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electrofix wrote:a lot of old modules have simple power supplies
I assume with your level of knowledge you have chacked this as when they start failing appliances do all sorts of silly things
Deciphering the circuit proved challenging due to the way the live (+) input taps of the bridge rectifier are split via a transistor and the +ve DC output linked to it through a 220µF electrolytic capacitor. Regardless, there are three electrolytics implicated (220µF 25V, 220µF 35V, 1000µF 16V) which I’ll replace. I’m working on the theory one or more have dried to the extent that the additional load when compressor is switched whilst fan is also switched caused a voltage drop on the microcontroller’s Vcc sufficient to cause it to reset. This would explain the constant cycling – theoretically the module is in a power-reset loop.
Cannot be sure since I’m unable to get the oscilloscope into position to apply probes on both sides of the PCB whilst it is connected to the freezer.
RasPi will be a relief after the hassle this has caused!
electrofix wrote:a lot of old modules have simple power supplies
I assume with your level of knowledge you have chacked this as when they start failing appliances do all sorts of silly things
It has a simple bridge rectifier circuit which I noted but haven’t done much analysis of aside from sketching the schematic. I’ll do some tests on that tomorrow. Thanks for the info.
Re: Typical AC evaporator fan power draw? [FF92]
As I said, evaporator thermistor is fine.
Having stripped down the evaporator motor I found there were hairline cracks in the moisture seal around the magnet where it enters the winding block. Painted it with Plasti-Dip and refitted and working fine again now.
Re: Hotpoint WMF740 Intermittent Random failures
There does seem to be a fault on the control module, I’m aiming to diagnose the root cause and replace the faulty components.
Re: Hotpoint Mistral Fridge AND Freezer not cooling
A pre-owned Fridge-Freezer containing the same parts. They can be got on Auction and classified advert sites. I recently got an FF92 for spares for £10.
Re: Hotpoint Mistral Fridge AND Freezer not cooling
The fan can be replaced, or it can be repaired. I’ve repaired fans successfully in the past when it is clear the alignment of the fan on the motor shaft can be set correctly.
I use fast-setting resin such as Araldite Rapid (~5 minutes) and Vaseline to lubricate/insulate the motor shaft from the resin so it is possible, later, to slide the fan off the motor shaft.
Apply Vaseline to the motor shaft, put the fan onto the shaft and fill the broken area with resin. Extend it over the surrounding plastic for strength.
As the resin is curing keep turning the fan steadily to prevent gravity causing it to gather in one place. This prevents the fan being off-balance once the repair is complete.Once the resin has set ensure the fan will slide off the motor shaft and wipe away excess grease.
If necessary, use needle files to clean up and balance the repair.Re: Hotpoint Fridge Freezer FF72 completely dead
The control PCB is pretty straightforward; if you’re an electronics engineer you’ll easily be able to decompose the circuit in your head.
One thing I misled you on. The three-way connector in the base of the rear cabinet is wired into the freezer compartment. There are two thermal fuses, one each in the live and neutral circuits, clipped to the right-top of the evaporator.
You should be able to trace the terminal connections from them up to the control PCB and back to the compressor using a continuity test, too.
Re: Hotpoint Mistral FF91, freezer only reaches -9°C any ide
cmthomp wrote:On mine I am getting error Eb which points at the Fridge Baffle motor running continuously. Sure enough when I’ve had a look it is continually cycling. The PCB appears to show a burnt Triac so I’ve ordered another to hopefully fix the problem.
The Triac for the baffle motor is TY3 marked “Z7S Y227” and is a Z0107MN 4-quadrant Triac.
Re: Hotpoint FF92 – Fridge Baffle Motor Running Continuously
On the Procond-manufactured control PCB the baffle motor is connected to pins 3 and 4 of the 4-way edge connector half-way along the front edge of the PCB. The baffle switch is connected to pin 3 with its return goes via the lamp (in series) to pin 1.
Re: Hotpoint Mistral FF92S Fridge Freezer not cooling
Sorry it took me so long to get back to you on this. The 2nd photograph you show is correct – the evaporator fan is behind that panel.
Measuring temperature accurately needs a digital thermometer and it needs to be touching some surface. I always stick the probe through the grill that has the freezer thermistor behind so they both should indicate similar temperatures.
I’ve found that the free air temperatue is often a lot warmer than the surfaces or the thermistors indicate, which makes sense.
As to the fluxtuations I’m still working on fixing our FF92 permanently. Yesterday, after a week of behaving correctly, I caught it when it went wrong again. Evaporator showed -39C, freezer -15C and fridge +18C.
This was caused by the evaporator fan not running and therefore the cold air not being circulated.
The cause is on the control PCB. The evaporator fan (pin 1 – rearmost – on the right-side 6-pin edge connector is connected in series to the positive thermal coefficient resistor (thermistor). This is a small grey-blue coin-like device about 8mm diameter on wire legs about 8mm long.
The voltage on the output (fan-side – rearmost leg) was ~7V whilst the input was 240V. The PTC was always very overheated – probably 50C or more.
Until I can replace it I’ve simply hot-wired it by wrapping a short length of 5 amp fuse-wire around its legs so it is bypassed.
I’m currently confirming the specification of the PTC and am planning on creating a kit of replacement thermistors that others can source themselves for less than £5, instead of the ridiculous £15 each that espares et al. charge.
Re: Hotpoint Fridge Freezer FF72 completely dead
Check if power is arriving on the control PCB in the top of the unit.
To gain access open the fridge door and release the three recessed cross-head screws that hold the control/display panel in place.
Lift the panel front edge and pull forward and up.
If the PCB has “Procond” on it you will see there are two edge-connectors on the right, one with the two leads, the other with six.
On the connector with six leads, counting from the *back* of the board is pin 1. AC live is on pin 6, neutral on pin 5.
These come directly from the compressor terminals via the three-way connector (leads coloured white, red, blue) that plugs into a socket in the base of the rear cabinet.
If you’re sticking multimedia prongs into the controller PCB connector be aware that you can think you’ve got a good electrical contact but in fact not have any, so you might need slim prongs and wiggle them about a bit.
Re: Hotpoint Mistral FF92S Fridge Freezer not cooling
radaronse wrote:The temperatures are now +21 Fridge, and -22 Freezer. -22 is the norm for the freezer (before the problem started), which is good. I’ll keep an eye on it, and if nothing improves with the fridge, I’ll try some testing on it tomorrow.
Has the compressor turned off?
Run a self-test. Turn the appliance off. Hold down the Save button and re-apply power. Allow the appliance to go through its self-test program. After about 60 seconds it should operate the evaporator fan for about 20 seconds.
Have you checked the state of the motorised baffle in the top of the fridge compartment when the freezer has reached -22? Remove the cover from the top-left of the fridge compartment around the light. Press the door-closed detection switch. Is the motorised baffle open? Can you feel frozen air coming in from the freezer? If not, and the baffle is open, is the evaporator fan in the freezer operating?
Re: Hotpoint Mistral FF92S Fridge Freezer not cooling
radaronse wrote:
Is there anything else I could do?I’ve recently been doing some discovery with this model and detailed it in a couple of other threads:
“Hotpoint Mistral (FF92) mis-reading temperature?“
“Hotpoint Mistral FF91, freezer only reaches -9°C any ideas?“In that latter thread check out my post entitled “Hotpoint Mistral temperatures and resistances” (Fri Aug 02, 2013 6:30 pm ) which describes how I discovered a thermistor positioning problem caused high temperature issues.
Hotpoint Mistral temperatures and resistances
Finally solved the mystery! I received the service manual (SM) today and checked it against all my observations and experience.
First thing I noticed is, the SM is not up-to-date (document # 5404310 Issue 3 March 2002 – part # C00220681). It covers the FF71 N/P/S, FF72 N/P/S, FF73 N/P/S, FF91 N/P/S, FF92 N/P/S, FF93 N/P/S, FF91 TC/LV/BL, FF92 TC/LV/BL, FF93 TC/LV/BL, and field trial models 896P, 8996P and FF73P
It describes an earlier version of the FF92 which had separate heaters for evaporator and gutter. As far as I can see on this appliance the gutter heater is an extension of the evaporator heater. It’ s a small difference but it affects the self-test mode since there is no gutter-heater test.
The SM also does not mention the self-test mode displaying the evaporator thermistor temperature when Zone is “Freezer” and Save button is pressed.
It also states that SM mode switches back to normal operation once done but my experience is that it keeps the compressor running constantly (freezer displayed -26C whilst fridge was at 2C when set to 3C; evaporator was -34C).However, it does provide a lot of detail including the expected resistances of the thermistors. It also confirms that all three thermistors (fridge, freezer, and evaporator) have the same specification and they match the readings I’ve been getting:
+25C = 2.7k? (component specification; all)
+21C = 3k? (defrost cycle max evaporator temperature; evaporator)
-15C = 19.5k? (evaporator fan trigger; freezer)
-19C = 25k? (compressor trigger; freezer)
-22C = 29k? (compressor trigger; freezer)Evaporator fan doesn’t operate above -15C (19.5k?) or immediately after a defrost.
I checked the components once again by freezing them in another appliance and measuring their resistance.
The solution, though, was discovered when dismantling the freezer down to the evaporator. Page 20 of the SM has a side-view line-drawing of the evaporator where the thermistor and thermal fuses clip on.
I noticed that it appeared different to the actual locations I’d refitted it after the thermistors were replaced the last 2 times.
The diagram shows the thermal fuses clipped on the lower returns of the evaporator pipes with the thermistor on the upper pipe. This is important since those pipes are set at a 60 degree angle with the upper pipe forward of the lower pipe. The thermistor *should* be clipped such that it lies in the U-shaped groove in the front of the evaporator fins so that its tip reaches the 5th fin from the right.
Let’s see if I can manage an ASCII diagram of that right-side view:
------------
| |
| O O |
) |
| O O |
| H |
| O O |
Q^ |
| O O |
| |
| O O |
) |
| O O |
| H |
~ ~In this diagram “O” is the pipes as-if seen end-on, the ” ” are the pipes in profile, “H” are the two heater thermal fuses, and “Q” is the evaporator thermistor with its clip “^”.
I had, at some point, refitted the evaporator thermistor such that it was on a lower return and it was pressed into the center of the evaporator (where things are much colder!) rather than laying across the front of the fins.
That would lead to the unit switching off prematurely since – as the self-test evaporator temperature display revealed – the temperature was -40C or below.
After switching on in self-test mode once more, with an ambient temperature of 25C, the freezer reached -22C in about 1.5 hours and the fridge +2C within 2 hours. The evaporator temperature displayed as -28C.
It continued cooling with the fridge at 2C and freezer at -26C with evaporator at -34C. At that point I switched off for 10 minutes then powered on in normal mode. It displayed 2C/-22C (digital thermometer showed -26C) and the compressor wasn’t started.
Re: Hotpoint Mistral FF91, freezer only reaches -9°C any ide
kwatt wrote:
It sounds really simple, the compressor may well be overheating and cutting out, restarting after it cools down. Happens a lot in warm weather as they get stressed out.I’d agree if weren’t for the fact that the compressor doesn’t stop working so the thermal cut-out nor the PTC are activating.
