Panel heater repair

[Kram]

We have a fairly expensive Fischer panel heater that was fitted 2019, 5 of them cost £6k After a few weeks of being switched off due to warm weather, it no longer turns on. Power is getting to the PCB but has no sign of life. This:

Premier Radiators - Storage Heaters

storageheaters.co.uk

Im well aware that a less pretty chinese version would have been under £100 each, and probably nicer controls.

I opened it up but forgot the first rule of getting pictures, I'll get some tomorrow.

Inside is a chinese style element, which is common in import heaters. There is a main PCB, bimetal thermostat switch on the storage brick/slab, and ribbon cables going off to the control pad.

Now Im a bit stuck as the main pcb has a lot of smd components but nothing obvious, to generate the low voltage for the control logic. No signs of burning or damage.

Before I start testing the element and switch, any ideas what to look for?

 

[voipio]

Any pics of the PCB?

 

[Kram]

 

[voipio]

Looks like there's no obvious switched mode power supply then. If that is the case, then the low voltage DC is likely to be generated by using the AC mains-induced charge/discharge current of the rectangular blue capacitor near the relay, to pass curent into a (probably bridge) rectifier circuit, with a shunt controller (Zener diode?) as the regulating element. Check for the presence of a low voltage DC supply voltage across the electrolytic capacitors, but take care probing, as the low voltage circuits might be live, and certainly not isolated from the mains. Look at the circuitry and the capacitor voltage ratings to get some idea of what's going on. The problem with the above method of generating a low voltage supply, is that mains voltage spikes, or switching the thing on at the peak of the mains cycle, can produce huge currents, which can pop or short the rectifier diode(s).

 

[gaz1]

go back to company as they should have a warranty

10 Year Manufacturers Warranty on there models

credit card payment and other reference would be where you bought them from

 

[Kram]

@voipio yes it seems to be a capacitive dropper, and seems to be the fault. I got some much better pictures and done some probing.

The small 3 pin low dropout regulator is an LD1117, pinout 1)Gnd/Adj, 2) vOut, 3) vIn.
vIn measures negative 0.176v.
VOut measures 0.1v
Red electrolytics are rated 35v, black are 16v.
So thats the issue, power supply failure and control circuit not being powered.

I couldnt easily unclip the PCB to look at the reverse, may have a go later on but I was impatient and powered it up.

The IC is an ST MZJH739, 14 pin, I assume its the microcontroller but no results on google. I expected a chip with more pins, as this heater is supposed to be radio control enabled by a seperate wifi box. Perhaps that functionality is built onto the display modue, there is an 8 pin ribbon cable connecting the two. 4 buttons on the display.
The display is inaccessable as the casing appears to have been bent shut after assembly theres a small hole that I could poke through to check the ribbon is fully seated.

 

[voipio]

D62 and D64 might be dual diode devices, with the pair possibly forming a bridge. Check for shorted diodes.

 

[Kram]

I need to get this fixed. I've removed the main PCB to investrigate and work on much easier than having the heater opened up on the floor.

@voipio I have checked all the diodes and the only questionable one is the zener D60 or D72, next to the relay, which seems to give the same 400R reading both ways, but its in the switch part rather than power section I think its not the cause .


Looking at the big resistor, R67, red-red-brown-gold, should be 220R 5%? Why didnt I check this first Its reading above 2000k R.
Any idea what power rating it is?

 

[gaz1]

your clutching at straws

if you base it like a dryer you will understand it more

heater element
power supply
thermostat
timer in the timer side of it based on when it being powered for on time off to for storage used more in storage heaters

main fault areas

heater element
thermostat
dirty magnetic switch

pcb board gone short circuit

power supply to the pcb board and to the high power magnetic switches then to the heater element
pcb feed to low power thermostat calling for heat on/off switch to magnetic switch

( take the pcb board out of the equastion and you have power into the unit then power to the thermostat which powers up the heater element )
same as a boilers turn on the central heating system

however both the boiler and the heater has magnetic switches in them

 

[Kram]

your clutching at straws

The fault is the power section of the pcb, specifically the above resistor is open circuit. New one ordered.

 

[voipio]

At a guess, R67 is a 1 Watt device. There's no sign of it heating up and affecting the board, so it may be there just to reduce the surge current at power on.

 

[slim_boy_fat]

Thinking outside the box (and knowing less than nothing about electronics ), does the heater come with a remote control thermostat and the fault lie there?

 

[Kram]

I noticed today some burnt wires/overheated mini spade crimps that go to the switch (before applying power, so happened before it broke) I guess I'll need to order some and open it up again to replace should have checked it better to begin with.

Repair done. The 2W is longer than the old, probably should have had it standing up for better airflow plus some heatshrink on the lead. Next time..

It works!

@slim_boy_fat your charger will be next on the electrical list, sorry I havent found time for it yet. Soon!

 

[Misterg]

Looking at the big resistor, R67, red-red-brown-gold, should be 220R 5%? Why didnt I check this first Its reading above 2000k R.
Any idea what power rating it is?

That looks like it might be a fusible resistor (or at least a flameproof one) - should be replaced with the same if so.

(Not all fusible resistors have a white band).

ETA:

Surge resistor data.

Given that it had failed, you might just have done the equivalent of replacing a blown fuse with a nail - You would need the schematic or BOM to be sure.

 

[Kram]

Interesting, Im aware of thermal fuses but havent seen combined with resistors.
The colour is a bit odd but I've not came across surge resistors before. I dont see any on ebay. Is it a cheaper alternative than using a MOV varistor across the input or some other purpose?

These look the same but dont say anything special about surge.
https://www.ebay.co.uk/itm/263172415913

Should I replace it again, and what with?

 

[slim_boy_fat]

your charger will be next on the electrical list, sorry I havent found time for it yet. Soon!

No problem, I appreciate your looking at it.

 

[Misterg]

These look the same but dont say anything special about surge.
https://www.ebay.co.uk/itm/263172415913

It could just as easily be one of them ^ :-/

A fusible resistor is often included as part of voltage dropper circuits for low voltage stuff powered directly from the mains; the idea being that in the event of a fault they die quietly in a controlled open circuit, rather than in a fiery, sparking, flaming mess. They *usually* (but not always) have an extra colour band that is white. (Flame proof are somewhat similar).

Should I replace it again, and what with?

We're both guessing without the schematic, unfortunately.