Perhaps the strangest problem you ever see...

Started by ikonsgr, 18:32, 05 January 22

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ikonsgr

 Recently i got some rather strange behavior with my fully remade Amstrad CPC 6128 (ALL ic's on board were removed and replaced with new ones). With specific games only i have sound problems along with erratic activation of tape motor relay.
Bellow there are 2 game videos:
Wairware from CNG, no sound & erratic relay activation: https://www.dropbox.com/s/jwsf0qfveryp4n9/wire_ware.mp4?dl=0
Subhunter, constant sound noise: https://www.dropbox.com/s/c4bj0danflbbgrd/subhunter.mp4?dl=0
In both cases, everything else works ok, game is playable, no crashes or hung ups occurs. There are other games that have similar sound problems (like orion), but most games seem to load ok.
I've checked all connections between involved ic's (e.g. 8255 I/O chip and AY-3-8912 sound generator and a coupe of logic gate chips) all seem to be ok. I've also replaced both 8255 and sound chip but i still got same behavior, so i'm feeling like hiting a wall with this...  :(
Does anyone has any clue of what might be wrong?

robcfg

Can you post a picture of your AY chip?


If you replaced the chip with an A variant, then you'll need to add a resistor array if I remember correctly. That may cause the erratic behaviour.

ikonsgr

Yes,i know that and a network resistor is installed. Afterall Amstrad worked ok for quite a long time, and afaik network resistors does not go bad "out of the blue"  ::)

robcfg


ikonsgr

I'm not quite sure but i have a feeling that it might have to do with games using 2nd 64k bank. Both subhunter and orion are 128k games, (but i don't know if that's the case with wireware too) ,although i've also replaced HAL/GAL  chip  with a known working chip but problem remains.
Another sypmptom i've noticed is that when a game finish loading from snapshot (most probable when CPU registers and 8255 ports are set), i get a "click" from the tape relay, something i don't get with my other CPC's. This suggests a problem with 8255 but as i noted, it is replaced with a new one and still got the same problem...  ::)

pelrun

It has to be the chip select logic for the 8255, making it read the bus when it's not supposed to. Check the CE,A0,A1,WR pins, which are connected to the A11,A9,A8,IOWR bus signals respectively.


Bryce

Try swapping the Firmware ROM, you may have damaged some content while removing it.

Bryce.

ikonsgr

Quote from: Bryce on 17:44, 06 January 22
Try swapping the Firmware ROM, you may have damaged some content while removing it.
You mean that firmware eeprom might loose some content and have such symptoms? To tell you the truth, since i can't find anything wrong with hardware, the only possible cause might be something like this...  ::)

Bryce

You didn't say whether you used the orignal 40015/40025 ROM's when you desoldered and socketed everything. If you used the original chips, there's a chance that you fried/flipped a few bits while desoldering the chips. If you burnt new EPROMs, then they should be good, but if you used the original chips, this could be the issue?

Bryce.

Jean-Marie

There's a diagnostic tool to check up your ROM & RAM on this thread :
Amstrad Diagnostics ROM (cpcwiki.eu)


Longshot

Hello,
A similar thing has already happened on a CPC whose power connector was not "tight" enough.
When using certain instructions that daisy-chained too many devices through OUT (0), A, it caused deficiencies in the AY or activations of the tape relay.
I don't know if this experience can help you.
Rhaaaaaa

norecess

My post may seem dumb but I love the font used in this Diagnostics ROM. Very clean and readable.  8)

ikonsgr

#12
@Bryce  Roms are C256 eeproms
@Jean-Marie all dignostics passed ok
@Longshot same thing happens even if i load from floppy disk without anything else attached to amstrad.
Another find i just discover: when amstrad with the problem is powered and a game is loaded, and i push motherboard to various places repeatedly, sometimes amstrad hungs or resets! This is not happening with the other 2 amstrad cpc i have, no matter how many times i push motherboard nothning happens!
Does this suggests a bad/loose connection or soldering somewhere? Because as i said, the specific amstrad had all ic's removed, solder sockets and place new chips!

Bryce

Sounds like a dry joint or a cracked trace somewhere.

Bryce.

ajcasado

Check that the new Z80 CPU is a NMOS one, not a CMOS
CPC 664

Empiezas a envejecer cuando dejas de aprender.
You start to get old when you stop learning.

Longshot

Due to a soldering problem, there may be a loss of energy.

Ajcasado's remark is judicious, because the Z80A CMOS do not do the same thing with the ED 71 instruction.
On CMOS it makes an OUT(C),#FF and on NMOS an OUT(C),0.
Significant impact! ???
Rhaaaaaa

ikonsgr

Finally,i found out the "root" of the problem: BAD Z80 CPU!
As soon as i replace the cpu, everything worked perfect! All wierd symptoms described above vanished!  :)


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