John; I do find it interesting to check out the old Monitor Top fridges. This ended up a very long message, and for that I apologize.
The DR models (those with the open coil condenser) were the first and were a bit of a learning curve for GE it seems. I've done a lot of reading on them and talking with other collectors and enthusiasts. It seems that GE learned about the shortcomings early-on and made a lot of improvements during production. A combination of two design elements together in the system makes the DR machines very finicky. They have a metering device which is normally closed, until refrigerant liquid is present; combined with the large volume of the compressor housing being on the high side of the system. This makes the system incredibly sensitive to any sort of noncondensible gases or air in the system. There is a trap in the line between the condenser and float, so unless the system is completely free of noncondensibles, they will collect in the float chamber, and with the uphill trap in the lines, this will prevent liquid from filling the float chamber. Since the float is normally closed in the absence of liquid, this results in a no-circulation (blockage) situation. The large volume of the high-side compressor housing will accommodate the entire refrigerant charge when this happens, leading to the rattling noise when the refrigerant is at a high level in the sump.
The manuals explain that SO2 systems normally never develop noncondensible gases except if the low side has a leak (it operates in a slight vacuum) or if the motor has been severely overheated.
This one seemed to sound OK initially, but I noticed some ominous signs. First, the bad contact between the control and the baseplate was the main winding circuit. If this circuit were intermittent or had a high resistance, it would provide low voltage to the motor all the time it was running. Another ominous sign was the amount of time it took for the compressor to build up to speed during a start. I have another working D2, and the startup takes a lot less time. It seems that with the DR Monitor Tops, this slow spin-up has preceeded winding failures in more than one case with other enthusiasts on the forum. There was a Kill-A-Watt meter in the circuit during startup, which was beeping indicating 15 amps or more current draw during the startup. That's also not expected. And finally, the ground resistance. I saw about 200KΩ if I remember, when making the first video. On the day of the second video before startup, this resistance was much less. So as much as I hate to say it, I wasn't really expecting it to live but wanted to remain hopeful.
Cotton wrapped windings fail in an entirely different manner than modern enamel magnet wire windings. Whereas enamel magnet wire windings fail suddenly as soon as the coating melts and the winding turns short - cotton never melts. The cotton will char and become hard and brittle, with carbonized oil and charred cotton remaining in position. The damaged insulation will still remain in place, keeping the winding from making a dead-short, but it has become partially conductive and leads to resistive shorts which cause ground leakage high current draw.
The SO2 interacts with the "vapors" from the chemical breakdown of the cotton, resulting in formation of noncondensible gases. This is by way of the cotton cellulose releasing water vapor as it degrades. The SO2 mixes with the water vapor and forms an acid solution. This immediately breaks down into hydrogen gas and some solid residues; in contact with metals.
From what I have found, cotton is considered a "Class "0" insulation system. It begins slowly deteriorating above 90°C (194°F). This would be the maximum hotspot temperature of the winding, inside the coils. So the overall temperature of the motor would not have to be 90°C to have hotspots above the winding insulation limit.
So, one theoretical sequence of events leading to the failure would be:
1 - Poor contact developed between base and control, causing motor to have intermittent voltage and reduced voltage.
2A - Over time, the motor winding operated consistently at or above its temperature limit, leading to slow deterioration and release of NCG.
2B - Or, alternatively, the motor was overheated severely, once; by a control malfunction or improper repair / testing methods. This could have been decades ago.
3 - The NCG built up to a level requiring excess pressure to obtain flow through the high-side float metering valve.
4 - The compressor pulled into a vacuum or near vacuum, resulting in excessive hot-gas discharge temperature on the high side.
5 - The motor is cooled by high-side hot gas, which was entering the housing at an abnormally high temperature and low flow volume.
6 - The overheating continued for a period of time (years?), exacerbating the deterioration of the motor winding and creating enough NCG to stop the refrigeration cycle. At this point, the unit was taken out of service.
7 - The refrigerator was transported across country, with vibration and position changes causing the charred winding insulation to mechanically break apart.
8 - The lost insulation allowed the motor to developed multiple turn-to-turn, and winding-to-ground shorts.
9 - Attempting to run the motor in such damaged condition led to a complete failure of the winding.
I think that the reason the DR machines are so hit-or-miss is because you never know how far along the path to failure they really are when you acquire them.
The CK models were the final revision of the Monitor Top. They did away with the high-side compressor housing and went with a forced-oil cooled winding. There is an oil pump which forces oil through the winding and then sprays it against the inside of the compressor housing as a heat exchanger. These are the most reliable, with winding failures being nearly unheard of. They also have a better control and protection system, with many having dual overload breakers.
The CK also has a direct connection from the compressor discharge, through the condenser, to the high-side float chamber. This ensures that a modest amount of NCG can be in the system and still allow the unit to function.
The Servel looks like it is in good shape, aside from the faded paint. The interior is immaculate due to the new door gasket. I did a little preliminary research and it seems that it has a natural gas burner orifice. I will have to locate an LP gas orifice in order to test it out. I've got too many things ongoing now to tear into it, so that will be a future project.
Feel free to text me the transmission pictures as well.
Sincerely,
David
