Repairing 110/220v hot circuits (Philips advance HR7978 kitchen appliance) - Page 1
Forum section: electronics / repair

author: Sky_hawk
author_rank: Contributor
timestamp: December 30, 2017, 06:11:06 pm
content: I have this kitchen appliance, the Philips advance HR7978:https://www.flickr.com/photos/philipsdesign/16903097411/in/set-72157651508777931It is stuttering when I try to turn it on, the stutter-frequency is slightly different on different settings (1-7)Unlike many of these devices, this one actually contains a small PCB with a microcontroller on it (Holtec HT 46R006)My first impression after optical inspection is that nothing has obviously released its holy smoke.there is some oxidation or deposit (or technical term: white stuff) on the bottom/solder side of the PCB.This causes me to suspect it might have been exposed to a liquid at some point. I tried cleaning the board with isopropanol 99.5% but this did not change the behaviour of the circuit.My question is this: as 220v is present without any isolation, I'm not keen to connect a scope to it.How to I debug such a at first look simple board ?Do I put a lightbulb in series? if so, what do I do then?Maybe it would be nice for an eevblog episode to go into how to deal with/debug/repair 220v (or 110v) hot circuits ?Photos attachedGreetings,Jan

author: Jeroen_Bezemer
author_rank: Contributor
timestamp: December 30, 2017, 06:34:49 pm
content: Quote from: Sky_hawk on December 30, 2017, 06:11:06 pmEither use a HV-probe and be carefull with ground connection , or a differential probe.Or use a safety Transformer. But even with a safety Transformer in place, the circuit still has high voltage, especially when you have a connection (probe!) with ground. The current probably won't be deadly, but a nasty jolt is stilll possible.So best option: take a bigger safety transformer, and keep both device under test and scope floating, that is:no ground connection nowhere.Verstuurd vanaf mijn SHIELD Tablet K1 met Tapatalk

author: Sky_hawk
author_rank: Contributor
timestamp: December 30, 2017, 07:12:15 pm
content: would it be safe to assume the motor is a 220v AC motor and the speed is controlled by PWM or dutycycle?

author: floobydust
author_rank: Super Contributor
timestamp: December 30, 2017, 07:20:11 pm
content: If you are not sure about technique on servicing live mains-powered equipment, let us know.Safety is first and there are checks to not get electrocuted or blow up test equipment or your pet cat.An isolation transformer is best but many do not have one, and take the great risk running it raw while doing measurements. I would not advise that.These are hard to troubleshoot because the entire board is hazardous live, and the MCU monitors motor current. So the motor can affect the board's operation.I would run the motor alone and see if current draw is not too high or problems with the brushes. You can use a series light bulb to run it at lower speeds, a mechanical load is best.Next I would measure MCU power across ZD1 with a multimeter and see if it is stable. TheHT46R006is a 3-5V part, probably ZD1 5.1V.The BTA312-800 triac is a high stress part, you could try change it. But we don't know the MCU's firmware, so still guess work involved.

author: Seekonk
author_rank: Super Contributor
timestamp: December 30, 2017, 07:29:49 pm
content: The motor is a universal commonly called a sewing machine motor, basically a DC brushed motor that will run on AC. This is a phase triggered triac circuit. Solder some wires onto the electrolytic, hook it up to a meter and stand back when you power it up. Repair is about thinking, not sticking your fingers in places. They used X2 capacitors for voltage dropping which is generally a bad idea as they will go bad over time. I suspect a power problem if it is flashing and then quits. Jumpering the triac with a lamp will likely get you some movement without setting it into high speed. That will test the motor.

author: SeanB
author_rank: Super Contributor
timestamp: December 30, 2017, 08:14:51 pm
content: My bet is C11, C3 and the little electrolytic by ZD1 are low in value, and a shotgun approach of replacing them with identical parts will fix it. Very common for the capacitors in a capacitive dropper to fail as open circuit from mains transients, and the electrolytic was likely cooked nicely by the warm running zener diode next to it. Change them all first then try it out, likely it will work fine again.Otherwise remove them and check capacitance, and all will have low capacitance and the electrolytic high ESR as well.

author: Sky_hawk
author_rank: Contributor
timestamp: December 30, 2017, 08:35:38 pm
content: > floobydust: ..not sure about technique on servicing live mains-powered equipment...well, I've repaired TVs, laptops/computers and 12v/5v devices (routers, etc, you name it), they are all 'live mains powered'. But usually they step it down a notch with some kind of transformer.(or as with Tv's the problem is more obvious by optical inspection)In this case, I don't have much to go on.Yea, want to buy my own isolated Variac, I used to lend it, because I so rarely need it, I don't repair that much anymore, but its so handy to have available.Any recommendations on ebay or otherwise on that?I've measured the triac, using this method:http://www.completepowerelectronics.com/how-to-test-triac-with-multimeter/it seems ok. the BTA312 was not available to me locally, so I will order it if I get nowhere.>floobydust/Seekonk: check 1: run the motor alone with a series lightbulb>floobydust: check 2: measure MCU power across ZD1>Seekonk/SeanB: I have a 'The blue ESR meter', so I can check the ESR on the caps and capacitance with a multimeter.Seekonk: which ones are 'X2' caps? I'm not familiar with that term?I will do some measurements and lets see where that gets me.Otherwise I will replace the caps and zener as SeanB said.

author: Sky_hawk
author_rank: Contributor
timestamp: December 30, 2017, 09:22:22 pm
content: To answer my own question about X2 caps:http://www.nxtbook.com/nxtbooks/hearst/ep1114/index.php#/20not sure if it is a clear article, but I think the gist of it is:- its a IEC 60664 installation category II general purpose capacitorto me this means approximately: its a capacitor that can handle peak pulses better then 'normal' caps. or as the article calls out "These are capacitorsthat pass safety approvals for use in ac-line EMI filters."

author: Sky_hawk
author_rank: Contributor
timestamp: December 30, 2017, 09:39:03 pm
content: caps measurements:0,33uf/ 310v has an ESR of 27 ohm, that seems low to me, but not extremely low?0,15uf/ 310v has an ESR of high (immeasurably high)I think: replace both.