WTB: cheap xenon ring flash

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I need, for my Olympus E-M1 mk1, a cheap, manual, slender, continuously circular ring xenon flash light.

The flash has to use center positive trigger and 24v or less trigger voltage (this can be measured with multimeter). Here is how to measure it: http://dpanswers.com/content/genrc_flash_measuretv.php

I think most Canon flashes made after 1980 will work, but probably not Nikon ones (as Nikon flash is said to use reversed polarity). Some detailed info here:

https://www.nikoncafe.com/threads/compa ... d3.307074/

https://www.mu-43.com/threads/continuou ... st-1297163

Yongnuo YN-14EX is cheap and otherwise fits, but its flash tube is not a continuous circle.

No LED ring "flash" please; I have one already.

This ring flash will be used mainly (if not exclusively) for cymatic (sound wave) photography, So I do not want to spend much on it.

Around USD $35 would be nice, including shipping (to Texas 77546) and Paypal fees. For $60 shipped, I can buy a new Yongnuo YN-14EX or a used Canon xenon ring flash right now. I know some vintage ring flashes are selling at around $35 on eBay now, but am unsure about center polarity and trigger voltage.

Thank you!