steve3000 wrote:My issue 3 beeb has a SN76489AN, however my issue 2 beeb has a SN76489N.
I'm very surprised and confused by this info as everything I've read suggests that the SN76489 max clock is 500Khz, (whereas SN76489A is 4Mhz max) so unless the issue 2 MB has a /8 compensation for the 4Mhz sysem clock, it seems it would be incompatible?
No. See the Wikipedia page I linked to earlier in this thread... or see the datasheet for these chips
steve3000 wrote:The only difference I've noticed with the issue 2 in use, is that it doesn't do the 'normal' beeb start-up sound. By 'normal', I mean the low-tone followed by a beep, which I get from the issue 3 (and other beebs I've owned). The issue 2, however, gives a high tone (sounds more like a chord, rather than a single tone) followed by the beep...
Could this be due to the different sound chip?
The first sound from the Beeb when powered up, is the uninitialised state of the registers in the sound chip being heard, before the OS ROM tells the sound chip to 'shut-up', then the OS ROM actually plays a tone. Hence why you only get one sound when you press and then let go of the BREAK key. Due to differences between each chip, you may get different sounds from them in their uninitialised state. The Memotech MTX computers use the same sound chip, and if due to a fault that prevents it's ROM code from running, it will make a similar 'uninitialised state' sound.
Due to the differences (see the Wikipedia page) between the SN76489N and the SN76489AN, it's not surprising that there is a difference in the 'uninitialised state' sound, as there will be differences in the chip die.Mark