Maker Pro
Maker Pro

Technics SA-818 FM Muting Problem

KilgoreCemetery

Apr 12, 2017
258
Joined
Apr 12, 2017
Messages
258
Everything on this receiver works great except for one little thing. The FM Muting cuts out to the beat of the music or when commercials get too loud. It's not dependent on the actual decibel rating coming out of the speakers, just the incoming FM signal. Like, if it's a quiet song or a mono signal, I can crank the volume knob up as high as I want and it never cuts out. It only does it with a strong FM Stereo signal.

My understanding is that the FM Muting sensor is supposed to filter out the audio and sample the signal strength somehow. From the way its acting, I would say the audio is not getting filtered out. I can't find any more information than that about how it's supposed to work, so please correct me if I'm wrong.

I pulled up a schematic for this model and found that the FM Muting adjustment is right next to an IC (NEC C1167C2), but I can't find a datasheet for it. I did learn that if I touch or wiggle the coil (L102), the signal cuts out completely.

I tried adjusting the Muting with VR101, but it didn't help, so I put it back to where it was.

I checked voltages on the IC. The only pins that were way out of spec are:
15: fluctuates between 1.2V - 2.8V quickly
13: 3.5V
12: -290mV

I don't quite know where to go from here. Is it probable that the IC has a short? Does the IC do the audio filtering? Is there some place I can read up on how FM Muting is supposed to work?
 

Attachments

  • FM Muting Clip.jpg
    FM Muting Clip.jpg
    264.6 KB · Views: 63

73's de Edd

Aug 21, 2015
3,640
Joined
Aug 21, 2015
Messages
3,640
Investigate to see if C125 bypass filtering tied into pin 11 has had a decline of capacitance. Also C119 on pin 7... . . . . .C117 pn pin 5.
 
Last edited:

KilgoreCemetery

Apr 12, 2017
258
Joined
Apr 12, 2017
Messages
258
Investigate to see if C125 bypass filtering tied into pin 11 has had a decline of capacitance. Also C119 on pin 7... . . . . .C117 pn pin 5.

I recently cleaned up and recapped the entire unit. I didn't realize anything was wrong with it until I tried the FM Muting. C117, C119, and C125 are all new Nichicon caps.

I'd be looking at trimming T101/T102
I found in the service manual that, when adjusting T101, the goal is to have zero volts when probing TP102 and TP103, so I did that and it seems to have fixed the problem! Thanks, Kelly!

The real question is, what is that actually doing? I definitely appreciate the fix, but I'm also interested in knowing what went wrong.
 
Top