BBC model B sound issues

[ed731pdh]

Might be me, might be the computer.

BBC model B restoration project, series 7 board, no speech chips fitted, just the plain vanilla B model. Speaker burrbeeps on power up, all works ok so far other than a sticky "R" key. Only issue is when trying to play notes using the SOUND command nothing comes out of the box.

Before I go hacking into the blasted thing, has anybody had this issue before in the past? There are four chips to dig into, a VIA (6522), sound (SN76489), preamp (LM324) and output (LM386) and a handfull of passives.

Paul.

[Mark Hennessy]

I suppose the analogue stages can be ruled out. I'm not sure what could cause the symptoms you describe, as I'm not completely sure about where precisely the start-up noises originate from... Or rather, I'm not sure where the "burrrr" comes from, but I'm fairly sure that the final "beep" is generated by the OS - it's the default beep you get when you press the Copy key, type VDU7 (or ctrl+G) or do a ctrl+Break. So, if you're sure that it is definitely producing that "beep", then it might be a rather more obscure problem.

Cheers,

Mark

[ed731pdh]

Managed to obtain a diag for the computer, what a mess. Each sound channel is fed to the final amp stage via a seperate op amp. There are four op amps in the same package so it is concievable that one of the amps could go down and remove that particular channel feed. Time to dig out an audio signal generator and go fishing.

The burrr part of the beep is apparently caused by stray signals on the data line that float around until the 555 timer initiates the data bus. Clever, but heavy usage of 74 series chips.

[Mark Hennessy]

Managed to obtain a diag for the computer, what a mess.

No it's not

Each sound channel is fed to the final amp stage via a seperate op amp. There are four op amps in the same package so it is concievable that one of the amps could go down and remove that particular channel feed.

No, that's not how it works. The sound chip (IC18) contains 3 tone generators and 1 noise channel. The combination of all of these emerges on pin 7. This travels through R5 into the summing amplifier IC17d. This sums other signals, such as the rarely-fitted speech synthesis chip (IC99), and an input from the 1MHz bus. Following on from the summing amplifier, IC17c is a low pass filter, ahead of the output amplifier.

The burrr part of the beep is apparently caused by stray signals on the data line that float around until the 555 timer initiates the data bus.

Yes, that's always been my assumption too.

However, you didn't answer the key point I raised: after the "burr", do you definitely get the "beeb"?

Mark

[ed731pdh]

Sorry, yes, got the full burr-beep power up signature no problems. Computer runs perfectly ok other than a sticky "R" key. Had to replace a couple of caps in the smps, they of the X2 type. A pair of honey coloured dipped types, long past life expectancy, one was split right down the middle, the other looked ok but gave up the ghost after ten minutes of soak testing. Out they came, now on soak test on the table and fingers crossed all ok.

Volts output on test meter reads, left to right, as follows. Left group -5v, 0v, +5v. Middle group 0v, +5v. Right group 0v, +5v. Circuit diag for smps shows a +12 volt supply available, but does not appear to go out onto the board so suspect it's for the aux socket underneath.

Thanks for the heads up concerning the amp section, it now makes a bit of sense.

[Mark Hennessy]

Very strange!

Does it produce a "beep" if you do a ctrl+Break? How about when you just press the Copy key, or ctrl+G, or type VDU7

All of those produce the same "system" beep that is produced at the end of the boot sequence.

On the BBC Master you could reduce the volume of the system beep, but I don't think there was a way to do this permanently on a Model B.

Mark

[ed731pdh]

Ctrl+break makes a beep, no probs.

[Mark Hennessy]

OK, so I think that rules out all of the analogue stuff...

Forgive the question, but are you sure you've got the right syntax/values for the SOUND command? Just in case, a good reference is here: http://central.kaserver5.org/Kasoft/Type.../Ch30.html

Have you tried all 4 channels?

Have you got any software you could try - games, for example?

Mark

[ed731pdh]

Sound gives nothing at all, using syntax SOUND 1,2,53,20 etc. Nothing from ports 0,2 or 3 either. Very strange.

[Mark Hennessy]

Try SOUND 1,-10,137,50