My friend has a simple system in his 1971 Mustang Mach 1 429 SCJ. His deck is a vintage 1982 Sony XR-75 FM/AM Casette deck. The volume fluctuates up and down within a noticeable range. The amperage draw fluctuates significantly, like from 6 - 10 amps at half volume. He told me that this only started within the last 2 years and he bought the deck new. The deck needs to run an amp and is hooked up to a newer MTX TC4004, (Which has a phantom draw problem I've asked about in another thread). How many variables could cause these symptoms or is the ole' deck just wore out?

 

Does the volume fluctuate on radio as well or just on tape? How about the current draw?

Does the tape speed up and slow down as well or just change volume?

Does either change depending on the RPM of the motor?

 

I only checked it with a good clear radio station. I will ask my friend to check all the variables you suggested on the tape deck to see how it works. The engine was not running when I did my tests.