Pokey Audio on Atari7800

[CraigsBar]

Hi all,

 

I have a French SCART A7800 whereby the Pokey Audio is so quiet... No issue with Ballblazer, you can just turn the telly up! but with Commando where both audio chips are used it makes it impossible to hear the music... or be deafened by the Soud Effects.

 

I read here ....

 .... that R5 needs to be changed from a 6.8k to 18k resistor to address this and balance the audio signals. However as the labels on the PCB are underneath the components I am unable to locate R5 on the PCB (REV C) can anyone assist so that I can fix this?

 

thanks

 

Craig

 

[juansolo]

Post #195 on page 8 (if it doesn't link properly). Might be that. There's been a couple of French 7800's that were wired incorrectly.

FWIW the NTSC 7800 has different value resistors than the PAL one as the PAL machine has only a single TIA audio line and the NTSC one has two. That post you quoted will work for the NTSC machines, yours is a PAL machine with an RGB encoder instead of a RF modulator essentially.

Edited January 4, 2022 by juansolo

[+-^CrossBow^-]

Actually on the NTSC machine it is:

 

R5 = 6.8K - Pokey / External Audio Line from cartridge port

R6 = 18k - TIA audio

 

Just wanted to make that clear. Due to variance in the resistors, even with these values at 1% tolerance I've had the reverse happy where after installing a second set of resistors for my 7800 services, the Pokey ends up being a tiny bit louder than TIA or the TIA ends up being a tiny bit louder than the Pokey still. Even when through RF all stock, the audio is more or less in balance. I stick with using 1% value resistors for that section just try and keep it as much within spec as possible without changing the originals as well.

 

[CraigsBar]

2 hours ago, juansolo said:

Post #195 on page 8 (if it doesn't link properly). Might be that. There's been a couple of French 7800's that were wired incorrectly.

FWIW the NTSC 7800 has different value resistors than the PAL one as the PAL machine has only a single TIA audio line and the NTSC one has two. That post you quoted will work for the NTSC machines, yours is a PAL machine with an RGB encoder instead of a RF modulator essentially.

Yep that link improved things. Looking at the trace on the board however it would be easier to just solder the blue wire to the AUD point as that is connected to the left most points of the resitors marked CA2 and TA 2 already. I am still going to tweak the resitor values however, as although the Pokey is now louder (Going through only one resistor and not two) is it still overpowered for my taste in Commando.

 

I might get a couple of variable resistors in there so it can be tuned more easily in the future

[juansolo]

8 hours ago, CraigsBar said:

Yep that link improved things. Looking at the trace on the board however it would be easier to just solder the blue wire to the AUD point as that is connected to the left most points of the resitors marked CA2 and TA 2 already. I am still going to tweak the resitor values however, as although the Pokey is now louder (Going through only one resistor and not two) is it still overpowered for my taste in Commando.

 

I might get a couple of variable resistors in there so it can be tuned more easily in the future

Depends what's populated further down the line. In an RGB machine there shouldn't be anything in there, so you'll probably be ok using the AUD pad, I'd imagine that's what it's there for. With a regular PAL machine it's easier just to lift the ends than go through and remove a load of other parts. Lifting the resistors just makes sure you're not going through anything else.

10 hours ago, -^CrossBow^- said:

Actually on the NTSC machine it is:

 

R5 = 6.8K - Pokey / External Audio Line from cartridge port

R6 = 18k - TIA audio

 

Just wanted to make that clear. Due to variance in the resistors, even with these values at 1% tolerance I've had the reverse happy where after installing a second set of resistors for my 7800 services, the Pokey ends up being a tiny bit louder than TIA or the TIA ends up being a tiny bit louder than the Pokey still. Even when through RF all stock, the audio is more or less in balance. I stick with using 1% value resistors for that section just try and keep it as much within spec as possible without changing the originals as well.

 

On a PAL machine it's a pair of 8.2K resistors IIRC.

[marauder666]

Pal is a pair of 6.8k

[CraigsBar]

15 hours ago, marauder666 said:

Pal is a pair of 6.8k

OK so I have now got beautifully balanced audio on my PERITEL Atari 7800. Using the picture from the linked post (Included here too) I have done the following mods...

 

 

1. Desolder the Audio Pickup (Blue Wire) from the right hand leg of TA2 and attach it to the AUD marked solder pad

2. Desolder resistor CA2 and replace it with a simple wire link

 

This leaves the 6.8k resistor in line with the TIA audio and no resistor on the Pokey audio, the 2 signals are mixed on the trace of the reverse of the board between the left hand points of the Wire link now in CA2 and TA2 and then sent on to the RGB board via the Blue wire.

 

I have now also done this same mod in the same way to my spare A7800 which just has a composite mod board and I can confirm that the quality of the balance between POKEY and TIA is greatly improved on this console too. In all honesty this seems to be a very simple way to greatly improve the balance between the 2 sound chips. *ALTHOUGH* I have not tested it with a RF Modulator as I do not have an A7800 with this still in place.

 

[CraigsBar]

7 minutes ago, CraigsBar said:

<snip previou post>
 

 

Sent from my CPH2089 using Tapatalk
 

 

Edited January 6, 2022 by CraigsBar

[jeff20]

Is a quiet POKEY due to the 7800 having a composite mod?  I'm having the same problem.   

[+DrVenkman]

1 hour ago, jeff20 said:

Is a quiet POKEY due to the 7800 having a composite mod?  I'm having the same problem.   

Read above - quiet (too low) POKEY audio levels are usually due to a poorly or improperly done AV mod that doesn’t balance the TIA audio line and the Cartridge Audio lines together using appropriate-value resistors. To get the sound levels balanced with one another, you will need to open the console to see how the mod was done, then un-do the current audio connections and re-do them correctly.

[+-^CrossBow^-]

2 hours ago, jeff20 said:

Is a quiet POKEY due to the 7800 having a composite mod?  I'm having the same problem.   

Like @DrVenkman stated it is likely that your TIA audio is being tapped from a point off the main board that is essentially directly off the TIA chip itself while the POKEY is also being tapped from directly off the external audio line. However, this causes an imbalance that may not be apparent until some games are used or some flash cartridges. 

 

This is a pretty simple fix and I'm happy to look into it and possibly correct it for the cost of shipping as I've offered to others in the past. Just send me a PM.