No signal lock on my new used Adcom DAC

Just received an Adcom GDA 600 DAC with mods, which I purchased from a fellow Audiogoner. I have a good coax cable on order, but decided to go ahead and hook it up to my Denon DCM 370 changer with a Monster Interlink coax cable which I had on hand. The Digital Frequency Lock LED will not light up and not much sound comes through the right channel--in fact there is a lot of distortion coming through that channel. So, it appears that the DAC cannot lock a signal from my transport (the Denon DCM370 changer). Is this possibly the Monster coax cable? Or am I going to have a compatibility issue with the mods on the Adcom DAC? These include:

Existing dual Burr-Brown PCM63P 20 bit DACs piggybacked with high performance Burr-Brown PCM63P-K 20 bit DACs (4 DACs total).

Dual LC Audio AD825 OP AMPS installed.

Any help would be appreciated.
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Depending on the length of cable or even not for that reason but some transports do not put out strong enough signals to clock in to certain DACS and need a booster hooked up between them . I have experienced this with my Kora on some transports. A Monarchy Dip or in my case the GW Labs digital booster fixed the problem. Audio Advisor has them and I guess other places also. See if you can get your money back on a trial if this does not solve the problem. Of course, as stated, you may also have a defective DAC.
Happy ending to this story--the DAC is fixed and works great. Kudoes to an extremely honest seller.
You forgot to tell us what the problem was, glad it worked out for you though. All sellers I have delt with on audiogon have been fine people.
The problem was that a small piece of sound dampening material inside the DAC came loose during shipment and knocked a pin loose. The pin was resoldered and the DAC works great.