MX-110Z L+R Output?

[David D]

Just became the proud new owner of an MX110Z. It sounds great paired with my MC225.

Currently, my system consists of a pair of small book shelf speakers and a single 15" JBL Sub1500 subwoofer being driven by a Crown DC300A. I'm going to use the L+R out on the MX110Z to drive the subwoofer amp, but I'm getting about 13 volts DC at the L+R out.

Am I correct in thinking C112 is the leaky suspect passing DC?

 

[ramjet]

Yes.

 

[David D]

Thanks for the confirmation. It was serviced prior to my taking ownership, but I'm sure the L+R out was never used, so never presented a problem. It's had several caps replaced already, so I'll go through it and see if I need to add anything else to the shopping list. The AC line filter cap is already on the list.

 

[ramjet]

Thanks for the confirmation. It was serviced prior to my taking ownership, but I'm sure the L+R out was never used, so never presented a problem. It's had several caps replaced already, so I'll go through it and see if I need to add anything else to the shopping list. The AC line filter cap is already on the list.

It is also possible that there are other causes.

 

[David D]

It is also possible that there are other causes.

Well, you were right. I replaced capacitor C112 and while the DC is gone, I'm getting some signal and hum out of the L+R out with the volume control all the way down.

 

[ramjet]

Are you getting hum out of the Left and Right Main Outputs as well?

 

[David D]

The left and right outputs are perfectly silent.

The schematic says I should read 96 VDC at the plate of the 12AT7 that acts as the output buffer tube for the L+R out, but I’m measuring 224 VDC. For what it’s worth, I have it on a Variac and set so the power supply voltages are correct.

 

[ramjet]

Change the tube.

 

[mangist]

The left and right outputs are perfectly silent.

The schematic says I should read 96 VDC at the plate of the 12AT7 that acts as the output buffer tube for the L+R out, but I’m measuring 224 VDC. For what it’s worth, I have it on a Variac and set so the power supply voltages are correct.

Check R144, if it's bad it could be putting the full 235V onto the plate. It should read 47K

 

[David D]

Check R144, if it's bad it could be putting the full 235V onto the plate. It should read 47K

I checked R144 and it measured 50.4K, so it was within 10%. Changed the 12AT7 and my voltages are good now.

In the process I discovered that a previous tech had changed the selenium rectifier to a modern bridge rectifier, but didn't take care of the now higher voltages due to the more efficient rectifier. I inserted a 200 ohm 25 watt wirewound resistor between the bridge rectifier and ground and all the voltages are within spec now. The resistor gets pretty hot despite it's 25 watt rating, but it works.

 

[mangist]

I checked R144 and it measured 50.4K, so it was within 10%. Changed the 12AT7 and my voltages are good now.

In the process I discovered that a previous tech had changed the selenium rectifier to a modern bridge rectifier, but didn't take care of the now higher voltages due to the more efficient rectifier. I inserted a 200 ohm 25 watt wirewound resistor between the bridge rectifier and ground and all the voltages are within spec now. The resistor gets pretty hot despite it's 25 watt rating, but it works.

Interesting. Could anyone explain how changing the tube would fix the voltage on the plate from 224V down to the correct 96V? I would have expected R144 to be the culprit. How did the new tube accomplish this?

 

[w3jn]

If a tube has weak emission, it won't draw current. The current flow through the resistor is what drops the voltage. It's a good way to test a tube without a tube tester. No/little current draw, probably a weak tube.