The problem is this topic has to do with Flash SCC+ cartridge.
The problem is in the cartridge.
What is the problem in the cartridge? I would like to know.
That is the question?
With all other soundcarts there are no problems, only with that cart the PSG is almost gone.
I'm not sure whether the problem is in the cartridge or not.
It works on all MSX models tested but not in some Philips, that have a well known mixer problems.
So I guess that the problem is in the computer, but with MegaFlashROM SCC+ you really notice it. With other sound cartridges it is also noticiable, but not as much as with this cartridge.
Lets turn that around: Every cartridge (e.g. FM-PAC, Music Module, Real Konami SCC and even a Manbow 2) seems to be fine, PSG is always audible. Except for the MegaFlashROM SCC+. To me, the problem seems to be the combination of the 2.
Note: the previous version of the Flash ROM cart did not have this problem.
Well, you can turn that around as much as you want, but the facts are the facts 
Manbow 2 and MegaFlashROM SCC uses the same IC that Konami SCC game (a real SCC chip). So no need adding them to the list.
In your own words "Weird thing is: with an FM-PAK or Music module inserted I can hear the PSG although the volume is slightly less when compared to no cartridge inserted at all. "
This means that external sound sources are not mixed right and the PSG volume is less when external sound sources are present.
Now, just read my former post again.
I've had someone else explain to me that a slight change in PSG volume is to be expected when a cart with a sound source is inserted. Bartl68 gives a possible solution, but repair_bas explains that its probably not gonna help (http://www.msx.org/forumtopic11709p30.html). So I'm kinda at a loss here. Dont really feel like soldering one or two resistor's if I'm not sure it is gonna help.
Bart68 contacted me some time ago and he said he fixed the problem. He says the sound is right after adding the resistors. I have not tried it myself (I have any 8280/8250/8255) but there is no reason to not believe what he says.
Repair_bas said "It looks like a the Flash cart uses the same adresses as the internal PSG" which is not right. The cartridge uses different ports. Moreover, using the same ports would cause that both PSGs sound at the same time (more volume). In any case one of the PSGs would silence the other for that reason.
After thinking about is, Bart's fix actually seems right. A current takes the way of least resistance. With the flash cart inserted, the current for the sound probably goes to the flash cart instead of going to the normal sound output. Adding two resistors to both slots should prevent the current to flow from the PSG to the carts (and thus via the normal sound output) and it should prevent the current to flow from slot 1 to slot 2 and viceversa.
So erh,
By cutting pin 49 on both cartridge connectors and reconnecting them with a resistor of 2,3 Ohm all mixing problems are solved?
And even a noob (like me) can do this?
So it seems. But it's a 2200 Ohm (2.2Kohm) resistor
