Attwood mixing valve

I have a Attwood water heater with a mixing valve. It appears the mixing valve has malfunctioned. We get a short shot of hot water then cold every time we turn on a hot water tap. Has any one tried to take one of these apart to try and correct this? Replacment is $285 Can$ so if I can tinker with it and make it work I would save some cash.
Thanks for any input
Stew