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I recently bought a S-Pixie transceiver kit from Amazon (more or less the same 40m kit that's been replicated on grand scale & sold for under $15).

I've seen its voltage requirements widely quoted as "9-12v".

Is it OK to power a Pixie with up to ~14.4v straight from 8 fully-charged NiZn cells, or would that run the risk of frying the Pixie's transistors, diodes, and/or LM386?

It seems like in ham radio, "12v" often has a very loose definition (unlike... say... USB, where .25v is officially the difference between 'brownout' and 'blue smoke'). I suspect this falls into the "14.4v is ok" category by virtue of being all-analog downstream from the Si5351... but I'd like to hear an authoritative opinion from someone who actually knows.

Why

Normally, I'd just use a cheap switchmode 5v or 12v power supply, and drop it lower if necessary using a $3 buck-boost converter board if I need lower voltages. From what I've read, Pixies really hate switchmode power supplies & almost have to run from battery power.

As luck would have it, I have a bunch of NiZn AA cells and an 8-cell holder handy... but my next-smaller AA holder is only 4-cell, and NiZn cells are the only ones I have in unused abundance at the moment.

The NiZn cells are nominally 1.6v, and measure ~1.8v when fully-charged with no load.

Schematic

For the 3 or 4 people who've never heard of a "S-Pixie", here's a schematic indicating the change I already made to replace its local oscillator with a Si5351a. The yellow line is a 47k resistor, the components highlighted in white were omitted, and the colored highlights correlate traces on the board with lines in the schematic.

schematic]1

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    $\begingroup$ You say you measured the nominally-1.6v batteries at 1.7-ish volts. Is that the open-circuit voltage, or with a load? Don't forget that a battery that provides current at load will have a much higher open-circuit (no load) voltage if taken out of circuit and measured. Try putting those eight cells in the holder and put a load across them, then measure it. How about using 7 of the batteries, and a "blank"? Experiment. $\endgroup$
    – Scott Earle
    Mar 18, 2022 at 2:22
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    $\begingroup$ The capacitors rated for 16 V will be fairly close to the limit, perhaps swap them for 25 V ones. Also note you're losing 1.2 V in the bridge rectifier, unless you remove that to raise the power a bit further. $\endgroup$
    – tomnexus
    Mar 18, 2022 at 18:05

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This well-known design is specified for 9V-12V operation, and 9V is recommended. Higher VDC may work just fine (I think you'd have to consult with EE.SE to be sure) but I suspect the life of some of the components would be shorter than you like.

For example, you might need to look at the LM386. I think it is a different part number for the variety that works up to 18V. If this is the 4V-12V part then it will not like its $Vs$ being higher than that. I am also assuming you can't just drop in that part without changing the other parts of the support circuitry as well.

This design was before 13.8V nominal (which is more or less what you are trying to use as a supply) was the ad hoc standard for radio equipment. I think you should consider using a 9V battery or cobbling up some cells to make your own 9V supply.

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