Zanussi ZX99/5SI fridge/freezer – Tricky to diagnose defrost problem

Hi All,

I have a Zanussi ZX99/5SI, product number 928405947-02. It has a problem which I need a little bit of assistance with. I know that it’s pretty old but it still keeps things cool magnificently, it’s in good condition, it’s been well-maintained and it integrates really nicely into our kitchen. Hence I’m reluctant to replace it. I’m an electronic design engineer with 35 years of experience and I design mains powered machines for a living.

The essence of the problem is that it does its defrost cycle only sporadically. Sometimes it will defrost on cue and stay ice-free but at other times it will go for months without operating the heater and so requires manually defrosting (I’ve become quite good at doing this). Things I know or have done are as follows:

1. The heater is intact (it was replaced about 18 months ago when the original heater went open-circuit).
2. The two thermal cutouts in series with the heater are intact. (replaced as a precautionary measure when the heater was replaced).
3. The electrolytics on the main PCB are intact (recently replaced because they were getting old, at least one had a much reduced capacitance).
4. The heater triac is intact (replaced as a precautionary measure when the electrolytics were replaced).
5. All subsystems operate correctly when using the diagnostic mode, including the defrost heater.
6. The overall power consumption does exactly what is expected during diagnostic mode when switching on/off the individual loads, and also during normal operation (except that usually the heater doesn’t operate when it should). I’ve monitored this with various test gear including a logger so to produce a long-term graph of consumption.
7. All of the various connections and cabling are intact, with no intermittent or high-resistance connections.

My understanding of the defrost cycle is that the heater remains switched on until one of the NTCs detects that the ice has melted (by virtue of the measured temperature rising) and that the heater is switched off once this point is reached. I also understand that if the NTC does not detect this condition, then the heater is switched off after a set time period.

A good candidate for the root of this problem therefore seems to be a failed NTC. If the NTC were indicating that the temperature were higher than it actually was then the heating cycle could perhaps be truncated prematurely or terminated immediately after it started or perhaps the cycle would not commence at all. I have measured the resistance of both NTCs at various temperatures and sure enough they both behave as NTC thermistors. What I don’t know however is whether they are behaving according to their correct specification as I have no technical data for them.

So, the questions which I would dearly like answers to are:

1. Is my understanding of the defrost cycle correct?
2. Is one of the NTCs indeed used to monitor and terminate the defrost cycle?
3. If the NTC has failed, could this explain the observed erratic operation of the defrost mechanism?
4. Is anybody able to provide me with technical data for the NTC so that I can determine by way of measurement whether it is faulty?
5. Is there any other likely candidate for the observed problem?

The Zanussi part number for the assembly consisting of the two NTCs is 2425155054. The two NTCs are connected to a single 4-pin connector and the 2-core cables between the connector and each NTC each has the number “SD 21466 1001” printed on it. Because the same number is on each cable, I suspect that the two NTCs may be identical. If this is correct then it offers a convenient way to test the suspect device as both NTCs should have the same resistance at the same temperature – easy to test.

Any assistance from you knowledgable people would be extremely welcome. I have indeed noted the forum conditions an I can confirm that I will not be undertaking any live testing as a result of any replies I receive. The most that I will be doing in terms of further testing will be measuring NTC resistance having first powered down the appliance and disconnected the NTCs.

Many thanks in advance and best regards to all.