Sakhr AX170 (Sanyo MPC-2) audio modification

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Door Omega

Master (217)

afbeelding van Omega

08-09-2019, 02:33

Hello all,

I was servicing an AX170 for a friend, and noticed the audio from the cartridge slots was very unsatisfactory. There's virtually no output of cartridge sound at all. I couldn't find service manuals or schematics, so decided to quickly reverse engineer the audio circuit.

This is what I came up with:


high res
As you can see, audio from both cartridge slots is fed almost directly to the audio outputs of the MSX. That can't be! The slot audio has to be amplified to normal line level first..

So I made this modification to the original circuit:


high res

This mixes the cartridge audio with the PPI and PSG sound in the same way as the original design does. The signal is amplified properly now and the balance slot / PSG is about 50/50 with the 1.5k ohm value. Less gives more slot sound.

How to:
- Remove jumper J213
- Solder one leg of a resistor of 1.5k ohm to the connection of J213 closest to the resistor array

- Solder a wire beteen the other leg of the resistor and the right leg of resistor R104

Interesting fact: the T7937 MSX Engine has separate outputs for all three PSG channels. In theory it should be possible to design some kind of pseudo-stereo circuit :)

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Van sdsnatcher73

Paladin (984)

afbeelding van sdsnatcher73

08-09-2019, 04:35

Good stuff!

Omega wrote:

Interesting fact: the T7937 MSX Engine has separate outputs for all three PSG channels. In theory it should be possible to design some kind of pseudo-stereo circuit Smile

Actually there are several Toshiba MSX models that have stereo output and probably use this feature of the Toshiba MSX-Engine. I've never had the opportunity to hear how that works out.

Van jltursan

Prophet (2282)

afbeelding van jltursan

08-09-2019, 09:26

Really good job!. I'll think about adding a little pot and having a variable output level Smile

Van Omega

Master (217)

afbeelding van Omega

09-09-2019, 01:54

sdsnatcher73 wrote:

Actually there are several Toshiba MSX models that have stereo output and probably use this feature of the Toshiba MSX-Engine. I've never had the opportunity to hear how that works out.

Models with a dedicated AY chip can also be modified. Not those with the ASCII engines, they all seem to have one PSG output..
I've got a Spectrum that was converted to stereo, very popular in that scene. It can sound very interesting with some PSG tunes, left-middle-right for the three channels.

Van Grauw

Ascended (9160)

afbeelding van Grauw

09-09-2019, 09:30

The Pioneer MSXes also have stereo output. However I think (iirc) that there was unfortunately no consistent mapping of the PSG channels to left/right, so there is no de facto standard mapping.

Van Omega

Master (217)

afbeelding van Omega

09-09-2019, 13:08

Grauw wrote:

The Pioneer MSXes also have stereo output. However I think (iirc) that there was unfortunately no consistent mapping of the PSG channels to left/right, so there is no de facto standard mapping.

In Spectrumland there's two standards; ABC or ACB (left-middle-right). Sometimes they use a switch/jumper to select between Smile

Van Grauw

Ascended (9160)

afbeelding van Grauw

09-09-2019, 14:09

openMSX wrote:

share/machines/Pioneer_PX-7.xml: <balance channel="1">0</balance>
share/machines/Pioneer_PX-7.xml: <balance channel="2">-100</balance>
share/machines/Pioneer_PX-7.xml: <balance channel="3">100</balance>
share/machines/Pioneer_PX-7UK.xml: <balance channel="1">0</balance>
share/machines/Pioneer_PX-7UK.xml: <balance channel="2">-100</balance>
share/machines/Pioneer_PX-7UK.xml: <balance channel="3">100</balance>
share/machines/Toshiba_HX-21.xml: <balance channel="1">0</balance>
share/machines/Toshiba_HX-21.xml: <balance channel="2">100</balance>
share/machines/Toshiba_HX-21.xml: <balance channel="3">-100</balance>
share/machines/Toshiba_HX-22.xml: <balance channel="1">0</balance>
share/machines/Toshiba_HX-22.xml: <balance channel="2">100</balance>
share/machines/Toshiba_HX-22.xml: <balance channel="3">-100</balance>
share/machines/Toshiba_HX-23.xml: <balance channel="1">0</balance>
share/machines/Toshiba_HX-23.xml: <balance channel="2">100</balance>
share/machines/Toshiba_HX-23.xml: <balance channel="3">-100</balance>
share/machines/Toshiba_HX-23F.xml: <balance channel="1">0</balance>
share/machines/Toshiba_HX-23F.xml: <balance channel="2">100</balance>
share/machines/Toshiba_HX-23F.xml: <balance channel="3">-100</balance>
share/machines/Toshiba_HX-32.xml: <balance channel="1">0</balance>
share/machines/Toshiba_HX-32.xml: <balance channel="2">100</balance>
share/machines/Toshiba_HX-32.xml: <balance channel="3">-100</balance>

So Pioneer uses BAC and Toshiba uses CAB.

Van mohai

Paladin (880)

afbeelding van mohai

09-09-2019, 18:45

S-1985 engine outputs audio in stereo format (BAC), so MSX'es with that engine can be modded to output stereo too.
Datasheet shows that option on page 26, but forgets to add PPI sound output. (That can be done easily).

I wonder why they designed the engine in such a way but never used that feature ? :-?

Van zett

Hero (606)

afbeelding van zett

09-09-2019, 18:52

great work omega!

Van wbahnassi

Expert (102)

afbeelding van wbahnassi

11-09-2019, 12:37

Hi omega, I have the same AX-170. So if I understand correctly, cartridge audio refers to things like SCC from games like F1-Spirit and Salamander? Because indeed the music I hear from such games sounds nothing like on other MSXs! It sounds like you're only hearing 3 channels instead of full 8.
So the audio is there, just at the wrong level?

Best!

Van Grauw

Ascended (9160)

afbeelding van Grauw

11-09-2019, 13:05

Yes indeed!

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