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pixelpedant

Weird sound problem on my TI-99/4A

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So I seem to be having an odd sound issue with my TI-99/A (non-QI, F18A modded, usually using a NanoPEB).  Certain sounds, but only certain sounds, are not being produced.  Sound aside, all games function normally.  But certain specific sounds seem to be missing.  For example: 

- Hunt the Wumpus: intro music 

- Star Trek: intro music

- The background beeping during surgery in M*A*S*H

- Most of the sound effects in Moon Patrol

 

But most music and effects work normally.  All music in Alpiner and Super Demon Attack, and the main background music in Moon Patrol, for example. 

 

Results are the same in the presence or absence of Speech Synthesizer and NanoPEB, and when using original carts or FinalGROM. 

 

Have I got a mildly embuggered PSG on my hands here, do you think?

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Thanks for the excellent suggestion.  Findings are as follows: 

 

- All periodic noise and white noise is generated successfully. 

- For any CALL SOUND value, the first value specified will not be audible, but subsequent simultaneous ones will be. 

 

That is to say, for 

 

CALL SOUND(500,200,0)

 

No tone is produced. 

 

For 

 

CALL SOUND(500,200,0,300,0) 

 

One tone is produced 

 

And for

 

CALL SOUND(500,200,0,300,0,400,0)

 

Two tones are produced. 

 

So it seems like one of the tone generators just isn't cooperating?  That would be weird.  But I don't know what else to make of it. 

Edited by pixelpedant

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Running the Sound Test on the 99/4 diagnostic tests cart corroborates the observation that Tone Generator 1 is not producing sound, while Tone Generator 2 and 3 (and the noise channel) are. 

 

So I guess at this point I end up asking myself whether my TMS9919/SN76489 somehow found a way to suddenly give up the ghost (or one out of three of them, anyway).  Or whether this could be somehow attributed to a memory issue. 

 

Ultimately, I guess the question is whether there's even any hope of determining if one of those scenarios (or any similar one) is provably the cause, and whether the responsible chip or component can be replaced. 

 

But regardless, it'd be good even just to hear if anyone heard of a single tone generator just giving up the ghost, before.

 

 

Edited by pixelpedant

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This really sounds (grin...) like a defect sound chip. I suppose it is not too hard to find another 76489 to replace it.

 

The sound generator is actually a pretty simple device; it is a counter that flips the output whenever it reaches 0. I guess the attenuator of the first generator is broken; each generator has its own, so this is not unlikely.

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If I remember correctly, the Speech Synthesizer does not pass through the audio line, so it would have to be put in the box, too.

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Can anyone recommend a source for SN76489s fully compatible with the TI-99/4A?  Don't think I could get another (even broken and console only) TI-99/4A for under $60, and once shipping and customs is considered. So it seems like a waste.  Also just because I might as well save the unit if possible, and ideally not by rendering another TI-99 inoperable. Even if nobody’s selling this chip in the present new or NOS, there must be some old, worthless sound card or something which could serve as a better source than “another TI-99/4A”. 

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13 minutes ago, pixelpedant said:

Can anyone recommend a source for SN76489s fully compatible with the TI-99/4A?  Don't think I could get another (even broken and console only) TI-99/4A for under $60, and once shipping and customs is considered. So it seems like a waste.  Also just because I might as well save the unit if possible, and ideally not by rendering another TI-99 inoperable. Even if nobody’s selling this chip in the present new or NOS, there must be some old, worthless sound card or something which could serve as a better source than “another TI-99/4A”. 

 

Unicorn Electronics under “Products”-->“Generators”.

 

...lee

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One note, the TMS9919/SN94624/SN76494 chips are drop-in replacements for each other in the console, with an input clock speed of 500KHz. The SN76489/SN76496 chips both have an additional divide by eight stage that allows them to be clocked at 4MHz, but that also makes them of limited use in the console, as using them would require additional console surgery to use the correct clock speed.

 

Here is a source for a supposedly tested chip.

Edited by Ksarul
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2 hours ago, Ksarul said:

One note, the TMS9919/SN94624/SN76494 chips are drop-in replacements for each other in the console, with an input clock speed of 500KHz. The SN76489/SN76496 chips both have an additional divide by eight stage that allows them to be clocked at 4MHz, but that also makes them of limited use in the console, as using them would require additional console surgery to use the correct clock speed.

 

Here is a source for a supposedly tested chip.

Thanks Ksarul!  Went ahead and bought it. 

 

Makes more sense to fix my existing unit, even if I do eventually replace it. 

 

And I do admit to eyeing auctions for good condition TI-99/4As, under the circumstances.  Since my long time TI-99's a little beat up from use, at this point.  A few dents here.  A bit of melted plastic from a brush with a soldering iron there.  But even if I do eventually replace it for one a little less dog-eared some day, it'd be nice to keep the old one fully operational. 

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Update on this: opened her up and popped the new SN76494 in, and all sound tests now succeed.  Tone Generator 1 will be silent no more!  Handy thing there was an Ebay listing with international shipping for a tested chip at the time. 

 

And thanks again to Ksarul for tracking it down and pointing it out!

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