Stepper not stepping

I recently recieved a rcsu4 for my LPC1. It had been modified. It is noted on it and the connections for the wallbox are overwritten in the colors of the 3W1. As i have both 3W1 and a Consolette, I thought I had both ends covered. So much for that theory. I will not respond at all to either wallbox. The 2050 plate resistor heats up to the point it started to smell before I got the power shut off. I assumed then that the 5mfd cap was bad, as this has not been recapped. I have used both a 4 and 5 mfd cap off appliances to temporaraly test that idea. No go, resistor still heats up. Not sure where to go now. Any help would be appreciated. Mark

There can be several causes here.

Examine if one of the stepper 'clapper" is pulled down. If the resistor is heating up, one magnet should be energized.

Try the following:

1) Test for bias voltage on pin 5 (2050). Should be at least minus 24vdc. Test with and without 2050 in socket. No bias voltage will cause the tube to conduct (and ruin it) all the time. If that voltage is absent it may be due to a dirty connection in the plug from the control center to stepper. This, and a shorted/ failing 2050 are prime causes. Do these tests with -no- wallbox connected.

2) If no clapper is down but resistor still heats, it is possible that the tube is conducting but a stepper coil is shorted. IIRC these later ones are around 500 ohms ea. This is an unlikely scenario but with equipment as old as this and from who-knows what source, anything is possible.

Remember that the tube is the last element in the chain that connects the stepper coil to ground thus completes the circuit. So the tube must be conducting for that resistor to heat up.

When I still used thyratron tubes (2050) commercially I used a 1/2 watt 100 ohm resistor in the plate circuit to act as a fuse and protect the coils in the case of shorted signal line ot other failure. I later went to 120 ohm when I started using an SCR instead due to the increased current flow.

BTW: that 5 mfd non-polarized cap --must-- be very near 5 mfd or the stepper will occasionally miscount. 4mfd or 6 mfd are out of range.

Do you have a schematic?

Rob-NYC

Thanks Rob
Your 1st test found the problem. I had lost the -voltage on pin 5. I traced back to the plug going into the control unit. it had never had a stepper and the socket was very stiff. When i plugged it in it pushed back one of the pins in the plug. Now it takes off and selects. Mark

Ah, ha--the old "plug syndrome"--BTW, that model uses A-MP brand, "Amp-loc", plugs--should any one have some of those pins, they wish to sell, PLEASE, PM me !
Ron Rich