Sinclair ZX80 / ZX81 / Z88 Forums

Just doing a bit of backward engineering and I discovered that two of my ZX81 boards have a 330 ohm resistor (orange-orange-brown; front and center in the photos below) that isn't on the schematics.

This resistor is connected to the node where X1 and C6 are connected. The other terminal is to ground.







Anyone know the history behind this mod?'

Thanks, Dan

Zoom x5: Maybe a montage error!
You can remove the component.

Pure speculation, but maybe this was an attempt to improve the reliability of the oscillator circuit when a particular batch of 6.5MHz resonators were being used.

Dan, which issue boards are those. I can see that they are not Sinclair ZX81 issue one boards, so are they Sinclair ZX81 issue three boards or are they Timex Sinclair TS1000 issue three boards?

The only TS1000 that I have, uses a Sinclair ZX81 issue three board and does not have that extra 330Ω resistor.

There are photos of a TS1000 using a Sinclair ZX81 issue three board with the extra 330Ω resistor here. Notice that this has chips with a 1982 date code.

And a photo of a Timex Sinclair TS1000 issue three board without the extra 330Ω resistor here. This board has chips with a 1983 date code.

Mark

I've seen similar on zx81 isue 3 boards with red solder mask but in my case they were 1K resistors

Andy

Both of them are issue 3 "ISS 3". Both have red solder mask on the underside, but one has a white top silkscreen and the other has a yellow silkscreen. The silkscreen is virtually identical except the yellow one has the number 8243 near the oscillator circuit.

Yeah, I figured as much, suspecting it was a bodge to help the ceramic resonator.

If the ZX81's internal oscillator circuit is like the Spectrum's (which is published in Chris Smith's book: "The ZX Spectrum ULA") then it's an design that I haven't seen used before. Creating a similar circuit in CMOS wouldn't be easy (for me anyway). Better off just ignoring pin 35 and putting a Pierce oscillator on the substrate -- just like Andy did.