No signal lock on my new used Adcom DAC

Doesn't sound good to me.
I own both an Adcom 600 D/A and an Adcom 700 D/A.
If I where you, I would try a different cable, any RCA 1 meter plus item will do. just to see if it might be the cable. Then have you switched to the second RCA input? have you tried the Toslink, or did the mods disable that?
Tell the truth, buying a "never heard of" modded box is a big risk, and you got what you wanted... a modded box... (I remember the ad, and no big name modder was mentioned)
Now you can have endless hours of fun putzing with it, till you get it to function.
(If I were you, I would contact the seller and explain the situation to him or her... and ask for a refund. You pay the shipping both ways, and hope they agree to refund your purchase price money.
Some mods are well researched, and talked about, some are experiments. Sounds like you got an experiment.

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.