ZX81 Save

Discussions about Sinclair ZX80 and ZX81 Hardware
rune
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ZX81 Save

Post by rune »

When I try saving using both of my Issue 3 ZX81s the screen display is lost (ZX8 Composite module on both), then it comes back. Im assuming the loss is the data being saved.

The cassette and PC capture do not record any sound. I've tried a couple of mono leads and also listened with headphones. As far as I can tell there is no sound at all.

Are there any common problems with the data at the MIC phono? I'm going to get a ZXpand module soon, but I'd still like to be able to save to cassette even if its just to have a fully working 'puter.

Dave
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1024MAK
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Re: ZX81 Save

Post by 1024MAK »

The ULA uses the same output pin for the picture signal and the save signal, so it cannot produce a normal TV picture during save.

The output level from the Mic socket is VERY LOW. Before feeding to anything other than an old 1980's mono-cassette deck, it will most likely need to be amplified. There is a thread on here somewhere about the low output.

Mark
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Moggy
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Re: ZX81 Save

Post by Moggy »

As mark comments the MIC output is very low on the 81 and seems to have been a backward step from the ZX80 which has a fairly strong output relatively speaking.
It's certainly not strong enough to drive the line level input of a capture card or have the current to drive headphones which will be stereo in any case.

I can't be sure with out looking but I believe some issue three boards had an extra electrolytic capacitor fitted near the output,standing out proud from the m/board as if badly fitted and orange in colour, as either part of the MIC or EAR circuit to improve the signal , maybe that has failed but I could be misremembering this one.

Here is a picture of one of Balfords issue three boards showing the two caps the zeddy normally has ie CPU reset and supply filter caps and the extra one down near the regulator which Balford has fitted much neater than the original.
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rune
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Re: ZX81 Save

Post by rune »

Cheers guys,

I'm using an old cassette player I got with a bundle of Speccy gear. I've never tried to save anything with it so maybe that would be the smart thing to do before assuming the ZX81 is bad.

When you say the output is low, would I be able to hear anything using headphones. I dont even hear static so maybe its not possible to hear the signal.

I'll also try amplifying the output and check the caps.
Moggy
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Re: ZX81 Save

Post by Moggy »

No the tiny signal from the MIC socket is not strong enough for headphones which being stereo would not make a proper connection to the mono MIC socket anyway.

Also remember that the leads from the 81 should connect EAR to the cassette EAR socket and MIC to the cassette MIC socket, connecting MIC to EAR and vise versa is a common mistake.
balford
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Re: ZX81 Save

Post by balford »

Thanks for the compliments moggy, but I put the replacement cap exactly where I found the original so can't claim style points :)

The reset cap on that board was leaking electrolyte so I changed them all as a matter of course. Not as critical to do this as on a Spectrum.

Finally, my experience is also that the ZX81 save signal is very poor. Par for the course.

B
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1024MAK
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Re: ZX81 Save

Post by 1024MAK »

rune wrote:When you say the output is low, would I be able to hear anything using headphones. I dont even hear static so maybe its not possible to hear the signal.
Lets put it this way. Between the ULA pin and the Mic socket there is a 1MΩ resistor. That is 1000000Ω. The resistance of a typical pair of lightweight headphones is 32Ω. So the ratio is 1000000:32
Now given that there is not much current from the ULA pin in the first place.... the 1MΩ resistor (and other components) then limits the output power to such a low value, you need a sensitive digital meter to detect it. Headphones, or earphones are useless for what you are trying to do. In normal audio gear, headphones are driven off a power amplifier capable of driving between 200mW to 1W of power into them.

Like I said, the ZX81 Mic output has a VERY, VERY small VERY, VERY LOW power output.

Mark
ZX81 Variations
ZX81 Chip Pin-outs
ZX81 Video Transistor Buffer Amp

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rune
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Re: ZX81 Save

Post by rune »

Thanks for the explanation Mark.

I've got the place to myself this afternoon and am going to set up the ZX81 and try to test with the old Murphy cassette recorder. Will report back how I got on.

Dave
rune
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Re: ZX81 Save

Post by rune »

Update on the SAVE. The old cassette player records nothing, so I guess thats not going to work.

Answer must be a ZXPand then...
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yerzmyey
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Re: ZX81 Save

Post by yerzmyey »

rune wrote: Answer must be a ZXPand then...
It always is, my friend, it always is.
IN NIHILUM REVERTERIS - a big text-adventure game for ZX81: http://tiny.pl/g2m6m
"MONOCHROME" issue 5 - (Spring 2014) free paper/PDF magazine about ZX81: http://tiny.pl/q2m44
ZX81 COMPETITIONS 2007/2009: http://zx81.republika.pl/
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