M100C - No Sound - Mute Circuit Issue?

[Brad]

I'm troubleshooting a 'no sound' issue on a M100C with the MRA3 amp. The mech is working fine. I removed the grid clip on the 6J7 and got rumbling, so I'm assuming the amp is functioning. When I remove the mute plug with a record playing I get audio (music) with a good amount of hum. According to the troubleshooting guide this indicates a possible problem with the mute circuit. I checked contacts M, MA, MB and they all look like they're opening and closing like they should. If I plug in an external phono to the MRA3 it sounds really good.

Any suggestions on next troubleshooting steps? Thanks.


Brad  

[Ron Rich]

Hi Brad,
Which cartridge is installed in your phono ? Using a meter on the resistance scale, read the pins on the mute plug while a record is playing--it should be open, on the ground side if all switches are correctly adjusted, closed on the AVC/squelch side (pin #3). As for "hum"-- cart. or wires to it--also check that the "ground side" of the Pickup, is OPEN, to the frame of the mechanism, but good at the input RCA plug ! Ron Rich

[Brad]

Thanks Rich,
The cart is a Varco ST2X. If I'm understanding you correctly on reading the pins on the mute plug, from #1 to #2 should read open, and from #1 to #3 should read closed. Is this correct?

Brad

[Ron Rich]

Hi Brad,
With a record PLAYING--yes--with no record playing, the opposite. Ron Rich

[Brad]

Jan 6, 2019 22:31:01 GMT Ron Rich said:

Hi Brad,
Which cartridge is installed in your phono ? Using a meter on the resistance scale, read the pins on the mute plug while a record is playing--it should be open, on the ground side if all switches are correctly adjusted, closed on the AVC/squelch side (pin #3). As for "hum"-- cart. or wires to it--also check that the "ground side" of the Pickup, is OPEN, to the frame of the mechanism, but good at the input RCA plug ! Ron Rich

Hi Rich,
I finally got back to this. The ground side of the pickup is NOT open to the frame of the mech, and it is good at the input RCA plug. The ground to the frame mech doesn't change in play or scan. The switches seem to be adjusted correctly. Does this indicate a short somewhere?

Brad

[Ron Rich]

Hi Brad,
Yes--this usually indicates "some dummy" has connected the record reject ground to the shield of the audio cable rather then the ground lug that should be beside it ! Or-the shield has been squished somewhere between the RCA plug and the terminal-- Happy hunting !! Please show your location as per request, in the User Introduction section-- Thanks, Ron Rich