Denon AU-320 SUT Left Channel Out

[pairustwo]

I hope someone can offer advice to an inexperienced DiY audio tech.

I've recently come to own a Denon AU-320 step-up transformer. When connected to my phono pre-amp the signal will pass through successfully. That is the Pre-amp is set to its usual MC setting and the signal from my tone-arm passes through the AU-320 to the pre-amp and on down the line as usual. This seems to indicate the wiring of the left and right channels, from input to output are correct.

When I switch the Au-320 to use the transformers (and my pre-amp to MM) - either in 3ohm or 40omh settings, the left channel of audio drops out.
Any ideas what might cause this?

Here is more information about this unit: https://audio-database.com/DENON-COLUMBIA/etc/au-320-e.html

I've opened it up and removed the transformer, cleaned what I can with deoxit. The wiring looks original and I don't see any breaks... but if there were a break... where might it be to impact both 3ohm or 40omh settings and not the signal passthrough?

Thanks everyone.

 

[rayma]

Could be the switch, dirty or worn out. Or a break in the "ground" lead of the left transformer.
Here's the schematic.

 

[pairustwo]

Interesting. I do see a ground wire coming from one transformer and going directly to the Right Channel input (it is bridged to both right channel inputs)

The left channel inputs are also bridged but there is no black wire to these or black wire coming from the second transformer...

Should I try to ground the other inputs somehow?

 

[rayma]

You can check continuity between all 3 primary leads of the left transformer with a DVM.

 

[pairustwo]

check continuity between all 3 primary leads of the left transformer with a DVM.

I have a multi-meter. I've been told that running a current through a transformer like this can magnetize it (https://www.diyaudio.com/community/threads/fs-denon-au-320-moving-coil-step-up.307925/). This thread warns against DMM.

What is the difference in settings between DVM and DMM?

And...how to identify the '3 primary leads off the left transformer' ?

Thanks again.

 

[rayma]

You can also signal trace inside the box while playing a record, if your meter is up to it. A scope would be preferable.
But perhaps an experienced technician checking it out would be a better idea, since it does need repair.