA runs on 3.3V, it boosts the 2-3.2V from the AAs to a stable 3.3V. You can't supply it with more than 3.3V because it is not a buck-boost, a well designed boost regulator will just shut down if you go over the set output voltage. The SPs and Micros use a buck-boost. The GBC and GBP also use a boost regulator, but for 5V, so you can run a lithium ion without modification.Also the Gameboy Advance is basically a Gameboy Advance system (16.8 MHz 32-bit ARM7TDMI) and also includes the CPU of the Gameboy DMG, Pocket and Colour for backwards compatibility (8.388/4.194 MHz Sharp LR35902). And the cartridge voltage are also different - 5V for the DMG/GBC/GBP and 3.3V for the GBA.

author: gcewing
author_rank: Regular Contributor
timestamp: April 25, 2021, 11:42:27 am
content: Quote from: amyk on April 25, 2021, 02:17:33 amIt looks like it's generating a couple of auxiliary voltages (+12 and -15?)QuoteI wouldn't entirely trust that schematic. I can see at least 3 places where I think there are erroneous dots: the connection between REG_EXT and GND, and between VOUT5, DA1-AC and DA2-AC. Also, the way it's drawn, the connections between pins 1-2 and 3-4 of the transformer seem to be shorting out windings, which seems unlikely.

author: Black Phoenix
author_rank: Super Contributor
timestamp: April 25, 2021, 12:05:49 pm
content: Quote from: gcewing on April 25, 2021, 11:42:27 amBy the way, here's the CPU part of the schematic.

author: amyk
author_rank: Super Contributor
timestamp: April 26, 2021, 12:06:52 am
content: Indeed if I ignore some of the dots then the schematic starts making more sense. They are deriving an extra +13/-15 supply for the LCD using that transformer in place of the boost regulator's inductor, and it is likely not so critical the exact voltage since they are just relying on the transformer ratio and the 5V sense.From that, we can guess that the transformer will likely have a similar inductance for the 5V coil as the 3.3V one, i.e. 100uH or thereabouts, and the requirement for -15 and +13 will determine how many turns of those are needed by the ratio.The gaming industry does tend to be very scared of cloning and such, so perhaps the extra dots and creating confusion were a pretty stupid attempt at obfuscating the function. This is definitely a case where reverse-engineered schematics will likely be more accurate...