Hi guys. As the title says, I have a 4004 with an arm that won’t drop. I got this from my 87 year old dad, and would like to get it running again. The only info I could get from him is that he always needed to put a penny on the arm to make it work. I figured it was a weight adjustment issue. Silly me.

Once I tested the unit, it appeared to indeed have a problem with the needle arm lowering. The motor was functional, and the belt was in excellent shape. The arms moved very nicely along the spindle. But I couldn’t get the arm to lower even with the up/down button.

As recommended by pretty much everyone, I started by cleaning and lubing every part of that assembly. It needs more adjustment, but I confirmed that it isn’t sticking. So I sighed and pulled out the schematics. I used to be an arcade/pinball tech, so I am at least familiar with old through hole PCBs.

Shorting transistor 6 most definitely makes the solenoid active. In fact, further testing revealed that it has no problems lifting the arm once it’s dropped. Buttons fully functional and make the voltage drop as expected.

Where I’m getting lost is the correct normal behavior of the arm. The schematic seems to show 30v coming from the spindle position sensor, and I have that. I can’t see a signal from the sensor for the arm, no matter what position it’s in. I freely admit I did all this with a multimeter, so it may indeed be sending signals I just can’t read. Before I break out the old scope, I thought I would ask someone who knows what they’re doing.

The service manual seems to call for the whole circuit to be replaced. While I am happy to do that, it won’t do diddly if that sensor is bad. Can someone walk me through how the arm operates and what normal behavior is? Thank you so much!