Pictures of my ZX81 fitted with a RECOM 5V Regulator
Posted: Fri Dec 16, 2016 5:53 pm
Here are some pictures of an issue 1 board ZX81, where I removed the 7805 series voltage regulator and replaced it with a RECOM R-785.0-1.0 switching regulator.
I also fitted two extra 100nF ceramic capacitors (marked "104").
I had originally intended on mounting the RECOM regulator vertical. But that did not work for me, so instead used some tinned solid core wire to make extra long legs at a right angle (90 degrees) so that I could lay the RECOM regulator horizontaly on it's back.
The first 100nF capacitor goes in the two holes in the PCB next to the 3.5mm jack socket for the 9V power input.
The second 100nF capacitor simply goes in two of the three the spare holes on the edge of the board where a switch can be fitted (provision for a channel / frequently band change switch for modulators that support this).
These holes may already be filled with solder. If this is the case, remove the solder with a solder sucker or desolder braid.
I decided to add these extra capacitors for two reasons, first the RECOM regulator is designed to be a straightforward replacement for the conventional 7805 series regulator chips. And having capacitors close to these is recommended to improve stability. This is good practice with any regulator. And secondly, they will help reduce electrical noise on the power lines.
I used thin heatshrink tubing on one capacitor. But this is not nessassary. If you want, you can also heatshrink the wires and legs on the RECOM regulator.
To make the extra leads for the RECOM regulator, I used stripped wires from some solid core telecom or ethernet cable that I had spare. I used a small set of round nose pliers to form a spiral ring on the end, so that it would slip over the leg of the RECOM regulator. Then I soldered it in place.
Note that when soldering the extended legs of the RECOM regulator to the PCB, enough heat may travel up the wire and melt the solder holding the wire to the leg of the RECOM regulator. So use something to hold the regulator in place while doing the soldering.
Mark
I also fitted two extra 100nF ceramic capacitors (marked "104").
I had originally intended on mounting the RECOM regulator vertical. But that did not work for me, so instead used some tinned solid core wire to make extra long legs at a right angle (90 degrees) so that I could lay the RECOM regulator horizontaly on it's back.
The first 100nF capacitor goes in the two holes in the PCB next to the 3.5mm jack socket for the 9V power input.
The second 100nF capacitor simply goes in two of the three the spare holes on the edge of the board where a switch can be fitted (provision for a channel / frequently band change switch for modulators that support this).
These holes may already be filled with solder. If this is the case, remove the solder with a solder sucker or desolder braid.
I decided to add these extra capacitors for two reasons, first the RECOM regulator is designed to be a straightforward replacement for the conventional 7805 series regulator chips. And having capacitors close to these is recommended to improve stability. This is good practice with any regulator. And secondly, they will help reduce electrical noise on the power lines.
I used thin heatshrink tubing on one capacitor. But this is not nessassary. If you want, you can also heatshrink the wires and legs on the RECOM regulator.
To make the extra leads for the RECOM regulator, I used stripped wires from some solid core telecom or ethernet cable that I had spare. I used a small set of round nose pliers to form a spiral ring on the end, so that it would slip over the leg of the RECOM regulator. Then I soldered it in place.
Note that when soldering the extended legs of the RECOM regulator to the PCB, enough heat may travel up the wire and melt the solder holding the wire to the leg of the RECOM regulator. So use something to hold the regulator in place while doing the soldering.
Mark
