McIntosh MR78 no multiplex/stereo

[mac1900]

Hi All,

I was wondering if someone can help give me insight into a problem I'm having with my MR78 tuner. It went totally quiet one day without warning - the "stereo" indicator went out and the tuning indicator would not move no matter where it was tuned to. This is a later serial number (after AD5165) and does not have tantalum caps. Long story short, I ended up replacing IC301, which is in the MPX section and drives the tuning indicator among other things. Now, I only get mono sound, even when "stereo" is lit. It's totally quiet when switched into "stereo only" mode. Is this because the decoder would need to be realigned due to IC301 being replaced? I've also noticed the 24V supply is measuring high at ~32V.

 

[clinic-audio]

Hello
Welcome on AK

Serial number ( AD5165 ) is not a late production one . The last MR78 in serial AD9355 was in march 1983

CG29XX is closer to the last produced in year 1984

The unit is more than 30 years old and need , whatever you replace something or not , to be realigned ( around 4 hours just for adjustments )
It maybe more than that if something need to be replaced .
Send it to Audioclassics in NY . Your MR78 will be back to originals specs .

 

[mac1900]

Seeing as IC301 is directly in the signal path, I figured an alignment would be necessary, especially given its age. I know there are several things that could cause this type of problem, but I've always had an interest in electronics and just wanted to know if there might be anything else that could be causing it.

Thanks,

mac1900