< Go to Part 4.

13.1 Installing the cooling unit.
It quickly became apparent that it was impossible to lift the cooling unit onto the cabinet by hand. Apart from the weight, the cabinet is 1300mm tall and the bottom of the evaporator has to be lifted over the top. Then the cooling unit has to be delicately aligned so that the seal and high side tube is not caught as the unit is lowered.
GE did make a portable hoist specifically for Monitor Top installation but as this was only used by installers, such an item would be impossible to obtain now.

For those that want to attempt copying it, this is the GE crane for the cooling units.

.

An engine hoist is an ideal substitute and is what I used. It is important that the hoist can be raised at least 2m.

The cooling unit was supported on milk crates which in turn were placed on the feet of the hoist. Once the hoist was raised, the crates were removed so the hoist could be pushed in front of the cabinet.
Lowering the top into the cabinet is a tricky procedure and requires a lot of care. As the high side tube runs right against the cabinet opening, it is easily damaged. The seal is easily caught on the cooling unit as it is lowered, so this must be done slowly checking frequently that everything is in the right position.
As my cabinet was slightly out of square, the top had to be lowered with the back lip on first and then the cabinet pushed into square as the front was lowered. Once on, the top lip keeps it square.

The restored CA-2-B15.

14.1 First operation in cabinet.
As always when the cooling unit has not been powered, it is necessary to run the heater first, to evaporate the Methyl Formate from the oil. It does not take long for the Methyl Formate to start boiling once the power applied. This is audible by listening close to the compressor. The boiling gradually gets louder, and after a few hours it stops. This means it is ready to be used.
It was found that ice cubes could be frozen in a plastic tray sitting in the evaporator, so it would appear the temperature was low enough.

14.2 Calibrating the thermostat.

Despite the cabinet temperature appearing to be sufficiently low, the compressor rarely switched off, even on the No.1 setting. Looking through the AK service manual (Canadian version of the CK), it appeared that the thermostat knob might need to be reindexed. The fact that I had removed the bellows from the rear of the thermostat could have upset calibration. Also mentioned was the effect of atmospheric pressure on the bellows, and that at higher altitude the calibration would be different. This was plausible given the elevation at my location.

 

My assumption was that given the evaporator temperature should be no more than 25 degrees F, then that should correspond with the warmest setting; i.e. 1.
Removing the knob requires the centre disc to be carefully prised out. I used the smallest size jeweller’s screwdriver I had. With the knob initially set to 1, I took it off and moved it to 6. Effectively, it had 6 positions more in the warmer direction. This seemed a good guess with the evaporator settling at around 25 degrees F. Cycling of the compressor was improved, but not optimum. This I assumed would be losses through the door seal.

15.1 Door seal.
I was able to purchase a reproduction seal from Don at Rusty Metal Rescue.
This has to be contact glued into position as it does not have the barbs that fit the holes around the door edge.

Unfortunately, upon reinstalling the door, I found the bottom left corner still had a gap. Further investigation revealed the cabinet was slightly warped in this area. It appears from the dent in the foot below this area that the cabinet has been dragged with all the force on this leg, thus applying stress to the bottom of the cabinet immediately above. Given the impracticality of getting the cabinet perfectly square, I simply added a thin piece of neoprene seal to fill the gap.
Reproduction foot pads were purchased to replace the originals.

16.1 Non condensable gases again.
Over the course of running the fridge, it had been noticed that after a few hours performance would drop off, with a low frost line and continuous running, along with warm cabinet temperature.
Yet, if the fridge was rested for a few days all would be reasonable again for another few hours.
I had developed a theory that maybe non condensable gases were present in the compressor walls and the motor and being driven out when warmed up.
Given that the motor windings would still be likely to contain moisture (the motor was not heated during evacuation) this seemed a likely scenario.
So, I purged the non condensables again; it took a couple of hours before the methyl formate appeared.

Purging the non condensables requires oil to be placed in the charge valve to ascertain positive or negative pressure.

Performance was now excellent. Within a few hours the duty cycle was perfect; 2-3 mins on and 10-13 mins off. On the No. 9 setting the evaporator got down to 3 degrees F which was more than sufficient, especially as the bottom of the cabinet was below freezing.
With this excellent performance still at 11pm, I decided to leave it on over night. Alas, next morning was a disappointment with the compressor running continuously, a low frost line, and ice melting off the evaporator.
My theory at this point was that not all the moisture had been driven out of the motor during the previous purge. Thus it would seem necessary to repeat purging until it is.

16.2 Float valve blockages.
A further purge was required, but performance was still poor. As non condensable gases were not the problem, it had to be a blockage. Tapping on the side of the float chamber’s needle and seat area brought forth a good flow. Several times the fridge had been run with a gradual decrease in performance after a day. A pattern was emerging, in that if the fridge had been rested for a day or more, it would run perfectly but always start to deteriorate after a day’s running. By this time, power consumption was under 200W (including transformer and oil heater), and the frost line would be very low.
With non condensables eliminated as the problem, another theory was looking plausible. Observing power consumption over the time the fridge ran correctly showed an initial draw of about 240W. As the fridge cooled and was cycling normally, consumption went down to about 200-210W.  It would appear that the density of methyl formate decreased as it cooled because of less strain on the compressor, which in turn relates to less power consumption. So, with less pressure, there is greater difficulty in clearing the blockages. This would explain why the blockages largely cleared themselves on initial power up, but not so after the fridge was down to normal working temperature.
Running the fridge for 10 days continuously showed up an interesting pattern. Blockages would usually occur at around 5am which is the coldest part of the day inside the house. Late at night, the power consumption would start to drop and the cycle time reduce. With the float seat starting to block, a higher than normal vacuum would occur in the evaporator which in turn means the compressor has less vapour to suck in. Thus, the compressor runs at less of a load. When this happens, the power consumption drops to about 185-190W. After this,  the frost level falls because the incoming liquid is not being replenished as fast as it is being evaporated. Short cycling then starts to occur. This is presumably because with less frost on the evaporator, it is less able to store the cold temperature. Eventually, the frost line drops so that it is no longer near the thermostat bulb, and the compressor runs continuously.

16.3. Compressor timer.
Not always being at home meant that in the event of the compressor running continuously, I would not be able to turn it off and/or clear the blockage. So, I built up a timing circuit that sensed when the compressor was running. If the compressor ran for more than 25 minutes (or any other desired time), power would shut off and remain so until manually reset. This would protect the motor from an excessive run time. Importantly, it meant I could leave the fridge running unattended and not worry about it. Operation is sensed by a current transformer. This actuates an Omron H3CR timer set for 25 minutes.
If the timer is not reset within that time, a relay latches cutting power to the fridge.The relay remains latched while ever the 120V supply is present.

16.4 Improvement in operation.
Running the fridge for several months did improve things considerably. The blockages became much less frequent. In fact, instead of hours apart, it was now days. It would appear that whatever was causing the valve to stick may have been gradually dissolved. Alternatively, the constant flow of methyl formate and valve operation may have “machined” the needle and seat for a less sticky fit. There is no difficulty in freezing ice in plastic trays and the cycling times are good; 2-3 minutes on and at least 10 minutes off. Correct temperature is with the control in position “3”. This suggests the calibration is satisfactory, although for correctness the knob should be moved to “5” before screwing back in.

17. Shelves.
Only two of the three shelves were present. They are made of steel and either galvanised or zinc plated. The finish is too dull to be nickel or chrome. A previous owner has painted over this with white enamel.

The shelves had been painted in white enamel over the original plating. The black objects on the lower shelf are the original foot pads.

I had the shelves replated by Artarmon Electroplating. It appears that they were originally zinc plated and this is what was recommended. Nickel or chrome was advised against due to rusting in the moist environment.

Like new again, the shelves were the final part of the restoration.
 

18. Acknowledgements.
This was my first entry into the world of refrigeration and would have found it considerably more difficult without the help of others. There is a lot of negativity about working on Monitor Tops around the internet, and when I found the Flickr group all that changed. In particular, due acknowledgements must be given to the following group members for their help:

• jhigdon2 - the guru of monitor top repair.
• worldsfair39, Coldspaces61, and Allan 1955 - enthusiastic members always keen to see how the restoration was progressing.
• and of course, litlnemo - for establishing the group in the first place.

19. Suppliers.

• Heatcraft - for the silver solder and copper fittings, etc.
• Sigma Aldrich - for the methyl formate.
• keep-n-time - ebay seller for the footpads.
• Don at Rusty Metal Rescue - for the door seal.
• Artarmon Electroplating - who did the zinc plating for the shelves.

Monitor Top Refrigerator Forum (replacement for Yahoo and Flickr Groups).

Yahoo group - GE Monitor Top Fridges. (no longer active due to Yahoo changes).

Flickr group - Monitor Top Refrigerators.

Postscript - Capillary Conversion.
Unfortunately, the reliability of the float valve in this machine turned out to be very problematic. It would block up frequently. After the success of the CA-1, the obvious solution was to do a capillary conversion.
See the details here.

This improved reliability considerably, but evidently the system had been badly contaminated because NCG build up was still frequent, requiring purging every month. The decision was made to confirm this was not due to leakage, so the unit was evacuated and filled with 50psi of R22. A leak detector could not find any evidence of leakage. Much to my surprise, the R22 must have had some kind of cleaning effect, because after evacuation of the R22 and refilling with methyl formate again, performance was really good. No purging was required for seven months. Cycling times were good; 4.5 mins run and up to 16 mins off with a cabinet temperature of about 34F.

The initial capillary conversion had to be redone because the length of the tube was too short for efficient use with the compressor running on 50 cycles. After much work, the optimum length of tube was determined.

Experiments were then done with the thermostat, April 2014, to see if the the temperature differential could be adjusted.

In January 2015, a year after doing the capillary tube conversion, a high side blockage occurred. The inline strainer was completely clean which was good news - no more corrosion evident. However, it indicated the problem of the blocked evaporator remained. Again, I am led to believe this cooling unit has been left open to the atmosphere for quite a long time. Once the evaporator was replaced, no further troubles were experienced.

The following is from postings on the forum giving details on the work done:

See forum post here for the following:

Thermostat Experiments.

13 April 2014:

The purpose of extracting the control is to adjust the temperature differential. The problem is the difference between on and off temperatures was 7.2F. It should be 9F. The reason why it needs to be increased is to lessen the frequency of cycling.
To measure the temperature, I've frozen a temperature probe to the bottom centre of the evaporator (as per the manual). I find the IR thermometer not sufficiently accurate for this kind of measurement, as it only has to be moved a few mm to give a different result.
As we all know, GE doesn't want anyone working on the inside of the 1933 and later controls. So, nothing is said in the manual about internal adjustment.
However, we do know the defrost facility is tied in with the temperature differential, which is increased considerably when the knob is turned to defrost. The cut in temperature is raised to above freezing point, while the cut out temp remains the same. Thus during the off cycles, the frost will thaw.
Luckily, in the DR manual it does talk about adjusting the cut in temperature. While they're quite different controls, the temperature differential setting appears to follow the same principles.
For the CA form B control, it is interesting to note that there's an adjustable nut associated with the defrost cam's lever. Yet, in the Scotch Yoke manual, the similar looking two knob control does not appear to have this adjustment.

My initial experiment was simply to bend the arm a little. Bad thing to do because it threw everything way out of calibration. Fortunately, I had marked a line at the end of the arm so by placing a steel rule against it, I could see how much I'd bent it. So, I bent it back, and am now experimenting with the nut.

The process is one of those things like the cap tube and charge experiments, of having to put everything back together and waiting for it all to stabilise, before meaningful results are obtained. A worrying thing is how many times I can bend the tube before it breaks - it has to be straightened out each time the control is withdrawn, and then bent back to clamp it to the evaporator. I'd really like some means of calibrating the control outside of the fridge.

14th April 2014: Turning the nut 1/4 turn clockwise has given an extra 3 mins off time. That's with less than 1 degree C extra differential. I can't see exactly what as my digital multimeter/thermistor set up only reads to whole digits.
This morning, the on time was 3 mins 16 sec and off time 14 mins 39 sec. Cabinet temp 37F. Quite acceptable, but as I want it the same as what the manual says, I still need one more degree C difference. So, I'll try another half or 3/4 turn.
I like the idea of using a chest freezer to calibrate the control, but alas I don't have one.

15th April 2014: I need to make a correction here. Closer examination has revealed the adjusting nut only sets the temperature differential for the defrost. Contrary to what I said previously, it has no effect on the ordinary running differential. This it seems is best set by bending the curved arm attached to the defrost linkage.

To try and give a clearer illustration, the pic on the left is with the knob in "on". On the right, it's in "defrost".  When in "on", the main arm that actuates the contacts has a hook shaped protrusion that rests against the curved part of the defrost arm. The defrost arm bears against the cam at the back of the knob.
When "defrost" is selected, the cam at the back of the knob releases the defrost arm, which now under spring pressure it now pushes against the main arm. The degree of this spring pressure is set by the nut. You can see the end of the spring going through the head of the screw to which the nut is attached.
One could assume if the cam wears that the temperature differential would change.
Obviously, the increase in differential that I got was caused simply by not bending the main arm back exactly to where it was in the first place. What I should have done is to bend the hook shaped protrusion of the main arm, which I've now done. Of course, I've restored the nut to the original position.
As for bending the bellows tube each time the control is extracted, I'm trying to bend it gradually and different points to try and spread the stress.

15th April 2014: How strange. Putting the nut back where it was gives the old cycle times; 2.5 mins on 11 mins off. I'll need to have a closer look at the cam - maybe the nut works on both defrost and normal running. I have to say, it does seem strange the nut would adjust the defrost and not the normal running...the defrost would be far less critical, as long as the off temp is something above freezing, doesn't really matter what.
So tantalisingly close to having it working perfectly....and yet I have to fully understand the control first to do so.

Cleaning out and leak testing with R22.

See forum post here for the following:

21st April 2014: This procedure of having to purge it every month is obviously indicative of a real problem. This time round it didn't fix it. Not sure if I went to excess because now the frost line is much less, so much so the thermostat sensor is no longer frosting to switch off. On a positive note, I can confirm the capillary works perfectly - evap down to -6F at least before I manually turned it off. Quite interesting to watch the film of ice at the bottom shatter like a pane of glass as it got colder and colder.

It's time to prove if there's a leak or not. I've got a cylinder of R22 at work and a leakage detector I'll use.
First I'll recover all the methyl formate and then vacuum it. Putting in some R22 and nitrogen to pressurise to about 80psi ought to show any leak.

If there's no leak then something must be reacting with the methyl formate. In which case I need to try and get some R601a which won't react with anything and produce NCG's or acid.
Something really bad must have happened to that fridge, because in comparison the CA-1 has been airtight since July last year, been running for the last 6 months and never needed a purge.

22nd April 2014:  The method I use is to slowly bubble the gas out until the float chamber has warmed. I've always done it like the training film says - bubble it as fast as possible without blowing the oil out. Perhaps my interpretation of "fast" needs to be a little less enthusiastic! Soon after, the methyl formate makes its appearance by visibly boiling in the oil when the valve is closed, and it soaks into the paper towel a lot faster than the oil. And, I can feel it evaporate off my fingers when I touch the paper towel. I have difficulty detecting it by smell unless it's really concentrated.

During the capillary tube experiments, it would take about 45 mins to purge the nitrogen out, but when I installed the last cap tube, it didn't seem to need as much purging. Hence, my decision to purge it longer this time, as I thought there might still be nitrogen inside.
One thing I have noticed is when I used to use sewing machine oil that the methyl formate droplets would react with it making it quite visible, but the refrigeration oil I've used of late doesn't do that. I think I might go back to the sewing machine oil.

The fact it has required this constant purging, about once a month when it's running, has to indicate a leak. I was quite annoyed by this last episode because I wasn't able to finish off my thermostat experiments.
It's easy to be trapped by this CA-2's charm, because after a purge it always works really well. After a few days one starts to think nothing could possibly go wrong. And then in a few weeks it happens again - the frost area contracts a little. The float chamber starts running cool under load. And then the bottom half of the condenser starts to run cool - you know you're not imagining it when that happens.

22nd April 2014: 22 Apr 2014 at 09:59 allan said:
"I gotta believe you are still purging out that nitrogen you put in on the last repair. The nitrogen probably behaves differently than the MF byproduct NCG. While it is a NCG to that system it is probably more difficult to separate and takes more time."

Good point and it's actually what I was thinking when I started the last purge. It could well be that having nitrogen in the system might be different to MF NCG's or air.
It is interesting to observe the operation when commencing the purge after working on it. Even with all the nitrogen inside, it does frost the evaporator. I find this interesting as the condenser does not appear to be doing anything. As purging continues, it all melts, and then frosts again as the condenser starts warming.
I suspect that the dome is actually working as a form of condenser to start with, because for a brief period about halfway through the nitrogen purge, the compressor rattles. This would suggest excess liquid in the compressor base. As the condenser continues to warm up it then goes quiet as the liquid now condenses there. This sequence of events occurs every time I purge the nitrogen after working on it.

The thing that does have me suspicious is that previous to the cap tube, it still needed purging every month with the float valve. Amazing how the time has gone, but I first had that machine operating December 2012. I ran it all of 2013 with the float valve and the purging frequency was much the same throughout the year; every 4 to 6 weeks that I actually was using it. You'd think that after all that time, the NCG's would have been bled out, even if I was using the wrong procedure. Something must have continued to produce them.

I really need to do the leak test just to confirm one way or the other for peace of mind.
If a leak cannot be detected, then it will narrow it down to methyl formate breakdown for whatever reason, or incorrect purging.

22nd April 2014: 22 Apr 2014 at 13:25 allan said:
"Could be a micro leak. Either the service valve or maybe the vapor line braze joint to the evap. I guess leak could be anywhere but check those two areas very closely. It doesn't take much r22 so use it pure with no nitrogen after a good vacuum for the best success with the leak detector. A vacuum leak is very strange,"
I have a suspicion it's going to be found on the low side. My reasoning is that the NCG build up seemed to only occur when the machine was in use. For example, if I purged it and then left it switched off for a month, I wouldn't need to purge it when starting up again.
There were two leaks on the evaporator when I first worked on it, but as the evaporator held pressure for a month after soldering them up, I assumed all was well.
At the time, I only used detergent bubbles to test - which I understand isn't so reliable for micro leaks.
For pure R22 only, how much should go in?

20th May 2014: The leak testing is now under way. For the last couple of days, the unit has been pressurised to 50psi with R22. It took just over 200g of R22 to get this pressure. No leaks are detectable yet, even around the charge valve. I know the leak detector works because it did find the leakage from the valve adaptor.

See forum post here for the following:

31st May 2014: After several weeks of being pressurised with R22, I'm inclined to think my CA-2 does not have any leaks. So, the only option was to start all over again and see what happens. I rinsed it twice with nitrogen. Interestingly, there was quite a lot of vapour coming out of the vacuum pump. Clouds of white vapour which I'd never seen before either when evacuating the CA-1, or the CA-2 first time round. I hadn't anticipated this so whatever it was settled on the kitchen table but I didn't see any residue. Just maybe after a year of running, the contaminants had been freed up and were now being removed.
Incidentally, I got a slightly better vacuum now using the O ring instead of the copper or lead washers with the charge valve adaptor. Over 29.9".
I put in 2.22 lbs of methyl formate (80.7% charge) as a starting point and ran the fridge overnight. I also added half a funnel of 4GS oil. From here things got interesting. I was surprised to find the cabinet at 32F this morning - it didn't used to get quite that cold at #5. Surprisingly, for that cabinet temperature, the cycling times are also very good - keeping in mind I hadn't finished optimising the control temp differential. About 4 min 30 sec run and 16 mins off! 16 mins off with 32F cabinet temp really surprised me. Never has this fridge been able to do that before. Any doubts about cap tube performance on 50 cycles are now settled.
As for the charge level, it could do with a little bit more just to get the frost to the ends of the evaporator, but tank level looks good.

2nd June 2014: It's been running since Friday afternoon with no deterioration in performance. The compressor is drawing a little less power than it used to (about 10-15W less), the off times are longer, and if I'm not mistaken it runs slightly quieter too (the extra oil?) - not that it was ever noisy.  At this point I don't see the need to adjust the charge level, but how high it is with only 80.7% charge surprised me somewhat.

6th June 2014: I've made the video so you can hear how it sounds www.youtube.com/watch?v=7QhsGV9DKTA&feature=youtu.be
The link is also in the Reference Section.
Seeing as it's performing so well after being cleaned out, I treated it to a new toroidal transformer. What a difference that made to power consumption. Now, compressor running is 160-170W instead of 200-210W, and when not running it's 14W instead of 30W. The previous E-I laminated transformer was terribly inefficient.

Blockages again.

See forum post here for the following:

18th November 2014: It's been nearly 6 months since I've said anything about my CA-2. Since optimising the cap tube length, adjusting the thermostat differential, and cleaning out the system with R22, it had been working perfectly. Seeing it's performance was so consistent and reliable, a bit over a month ago I started using it as my main fridge and even with the warmer weather and a few hot days I couldn't have been happier with its performance. When it's working, this fridge has a certain charm about it - one that entraps you into thinking nothing could ever go wrong with it again. I hadn't said anything here about how well it was working in case I jinxed it.

Well, I needn't have bothered, because in the last couple of days I sensed something wasn't right. Some of the run cycles seemed to be a lot longer than they should be, despite the hot weather. Then, two nights ago I was looking at the power meter and saw it fluctuating from 178 to 151W and I could detect a change in compressor sound while it was doing this. Normally, the consumption is steady with a gradual decrease towards the end of the run cycle. I had never seen it get below 161W.
It seemed to come good again, and then lulled into a false sense of security it did it again last night. Now the power consumption was staying low, and getting down to 138W, and the evaporator started thawing.
I moved all the food into the ever faithful and reliable CA-1.

As to the problem, I believe it's blocked up on the high side again. Back in its float valve days this is exactly what it would do. The float seat would clog up and the the low side goes into a deep vacuum. With nothing to suck and compress the current drops markedly. It would more often than not do this in the evening as the room temperature cooled, and presumably the methyl formate pressure dropped to the point it couldn't force itself past the blockage.
Of course now the float valve is long gone, the likely place for blockage is going to be the inlet of the filter, and presumably it will be the same copper sulphate crystals. Perhaps some has got through and is clogging the cap tube.

So what to do? First to take off the filter and cap tube and confirm either or both are blocked. Depending how bad the blockage is, I might just replace the filter and try again. It has after all taken nearly six months for this to happen. Unfortunately, the system has to forever live with the crap circulating through it, because short of cutting open the compressor and evaporator and cleaning it all out, there's nothing I can do to get it all out. At the moment I'm thinking about a larger filter so there's more room for the crap to collect, then have that feed the existing small filter and cap tube. A sort of dual stage arrangement.

That takes care of what I'll be doing this weekend. Yet again set up the engine hoist and turn the kitchen floor into a refrigeration workshop.
If I could simply replace the top with one that hasn't been open to the atmosphere I would, but that's not going to happen in this part of the world.

18th November 2014: First thoughts (hopes?) were it would just be an oil plug but I defrosted and heated the evap and still no go. In its moments of coming good the only indication was the power consumption going back up to normal. There was never the sound of an oil plug breaking free. I will try again before I open it up. The thing is I have no idea how much contamination is in it. Back when I did the cap conversion in January, all I saw was the copper sulphate crystals in the float valve - there could be more in the system that I don't know about. It occurred to me that the first (coarse) screen in the filter drier isn't needed, as the pellets are removed, which would give much more room for accumulating the contaminants.

I'll do the repair with the unit charged, and purge the NCG's as required afterwards. At least this time round I know any NCG's won't be from leaks and should just be from the nitrogen separating from the methyl formate.

20th November 2014: The patents mentioned lard and methyl alcohol. The early patent mentions methyl alcohol to reduce the formic acid. The next patent was later on when they discovered the NCG problem. It appears that the lard reduced that, as well as the formic acid, so it seems the methyl alcohol was no longer required. Although some vague mention of quantities is given, it might not be exactly was put in for production.
As blackhorse says, if one was to remove the old oil, I'd be wanting to put in R11, R123, R601a, etc. as these are 'known' refrigerants that use 'known' oils.

Anyway, I have some interesting news.
I turned on the machine last night, after it had sat idle since Monday night, and surprisingly everything seemed normal again. The power consumption started off at 221W and gradually reduced down to around 180W and eventually down to about 165W. The condenser was warming nice and evenly. By the sound of boiling in the evaporator, you certainly wouldn't think there had ever been a blockage. After about 45 mins, it cycled off and all was good again.

Now it's time to start theorising...two options come to mind. One is that the inlet screen to the modified filter drier was blocked, and because there's been no circulation for a few days, the blockage has fallen away from the screen. If this is the situation, one might expect another blockage soon.
The other option is what Allan suggested; oil slugging. This is what I initially suspected because when it first happened I hit the bottom of the evaporator and it did seem to do something temporarily. There was also 5 1/2 months ice build up on the evaporator so this may have had some effect. When I restarted the fridge last night, the room temp (and cabinet temp) was about 25C/77F. So, it was having to work extra hard to come down to temperature.

I haven't read much about oil slugging except to know some DR's are prone to it, but I assume that a flooded evaporator would be more susceptible.
So, what does everyone know about oil slugging of evaporators? I have to say the run time improved markedly with an ice free evaporator. I'd just been putting the gradual increase in run time down to the warm weather, but it's back to its normal 4.5 mins. Perhaps more frequent defrosting is in order.

24th November 2014: Room temp yesterday was 33C/91F, so that was an interesting test for both CA's. First time both have been run in such a hot environment.
I can't recall such a hot temperature at this time of year - normally it doesn't happen for another couple of months, and last summer we didn't get any such hot days. Outside it was 37C/99F in the shade.
They coped with it, although of course the cabinet temperature increased, maybe by about 8F. The "off" times, particularly for the CA-2 were much reduced. It seemed happier once the room temp got below 30C.
I forgot to put my bottles of methyl formate under the house in the cooler air - I'd hate to think what pressure they must have got up to.
The CA-2 has been running normally since restarting it five nights ago, so the oil slugged evaporator theory is still looking good.

Still keeps blocking.

See forum post here for the following:

11th January 2015: In this episode of the CA-2 drama series is a repeat of what happened three months ago, in November 2014. This time I went into the kitchen and found the 30 min compressor timer had tripped. As the fridge had been working so well I just thought all the door opening and warm weather had made for an unusually long run time. I reset it and all good again for the rest of the day. Then, mid afternoon next day, I noticed a few short cycles. By evening it was just continuously running with the wattage dropping as it went into a deep vacuum. Normally, it would be drawing about 165W. It got down to 144W before I turned it off. Knowing an impromptu defrost was going to occur, all the contents were moved to the CA-1 for safekeeping.
Hitting the evaporator did give a few short changes in compressor sound and rises in wattage but only temporarily. A saucepan of boiling water did likewise.
One interesting thing I noticed was that with the compressor switched off there was absolute silence from it. Normally there's a crackling sound from the methyl formate inside the compressor dome whenever the heater is on. It was like all the methyl formate had left the compressor and gone somewhere else.

Given the previous time this happened, I decided to rest it overnight. When restarting this morning, I got some nice high wattages and rattles as what sounded like the oil slugs broke free. All seemed promising again. Alas, it's not completely cured. It's been partially blocking throughout the day, and listening to the refrigerant flow along the high side tube around the cabinet top it doesn't seem as lively as normal.
I'm starting to wonder if, assuming the oil is collecting in the evaporator, I should add more refrigerant. As it is, it has 80% charge which gives a good frost line with my particular cap tube. Maybe a higher frost line would get more oil going back into the compressor. The catch is the cycling times might be worsened.
But then, the CA-1 has a lower frost line and never has this trouble.

12th January 2015: Thinking more about the scenario, its behaviour has been exactly like that when the float valve would block up. The same good and bad days, intermittent power fluctuation, etc.
At this stage, I'm thinking a blocked filter is more likely than the oil slugging theory. When I did the capillary conversion it ran for 10 months with no blockage. And now the second blockage is only 3 months later. That does make me think the screen is filling up with crap. The fact it hasn't completely unblocked this time round also suggests that.
If oil migration to the evap was the cause of the problem, it seems that it would have been evident a lot sooner. If anything, the heavy use of the fridge of late should have stirred up the oil more.
Next thing to do is to order the larger Supco strainer and install it. It will also give an opportunity to find out what the contamination level is currently at. The cap tube conversion was done  just on a year ago.

New filter and capillary tube.

23rd February 2015: Following on from here monitortop.freeforums.net/thread/443/latest-ca-high-side-blockage, the new Supco S210 strainer arrived so I've installed it, along with a new 44.5" length of .026" capillary tube.
As before, I charged the unit with nitrogen to 0psig in order to work on the unit fully charged.
Anyway, silly me forgot to keep the charge valve open whilst desoldering the old filter drier. So, with a torch waving about the joint, the pressure built up didn't it!
As it came away there was a good amount of hissing and the green/yellow flame until it equalised. I guess I lost a little MF, that which was still in the filter drier anyway.
New Supco S210 strainer and cap tube went in without any problems, and I spent about 50 minutes purging the nitrogen.  So far all is good, and despite the loss of MF, the frost line is much the same as before. There should be another minute or two off time, so I'll probably add a little MF, say .03lb.

Without actually cutting open the old filter drier, it is completely clean from what I can see inside - like new. The production of copper sulphate crystals has not occurred since before I did the initial cap conversion just over a year ago. The corrosion appears to have stopped. As to what the actual blockage cause was, it's still unknown. Certain things make me think next time this happens I'll be wanting another evaporator.

Evaporator Replaced.

See forum post here for the following:

29th March 2016:  Bob had kindly sent me a CA-2 evap at the end of last year, and with the last blockage of the original being the last I was prepared to tolerate, it was time to install the replacement.
First thing was to recover the methyl formate. I used my car fridge/ice bath set up of previous times:

It took about 8 hours to extract all I could. About 1.8 lbs was recovered. With my set up it's impossible to recover it all - the recovery bottle would have to be in a vacuum to do that. The extractable MF is almost out as shown with the last remaining frost:

Next is to vacuum out the remainder ready for a nitrogen refill. I wasn't going to do this job with the machine fully charged because of the new evaporator having absorbed moisture, and possible other contaminants - it would have to be cleaned out. Because of .4lbs of MF remaining, the vacuum pump is placed outside:

Next is to test the new evap for flow. It was interesting to see how freely the nitrogen flows through an evaporator with no blockage. There was absolutely no impediment to flow.

One last look at the original evaporator:

Next is to remove the evaporator. The top plate screws were removed and then the 1/2" low side tube connection heated. Once hot enough, the top plate with evap attached could be lifted away, allowing access to the evap mounting bolts:

The replacement evap was then bolted to the top plate ready for soldering. I used a spray gel around the low side grommet to prevent heat damage.

The new evaporator is in!

Now to make the copper tubing look pretty:

Then to vacuum and fill with R22 at 25psi. This is to check for leaks at the soldered joins, including the delicate low side tank connections, and the charge valve. It also helps clean out the system:

With no leaks detected after three days, it was time to remove the R22, fill with nitrogen, and vacuum again. Then to refill. I put in 2.215 lbs of methyl formate to start with:

I was quite surprised that with 80% charge, it was not as frosted as with the original evaporator. But then again, with a transplant like this, and a properly working evaporator, I shouldn't be surprised. So, I increased to 2.4 lbs:

That's closer to what it should be.

The new personality of my CA-2 is different, and will take a while to get used to. The initial temp pull down was considerably faster with the new evap, and the thermostat started cycling within about 20 mins, whereas before it might take 40 mins or an hour. The increased flow of refrigerant was clearly audible - nice loud gurgling sound. No wonder the cabinet temp was coming down so quickly :)
Overnight, it has stabilised at 31F bottom of the cabinet temp, and at present the run time is 3.5mins and off time is 10.5 mins. This would roughly equate to 4.5mins on and 11.5 mins off which is comparable to what I had before, in the same ambient temp. I'd like to extend the off time another minute or so, and apart from experimenting with the MF level, I think the mounting position of the thermostat cap tube has something to do with it. Looking at my old pics, the thermostat cap tube end was closer to where the shelf is rivetted to the side of the evap. The extra thickness here would surely add thermal mass and thus slow down cycling frequency. So, I'll check that first, and then if necessary experiment with the charge.

To finish off, I put nitrogen through the old evap. Even after 3 years of MF flowing through it, the obstruction is still there. Obviously, the blockage cannot be dissolved by MF which is what I had hoped. The difference in flow between the two evaps is very obvious - like night and day.

11th April 2016: It's been running for just over two weeks now with the new evaporator. I added .095 lbs of MF at the end of the first week, which brought the frost level up on the right side side tank to where it should be. There are no unfrosted areas around the bottom and sides, unlike the original. The appearance of the frost seems to be different - more fluffy looking rather than looking more like a frozen sheet of water. Cycling times seem quite good; a bit over 4 mins run and a bit over 10 mins off with an ambient temp of 23C and a cabinet temp of 30-31F. I could probably improve those very slightly by removing say .04 lbs, but hardly worth it. Of course I could also back off the control because it really doesn't need to run that cold. As winter draws closer here, I'll expect to see off times of at least 15 mins.
Maybe at last, after 4years persistence, this fridge will now give me the same reliability as the CA-1.

9th May 2016: The replacement evaporator has been in for just over five weeks, and the system has completely settled down. Soon after I added the extra charge(it's now 90.7%)there was a slight rattle, for a few seconds just after it came up to pressure after starting. My theory is that the extra oil needed to be pushed out of the evaporator. From what I could hear sloshing around before I installed it, I gather there was a good quantity already inside. After a few days the rattle went away and the off times started to improve also. At present, in a 20C (68F) room, and a bottom of cabinet temperature of 30F, the off times have been just exceeding 16 mins at times.

9th May 2016: There's been no further change with the frost pattern. It's all quite even over the frosted areas. If anything, there is just a very slight dip in the level on the header tank near where the oil skimmer location is.

10th May 2016: This is the frost pattern with 2.495 lbs charge:

 Since the new evaporator was installed, performance has been completely reliable. This is now my main fridge.