low volume & distorted fm-pac sound when scc cartridge is in other slot

Resident (33)

28-05-2022, 23:02

Hey Guys,

On a msx nms8250 if i only insert the fm-pac the sound is good. If i only insert the scc sound cartridge the sound is good.

When i put both fm-pac (slot 1) and the scc sound cartridge (slot 2) you can hear the fm-pac music very low volume and distorted. Is this a common problem?

This only occurs with the fm-pac by the way, when both sound cartridges are in both slots SCC still sounds ok.

Por Pencioner

Scribe (1546)

28-05-2022, 23:45

I think the issue is related to the very famous audio issue on those machines. There are fixes available (require soldering skills)

Por Manuel

Ascended (19303)

28-05-2022, 23:45

Did you try to change the volume switch on the FM-PAC (if you have an original Panasonic one)?

Por spanjed

Resident (33)

29-05-2022, 00:58

Hi guys, thanks for the replies.

I do know about the famous sound problems on the nms8250 but that's not the problem in this case.

The fm-pac is a dutch clone from msx club Gouda
The scc cartridge is a clone also

If i use only one of them (either scc or fm-pac) the sound is fine. The problem only occurs when i put them both in the 2 slots and i want to play a fm-pac game :-?

Por sdsnatcher73

Prophet (3849)

29-05-2022, 04:59

You have to check this audio modification for the NMS8250. Only the part on page 5 and 6 are needed (modification of the slot board). I got this info from member Omega here on MRC and have confirmed it works well with help from Alexey.

Por spanjed

Resident (33)

29-05-2022, 14:50

sdsnatcher73 wrote:

Thanks for the reply.
Perhaps that's the solution, i will look into it or go something like a carnivore to use the audio jack from that card (with the downside that original scc roms don't use that card ofcourse)

Por sdsnatcher73

Prophet (3849)

29-05-2022, 15:42

Yeah, even original SCC benefit from this modification because the PSG audio is muted (compare the sound on your 8250 with an SCC game to what you hear on an emulator or even YouTube videos of that game). The drums will be missing among others.

Por spanjed

Resident (33)

29-05-2022, 15:56

I see, well i already had the 2 resistors changed like in this video

https://www.youtube.com/watch?v=idSN1l9TwRM

So i was hoping that would solve the problem, i cannot play SD Snatcher with no psg drums ;-)

Por Daemos

Prophet (2051)

29-05-2022, 17:03

Mixer failure in nms8250/55 series can only be resolved by replacing the mixer. There is a treath on this forum where schematics and board files are available. All you need is a opamp and a couple of resistors.

Por sdsnatcher73

Prophet (3849)

29-05-2022, 19:36

@Daemos. That is incorrect information. The mixer is really not the problem, the slot board is the problem. After fixing the slot board the sound is mixed properly (PSG and 2 external cartridges). Changing the slot board effectively disconnects the two audio inputs (which are shorted in the original design) and changes the way they are connected to the mixer (making for an equal playing field between all inputs).

Por Daemos

Prophet (2051)

30-05-2022, 12:33

Partially incorrect. That fix just adds resistors in the slots signal paths which in analogue audio terms is well asking for many problems. This topic has been extensively been discussed before. Adding a few resistors here and there will solve your problem but not the way you should. Have a look at schematics of amplifiers and see for yourself. Offcourse you could use the one transistor for that to get the job done. But then there is the effect of input impedance.