Repairing the ZX Spectrum Coil

For the past few days I’ve been trying to get the DC-DC circuit working on a ZX Spectrum Issue 3. The problem was TR4 kept drawing too much current which damaged the transistor.

The DC-DC circuit is really a horrible design. Even after applying the DC-DC mod which makes the circuit more stable I still had issues with the transistor blowing.

I then isolated the -5V and 12V circuits, still the same issue.

Turns out the issue was a faulty coil. The coil seemed to test fine. No shorts between the primary and secondary and the resistances measured ok on both windings, but from looking at the wire coating, some of the enamel has melted which caused oscillation issues.

On the picture below you can see lots of dark purple spots on the wire where the coating has melted.

After looking online for spare coils and seeing the insane prices (£14) I decided to rewire it myself. 1m of 28AWG wire from eBay is £1

I haven’t been able to find any details on the winding so took the coil apart and took the following measurements:

  • The wire used on the coil is 28 AWG (0.32mm).
  • Outer winding is the secondary. 39 turns. 74cm of wire.
  • Inner winding is the primary. 15 turns. 21cm of wire.

With the coil rewound I put it back into the circuit and the problem was fixed. A stable -5V/12V with no large current draws.

6 comments

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    • Ronny on May 5, 2021 at 19:20
    • Reply

    Hi,

    Nice info on the coil repair. Do you have some info around the core itself?
    I want to make a new coil and have found coils that are measured 6mm inside diameter.
    I have no idea of the hight of the coil. So, is the inner diameter of great importance to make the coil ( i think it does to make the correct number of windings) and does the high be of any importance. Is it also imortant how to wind? lefthand wind or righthand wind.

    Thanks in advance
    Ronny

  1. I think the direction of both windings will be important. It will effect the magnetic field if one of them is the wrong way around. I dont have any faulty coils at the moment to take apart and unfortunately can’t remember the direction of the windings. Will update this post when I next have a faulty coil.

    • Ronny Verminck on May 14, 2021 at 11:45
    • Reply

    Thank you!,
    Would there be a way of testing this outside the Spectrum, like adding a low voltage on the prmary winding and check output on secondary winding?

    1. You would need to remove all the PSU section of the components and build this circuit outside of the Spectrum. I did this on breadboard as needed to test the -5V as faulty RAM kept killing the -5V rail.

    • Ronny Verminck on May 18, 2021 at 08:18
    • Reply

    Hi again,

    I have found a shop for 0.32mm² wire.
    Would this be sufficient?
    https://www.dunebeads.com/nl/artistic-wire-koperdraad-0-32-mm-rood-email
    Thank you 🙂

    1. That looks like it will work perfectly.

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