Either it cannot read from the tape, or it is not recording.
Hold something with an AC field near the playback head to see if it picks it up. A soldering GUN, a head demagnetizer, a small transformer - all emit a field. In fact, the old tape ought to have old material or noise on it.
It is just a tape recorder after all, so is there a large oscillation on the erase head? SHould be on the record head as well. I would guess 50kHz to 100kHz at substantial level. We call that bias. No bias oscillator, no record.
So first, think of it as a reverb not working. is it the drive end or the pickup end?
And send me an email with Unicord EC-100 schematic as the subject line.
tmenzo at msn dot com