Hi everyone,

I would like some help in fixing a Hotplate+Stirrer which has slowly developed a fault with the temperature control. Over time the variable resistor that controls the heat and the range has become more and more compressed. As it is ATM the heat is little better than on and off, slightly warm to boiling. This has slowly increased over time to it's current state.

BTW two pictures are opposite sides of the circuit board controlling the heat to the hotplate and they have been rotated and flipped so that the connections are all in the same location ie connectors etc. C3 which is the large capacitor on the upper left next to the LED which is 0.22 uF 250V / 275V. The 6 pin IC is labelled with MOC3023 443Q and looks like a fairchild component. The 8 pin IC you can see in the picture.

My question is do you think that it is feasible to repair this? I have some basic electronics knowledge, a variable temperature soldering iron and a Tenma 72-10415 Multimeter which is fairly accurate at around £120.

Carl

 

Welcome to AAC!

I have some basic electronics knowledge

Can you trace the board to make a schematic?

I assume VR3 is what controls the temperature. Have you removed it to see if it's still working?

 

The chip LT1007 is an op amp, and will be configured as a Comparator , it looks like there is a bead thermistor on the pcb (TH1) is this the main sensor?.
The Moc3023 will be used to trigger the Triac.
The op amp supply is pin 7 +, and pin 4-, you should be able to measure them with the meter on DC 20V range.

 

I am still trying to solve this. Initially I decided to contact the company to see if they could provide a replacement circuit board but this was over Easter which delayed things with the holidays. They did get back to me but the cost of the replacement circuit was higher than I wanted seeing as it was close to 40% of the cost of the complete unit and well over a 1/3 the cost of a new unit. It is already about 5 years old and therefore the hotplate element has had a fair bit of usage so I was unwilling to pay out so much when a brand new unit might of been a better option. It does seem a bit silly if the cost of replacement components would be much smaller. I wonder what the cost of them supplying just the components rather than a replacement board seeing as they said that they would need to make a new PCB and assemble it which is probably where the cost is coming from.

The question is can the components be identified and purchased? I don't know whether the capacitors can be identified that is the question. u= microfarad as follows. The small capacitors, C2 (u47 J63 1B7) and C4 (u1 J100 1B3) have some information on them which might be identifiable but C1 is much smaller and might be more difficult to identify because it only has T101 on it. It is these in particular that I do not know whether they can be identified. The electrolytic capacitor C1 rating is clear from the side of the component, it's just whether I can find the same sized component.

I would also need some help in identifying the resistors because one or two I cannot identify the colours.

Welcome to AAC!
Can you trace the board to make a schematic?

I assume VR3 is what controls the temperature. Have you removed it to see if it's still working?

I said that I have some basic electronics knowledge, I do know one end of a resistor from another lol.
When I was in my late teens, around 19 I applied for an electronics course and following a colour blindness test which everyone needed to take, I was not allowed to do the course because of partial colour blindness. I do struggle with some colours, even some resistor colour codes is extremely difficult for me to make out and some I just cannot work out.

I have never reverse engineered a PCB before, the PCB is not too complex so maybe I could do that. Can you recommend any software which I can use to construct the schematic? Is there one which supports the components on this PCB, the two IC's in particular?

The VR3 does control the temperature and I have not removed it but I don't think that is where the problem lies TBH because the problem began only around 12 months after I purchased it so I doubt that a variable resistor would fail so quickly. The usable range became more and more compressed over time until it got like it is now. I suspect that the problem lies elsewhere TBH.

The chip LT1007 is an op amp, and will be configured as a Comparator , it looks like there is a bead thermistor on the pcb (TH1) is this the main sensor?.
The Moc3023 will be used to trigger the Triac.
The op amp supply is pin 7 +, and pin 4-, you should be able to measure them with the meter on DC 20V range.

Thanks for the information.
I believe that TH1 is only to detect that the hotplate is heated to show that it should not be touched which is what one of the LED's indicates. It does not provide any monitoring to the circuit. That thermistor is a great distance approx 13+ centimetres away from the hotplate and there is an air gap between the chamber holding the circuit boards and the hotplate itself and hotplate is mounted on legs.

I am a bit wary of touching anything connected to the mains and add to that the extreme heat from the hotplate if the switched variable resistor is turned on adds to the danger. On top of all that there are two strong neodymium type magnets that are attached to the motor which attracts the spinbars and those magnets have a habit of sticking to anything metal including the case whenever the top of the case is removed. The last thing that I want is to turn it on, while trying to avoid being burnt and having those magnets strongly attracting themselves to any part of the case.

I have seen a number of guides on testing components on PCB's and I have seen that capacitors can only be checked if using an ESR meter when the capacitor is in a circuit. I have tried my multimeter on an old digital alarm clock PCB that I have where I checked some capacitors on the PCB. Many of them do give readings very close to their stated values. There is a 4700uF electrolytic capacitor which gives a reading of 4.65 mF ie 4650 uF which is very close to it's stated value. Other capacitors 100uF also give readings very close to their rated values. Two capacitors which have what looks like electrolyte leakage, a brown coloured coating on the case although the top looks okay and I cannot see where they could of leaked from. Those two give very wrong readings with my multimeter. Not such good readings with resistors though. I do understand that resistors and capacitors in parallel does dramatically alter their values.

I do remember a man who used to fix my parent's CRT TV, he would come around and take the case apart and check some components and always managed to fix it in our home. He was quite an old guy. He seemed to manage to read component values with what I remember was an analogue swing pointer type meter.

What are your thoughts on this?

 

Don't bother about trying to read resistors with their bands ,i know the colour code like the back of my hand but i use a dvm as nowadays you have 5bands sometimes, have you measured the supply voltage across the chip on DC pins 4 Negative/7 Positive, , and pins 2,3 and 6 , to see what happens when it's cold and warm?

 

What are the markings on the yellow rectangular component (capacitor?) top left of the first picture?