Sinclair ZX80 / ZX81 / Z88 Forums

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

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 eve...

Ian, the vertical stripes look like a memory fault with the ‘lower’ 4116 (or equivalent) RAM. You say you have replaced all of them. But it looks like bit 2, which is provided by IC8 is not working correctly. First double check that all the pins are correctly in the socket. Then try swapping IC8 wit...

Well done on unshorting it!

Mark

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

Without a valid clock signal on pin 6, a Z80 will just site there doing absolutely nothing useful, and worse, it’s control outputs will not be in a valid state. For it to have been working, and now not working means something has been disturbed. The circuitry between the ULA CPU clock output pin (IC...

There are plenty of other places where you can connect the negative probe of the multimeter, such as the modulator case, the appropriate DC input connector pin, the 0V fingers on the edge connector, various capacitor legs, etc...

Mark

See this post for ideas.

Mark

The ZX Spectrum Next is in production. See this post. If you want one, best indicate your interest here.

Mark

For your 4A board, what is the situation with the 4116 DRAM chips? Are they fitted or removed?

Mark