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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 13th Mar 2020, 11:49 pm   #41
John Earland
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Default Re: ZX Spectrum Tuning Issue

Ah! No-these have three segments! I guess that’s the problem!! How embarrassing!!
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Old 14th Mar 2020, 12:02 am   #42
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Default Re: ZX Spectrum Tuning Issue

It might be, depending on the 'geometry' of the sockets. I have a feeling those sockets on the ZX machines do not play well with typical stereo plugs due to the spacing between the socket contacts.

Do you maybe have a 3.5mm mono to 3.5mm mono jack plug lead, or do you have a couple of solderable mono plugs which you can use to make up your own lead?

Ironically, I have a nicely made 3.5mm to 3.5mm mono lead sitting within arm's reach here. I could send you one, I know I have plenty, but you could probably order one yourself online just as quickly.
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Old 14th Mar 2020, 12:18 am   #43
John Earland
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Default Re: ZX Spectrum Tuning Issue

Thank you so much for your kind offer-I’ve just ordered a couple from Amazon! Thank you though. When they arrive I shall let you know how I get on!!
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Old 15th Mar 2020, 9:00 pm   #44
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Default Re: ZX Spectrum Tuning Issue

Have now tuned the Spectrum into both a CRT TV and LCD TV both via the SCART socket and both give excellent results. The border is still a little fuzzy but totally acceptable. I am hopeful that using the correct audio cables will allow me to SAVE and LOAD programs as before!

Finally, I have discovered that this is a 48k Spectrum. I do remember it used to have this somewhere on the case but this one doesn’t. I found on the internet a PEEK command that would return a value of 255 if it was a 48k and that’s exactly what it did do - so doubly happy.
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Old 27th Mar 2020, 12:10 am   #45
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Default Re: ZX Spectrum Tuning Issue

Brought over from the ZX81 thread:

Quote:
Originally Posted by John Earland
I had modified the ZX81 using the transistor and two resistor method-still awaiting the diode. I know this works for the ZX81, would this also work for the ZX Spectrum? I'm assuming it will.
All the composite mods I have seen for the Spectrum so far just use the straight-through approach, ie, the way your Spectrum is already modified for composite. I don't think there is really a need, therefore, for the transistor buffer mod on the Spectrum.

In an earlier post you seemed to find the image quality over composite on both of your displays acceptable - is there a particular reason why you think it needs a transistor buffer? If you can find a documented instance of a more complicated composite mod for the Spectrum by all means try it, but if it is working really well, it might be best to let sleeping dogs lie...

Did you get those audio cables you were waiting for?
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Old 27th Mar 2020, 12:02 pm   #46
John Earland
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Default Re: ZX Spectrum Tuning Issue

Thanks for that-I completely agree with you. I shall leave it. No, I was just thinking it might enhance the picture but I’ll leave well alone.
The audio cables have arrived now and all working well. The cassette recorder is being picked up by both the spectrum and ZX81(when it works!) and I can read and write to it.

Last edited by John Earland; 27th Mar 2020 at 12:17 pm.
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Old 27th Mar 2020, 12:18 pm   #47
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Default Re: ZX Spectrum Tuning Issue

I thought the idea of the buffer was more one of protecting the internal circuitry rather than enhancing the quality.
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Old 27th Mar 2020, 12:33 pm   #48
John Earland
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Quote:
Originally Posted by Dave Moll View Post
I thought the idea of the buffer was more one of protecting the internal circuitry rather than enhancing the quality.
Yes - it is - I am new to this world of electronics! I don't really know what I am doing - hence I greatly value people such as yourself to keep me on the right path!
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Old 27th Mar 2020, 12:35 pm   #49
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Default Re: ZX Spectrum Tuning Issue

I think that on the ZX81 the ULA is not designed to drive the rather low impedance 75R load of a typical composite video input, it is designed to drive the input of an ASTEC modulator.

The Spectrum ULA seems more tolerant / more capable as shown by the fact that few people seem to bother with a buffer stage, although of course a unity gain buffer would give added protection as you say.

I notice John's new CPLD based ZX81 ULA which he has on the way has a switchable setting to optimise the output for modulator input / for direct composite video out.
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Old 27th Mar 2020, 1:21 pm   #50
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Default Re: ZX Spectrum Tuning Issue

Yes - it's a really neat solution. I've seen images where this little vLA81 pcb fits inside the modulator. I'm not sure I like that though, not sure why because I would have no intention of putting it back to UHF output.
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Old 27th Mar 2020, 1:24 pm   #51
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Default Re: ZX Spectrum Tuning Issue

Quote:
Originally Posted by SiriusHardware View Post
I think that on the ZX81 the ULA is not designed to drive the rather low impedance 75R load of a typical composite video input, it is designed to drive the input of an ASTEC modulator.

The Spectrum ULA seems more tolerant / more capable as shown by the fact that few people seem to bother with a buffer stage, although of course a unity gain buffer would give added protection as you say.
The Spectrum's video output is already buffered, because the luminance (which comes more or less straight from the ULA) has to be mixed with the chrominance from the LM1889 encoder chip, and there's an emitter follower after this. Adding another buffer wouldn't do anything useful.

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Old 27th Mar 2020, 1:49 pm   #52
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Quote:
Originally Posted by John Earland View Post
I've seen images where this little vLA81 pcb fits inside the modulator.
I think there might be a degree of confusion there - the VLA81 PCB is the same size / has the same footprint as the original ULA, that is, it is about the same size as a 40-pin IC.
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Old 27th Mar 2020, 1:51 pm   #53
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Originally Posted by cmjones01 View Post
Adding another buffer wouldn't do anything useful.
Indeed not, now that you have explained why.
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Old 27th Mar 2020, 3:10 pm   #54
John Earland
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Default Re: ZX Spectrum Tuning Issue

Just an update: the new vLA81 has just arrived! I shall have a long think about whether I will try the swap myself. If I do I will do as you suggest and cut the old ULA out as close to the chip casing as possible. I've also been advised that as you melt the solder on each pin make sure you pull the pin out of the slot in a direction where you don't have the potential of tearing the printed circuits.
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Old 27th Mar 2020, 3:23 pm   #55
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Default Re: ZX Spectrum Tuning Issue

The main thing is just to be sure that the solder on the pad is 'wet' (ie, melted) and then withdraw what is left of the cut-off pin straight upwards away from the top side of the PCB with something like a pair of fine pointed tweezers.

Don't worry about clearing the holes at that stage, you don't do that until you have removed all 40 of the original pins.
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Old 27th Mar 2020, 5:02 pm   #56
John Earland
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Default Re: ZX Spectrum Tuning Issue

Quote:
Originally Posted by SiriusHardware View Post
The main thing is just to be sure that the solder on the pad is 'wet' (ie, melted) and then withdraw what is left of the cut-off pin straight upwards away from the top side of the PCB with something like a pair of fine pointed tweezers.

Don't worry about clearing the holes at that stage, you don't do that until you have removed all 40 of the original pins.
I am going to have a go! If I get the wobbles I will stop and wait until I can see my friend once again!
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