I have saved text from the archives that I have found helpful in the past. I will post this because it is very helpful.
I sincerely apologize that I couldn't post a link to whoever made it, but our 10ees good health comes before all,
from the archives of PM,
Module out test
First, let's deal with the polarity of the C16s. If you look on the bottom of the tube, the three wires coming out are labeled G, F+, and F-. That is the final word on which filament lead should be positive and which should be negative, I would not pay too much attention to the color of the sleeves. On mine, the red sleeves are negative, the positives are yellow. It may very well vary between different tube manufacturers. As to the Transformers, the 440 V Transformers I removed were not marked, nor were the 220 V ones I put in. You need to realize that being AC, the Transformers output does not truly have a positive or negative. That said, what the tube wants to see is the negative lead hooked to the wire which is most negative during its conduction time. The tube will only conduct when the voltage on the anode is positive with respect to the filament, at this time the negative lead should be at a lower voltage than the positive, with respect to the anode. So, how do we determine this? Actually, it's very easy to figure out. Set your meter to AC, and put it on a scale which is good for at least 350 V if it is not auto ranging. Connect one of your leads to the anode of the tube in question. Now, check the voltage between that anode lead and each of the two filament wires. You will see about a two half volt difference between the two leads, the filament lead which gives you the higher voltage is the negative lead. This may sound wrong to you, but trust me, it is correct. I will give you the explanation if you really feel you need it.
Second, I'm not sure I have ever seen anybody explain what actually happens when you remove the module, so let me fill you in on that detail here. Believe me, it is nothing magical. First, a little lesson on the schematic which I have seen presented here before. The areas where you see --> are the module terminals at the module end. The >-- is what the module plugs in to. Knowing this, you can see that when the module is pulled, there is no connection to the grid terminal of any of the tubes. You can also see that the field's freewheeling diode is also not connected to anything. So, here is what happens when you apply power. With no signal on their grid, each of the thyratrons will turn on as soon as the anode voltage is high enough to cause ionization. They will all act as simple half wave rectifiers with no control. This will apply full Armature voltage of around 240 V, while the field voltage will be reduced(due to the lack of freewheeling diode) to around half of normal(actually a bit less than half, mine has a bout 53 V on the field with the module removed.).
What is the purpose of this module out test? Well, by removing the module, you are removing all of the control components from the circuit. The only thing left connected is the main anode transformer, and the various filament windings on the other Transformers. All this test really does is let you know that all your tubes are good, and that power is available to the anodes and filament windings. Oh, yeah, it also kind of tells you that your motor is not burned out
Third, the filament polarity on the C16s will not affect operation of the machine, it will only affect the long-term life of the tubes. You should correct this before you put the lathe into service, but doing so will not fix your problem now.
Fourth, there are two reasons the control could be dropping out. One is the field failure relay, the other is the supply voltage could just plain be dropping somewhere to the point where the contactor cannot hold. Actually, there are probably others, one of these two is your problem however. I would like you to post your field voltages at the recommended test RPMs, as well as with the module removed. However, I think the voltage drop may be your problem. I say this because of the fact that it seems to drop out whenever the motor tries to accelerate rapidly. In other words, when ever the motor pulls a lot of current. The first thing that comes into my mind is you may have hooked up to a phase converter. Is this the case? If so, I believe you hooked the generated leg up to either L1 or L3, which would cause the voltage on those leads to drop when heavy current is pulled, which would drop out your main contactor. If you are hooked up to a converter, try making sure that L1 AND L3 are hooked up to the original single phase input and the generated leg goes to L2. You only need the phase converter for running the coolant pump, if you have one, anyway.
If you are hooked up directly to single phase, you may have a high resistance connection somewhere in the machine. Check the field voltage first, if it appears normal, I suspect a large voltage drop somewhere. If it is low, you need to find out why.
I am a little concerned when you say guesstimate the RPM, is your tach inoperative? I have one suggestion for you to try, but I will not even suggest it if you don't have a functional tach.
