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Sound chip on cpc plus (AY38912A/P vs AY38912/P)

Started by makinavaja, 20:10, 12 January 20

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makinavaja

Hi!
Lately I've been "fighting" against a cpc6128plus, trying to repair some issues.
The issue I'm working now is about sound.  One of the channels didn't work. On cpc plus, one of the channels goes to one of the "pseudo" stereo channels, and the other two goes to the other one.  Testing the board, I didn't found anything weird so I decided to replace the sound chip.
The sound chip on cpc is used also for the keyboard, so if the chip is dead, no sound and no keyboard.
Well, last week I purchased a AY38912A/P chip.  I believed it should work since the pin out is quite the same compared to the one used on en cpc plus, the AY38912/P, only one pin is different (test/no connect)
Well, carefully I remove the the AY38912/P, then I install a socket and with multimether I see that all the tracks survived, nothing was broken during the desoldering process. Now, testing.
- I place the new AY38912A/P chip on the socket: keyboard doesnt works and an also I've got horrible noise sound on the tv-set speakers.
- I place its original AY38912/P: the horrible noise dissappears but now the keyboard doesnt works


I supose that the original AY38912/P was completly killed during the desoldering.
Now, the questions:
- should I try find an original AY38912/P? is it compatible with AY38912A/P?


Thanks!

TotO

Because the "A" version require an external 2K2 resistor network to pull-up the GPIO pins.
On CPC there is the room for it. I don't know about PLUS.


EDIT: It doesn't have. You require to add it from pin 7 to 14 and took VCC from pin 3 or a closer source. (like capacitor pin)
Best to add from the Amstrad PLUS PCB back to be more easy to solder it. (or use a not "A" IC)
"You make one mistake in your life and the internet will never let you live it down" (Keith Goodyer)

pelrun

That sounds less like the AY was broken (since you can hear all three channels) and more like the mixer path for the centre channel is broken at some point. That's through R165 (to the left channel) and R166 (to the right). Whichever side is the one you can only hear one channel in is the one you need to look at.

makinavaja

#3
Hi! first of all, thank you very much for you answers!
Quote from: TotO on 00:14, 13 January 20
Because the "A" version require an external 2K2 resistor network to pull-up the GPIO pins.
On CPC there is the room for it. I don't know about PLUS.


EDIT: It doesn't have. You require to add it from pin 7 to 14 and took VCC from pin 3 or a closer source. (like capacitor pin)
Best to add from the Amstrad PLUS PCB back to be more easy to solder it. (or use a not "A" IC)
I've got enough soldering skills for that, but maybe I preffear getting a not A chip.  Does anyone knows where to get one with guarantee that it is not a chinesse reprinted fake?


Quote from: pelrun on 02:24, 13 January 20
That sounds less like the AY was broken (since you can hear all three channels) and more like the mixer path for the centre channel is broken at some point. That's through R165 (to the left channel) and R166 (to the right). Whichever side is the one you can only hear one channel in is the one you need to look at.
Well, back then I couldn't hear all three channels, only 2.  But now I listen nothing, so now I supose that the original AY is really dead since keyboard doesn't works (before removing the original AY keyboard worked perfectly)  Anyway, if its the resistor, that would be my first time seeing a broken resistor, weird weird weird ;-)
By the way, you said its R165 and R166, but can't find those on the board.  The higher number was R98, but I did find near de AY R66 and R65. Are those what you wanted to say? wich kind of resistors are those?
Also, maybe I should mention it, before all this I changed all electrolytic capacitors since two of them were dead and computer didn't started up before.

Bryce


TotO

I can send you one to test and you will let me know if it is OK.  ;D
If yes, will be kind to send me next the A version in exchange.
"You make one mistake in your life and the internet will never let you live it down" (Keith Goodyer)

makinavaja

Hi again
Quote from: Bryce on 11:01, 13 January 20
https://www.ebay.de/itm/AY-3-8912-SOUND-CHIP-GENERAL-INSTRUMENT/283441064745

Bryce.

Just now I purchased it.  Lets see what happens!
Its supposed that the manufacturer its not important.  I mean, no matter if it is from Microchip or from GI...

makinavaja

Quote from: TotO on 11:09, 13 January 20
I can send you one to test and you will let me know if it is OK.  ;D
If yes, will be kind to send me next the A version in exchange.

Aaaargh!, I was too fast purchasing the chip! :-(   
Anyway, do you know if it works or I do have to test it?
If I'm able to repair this, I'm planning on purchase some cheap chips from china and test them now that I've got the AY socketed.
Since its ebay: "doesnt works -> f**k you seller, works? I can offer them to people with same problems.

TotO

The time to search the IC and took the picture... But, if you was OK to buy a old AY IC for 15€ + 6.5€ shipping, may be it was first better for you to add over the "A" chip a 5cts resistor network to see if it fix the issue.  ;D 



"You make one mistake in your life and the internet will never let you live it down" (Keith Goodyer)

makinavaja

Quote from: TotO on 11:17, 13 January 20
The time to search the IC and took the picture... But, if you was OK to buy a old AY IC for 15€ + 6.5€ shipping, may be it was first better for you to add over the "A" chip a 5cts resistor network to see if it fix the issue.  ;D 
Yes, thats another possibility I dont want to refuse completly, but my priority now is fully repair the computer and then make tests about it.  If I add the resistors and something is not ok because I make a mistake, everything will become more confusing :-(

TotO

Quote from: makinavaja on 11:23, 13 January 20my priority now is fully repair the computer and then make tests about it.  If I add the resistors and something is not ok because I make a mistake, everything will become more confusing :-(
Sure, I understand. Now, you will got an IC for testing.
"You make one mistake in your life and the internet will never let you live it down" (Keith Goodyer)

makinavaja

Yesterday I received the chip, and I still have the same result: no keyboard and no sound.
Also, with both chips (the original and the one I purchased) if I leave the computer on for a while, automatically burning rubber will start with a qualifying race.


Any idea?

Bryce

Burning Rubber does Autostart if you don't press any button, so that's normal.

If there's still no sound or keyboard, you need to investigate if the AY is getting all the signals it requires, starting with the 5V power rail and clock signal. Do you have the equipment to check these?

Bryce.

makinavaja

Quote from: Bryce on 14:50, 21 January 20
If there's still no sound or keyboard, you need to investigate if the AY is getting all the signals it requires, starting with the 5V power rail and clock signal. Do you have the equipment to check these?

Bryce.

I only have a multimeter, So the only stuff I can investigate is the 5V

makinavaja

5v tested: 4.6v arrive.  The clock signal, dont know it :(

Bryce

4.6V is too low, the chip will stop working at anything below 4.75V. You need to find out why it's so low. Check if it is still 4.6V when the AY is removed from the socket.

Bryce.

makinavaja

Sorry, a mistake writing:
4.3V, not 4.6, so its even less!
And also, the same result with the chip removed.  Anyway, I'm not using the original monitor, Ii dont have it, instead I use an external power supply that works withouth issues on the other cpc6128plus board I have.

Bryce

Then something close or connected to the AY has an internal short. Unfortunately, the most likely culprit is the ASIC.One final test you could do is check whether the sound works (ie: Let burning rubber start) if the keyboard is completely disconnected.


Bryce.

makinavaja

Tested burning rubber with the keyboard disconnected: no sound :(

Bryce

Then there's a very high possibility that the ASIC is no longer doing its job. Unfortunate, as this part isn't replaceable.


Bryce.

makinavaja

Argh! :-(
Anyway, thank you  everybody for your help then.
It curious, because before removing the original AY, at least the keyboard and two audio channels worked, but after that... could the ASIC die from that?

gerald

Quote from: Bryce on 15:58, 21 January 20
Then there's a very high possibility that the ASIC is no longer doing its job. Unfortunate, as this part isn't replaceable.
Or just the power switch that need a good cleanup  ;) @makinavaja can you test this CPC using the keyboard/power switch of your other plus ?


makinavaja

Hi!
Quote from: gerald on 19:21, 21 January 20
Or just the power switch that need a good cleanup  ;) @makinavaja can you test this CPC using the keyboard/power switch of your other plus ?


Yes I can, tomorrow I will try it.
By the way, how can a power switch affect the AY, the keyboard or the asic?

Bryce

Quote from: gerald on 19:21, 21 January 20
Or just the power switch that need a good cleanup  ;) @makinavaja can you test this CPC using the keyboard/power switch of your other plus ?

If it was just the power switch, then it would be very unlikely that Burning Rubber would run. That would mean that every chip on the PCB was getting enough voltage, but the AY wasn't??

Bryce.

gerald

Quote from: Bryce on 08:24, 22 January 20
If it was just the power switch, then it would be very unlikely that Burning Rubber would run. That would mean that every chip on the PCB was getting enough voltage, but the AY wasn't??
For an other thread about supply for GX4000, I've bee testing one down to 4V : sound is the first thing to fail.
Knowing that the  measured supply on the AY is about 4.3V on this plus and the other one is fully working, if the supply provided is indeed 5V, the power switch is one of the possible culprit for the voltage drop.

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