Sinclair ZX80 / ZX81 / Z88 Forums

Does anyone know of a source for the 200ohm speakers like those used in the Issue 1 and 2 Spectrums? The newer (Issue 3 and later) 40ohm speakers are easy to find as new versions, but seemingly less so for the 200ohm.

200 ohm sounds more like a piezo than a speaker, googling "200 ohm piezo" throws up quite a few answers.

I would suggest upgrading the circuit to the issue 6A board standard, then you can use a 40Ω speaker.
Mark

It certainly looks like a speaker in that it has a coil/magnet and membrane. It's just one that I'm fixing up to sell, so an entirely new circuit is probably overkill, and perhaps a non-beeping Spectrum might actually appeal to people that like to shoot-things-up in the dead of the night or whatever (you can't actually turn the speaker off normally of course).

Electronically, is it viable/what would happen if I used a 40ohm speaker with a resistor added to bring it up to about 200 ohm resistance?

Hi,

In case of 40ohm speaker, the resistor will pull up the ZTX450 main currant and will destroy it by overheated.

You had to make a serial pod made with five 40ohm speakers…
Or add a 150 ohm resistor.



Wiring diagram:

The reason that Sinclair changed the circuit (which occurred from the issue 3 board, but the schematic is wrong) is because the sound output was too low (too quiet).

If you use a less sensitive speaker, such as a 40Ω one, and then wire a resistor in series with it, you will only end up making it even less loud! In fact with a 160Ω resistor, more power will be wasted in the resistor than used in the speaker!

Changing to the later circuit just involves removing diode D10 and replacing it with a transistor and some patch wires.

Mark

1024MAK wrote: ↑Wed Feb 05, 2020 9:01 pm Changing to the later circuit just involves removing diode D10 and replacing it with a transistor and some patch wires.

Oh yes, so it does! I only need to pinch the 9v from somewhere. I'll give that a go then.

XavSnap wrote: ↑Wed Feb 05, 2020 8:15 pm Hi,

In case of 40ohm speaker, the resistor will pull up the ZTX450 main currant and will destroy it by overheated.

Xav, the issue two circuit does not use a transistor: And the speaker is a higher resistance type.

Mark

So we cannot have a DOLBY 5.0 standard on the Spectrum.
it's really sad...

Sorted. Half neatly done - but it doesn't matter as it's hidden by the heat sink. I stole a TR7 from a parts only Issue 6 board.

Thanks Mark and Xav for your comments.