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.