vntgstereo
New Member
The Problem:
My Denon DP-37F turntable was experiencing the classic "reverse creep" issue. The power supply rails (+5V and +10V) measured completely healthy, ruling out the standard voltage regulator failure or collapsed power stage, but the motor would only spin slowly backwards.
The Fix:
The issue was entirely caused by the physical gap between the magnetic tape reader (speed sensor head that can be seen thru platter cutouts under the rubber mat) and the inner rim of the platter. Over time or during transit, the head had shifted very slightly away from the platter's brown/black magnetic stripe. Or possibly the platter magnets got weaker over time.
When this gap is too wide, the motor control IC receives no feedback pulses from the platter. The microprocessor logic interprets this silence as an uncontrolled forward runaway speed condition and fires the directional brake loop indefinitely.
Loosening the tape reader mount, manually readjusting the tape reader head so that it sits microscopic millimeters away from the platter's magnetic area without rubbing the platter (using a standard piece of paper or thin business card as a gap gauge), and re-tightening it instantly restored proper forward quartz-locked operation.
If you are diagnosing a Denon direct-drive that the platter starts creeping backward very slowly and your board 5V and 10V voltages check out fine, check the tape head physical alignment before replacing any ICs or driver transistors!
My Denon DP-37F turntable was experiencing the classic "reverse creep" issue. The power supply rails (+5V and +10V) measured completely healthy, ruling out the standard voltage regulator failure or collapsed power stage, but the motor would only spin slowly backwards.
The Fix:
The issue was entirely caused by the physical gap between the magnetic tape reader (speed sensor head that can be seen thru platter cutouts under the rubber mat) and the inner rim of the platter. Over time or during transit, the head had shifted very slightly away from the platter's brown/black magnetic stripe. Or possibly the platter magnets got weaker over time.
When this gap is too wide, the motor control IC receives no feedback pulses from the platter. The microprocessor logic interprets this silence as an uncontrolled forward runaway speed condition and fires the directional brake loop indefinitely.
Loosening the tape reader mount, manually readjusting the tape reader head so that it sits microscopic millimeters away from the platter's magnetic area without rubbing the platter (using a standard piece of paper or thin business card as a gap gauge), and re-tightening it instantly restored proper forward quartz-locked operation.
If you are diagnosing a Denon direct-drive that the platter starts creeping backward very slowly and your board 5V and 10V voltages check out fine, check the tape head physical alignment before replacing any ICs or driver transistors!
