SNES-Mini RGB Measurements

Okay, the cap to resistor to output way worked somewhat. The scaler doesn't like the signal much (no auto sync lock-on) but it works
with the signal as well as with any other method. Seeing how it's much safer for the system, I'll go with this

Any idea what the 47pF cap to ground is for?

 

It doesn't matter which is first, as long as the resistor is relatively close to the source (the S-RGB) in relation to the total length of the line (including coax cable). What's more important is how you wire the resistor and capacitor to the connector, ideally you want an ideal wire without any impedance characteristics (such as directly soldering the components together and to the connector), or if a PCB is used, the impedance should match the connector and coax's 75 ohms specification.

Yes, that's a discrete sync driver. There must be a reason they chose to use one over the S-RGB output however.

It could be that the S-RGB doesn't put out a sufficiently large signal. Maybe it outputs ~300 mVpp with the intention of it being added to other signals instead of ~600 mVpp intended to directly drive a 75 ohm coax. Or it could be that your scaler has a larger termination resistor than 75 ohms (so it may receive large dc-coupled sync signals), in whcih case the 220uF capacitor would distort the low-frequency components of the signal.

It's a very cheap method of transient suppression (when inserting and removing the cable). If you look at the circuit literally it creates a low-pass filter with a cutoff frequency way outside of a sync application, and since the filter is directly followed by a coax cable with variable capacitance (determined by quality and length) it makes even less sense as a filter.

 

^ thanks for all that , makes sense why they went another way for c-sync... so according to the newer schematic one should build this to get a proper signal:

 

Yes but note that the circuit's input source is the digital output from the PPU, not the S-RGB's sync input.

Something I wonder is whether the 1.8k and 3.3k are reversed in the schematic. It would make a little more sense the other way.

 

^ the way I read it s-ppu2_pin100 goes to 2 paths: one leads to the S-RGB chip and the other to the sync driver - both are deriving from a straight connection...
Shouldn't it be possible to grab the signal from s-rgb's pin since it's basically the same signal coming from s-ppu2?
Am I reading it wrong?

 

Yes, it's the same thing. The schematic isn't how the actual circuit is laid out either
Do you want to rebuild that driver btw? I'd be interested to hear about it.

 

I have it in my plans to try that, need to order the 2sc2412k transistors though. At some point I'll report back about it

 

Does it have to be a special transistor? I haven't worked with them yet so excuse my ignorance please

 

^ no idea... maybe someone here knows more , it would be nice if a more generic transistor could be used

 

I looked up the BA7232FS Datasheet, which Link83's thread over at the NFGGames forum says is similar to the BA6596F (S-RGB A).

Link to Datasheet

The last page shows an application for composite video. It also shows the composite sync output with no extra components. The TV or upscaler should have a 2k resistor to GND on the sync input.

If someone could trace pin 3 on the multi-out back to the S-RGB A pin 18 on their 1-CHIP, that would be great.

 

Which version? NTSC or PAL?
The PAL one doesn't have CSync on the AV out and the NTSC one is similar to the schematic shown above?
(IE: It doesn't use pin 18 but pin 7 with transistor)

 

Any small-signal NPN transistor.

If the datasheet is any indication, it's showing that SYNCOUT is a 2.5V logic level sync output--nothing more than a buffered copy of the input.

The 2k resistor in the diagram isn't the TV's termination load, it's either a generic load to demonstrate the pin's current-sourcing ability, or it's a recommended emitter pull-down resistor for the output driver (emitter-follower stage) so it may sink current to drive a logic gate.

TVs terminate inputs with 75 ohms almost without exception, so it's definitely not proper to wire pin 18 directly to the multi-out/drive a TV with it (assuming the S-RGB is comparable). That would be why Nintendo used a discrete sync driver. If I'm right about the 1.8k and 3.3k being switched, the discrete driver will yield about 400 mVpp into 75 ohms.

 

would a 2N3904 be OK? just found it via google "commnon NPN transistor" ... or do you have a part in mind for that job?

 

Yes, perfect.

 

2N3904 is available here and costs 16 Cents

 

The NTSC 1CHIP has the same sync circuit as the previous NTSC consoles, the same one in the schematic above by Darthcloud.

 

I have all the needed parts now to test the c-sync driver , is there a way to measure output with a digital multimeter in order to test this with the 1.8k/3.3k resistors swapped or as pictured?

 

No, you will need an oscilloscope. Multimeter will not work.

 

You can measure which resistor is which with a multimeter in-circuit, though accuracy is not guaranteed unless you measure them out-of-circuit.

Also you can roughly measure the output level of the driver by attaching a 75 ohm load to GND on the output then measuring the DC voltage across the load while switching the input between 5V and GND.

 

^ I won't lie, I can't do that

Anyone knows of a way to isolate pin9 on the av port (composite) so I can safely attach c-sync there?
Here is a photo of the back of the mobo , the pink line is the composite trace , there is a hollow rectangle there that depicts a resistor, I thought that by removing it the pin would be isolated but sadly there is a trace on the right that connects it... :\



So looking on the other side of the board I found that there is connection to the (-) pad of C18 capacitor - a 220uF hole-thru component. (+ is connected to the composite output pin of the s-rgb chip)
So am I right to think that removing C18 will prevent any composite signal going to the av port and safely test c-sync?
On the line there will be left at least a resistor, a cap and a filter, would they interfere with the new signal assuming they are not fed anything from the original source?