1992 genie cm7500c 1/3 hp screw drive door goes up but doesn't come down

I installed a GENIE BLUE MAX CM7500C 1/3 hp screw drive garage door opener on November 21,1992. It has a receiver board 25689r with 12 dip switches. The opener was sold to me before safety beams became mandatory. It came with a wall push button 37644R model GWCR and a remote BLUE MAX CT90C (freq 315) with 12 dip switches & 9 volt battery. It has a GD-1 DOMINO keypad outside the garage and DOMINO control box inside the garage using three AA batteries. DOMINO power box & wall push button are hard wired to terminals 1 & 2 of the opener. Over the years I have had few issues but which I solved. Around 2000 I replaced the keypad. In 2012 I replaced a leaky capacitor 18004B. Just recently I replaced wall push button, keypad & power box & capacitor. I cleaned grounds on motor, checked cold solder joints on 25689R and sequencer board 24350S, checked for shorts in all wiring to opener but I still have the same problem. The sectional wooden door (7'x 8') operates up to the limit stop but once I press the on/off button on the DOMINO power box or wall push button to bring the door down nothing happens. I then have to place the opener on manual and lock the door with a nail. What can I personally do next to get the opener working again?