TI99/4A Sound Chip Replacement

[widowsson]

Hi:

I need to replace the sound chip in my 99/4A (Unit was working well, now I still have video but no sound at all)

I did do what others have suggested and checked the internal 5 pin DIN connections for breaks and all looked well.

Besides pulling a replacement chip from a spare board, are there any sources/alternative aftermarket chips that can be used in place of the original?

Thanks

[pixelpedant]

I was in this situation a while back.  Tone Generator 1 had failed on the TMS9919 in my main TI-99/4A (weirdly).  I was able to successfully replace it with an SN94624 from this seller.  Exactly the same item/auction as listed there, actually.

[widowsson]

Excellent! Thanks 

[HOME AUTOMATION]

Some '4a boards use the SN76494, which I believe uses a different timing frequency! A small change in one of the traces on the PCB should allow for this.

[pixelpedant]

1 hour ago, HOME AUTOMATION said:

Some '4a boards use the SN76494, which I believe uses a different timing frequency! A small change in one of the traces on the PCB should allow for this.

My impression had been that the SN76494 used the same frequency divider as the TMS9919/SN94624 and it is the otherwise pervasive SN76489 which differs.  Is that not the case?  Are there TI-99 units which were originally shipped with SN76489 (or compatible) chips, and wired to accommodate the higher clock input?  If I'm reading Thierry Nouspikel's always invaluable tech page on the subject correctly, it seems like both the TMS9919/SN94624 and SN76494 divide by two. 

[HOME AUTOMATION]

Well, I may not recall the # anymore, but, a couple decades ago...

In desperation, I replaced a SN76494, with something I pulled off of a Coleco board. The frequencies were all much lower ...wish I had known then, what I do now.

 

The choice of what CLK to use comes down to W104 vs. W502. I'll see if I can fine a "board pic" of this.

[AwkwardPotato]

The SN76494/94624 (haven't come across a TMS9919-labelled chip yet) share the 447kHz clock with the GROMs on most of the TI boards I've seen. The SN76489/76496 take a 3.58MHz clock input, and I've only seen them installed in 99/4 and some very early 4A boards. Correct me if I'm wrong, but I believe you can tell if you need a 3.58MHz-capable chip if your board has the long coaxial bodge wire between the VDP and PSG.