Omega mild short?

[Joao Miguel Duraes]

Hello all,

I've just finished building a v1.5 of Sergey;s Omega board and before deploying any ICs on it or powering it up, I checked the resistance between the +5V and GND lines on J10. It was 260 Ohm - not good. 

After some digging around I can see that R36 (120R) is pulling it to that figure. Without it in circuit, the resistance is actually around 1.6K Ohm. But the resistor on R36 is good (measures a dead-on 120 Ohm) and I think that I have a problem elsewhere, but have so far not really been able to figure it out.

Can I ask of someone who has one to hand, would you mind checking your resistance between 5V/GND on J10 so I have a "this is what good looks like" baseline?

I have already removed all the electrolitic and ceramic caps, checked the RNs, inspected for solder bridges around the sockets, but nada. But then again, am I just being too cautious and 1.6K is ok? I don't think so - nor that with R36 on it should go down to 260...

Thanks,

Miguel

[T Gerbic]

If the resistance is 1.6K with R36 removed and you put the 120 ohm R36 back in and it is 260 ohms then there is some series resistance between the ohmmeter leads and R36  because if you bridge 1.6K and 120 ohms it must be lower than 120 ohms. There must be some resistance coming from a diode, resistor or maybe even a voltage regulator somewhere on the board. The circuit should show open or at least a very high resistance. 

Where is the schematic for this board? It would be easier to give a more exact answer.

[Joao Miguel Duraes]

Thanks, T.

Sergey publishes the schematic here: https://github.com/skiselev/omega/blob/master/Mainboard/KiCad/Omega-Mainboard-Schematic-1.5.pdf

I am measuring on J10, pins 3/4 (GND/VCC) which you can find on page 1. R36 is on page 2, in the "Optional: RGB Output" secion.

TG>  There must be some resistance coming from a diode, resistor or maybe even a voltage regulator somewhere on the board. The circuit should show open or at least a very high resistance. 

That was exactly my feeling, hence why I decided to stop and go hunting for the fault. But so far, I haven't really been able to figure it out.  

That said, I am now looking dead-on page 2 of thew schematic, on the right (where R35 and R36 are) and those two are resistors in series btween VCC and GND amounting to 300 ohms (not too far from 260). 

Hmmm...

[T Gerbic]

Looked at the schematic and there are a bunch of voltage dividers and depending on the strap settings, there are some pull downs. So quite a few resistors in parallel across VCC and ground. I would say based on all the connectivity between VCC and ground that a low resistance would be expected. I see a premanent parallel resistance of 3.24K and if the 120 ohm resistor is in place the 3.24K is in parallel with 300 ohms. Depending on other straps there could be 10K and or 4.7K in parallel with that. So with the 120 ohm resistor in or out you could see the symptom resistance you stated above. I was not aware of the analog circuitry on the board. 

[Joao Miguel Duraes]

Thank you.

I guess this might be as designed then.

I'll wait for someone who has one to chime in what they can see in J10 on their active working system, as I do not want to risk damaging a V9958 - these are expensive nowadays.

And if Sergey sees this and could comment on the design decision, that would also be great so we can learn a bit more. I can put things together and understand the basics, but still struggle with some design principles, which is why I like to play and learn with retro.

Cheers,

Miguel

[Sergey Kiselev]

That's been a while...

If I recall correctly, R14/R33 and R35/R36 voltage dividers are intended to set the voltage levels for so called SCART "slow switch" and "fast switch".

These signals on SCART connector would switch on the TV and switch it to AUX input or something like that. They are completely unnecessary otherwise.

There's some info on these signals here: https://www.msx.org/wiki/RGB_(8-pin_DIN_45326)

Regarding your observations. If you measure resistance on the board between GND and VCC with all components except of the ICs installed, when you're going to get less than 300 ohm. The R35/R36 alone is 300 ohm. And there are several more places with resistors between GND and VCC, e.g., R21/R22 and R23/R24.

--Sergey

[Joao Miguel Duraes]

Brilliant Sergey.

As it turns out, I was reading exactly about that (SCART) in msx.org so was reaching the same conclusion. I will be using the RGB function, but only literally the RGB signals, so I guess I can leave that section off.

It is reassuring to hear that this is normal and I can continue with the build.

Thank you so much for chiming in and thanks for designing this board (and everything else from your collection, which I also have).

Cheers,

Miguel

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