DC where it doesn't belong

[WQ9E]

I ran into a little problem while providing some vintage audio sounds for the season.  In the room with the Christmas tree is a Marantz SR-820DC receiver with Akai GX-400D SS and GX-747 DBX reel to reel decks connected.  I have been listening to some tapes I picked up from a radio station and also transferring some formats over to the more common 7.5 IPS quarter track format.  Some of the tapes are 15 IPS and others are old half track mono.  The GX-400D handles the 15 IPS speed and with its 4 channel capability it also does a good job with the old half track tapes.  All was happy with the vintage stuff.

The problem occurred when I hooked up an XM receiver for the Radio Classic channel.  I noticed after a few minutes the receiver volume control had gotten noisy.  Switching to either of the tape decks made the problem disappear.  I checked the XM receiver and it puts about half a volt of DC on the audio output.  This one doesn't have a line out so I was using headphone out to line in.  Adding blocking capacitors took care of the problem and after Christmas I will do some surgery and add line output to the XM receiver.  I imagine with dynamic headphones connected the DC isn't an issue but with the high resistance/impedance line inputs it was.  So with a tip of the hat to "madman Muntz" I added a couple of capacitors  

Now I can enjoy Dragnet and Johnny Dollar without a crackle.

[W2PFY]

It would be interesting to know why that voltage is present? I wonder if it is there to forward bias an external amp of their manufacture? It would seem to be too low to power something.

Good find nevertheless.

[WQ9E]

I don't have a schematic and haven't done a thorough investigation yet but I bet the voltage is just an artifact of poor design/cheap components.  I haven't tried to load it down but I bet the voltage is just "leakage" and disappears once there is a low resistance load like headphones.  The line inputs of the Marantz receiver are at least 10K (and probably higher).  The audio gear I have worked on from the 70's have discrete output transistors and an adjustment to minimize DC offset.  I am sure the XM receiver uses IC output amps and there probably isn't any adjustment for DC offset.

I think my Marantz receiver is DC coupled straight through to the output so I am lucky I didn't have actual damage from this incident. 

I will just chalk this up to another of the many hidden costs of modern cheap electronics.  Now back to my Tobe Deutschmann receiver which is not modern (and probably wasn't cheap when it was new).

[W2PFY]

Is this the one?

[WQ9E]

Yes except my receiver isn't nearly as clean and shiny as yours!  Hopefully it will be soon.