Another 8250 sound + video issue - mod already done

Champion (354)

25-03-2023, 10:57

*sigh* yet more 8250 issues.

Audio
I recently received my Carnivore2, what a great thing that is! However, I have issues with the PSG mix with the sound from the cartridge, PSG volume is low and barely audible. Now I hear you all scream 'what a noob, way back fixes were already described in the MSX Gids and MCM and the like'.
So yeah, actually I already did https://hansotten.file-hunter.com/uploads/files/PSG%20vervor..., for the PSG distortion, which I had very badly. Reversed capacitor C119 (which measured out still perfectly fine). Didn't help at all.

So I used https://hansotten.file-hunter.com/uploads/files/nms82xxgelui..., changing the value of R174. That actually helps, it's much better now, still not perfect. Might need to tweak the resistor value.

I also did https://hansotten.file-hunter.com/uploads/files/Sound%20Fix%..., replacing R428 and R429 with 100µF.

But still the PSG volume is way lower than SCC/FM from the Carnivore. Any clue what value to change? By the way, yes I know the Carnivore has its own mixer and I can use that. But I still would like the MSX own output to be working well too.

Video
I've had a topic before, https://msx.org/forum/msx-talk/hardware/nms-8250-video-issue..., which shows a weird issue when RED comes into play (on RGB output) on a specific TV, which works fine with all other computers I have and vice versa. Now I just switched TV's for another one (same type btw) for my MSX and it works now. But still, I can clearly see the red signal is actually too high. And it is on a completely different tv with RGB as well. Could I just add a resistor to the red output to lower it's signal a bit? Or is that not the way to go here? What value range are we thinking of?

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By AndreV

Champion (272)

25-03-2023, 12:04

I have this audio fix in my computer, (din't do it my self) and the psg-scc fm pac sound is good.

https://www.retromsx.com/wp-content/uploads/2019/04/Fallo_NM...

lot of work i have a dutch translation, by deepL online translator

By meits

Scribe (6577)

25-03-2023, 12:09

The mod on page 5 is very important.

By AndreV

Champion (272)

25-03-2023, 13:22

I have my msx also put to a B&O and the red on that tv is also to red, in service menu you can turn down the red.
I have modifide the mx40000 witch a toggle switch to acces the menu
vanaf 6.00 minute:

https://www.youtube.com/watch?v=hSPuT_t7Lnw

http://forum.arcadecontrols.com/index.php?topic=134556.0

I've got a othter nms 8250 secently with has also have psg-scc issue, and couple a day ago change the R428 en R429 also no result, And rechearge my other working msx, and had that spanish mod. mayby changing g R428 en R429 doesn"t work.

By Rataplan

Champion (354)

29-04-2023, 12:28

Coming back to this; I just tried yet ANOTHER TV, although also a B&O, which also gives me issues on the red signal. I'm currenty building my retro-corner, and actually I wanted to use this particular TV for MSX, as it has the best picture quality, ie. 80-column text is perfectly fine al over the screen. So far I have one Sony TV that works fine, and of four B&O TV only one works fine. What I notice though is that the image colors from the MSX are simply more intense than with other RGB consoles I connect to the same TV. So MSX signal seems a bit to high for them. Now, I suspect the issue is a bit of a combination between my MSX and the B&O's but I still want to fix it.
In the service manual (NMS8250,page 32 from 49)) I see for RED we have C350 (470µF) and R344 (39Ω). Same for GREEN (C351 and R345) and BLUE (C352and R346). They aslso all have a 470Ω resistor to ground after the resistor.

Now if possisble I'd rather put a 'mod' in a scartlead I would then designate to MSX vs B&O. But my calculation skills are rusty, I'm not sure what resistor or capacitance to increase in order to lower the signal. If I just put another resistor just before the scart connector (or in the scart lead for that matter) I'd lower the signal, but I guess the impedance gets distorted? So what value to change or add?

For the record, I've checked all settings, also in the service-menu's of the B&O's. Tons of settings, but unfortunatley no settings for input levels.

By Rataplan

Champion (354)

29-04-2023, 14:50

I now see I was looking at the 8280 service manual, which seems to have a slightly different video output circuit. In the 825x service manual it's page 15 from 40. I'll paste the relevant piece below (not the best readable scans). Anyone with some insights?

By Daemos

Prophet (2170)

29-04-2023, 20:15

MSX and audio are not friends when it comes to the mixer and analogue parts. There is a audio mod somewhere described that actually works 100% using a opamp. Most solutions will eventually introduce crosstalk and other problems. Do not overdo the modding as well as the output of the MSX chips have a limit to what quality of amplification is required. The amount of harmonics from the PSG alone is allready shockingly high so you don't need PP caps and all that.

By Rataplan

Champion (354)

30-04-2023, 00:07

Well, audio seems rather fine now, while not having done additional steps. I need to look into the 'page 5 mod ' from https://www.retromsx.com/wp-content/uploads/2019/04/Fallo_NM... still. But at the moment I'm rather good on the audio level. The video is what bugs me more now, as that really corrupts the screen. I think I'll just introduce slightly higher resistor to the RGB outputs, to lower the signals a bit. As soon as family grants me some time again oO