Sinclair ZX80 / ZX81 / Z88 Forums

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

It is the oscillator which causes this and may need replacing (assuming it is not your power supply!)

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

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



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

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

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.

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....

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.

Mark

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

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?