Understanding ZX81 signals to Timex/Sinclair Printers

[1024MAK]

From what you describe, it sounds like both the enamel insulation (that covers the copper wire) and any lacquer or varnish type coating (used to reduce the vibrations and keep the coil fixed in place) have been affected by the overheating. It’s also possible that the former (base) may have distorted if it’s made of plastic.

Ideally, I would suggest replacing it. I think you are in America?

I don’t have a ZX Printer to hand to check at the moment, so can’t remember what it actually looks like. If/when I find mine, I could measure the coil. If I do, I will post the results here. But it may be a while, as I have to find it (it’s in storage).

I do know that a similar coil as used in the ZX Spectrum, can be repaired by carefully removing the existing windings (counting the number of turns, the start and finish and measuring the wire diameter), then winding new enamelled (transformer) wires on. Then coating with nail varnish or similar.

Having said all that, the printer should work even with L1 being damaged. The downside is that the computer may crash occasionally if the level of interference from the printer is too high. But that’s very hard to predict without doing any testing. It’s possible that you won’t notice any problems at all.

As to the cause, it’s more likely to have been a short circuit between the positive supply rail (+9V / +10V) and a 0V/GND connection. A short that bypasses or partly bypasses a coil would not cause damage like you describe, as it would not overheat. It’s possible that the heat caused by the short circuit or you dismantling it distributed the cause of the short. Hence why it’s no longer able to be picked up by testing with your multimeter.

Mark

[bwinkel67]

Thanks for all the feedback Mark. I checked by two spectrums and there is no inductor...just a coil looking thing by the power adapter. The ZX Printer has one of those too, but that's not the thing that overheated and is not shown as an inductor on the schematic (at least not the simple repeating half circle one).

A couple of other questions. When I plug it to my ZX81, I can hit the paper feed button and the motor moves. If I press LPRINT 1 on the ZX81 it also moves the motor, but non-stop. Note that I don't have a band in there with the styluses, so it's never completing the electrical circuit (white wire to stylus to paper to conductive roller to green wire). Can I just connect the white and green wires (they both slot into connectors on the printer) to complete the circuit? Would that cause the printer to behave normally...other than not printing obviously. That seems to be the circuit that's complete when it burns off a dot on the paper.