MPC-100 no sound

By timharris

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timharris's picture

17-02-2017, 01:24

hi there,

Found this forum looking for some technical info's and it's a massive resource!

I myself have been focussed on C64 repairs since that was my first love Smile but my local buddy had a msx when we were kids, so it's been great fun so far getting a few machines (mainly 8020's) up and running again lately.. and sure enough, I've enjoyed a few classic games to test them too Smile

Been checking this site a lot for all sorts of things, I think the biggest smile I got sofar was figuring out how to build a quick din -> jack cable and use a windows laptop to slave as a tapestreamer for the msx Smile

Anyhow.. I had picked up a very nice boxed Sanyo MPC-100, which has been standing in a corner for a month or two.. I had initially quickly inspected it and all seemed fine, so I only noticed now that it does not put out any sound whatsoever Sad

Sadly I can't find a service manual (or any technical manual) on this MPC-100 machine (many thanks though for this forum to pointing to other sites which contained a lot of manual on other machines!), which makes bug hunting a bit tricky.. but being used to the c64 sound chip - which is known to die sometimes - I thought.. right, lets google some more, perhaps the sound dies as frequent as a c64 and it must be documented SOMEWHERE Smile but was surprised I could find a single post, video etc. anywhere on a dead sound chip on the MSX, on any MSX machine for that matter..

So.. curious for a look inside.. I've popped up the hood.. it had minor dust inside it.. on visual inspection I can't fault it.. caps all seemed fine, etc. so I noticed the AY-3-8910 chip, which luckily says SOUND Smile which is still available for a great price, great.. so intend to give replacing that a go.. without the schematics however, and no scope at hand right now.. wondering if anyone has the scematics and/or if there's a common TTL in the audio section that is known for killing the audio that I should also order just incase? Or.. perhaps some other tip/advice on something that could kill the sound on this msx?

thanks
Tim

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By RetroTechie

Paragon (1385)

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18-02-2017, 00:26

Welcome here, Tim! Big smile

I've seen (& heard) quite a few MSX machines where audio went bad somehow. But I've yet to see a dead PSG chip. Not saying it can't happen, but well those AY chips (or PSG output on MSX engine IC) just don't die easily afaik.

Much more likely is a missing power line, a fried transistor in audio circuit, connector issue, or something like that.

I'd start with checking voltages - on the main board itself. Most likely suspect would be +12V in this case. Although if that goes out, usually so does the video output... Sad Anyway check voltages first.

Next: if you're using RF output, see if sound comes from the RCA audio out. If using that, check RF output. If one's out but the other not, that's a good clue. And check soldering on the connectors involved!

Beyond that it's basically trace the circuit, see if you have a signal @ each input to it, and follow the circuit until you find the signal gone. Maybe you can improvise some sort of 'probe': say, a length of coax cable with RCA on the end plugged into amplifier. On the other end, solder mantle to a 0V point (GND). And a series resistor (some kOhms) soldered to the core of that cable + some wire on other side of that resistor. Then (with some sound-producing software running) probe around the sound circuit if/where you can find a signal. Should be easy enough to locate the problem then.

In any case you'll have to trace pcb tracks & make some drawing how the audio circuit is constructed. Good luck!

By timharris

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18-02-2017, 06:41

hi RetroTechie,

many thanks for the reply!

I did do a quick check, yesterday if it had any voltage at all.. I recall it being somewhere around the 1v , most machines I tinker with are 5v or 12v, so that was a suprise but was the same on other chips too, so without a shematics I assumed this is normale.

Hmm, yeah that's what I was worried about, there simply not being schematics.. Is not that I couldn't draw it out.. it's more that my experience on drawing scematics is very poor, i've self tought myself how to use eagle only a few weeks ago :S

thanks for the tip.. I might just do that, sounds plausible.. I assume you meant backtrack from the RCA?

Tim

By RetroTechie

Paragon (1385)

RetroTechie's picture

18-02-2017, 08:49

timharris wrote:

I did do a quick check, yesterday if it had any voltage at all.. I recall it being somewhere around the 1v , most machines I tinker with are 5v or 12v, so that was a suprise but was the same on other chips too, so without a shematics I assumed this is normale.

Noooo...!! If machine -apart from sound- works, then you'll have something resembling 5V in there. Must be some measurement mistake there (or your voltmeter is broken Wink ).

Usually there's some 74LS... IC's inside. If so, you can check +5V between GND/Vcc pins of that. GND pin can be used as reference for +12V and -12V too, they use the same GND. Other easy to find test points are GND/Vcc pins of ROMs, or on a cartridge slot. So you don't even need to open an MSX to check supply voltages.

Most MSX1's have a 7812 for the +12V line, pinout of that should be known to you I presume? Is power supply integrated on main board or a separate unit? Maybe you can post some photo('s) of the area, and/or some info on what parts are in there?

EDIT: Never mind! You may want to check this thread. I'd guess that black wire in power connector is GND, red is +5V, yellow is +12V, and orange would be -12V then? Measure while plugged into mainboard. Also there's a useful hint at the end of that thread!

By timharris

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18-02-2017, 15:04

cool, I'll get it popped open later today and check.. Mailman just arrived, got a big pile of CDI to sort for that I've sourced for a few friends abroad since it's well priced here in the Netherlands.

As for the voltage.. odd.. I specifically remember being surprised all voltages were at 1.2 and thinking m'kay.. this is odd, but shrug.. at least there's something there... I will be sure to check this evening, thanks for the help!

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