Mcintosh 427 Problem!!

[selmervi54]

Hi I was wondering if anyone had any experience with McIntosh amps.
I have a MC427 that is acting up.

When the amp sees 12v and the car is not running it works perfectly, and outputs sound. When I start the car, amp running at 14v, the amp outputs a high pitched squeal that is constant and not any music.

I disconnected the RCA’s and the squeal is still there when the RCA’S are disconnected. It is a really high pitched ear piercing noise that really hurts your ears.
If I turn off the car, and turn the amp on, it then works perfectly, at 12v.
Do you know what could cause this?
I have sent it into McIntosh twice and they replaced the IC chip (first repair) and two capacitors and 1 regulator IC (second repair)
Could this be an op amp, it is really strange that it only happens at the higher voltage. The Power Guard is not coming on.
I was thinking about sending it back to MAC again, but I wanted to see if anyone had some suggestions or had this problem.

Thanks,

Brandon

[1moreamp]

The only thing I know of that will squeal is the power supply toroid transformers but these are H frame style and that is not typical for this design.
Without your amp being scoped to see whats going on its gonna be anybodies best guess. Any average shop tech will have the ability to vari the power feed to test such operation on the bench.
The bottom covers remove fairly easily but only expose the underside of the amp board and the power devices connected along the side rails, so it will have to come all apart to get a fair view inside to trouble shoot the amp.
These are getting pretty old now so don't be surprised if it needs new filter caps in the power supply. They do take a beating over the years. but here again I am best guessing due to the nature of your troubles.
If mac can't satisfy your repair needs try someone local that has a decent level of experience.
Any tech should be able to operate the amp from 10 to 15 volts on his test bench with ease.
Sorry I'm not over-flowing with answers but mac seems to hold their products with very high levels of product secrecy, even though they no longer even sell car gear.