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Dead TIA in composite modded 7800, started as bad audio.


marauder666

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We've been sent a 7800 from a forum member with audio issues.

Audio has been wired without the mixing resistors, no cap on the output.

Recently, its started misbehaving with distorted TIA audio and no POKEY audio.

 

Rewired the audio with output cap and mixing resistors, the TIA is still distorted, but the POKEY is now working.

 

Thinking its a faulty TIA, I tried Combat.  Wow.   No audio at all, and one player randomly moving around.  See video.

Socketed the TIA and put a spare TIA I've got in and all's working fine.

 

I'm coming to the conclusion that without the mixing resistors on the audio lines, it allows too much current to flow, and the TIA output circuits are not up to the ask and are damaged, including damage to parts not directly connected to the audio output. 

 

Be careful, when modding your 7800 wire up the audio with the mixing resistors, no need to remove them, just lift one end of them, tie them together and add a 1uf-10uf cap to the audio output jack.  This console did have good TIA audio for a while, damage occurred over time.

 

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3 hours ago, marauder666 said:

I'm coming to the conclusion that without the mixing resistors on the audio lines, it allows too much current to flow, and the TIA output circuits are not up to the ask and are damaged, including damage to parts not directly connected to the audio output. 

 

Be careful, when modding your 7800 wire up the audio with the mixing resistors, no need to remove them, just lift one end of them, tie them together and add a 1uf-10uf cap to the audio output jack.  This console did have good TIA audio for a while, damage occurred over time.

Unless the owner of the console also wants to keep the audio working for the RF modulator in which case, a second pair of mixing resistors is needed as I usually do.

 

But, if this is true, then it means that many of those simpler composite upgraded consoles that bypass all of the resistors are ticking bombs for their TIA chips. And those aren't exactly made anymore either.

 

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