The lagrangian density of a massless vector field is
$ \mathcal{L} = -\frac{1}{4}F_{\mu\nu}F^{\mu\nu}$, where $F_{\mu\nu}=\partial_{\mu}A_{\nu}-\partial_{\nu}A_{\mu}$
Expanding out gives
$\mathcal{L} = -\frac{1}{2} \left( \partial_{\mu}A_{\nu}\partial^{\mu}A^{\nu} - \partial_{\mu}A_{\nu}\partial^{\nu}A^{\mu} \right)$
In order to solve the equations of motion, at some point you need to take the derivative wrt $\partial_{\alpha}A_{\beta}$. As far as I can see, this should be, by recognizing
$\partial_{\mu}A_{\nu}\partial^{\mu}A^{\nu} = g^{\mu\rho}g^{\nu\sigma}\partial_{\mu}A_{\nu}\partial_{\rho}A_{\sigma}$
and then fixing $(\mu,\nu)$ or $(\rho,\sigma)$ to $(\alpha,\beta)$, or similarly for the second term,
$\frac{\partial \mathcal{L}}{\partial(\partial_{\alpha}A_{\beta})} = -\partial^{\alpha}A^{\beta} + \partial^{\beta}A^{\alpha} = -F^{\alpha\beta}$
However, the particular example problem I'm working through will only lead to the final answer if the solution is $+F^{\alpha\beta}$. This positive solution is also what is found in the QED wikipedia page.
Can someone please explain why I have the wrong sign?
EDIT: In response to Dox below.
The exercise is actually for a massive vector field $C_{\mu}$, with
$\mathcal{L} = -\frac{1}{4}F_{\mu\nu}F^{\mu\nu} + \frac{1}{2}m^{2}C_{\mu}C^{\mu}$
So the E-L equation I get is
$-\partial_{\alpha}F^{\alpha\beta} + m^{2}C^{\beta} = 0$
But both terms should actually be of the same sign. The exercise goes further to show some explicit relationship between $C_{0}$ and $C_{i}$, which I would be able to derive correctly if I had the proper signage in the E-L equation. Ultimately, my problem is still how to get $+F^{\alpha\beta}$ and not $-F^{\alpha\beta}$.