Sega Saturn - bleeding only in RGB ..

I do a lot of Sega Saturn region free and CD-R modchip installs. I noticed a couple of customers now that bring their Saturn to me with the same issue.

I always test all consoles before working on them. The problem is as following:

- Saturn looks great using composite video
- Saturn looks horrible using CSYNC RGB SCART cable on my Sony PVM. (It's not the cable. The cable is a high quality cable made by that lady on eBay. A vast majority of Saturns looks amazing using this cable on my PVM monitor).

The picture is dim. There is horizontal color bleeding. Looks almost like color rolling at the top.

Anyone else experience this?

 

Well.. figured it out. In case anyone else runs into this in the future, there's a little IC between every pin of the port and the rest of the PCB.

This IC has marking "101" on it. Middle leg is common between all of them. I think it's some sort of filter. The one for CSYNC went bad. Swapped it with the one for Chroma, and RGB looks great now!

If you look at the top of the main board, it's the row of black colored square chips. They look like square flat capacitors.

 

Is that "IC" any of the coils marked Lx by chance?
https://upload.wikimedia.org/wikipedia/commons/9/9d/Sega-Saturn-Motherboard.jpg
Or is it the EMI filters?

 

It was the EMI filter. Second from the left is CSYNC in that picture.

If you trace the circuit, you'll discover that Saturn gets CSYNC directly from the 315-5687 GPU chip. RGB and other outputs are generated by the CX clone chip just under the EMI filters (IC22) Given other output are good, I knew the CX chip was good. Given CSYNC came directly from the GPU and I have video, I knew it was a passive component that went bad.

The EMI filter is the only passive component in the circuit! So I swapped it. Not sure if I can find a replacement on digikey.

 

I can't find that specific part anymore but since it's a 3 pin EMI filter, I suppose (guess) it's an LC or LCL part.
Aren't those also used in the Genesis btw?

 

Was it limited to a particular revision board?

 

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Thanks for finding the correct EFI filter. At this time I'm using the EFI filter from the Chroma pin in place of the failed CSYNC one.

Chances are that S-Video connection will never be used on this Saturn. So I can leave the IC out, or I can just jump pin 1 to pin 3 (hence not using an EMI filter, similar to how many other consoles have it!). Didn't decide yet.

 

@Druid II are you sure the values and LCL configuration are correct? This spec seems far too high for the purpose:
Attenuation Value 25dB @ 350MHz ~ 500MHz

If that component dies a lot (or unusually often), there's probably some reason for it.
Maybe people are drawing too much current through the CSYNC line, frying the small coils.
I don't think it's the capacitor anyway.

By the way: Many consoles just omit this filter and they work fine.

 

Funny thing is, the 2 consoles that I saw which had this issue .. I'd be SHOCKED if CSYNC was ever used on them. They both were only used with composite video. Unlike all the other pins, CSYNC feeds from the GPU directly with no other passive components down stream.

Also, when the coils die, shouldn't they just open? This is not the case here. CSYNC was still "ok" but seems to not be at the correct level. I think what happened was the small cap inside the EMI filter somehow died.

 

CSYNC probably got used lots more in recent years, by retro gaming people with potentially bad homegrown equipment.. Just a hunch, really
I don't have the proper background but I remember reading about coils going bad with their symptom being much higher impedance.

 

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Yea, okay. We need someone with the proper background then to make sure.

I found a datasheet for an EMI filter series and the capacitance value is the only differentiation there (range from 22pF to 22nF).
So if we go by that, the 100pF version would start affecting something at about 100 to 200Mhz.

 

The original part has "101" marking on it if that helps.

 

Another Sega Saturn model 2 came in for mod work today. Once again my CSYNC cable had all washed out image! (3 in 1 month!?) give me a break. I think there's a design flaw with these systems! Removed EM4 (TDK blue with 101 marking) and swapped with EM10 (Chroma). CSYNC problems went away!

This board is VA7.

This is a USA Sega Saturn. Pin 1 of the 10 pin mini DIN is CSYNC. CSYNC SCART cables is recommended by many cable sellers for Saturn 1. Sync on Luma recommended for Saturn model 2. Well, that's just BS / Hack! Sure, sync on composite video or luma will work, but it doesn't solve the inherit problem.

I believe the Sega Saturn model 2 has a design flaw that causes the EMI filter to fail prematurely.

FWIW.. Tim Worthington (Maker of NESRGB) posted this on another forum:

The Saturn needs a 470 ohm resistor in series with the TTL sync output to make it compatible with a standard 75 ohm video input (SCART, XRGB, JP21, pretty much any monitor related to television). Otherwise the signal will be much to large and excessive current will flow from the sync output, through the cable, into the TV's video input. It causes unreliable operation of the TV's sync separator, buzzing in the audio, and possible damage to the c-sync output (leonk, your popped EMI filter was probably not a coincidence).

If that's not bad enough, if you unwittingly connect a PAL Sega Saturn, which has a 9V DC power supply in place of composite sync, you end up with it connected directly to your TV's video input. That will damage the TV, scaler, etc. It's no problem at all if there's a resistor in place to limit the current.

So don't forget to put the resistor in place. It's better it put it into the console end, as that reduces buzzing audio, but it will still do the job in the SCART end.

 

Sounds exactly like what happened here.
People probably use self made or after market cables for RGB today and chances are that many of them don't have that resistor.

I only wonder why Sega choose to omit this single component. Compatibility with non-terminated devices?

 

I wonder if the solution is as simple as:
- remove EM4
- install 470 ohm resistor in its place (between legs 1 & 3)
- use same csync scart cable on model 1 and model 2 saturn (simple pass through)

 

On consoles where the resistor is missing: Yes, that would be the solution.

(I understand that "model 1" consoles have this resistor and "model 2" do not?)

 

I had issues getting my model 1 to sync in 576i (all other modes worked), installing the 470ohm resistor fixed it. I'd argue no saturn has it, there is no 220uF coupling capacitor either.

 

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