CPC 6128 Sound issues

[VincentGR]

Two years ago, I bought a real CPC bargain.
A 6128, CTM, GT monitors plus a TV Tuner for 10 euro.
After the usual switch repair, I brought it to Digital Universe for public display.
Games were running fine, drive and tape input too.
Joystick and sound also work great. But after a while I've noticed that some games had white noise instead of sound.
I saw that 128K games did that (is relentless 128k too?)

So I thought it was the page ram. Multimeter gave me 5V to all ICs and none of them was hot.
I ran 6-7 ram tests from disk but not any rom based.
Don't really know what is going on, maybe some LS?

A couple of days before Christmas, I will return home to search more.

Any help would be great.

[Audronic]

What sound do you get after Power up by pressing the "DEL" Key several times.
What sound do you get if you shut down the Game that produces White Noise and press the "Del" Key several times ?
Good luck    Ray

[Bryce]

It's more likely that your AY is failing. This happens quite often.
If RAM had failed the games would be crashing, not just white noise problems.

Bryce.

[VincentGR]

Amstrad is fine, no probs with del key or in game music.
This happens only on 128K games 

[Bryce]

Amstrad is fine, no probs with del key or in game music.
This happens only on 128K games 

There's absolutely zero physical or electrical reason why accessing Bank1 would have an effect on the sound. I would do further testing if possible and also try a different PSU.

Bryce.

[VincentGR]

I used a PC PSU and two different CPC monitors.
Probably I'll replace the AY although as I mentioned above it plays perfect with no noise or missing channels.

[arnoldemu]

which games other than relentless cause the noise?

[VincentGR]

Can't really remember as it was two months ago but in a few days I will make a list when I go back home.

[VincentGR]

So, I've changed the AY with a new one plus a socket.
The problem remains 

[VincentGR]

Bump!


Well today I did check him once again and discovered something else too.
There is a problem with the keyboard when a joystick is connected.
I also tried a different joy with the same results.



https://www.youtube.com/watch?v=_WH1gdMHVmk


The funny thing is that Shinobi runs just fine and the music is playing normal.

[Bryce]

The analogue (sound) and digital (Keyboard/joystick) sections of the AY are completely seperate from each other. One can fail and the other part will still work perfectly normal. But, it could also be the scan generator (IC101 - 74LS145) that's causng the problem.

Bryce.

[TotO]

If your new AY is a "8912A" version, it will require a network resistor on the GPIO. (see the picture)
http://www.cpcwiki.eu/imgs/f/f1/CPC6128_PCB_Top_%28Z70290_MC0020B%29.jpg

[vasilisk]

If your new AY is a "8912A" version, it will require a network resistor on the GPIO. (see the picture)
http://www.cpcwiki.eu/imgs/f/f1/CPC6128_PCB_Top_%28Z70290_MC0020B%29.jpg

Upon cleaning my second 6128, I noticed that the AY chip is the only one with some strange modifications. Maybe it has been changed in the past. Is this what you mean by "network resistor"?

[TotO]

Yes it-is. The picture show a "homemade" 2.2KOhm resistor network, because the AY-3-8912A do not embbed it inside the circuit to pull-up the GPIO to avoid the bus to be random.

[vasilisk]

Yes it-is. The picture show a "homemade" 2.2KOhm resistor network, because the AY-3-8912A do not embbed it inside the circuit to pull-up the GPIO to avoid the bus to be random.

Why it was replaced with 8192A and not with 8192? To avoid all this "trouble"?

[TotO]

Why it was replaced with 8192A and not with 8192? To avoid all this "trouble"?

I don't know... Should be only A version in stock, cheapest part... When you put 8 resistors together instead of using a real one (like on the right side on the 3rd picture), I think that is more a handyman solution to fix the CPC.

[VincentGR]

Will check on Monday, thank you all.

[TotO]

Will check on Monday, thank you all.

Here, a good part for the AY-3-8912A version. (8x 2K2 resistors network with 1 common pin)
https://www.reichelt.com/fr/fr/r-seau-de-r-sistances-sil-2-2-ko-2-sil-9-8-2-2k-p18029.html?&trstct=pos_0

I should have it if required... Have fun! 

[VincentGR]

Solved.


Can't remember the exact post about the incompatibility of the Z80 but yes, this was also the problem here.
I swapped the CPU with another CPC and the problem moved to the other 


This is the suspect:

[gerald]

Solved.


Can't remember the exact post about the incompatibility of the Z80 but yes, this was also the problem here.
I swapped the CPU with another CPC and the problem moved to the other 


This is the suspect:

Auto referencing myself  : http://www.cpcwiki.eu/forum/hardware-related/cpc-464-noise-after-loading-game/msg170591/#msg170591
and http://www.cpcwiki.eu/forum/amstrad-cpc-hardware/z80-cpu-nmos-or-cmos/

[VincentGR]

Aaaahhh yes!


It was that thread by catenaccio


Thank you.