Modular with AP chatter

A modular has developed a chatter in the AP relay. The spindle thyratrons glow dimly and can be influenced by the speed control but will not move the spindle. The field thyratron goes dim in the second half of the speed control range, implying that field weakening is working. The AP relay chatters and the thyratrons blink. If I hold its contact open, that is like the relay was energized, the thyratrons continue the low glow but the spindle doesn't turn. I found 4 bad diodes in the control box and replaced them but it didn't change anything.

I seem to recall a thread on this but a search didn't help. BTW, I don't have the spindle lock on.

Bill

Ammendment- not AP relay, but QSD. Chatter rate varies with speed setting.

Sounds like you need some 3-500z's in there

"It is suggested that a spare module box is purchased so that if the module is suspected of being the cause of a malfunction it can be replaced with one known to be good". Monarch Lathes, L.P.

Amendment- AP is not the relay chattering. It is QSD. the rate of chatter varies with the speed control.

Bill

The earlier large coil type QSD can often be the problem. It has been several years since I worked on one, but I found posts here about fixing the OSD relay-it can become magnetized, and some other adjustments. I remember they can chatter and cause the switch to burn.
Some time around 1979, Monarch replaced the large coil QSD with a board using one of the smaller coil relays,a heavy duty bridge rectifier, and a heavier switch, I have seen that newer unit installed on earlier drives.
I dont know if it is a common problem, I have had the safety solenoids-one on the left hand door, and the back gear solenoid go bad, cause a load-blow diodes.....

Donie, this lathe has a large coil QSD, but that was not the problem. I slipped a piece of paper in the QSD contacts so it would not cycle and looked at the output with an oscilloscope connected to one of the filament transformer's center tap. Despite the whole circuit floating, there was not that much noise interfering with the reading. BTW, I used a 10:1 probe on my Tek 453 with 2 more 10 meg resistors in series with it. The anode voltage on the tubes will fry a normal probe. I could see that the tubes were firing over a large part of the cycle but the motor wasn't running. The bottom line was that the motor had a stuck brush and a strategic wiggle cured the problem.

The reason for the cycling was that with no motor load, the QSD relay is not energized. The contact is closed so the braking resistors make a load. That pulls in QSD, disconnecting the resistors and releasing QSD. Da capo al fine. The AP relay may chatter because it is seeing a fluctuating voltage but that is just a result, not a cause.

Anyway, the machinist who had a hard turning job and wanted to do it on his favorite machine is happy and peace and tranquility reign.

Bill