Zx Spectrum 48k Noise

A place to discuss the ZX80's and ZX81's younger brother - the Sinclair ZX Spectrum
Jacobartaylor
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Zx Spectrum 48k Noise

Post by Jacobartaylor »

I have recently dusted off my old rubber keyed ZX Spectrum 48k the machine has an issue 2 board with the resistor/ diode timing fix. I have also replaced the capacitors on the board and have converted the machine to output a composite video signal, however when i plug in the machine, i can hear this annoying high pitched whining noise, which sounds almost like coil whine in a modern gpu. I was wondering if this is 1 normal and 2 if there is a way to remove this noise. Cheers
RWAP
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Re: Zx Spectrum 48k Noise

Post by RWAP »

It is the oscillator which causes this and may need replacing (assuming it is not your power supply!)
Jacobartaylor
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Re: Zx Spectrum 48k Noise

Post by Jacobartaylor »

Thankyou :) do you know where i can purchase this part? I assume it isnt the power supply as i have 2 sinclair 1400 units and it makes no difference what one i use
Last edited by Jacobartaylor on Thu Jun 16, 2016 9:45 am, edited 1 time in total.
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Fwapp
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Re: Zx Spectrum 48k Noise

Post by Fwapp »

It just the coil buzzing in the power section and the noise is perfectly normal. It's the highlighted part in the pic below.

Image

There's no point in replacing unless the former is physically broken, but the whining can be reduced in intensity by removing the coil and dipping in varnish or wax a few times, making sure not to get any on the legs - allow to dry before refitting :D
ZX80, ZX81 - 2C158C Bugged Rom iss.1, 2C158C "kludge fix" iss1, 2C184E iss1, 2C210E iss3, US NTSC iss1 "straight traces", US NTSC iss3 TS1000
Jacobartaylor
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Re: Zx Spectrum 48k Noise

Post by Jacobartaylor »

Ok thankyou :) if its normal and safe to use it i think i will leave it as it is, i was mainly concerned about causing damage to my machine if it was faulty, plus i cant seem to find any replacement for the coil online
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Fwapp
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Re: Zx Spectrum 48k Noise

Post by Fwapp »

One thing to check is that when you replaced the capacitors that you put C46 back in the "wrong" way around - the PCB is marked up incorrectly, so you need to put the -ve lead of the capacitor to the + mark on the pcb.

Check all the voltages as well - 12v, 5v, -5v (easiest on the lower memory chips, IC6-13) - as long as they are there or there about you'll be fine.
ZX80, ZX81 - 2C158C Bugged Rom iss.1, 2C158C "kludge fix" iss1, 2C184E iss1, 2C210E iss3, US NTSC iss1 "straight traces", US NTSC iss3 TS1000
RWAP
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Re: Zx Spectrum 48k Noise

Post by RWAP »

RWAP wrote:It is the oscillator which causes this and may need replacing (assuming it is not your power supply!)
Oops - yes, I meant the coil - it was early this morning.... :lol:
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1024MAK
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Re: Zx Spectrum 48k Noise

Post by 1024MAK »

The coil is actually a small transformer. To generate the +12V regulated and the -5V supplies, the Spectrum uses an early form of DC to DC converter (also known as an inverter). This transformer "coil" is part of this circuit. It switches at high frequency. Which results in the copper wire formed around the core vibrating. It is this vibrating that results in an audio buzz to human ears. In actual fact, for issue 1 and issue 2 boards, it's a good sign that the DC-DC converter is working correctly if you hear the buzz :(

The later issue boards use a simular circuit, but the transformer "coil" makes less noise, as over the life of the production run of the 16k/48k Spectrum, this part of the design had various changes in an effort to improve it.

The only source of replacement transformer "coils" is from dead Spectrum circuit boards. More modern DC-DC converters exist, but fitting them means lots of changes to the circuit board.

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balford
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Re: Zx Spectrum 48k Noise

Post by balford »

If the coil is too noisy for your tastes, a little varnish over the windings helps reduce it considerably.

Me, I quite like the buzz :)

B
pistolhamster
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Re: Zx Spectrum 48k Noise

Post by pistolhamster »

balford wrote: Fri Jun 17, 2016 5:41 pm If the coil is too noisy for your tastes, a little varnish over the windings helps reduce it considerably.

Me, I quite like the buzz :)

B
Oh, that is interesting. I got a + model today imported from a UK seller, complete with the Plug of Doom with the fuse in on the original PSU. It buzzes quite loudly, almost rattles. But I measured the voltage to about 11 when not under load, that should be fine I guess?

The rattling could be from the coils? What kind of varnish would you suggest?
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