Philips NMS 8250/8255/8280 mixing issues

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By Wild_Penguin

Hero (628)

Wild_Penguin's picture

27-10-2010, 21:56

Hi all!

And sorry for bringing up this subject in Yet Another Thread but I've read the others, and also this - and still my 8250 is not fixed ;) I'd like to fix this, and perhaps some other Philips NMS's can be also fixed if I can get this fixed with someones help =)

To summarize the problem - PSG is nearly silent when ANY SCC cart is inserted - :

  • I've changed R428 and R429 to elco's
  • C119 polarity was already correct (different to C121 on the PCB) and R174 is already 910Ω.
  • Sometimes there ware quality problems in SCC audio (see link)
  • Philips FMPAC does not change the PSG level (noticeably)
  • Philips FMPAC is quiet (as PSG) if used together with an SCC cart

Possible remedies:

  • I'm thinking about changing R170. Does anyone have any idea, what value I should put there?
  • I've read in other threads to put a potentiometer in pin 49. But I don't like that fix, I'd rather put some resistor with a fixed value; but also, why is FMPAC level correct? Will the potentiometer make the FMPAC level wrong? But if this is the only way, then I guess that's what I'll need to do.
  • ???

I've made a recording on the SCC quality issue (notice missing PSG, but also the crackling sound). I have not noticed any quality issues on other software than the SD Snatcher, and in SD Snatcher only in the recorded part (actually there might be very slight issues earlier in the intro, too, but they are nearly inaudible). On an emulator the sound is nice in SD Snatcher. But also I have no other MSX2 to compare with, what SD Snatcher should sound like in a real MSX2...

Thanks! Hope someone has an idea what to do next. LOL!

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By Samor

Prophet (2070)

Samor's picture

27-10-2010, 22:54

Are you sure the FMPAC level is correct? or does it just seem to be so?

you could add the potentiometer and make it controllable (by letting it stick outside of the case or something). That way it'll be like a mixer control which you can set as you like.

Alternatively you could MOD one SCC cart and add a potentiometer in there.

I don't have audio on the pc I'm at atm, but AFAIK SD Snatcher makes a crackling noise in some parts of the music when you use a normal SCC instead of SCC+, since it has to merge 2 channels.

By Wild_Penguin

Hero (628)

Wild_Penguin's picture

28-10-2010, 16:31

Are you sure the FMPAC level is correct? or does it just seem to be so?

"correct" as in there is no audible difference in PSG sound level when I plug in the PAC, compared to when I don't and the PAC sound level is "about the same" as PSG (I don't need to touch the volume knob). Also, what is "correct" level, is a question without an answer, according to what I read in other threads. Every MSX model mixes a bit differently - there's difference even within the same model. But I find this weird indeed, why is the PAC behaving differently than the SCC? There is a switch on the PAC, that AFAIK is for the sound level - maybe it was put there because of these mixing "faults" in the first place, and I luckily have it in the right setting for my MSX... but then, why does my SVI728 mix both SCC and the PAC well with it's PSG?

you could add the potentiometer and make it controllable (by letting it stick outside of the case or something). That way it'll be like a mixer control which you can set as you like.

Alternatively you could MOD one SCC cart and add a potentiometer in there.

These are exactly what I would not like to do =).

I'd like a proper fix in that way, that SCC and PAC are heard at a usable level (in fact, I have an SVI-728 for which I have changed the resistor, it has no problem in mixing SCC or the PAC). If there's a resistor which I could change, but the value is not known, a potentiometer would be nice temporarily in finding the right value.

As for the second option, I can't open every SCC cart I have; nor do I know if installing a potentiometer in every cart is even possible.

I don't have audio on the pc I'm at atm, but AFAIK SD Snatcher makes a crackling noise in some parts of the music when you use a normal SCC instead of SCC+, since it has to merge 2 channels.

I tested it with the MegaFlashRom, which has SCC-I (SCC+). Of course, the translated version of SD-Snatcher could have a bug and use it as an SCC, not an SCC+ - that would explain this issue. OpenMSX can't emulate MegaFlashRom (as of yet, I red somewhere they were just working on it?), so I chose an emulated SCC+, so that would explain why it works nicely there. I don't have any other SCC+ cartridges to test (not to mention, an original Snatcher/SD-SNatcher cart Sad ). Hmm., maybe I should test it on OpenMSX with a regular emulated SCC instead of SCC+ Tongue

Put thanks for your suggestions, though. I'll report here if / when I attempt to fix this =).

By Wild_Penguin

Hero (628)

Wild_Penguin's picture

28-10-2010, 17:55

About the SD-Snatcher audio issue: I tried again and now I can hear the same issue in OpenMSX no matter what I choose for the SCC emulation Shocked!. Maybe it sounds like this even originally, so never mind about that - I must have had some earwax, I could have sworn it sounded better at that part in OpenMSX Wink

By Samor

Prophet (2070)

Samor's picture

31-10-2010, 20:52

heh, I finally downloaded that .wav, it's actually nostalgic. The SDS intro demo sounded almost exactly like that on my 8250 with a normal SCC (not SCC+) inserted. That part is fine. On the other hand though, the PSG level is even lower than on my original 8250; actually, its pretty much non-existant in that recording

And btw, I think there's no perfect "scc/psg" balance setting.. well perhaps there is when you use the same MSX model the composer used....

By bartl68

Expert (86)

bartl68's picture

01-11-2010, 08:22

for a good mixing psg/scc i thought i used 2 times a transistor (value 2,3 Ohm) from the pin 49 leg of the cart to the pcb, with good results!!Big smile

see link : http://www.mijnalbum.nl/Foto-NPHJDBPA.jpg : notice the red circle !!!

By BeauDiGi

Expert (72)

BeauDiGi's picture

05-11-2010, 11:46

maybe I buy a philips msx computer, but have al this computers sound issues?

By Samor

Prophet (2070)

Samor's picture

05-11-2010, 12:14

NMS82XX series tend to have issues, the VG8235 (another common model) is fine.

By Repair-Bas

Paragon (1101)

Repair-Bas's picture

05-11-2010, 13:16

No problems with the 8250 computers.
Only problem is mixing external sound with the internal sound. But most computers have a modification, so problem solved.

The problem is this topic has to do with Flash SCC+ cartridge.
The problem is in the cartridge.

By Samor

Prophet (2070)

Samor's picture

05-11-2010, 14:57

No problems with the 8250 computers.
Only problem is mixing external sound with the internal sound. But most computers have a modification, so problem solved.

The problem is this topic has to do with Flash SCC+ cartridge.
The problem is in the cartridge.

I meant unmodified (standard-out-of-the-box) 8250's, and the mixing part in particular.
I know's there ways around it, but if you buy a completely original 8250 it would have the mixing issue, at least.

By Repair-Bas

Paragon (1101)

Repair-Bas's picture

05-11-2010, 15:22

Not that problems they discribed in this topic.

There are problems with the Flash SCC+ cart, then there is almost no PSG anymore.

The mixing isseu where you talked about is almost nothing and many users do not hear that.
If somebody has problems with that then you only need 2 little parts.
That's all, no problem I think....

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