Does hashing a compromised rng and a secure rng give cryptographically secure random output?
If you have a cryptographically secure hash function and you use as much input from each of the sources as there is output i.e.

for $H: \{0,1\}^* \rightarrow \{0,1\}^n$ you use $n$-bits of input from each RNG it will work.

However, in your specific example if you only use 3 letters to generate 512 bits of output, when the DEF repeats the entire 512 bits of output will repeat and this will happen with pretty high probability and after detecting it the rest of the 512 bit are known to an attacker.