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Re: BBC model b serious sound issue

Posted: Thu Feb 20, 2020 2:40 pm
by TimbukToo
Hello again. Thanks to Dave the VIA problem is fixed :D

Now the bad news :(

Sound on switch on is monotonal 2 sounds (the second is like a horn) not a beep and a chord like my other working machine.
Now I can load games, games with simple sound effects and beeps run fine, whereas games with complex sound or speech like exile (hangs at loading screen with a constant beeeep) or wont run at all. Positron works but runs at double speed :?

I'm suspecting IC18 76489N or IC19 LM386N-1 or both but before I start ordering ic chips are there any tests or things to look for?
Thanks Ian.

Re: BBC model b serious sound issue

Posted: Thu Feb 20, 2020 2:49 pm
by johnkenyon
If the computer is freezing then that indicates something on the "digital side" of the sound generator
First port of call is the system VIA

Then working your way from the CPU towards the speaker
IC3 System VIA
IC32 74LS259
IC99 (Speech processor - if installed, extract it to test)
76489
IC17 LM324
IC19 LM386

jk

Re: BBC model b serious sound issue

Posted: Thu Feb 20, 2020 3:05 pm
by TimbukToo
johnkenyon wrote:
Thu Feb 20, 2020 2:49 pm
If the computer is freezing then that indicates something on the "digital side" of the sound generator
First port of call is the system VIA

Then working your way from the CPU towards the speaker
IC3 System VIA
IC32 74LS259
IC99 (Speech processor - if installed, extract it to test)
76489
IC17 LM324
IC19 LM386

jk
Thanks John,
The machine isn't exactly freezing I am able to break the operation and caps lock or scroll lock work.
I will do as you suggest and swap out IC3 with 69 and see if it makes a difference, then go from there.
Cheers Ian

Re: BBC model b serious sound issue

Posted: Thu Feb 20, 2020 4:48 pm
by 1024MAK
The 76489 sound chip is a fairly dumb device. In fact, the CPU can only write data to it, there being no facilities for the CPU to read any data from it. So it is not possible for it to stop or disrupt the running program unless a fault with it’s data lines mess up the keyboard data on the ‘slow bus’ (System VIA / IC3 pins 2 to 9). So if programs/ games crash or misbehave and the keyboard is working correctly, it’s not the 76489 sound chip or any of the sound amplifier circuitry that is the cause.

Mark

Re: BBC model b serious sound issue

Posted: Thu Feb 20, 2020 5:02 pm
by BeebMaster
I'd certainly suspect IC32 (74LS259) as I had a BBC Model B which did almost exactly the same thing till it was replaced.

Re: BBC model b serious sound issue

Posted: Thu Feb 20, 2020 10:14 pm
by TimbukToo
BeebMaster wrote:
Thu Feb 20, 2020 5:02 pm
I'd certainly suspect IC32 (74LS259) as I had a BBC Model B which did almost exactly the same thing till it was replaced.
Thanks mate, I've ordered some of those off ebay and will update :)