MC152 - Power Guard Light issues and power off Subwoofer pop

[sr20022002]

Hello,

I have an MC152 that I've owned for over 2 years, it had this problem where the flat ribbon cable that connects to the front display was not seated and my left Meter would randomly stop functioning (this was pointed to me by a Mcintosh dealer tech when I got this checked wiht them). The other day again this issue happened so I popped the top cover and tried my best to push that ribbon cable in and added a small piece of tape on top of the transformer cover to keep it from moving.

I'm not sure if I bumped something during this process because ever since then, the Power Guard Led for the left channels does not momentarily come on when I power the unit ON, the one one right comes ON. During operation both the power guard LEDs are off as expected - no issues with any of the front LEDs including the Power LED. The Amp works normally otherwise, both channels sound good to me, both front panels light up and the meters work. Should I be worried about this left power guard LED no coming on at startup? Is it just an issue with the LED itself or is there a chance the power guard circuit is inactive on the left channel?

Second thing I've noticed (not sure if this has started happening recently or not), is that when I power off the amp, my subwoofer gets a small low freq pop - is this normal - Any reason its connected to the issue stated above?

Please let me know if any pictures would help. Cheers!

 

[Bob the Tech]

The L PG signal is adjacent to a ground pin on that cable/connector, so you've probably accidentally shorted the L PG signal.

After a lot of fiddling, which it sounds like your cable has had, the contacts can delaminate and may be wandering around a bit when you insert the cable into the connector.

Even if the cable is in good shape it's easy to shift it just far enough left or right in the connector to cause issues. Usually you'll notice a corner of the cable is dinged up a bit if you do that.

Less likely the connector may have actually been damaged. I've had one case where one of the little pins in the connector was shifted to the side and shorting to the next one over.

I'd pull that cable again and inspect it and when you insert be absolutely certain you're getting it inserted without dinging a corner and shifting it one way or the other.

 

[sr20022002]

The L PG signal is adjacent to a ground pin on that cable/connector, so you've probably accidentally shorted the L PG signal.

After a lot of fiddling, which it sounds like your cable has had, the contacts can delaminate and may be wandering around a bit when you insert the cable into the connector.

Even if the cable is in good shape it's easy to shift it just far enough left or right in the connector to cause issues. Usually you'll notice a corner of the cable is dinged up a bit if you do that.

Less likely the connector may have actually been damaged. I've had one case where one of the little pins in the connector was shifted to the side and shorting to the next one over.

I'd pull that cable again and inspect it and when you insert be absolutely certain you're getting it inserted without dinging a corner and shifting it one way or the other.

Thank you very much for the detailed response. I’m adding a picture from top down for your context, the cable marked with the red arrow was the one that I had to fiddle with (you can see it’s a little crooked still. It’s kind of weird that it doesn’t have a latch on the PCB it seems like just a friction fit connector it’s very stiff so I’m scared to mess too much with and destroy the ribbon cable.

Is this the same cable you were referring to that could be shorting the LED to ground or could it be one of the black 2-pin connectors to its left? Would be awesome if you are able to markup on the photo.

Also will it be okay if I just leave it alone? Or do you think it could be an issue/ I wouldn’t know if the power guard comes one for some reason?

 

[Bob the Tech]

Yes that’s the one, and it does seem to be misaligned. Each pin on the board has a little groove in the connector just above it and that’s where the connector contacts live. You can see the foil strips on the cable are shifted left a bit from those grooves/contacts.

There’s not a lot of room to maneuver fingers in there but I’d pull that back out, inspect it for dinged corners and straighten those, and reinstall making sure to align it properly. If you see lifted traces on the cable you’re going to need a new cable.

If you have to force it it’s not aligned right.