60 Hz noise shielding suggestions?

effischer, a steel plate connected to ground might help but you won't know till you try it. You can get cut offs from a local metal supplier and they might even cut it to size for you.

I thought you said it was a solution.
Seems more like a crackpot idea.
Try a cone of silence.
Get smart

Can you put the TASCAM on top of the Denon? That will made the  TASCAM a little farther apart from the Denon's tape head. 

Everything is electrically isolated, and the hum is audible as soon as the tape heads are energized.

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If every piece is isolated that can be the problem. They ALL need to be on the same circuit. If you don't, there will most likely be noise.

IF you are plugged into a common cleaner, try a cheater plug on the Cassette.  If it's a two prong already, try a 10.00 RCA noise suppressor.

I think we're up to 12.00 usd with the 2.00 cheater..

You know tape heads can act much like a TT/cart/phono stage/ with  unshielded RCAs. RtR are the same way unless your Tape units are balanced. You need to be aware of that. GOOD, Shielded RCAs are kind of a must.

 Regards

Right. Either ground loop hum like Dave says or it could be a fault within the deck itself. Easy way to check, move the deck around. If the hum diminishes with distance then yes you do need shielding. But if not, if you can move the deck a few inches in and out and no difference, then look elsewhere. AC connections being the first most likely suspect.

Thank you all for the suggestions!  They gave me a simplified path to further isolate the problem.  Here's the current status:

@imhififan Swapping positions of the 2 units had no effect.  Had tried that before I posted yesterday.  60 Hz hum equally audible either way.

@oldhvymec The Denon has a polarized 2 prong plug and by electrically isolated, I mean everything at the line level is plugged into the same power conditioner.  RCA cables are all shielded with the Denon using a matched pair of older Monster directional cables (output only) and the TASCAM two pair of new Mogami (don't have a pair of XLR input connections left on my preamp to use those instead).  

@millercarbon Moving the Denon about 3 feet away from the TASCAM eliminated the noise, and it stayed eliminated whether the Denon was powered through the conditioner or via a totally different circuit.

Finally, the 60 Hz noise is there regardless of whether the RCAs are connected to the Denon and the only thing that eliminates it is moving the deck further away from the TASCAM.

So unless this update spurs another approach, I'm going to deploy the advice from @mijoystn and @millercarbon next.  Getting a piece of magnetic steel is both easy and inexpensive as is screwing a ground wire down to it.

Thanks again and I'll post a follow-up once I've had a chance to to try the steel shielding out.

That Denon was made in 1991, don't get to crazy. I'm really wondering if something else is wrong.. 30 years old. The electrolytics in that unit have to be going south..

Again GOOD shielded RCAs or XLRs are a must with most tape heads. I haven't used monster cables in 20 years. How old is your cabling? 

I'll repeat it at the risk of being a little rude. :-) GOOD shielded RCAs or any XLR if you can just swap them out. AC cable, or a simple GL eliminator on one end or the other of the RCA  IC. The GL eliminator is 5-10.00 usd the cable is a swap with something else.. Some gear likes XLR better than others..

the old gold shield w clear plastic monsters are prone to ground failures. Swap in some different RCA. the plate w ground is also a good idea. Denon made a nice deck, especially the 3 head gear, hope this works.