C28 intermittently loses right phono signal

[CDinMB]

When playing records in channel 1 or 2 my C28 occasionally loses the right channel sound. I unscrew the lid and lift it and the sound returns. I lower the lid and it will play for a 3-7 minutes and go out again. I lift the lid, lower it and sound returns. Adjusting the cables does not affect it, so not a bad cable. Cleaning the selector or moving the selector to other inputs does not impact right channel recovery.

Another symptom is channel 2 phono input can be heard when phono 1 is selected, albeit with some hum and grounding noise.
Any input on where to look for a cause? Grounds, phono logic board, selector switch?

I love the sound through my system when it’s working. Should I just sent it out for a thorough refurb?
TIA!

 

[jcidave]

One of my C28s does something similar. The left channel of phono 2 will fail after playing for a short time. If I unplug everything and move the unit, then replug everything it will play for awhile then fail. My phono 1 is fine and never has a problem. My local tech, who is pretty good and has worked on lots of MAC but is not formally trained on this gear cannot resolve the problem. I think its in the selector switch wiring which is a bear.
He did, however find a MAC tech bulletin that involves running a ground wire to all the grounds of the low level inputs then attaching this to a chassis ground lug. This solved a noise problem on phono 1 that works properly.
So for me, my bail out was to get an outboard phono preamp. Otherwise, for me, it would be a trip back to MAC which I don't want to go through right now.
One other thing you want to do from time to time is exercise all your knobs and buttons on the face. push them in and out a few times and turn the loudness knob just to make sure all the contacts are working.

 

[twiiii]

I had similar issues with my C-28 when new. When Davie came for an amp clinic he opened the unit and lined up the control and loosed the extension shaft and then centered the switch section and retightened the shaft coupling. I also had a similar issue with. the function selector. The volume controls and calibration controls were always an issue becoming noisy and intermittent. When the C-29 news reached my ears I ordered one immediately. No headaches for the next decades. . Best move I ever made as far as pre-amps are concerned. If I had the room I would upgrade to a C-40 vs. my current C-34. But the MP 100 solved my phono section issues so I am happy.

 

[CDinMB]

Thanks for the input. I will take a look at the controller alignment and grounds. I did replace the ground on one channel but did not tie them all to a single chassis point. One nice thing about sheltering in place is plenty of time for projects.

 

[twiiii]

The low level in put connector plate can be an issue too. I have forgotten the particulars.

 

[CDinMB]

I opened the unit today and found it already had the input ground mod. I checked resistance and there was high resistance between the input grounds and the chassis. I removed the ground point chassis bolt and found it corroded. I cleaned all the contact areas and retightened. Resistance to ground fell and both players work without losing a channel for about 5 hours. I will continue to monitor and enjoy. My turntables never sounded better! I hope this proves to be the solution. Thanks for the help!
CD

 

[jcidave]

I opened the unit today and found it already had the input ground mod. I checked resistance and there was high resistance between the input grounds and the chassis. I removed the ground point chassis bolt and found it corroded. I cleaned all the contact areas and retightened. Resistance to ground fell and both players work without losing a channel for about 5 hours. I will continue to monitor and enjoy. My turntables never sounded better! I hope this proves to be the solution. Thanks for the help!
CD

Good troubleshooting on this. Glad your 2 phono inputs are working better. I have a couple other C28s that need this modification. Someday I'll pull the whole rig apart and get it done.