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.

[Zebra Systems]

Hi. My name is Stewart Newfeld. I live in Brooklyn, New York. My company Zebra Systems, Inc. supported Timex/Sinclair computers in the 1980s, but I have not played with them in decades.

I want to start an open source project to understand and document the signals that the ZX81 provides to the AlphaCom 32 and Timex/Sinclair TS2040 printers. That would be Phase I. Putting a logic analyzer on the bus between a ZX81 and a printer would be one approach. Studying the ZX81 ROM code for the printing functions might be another. Or maybe, someone on this forum knows something about this subject.

Phase II would be to generate those signal signals from Arduinos, Raspberry PIs, microbits, etc., to drive the printers from those microcontrollers.

Phase III would be to design an ATMEL or ESP32 based serial to AlphaCom 32 and TS2040 adapter to make it easy to interface almost any comptuer / microcontroller to those old printers.

Everything learned would be open source, software and hardware. If the project ends of with an adapter, the KiCad PCB drawing and schematics, the software ... all completely open source. I am prepared to spend some money to fund prototypes.

If you know anyone who might be interested in this project, please pass on the message.

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If anyone wants to understand the ZX printer signals, I suggest studying the open-source Arduino C-Code Libraries for the follow product.

FOR IMMEDIATE RELEASE
ZX Printer Shield now available
Connects Arduino UNO to Timex/Sinclair Printers
by Stewart Newfeld
                                                                       November 5, 2024,  Brooklyn, NY   

Distribution for the ZX Printer Shield designed by New Zealand-based Timex/Sinclair enthusiast Tim Horner is now available.  Tim originally designed the ZXPS for his personal use and to satisfy his desire to fully understand the signals the ZX81 and other T/S computer use to print.  Stewart Newfeld, who started Zebra Systems, Inc. in March of 1983 as a mail order business for Timex/Sinclair products and who has been selling T/S products on ebay since 1999, learned of Tim's efforts in a Timex/Sinclair Users ZOOM session, and has collaborated with Tim to get the word out about this project and make it easy to obtain ZXPS PCBs and parts in kit form. 

The ZXPS allows an Arduino UNO to print to the TIMEX TS2040, ALPHACOM 32, and Sinclair printers that were originally designed for the Sinclair ZX81 and Timex 1000, 1500, and 2060 computers.  Tim's open source Arduino Libraries for the ZXPS are on GitHub. They are very well commented and include example Arduino sketches. They serve as the software documentation for using the ZXPS.

A comprehensive support website at www.whattocallit.com/shield includes a review of the software, the schematic and parts list, assembly/soldering instructions, example applications, and more.  The website also has links to purchase the ZXPS on ebay.