Are you sure they are really fuses and not a self resetting device? On the one you took apart, does it look like there are movable contacts somewhere on that bimetallic strip? Have you looked at the manufacturers specs of the device?
Yes, no and yes. They are fast-acting fuses specifically intended for SCRs and the like.
I have never really run in to "shelf life" problems with fuses. Shipping could be another issue. I don't suppose the guy selling the pulls checked them before sale. A warranty on a fuse would be real close to silly.
I believe they are NOS rather than pulls, and the guy sent a message today saying the rest of them had been checked and about half were definitely bad. He is willing to give me a complete refund, but I told him to go ahead and send four more since they have now at least been checked.
It really does feel like something has failed in the controller. Do you have a resistive load that you can use that would not require removing the floor if it breaks?
Yes, I have another piece of heating wire I can use, and I thank you for that suggestion since I had not thought of that.
Attach a load and see what the voltage is. Start low and see if you have any control. Don't put a fuse in the output leg.
Yes, I plan to remove the fixed resistor and use the original potentiometer to begin at zero and go up, but I will have to have a fuse in the SCR output to get anything at all.
Is the primary side of the controller fused or on a breaker?
It gets its power from an electronic thermostat that gets its power from a breaker. The thermostat "pulses" or whatever to conserve power when it believes it can, but the tech at Payne told me that is no problem for the SCR.
If you try this be careful you don't make a big mess in the controller. If the SCRs failed (or the control of the SCRs failed) so they are full on you would get 120V across the load and this would be consistent in terms of blowing 10 A fuses.
"Full on" would be too much for my 14-ohm heating wire, but the total load at 120V would still be less than 10 amps.
Have you measured the output voltage of the controller? You could use a regular incandescent light bulb (hundred watts) as a test load. The bulb will keep the output current from exceeding rated value. You will probably be able to adjust the voltage; if not and you just get 120V it is the controller.
The SCR needs a minimum resistance load to work at all, and I do not know that number at the moment. Maybe a couple of bulbs would be sufficient, or maybe not. The last (and only) time I have tried the SCR after its OEM fuse blew, the output was the usual 70 volts as established by the fixed resistor. The NOS-salvage fuse being used at that time blew in about 4 or 5 seconds, but that was before I had discovered the loose output connectors.
I will have some more fuses late next week, and a distributor in Louisville has told me where I can get (or probably order) some truly new ones locally. For now, I will check prices at those places while hoping at least one or two of the NOS replacements being sent are actually good!