Coffee grinder failing motor ok but electronics ?? - Page 1
Forum section: electronics / repair

author: wedgetail
author_rank: Contributor
timestamp: March 16, 2022, 11:02:00 am
content: My wife's favorite grinder failed with very intermittent operation and grinder blade only seem to rotate 180 degrees.I am trying to find a schematic to give information on the 8 legged IC. I will attach photos front and back of pcb.Coffee mill made in ChinaPCML2013T-7502V1.2 2014-05-19There is a BY134 600V on the corner of the boardThere is a slider switch regulating speed I guess to grind from coarse to fine in 5 stepsIt took long time to figure out how to dismantle the unit, being retired I took it as a challenge. Dremmel with cutting wheel allowed me to remove the stainless steel wrap. Then I realised how I possibly should have done the disassembly. It was nothing like a number of Utube repairs of slightly simpler grinders that seemed very easy.I am hoping somebody may recognise the pcb and perhaps already has found a diagram.

author: kripton2035
author_rank: Super Contributor
timestamp: March 16, 2022, 11:34:51 am
content: simple pcb so easy to reverse the schematic yourself...

author: Psi
author_rank: Super Contributor
timestamp: March 16, 2022, 11:40:49 am
content: Intermittent operation could easy be a simple dry solder joint.

author: Gyro
author_rank: Super Contributor
timestamp: March 16, 2022, 01:04:58 pm
content: Is the grinder left plugged in the whole time?It looks as if it has a capacitive dropper power supply. It could be that the dropper capacitor (the Yellow one from the look of it) has reduced in value over time due to self-healing on mains transients.If you have the means to measure capacitance then it is worth checking. If not, a suitably X2 rated replacement may fix the problem.The above is a lot less likely if the grinder is only plugged in when it is used.

author: TheMG
author_rank: Frequent Contributor
timestamp: March 16, 2022, 01:18:18 pm
content: How do you know the motor is ok did you put power directly to the motor to test it?It is very likely a universal motor and this type of intermittent operation could very well be caused by worn down brushes or commutator.

author: fzabkar
author_rank: Super Contributor
timestamp: March 16, 2022, 05:19:31 pm
content: Here is one dry joint.

author: wedgetail
author_rank: Contributor
timestamp: March 17, 2022, 12:44:52 am
content: Thank you all for posting. Just a few comments about what I have done so far.1.. To get the motor out I had to cut the two wires [later I realised it is possible to remove electronics and motor connecte]. Then very carefully applied mains via a 60W bulb [just in case] it ran at a guess at 60-80% full speed then removed the bulb and sounded just right at its usual very energetic rpm. Let it run for a minute or two.2.. No it is not connected all the time, power lead removed as mains socket need for other appliances. If lead not removed power is turned off at the wall outlet.3.. Dry joint, my first look was the soldering looks better than on the photos. When I inspected the suggested specific dry joint with my normal glasses and bench top magnifier I still thought it looked ok. I did however put on extra strong glasses and there was a clear but thin darker ring around the the wire. The wire is the leg of the yellow cap.Trying to push on the cap I noticed the leg moved a fraction, ring got more obvious. So yes it is a dry joint no doubt. The motor torque certainly strong enough to shake the cap and wire.My wife did say that if she shook the grinder it would run for a little while initially. When I asked for a demonstration it did not work for her but when I tried occasionally it would just kick alive.Now it make sense even what has happened. My wife has a small hard plastic ball on the end of stick she likes to tapp the grinder firmly to shake the grounds free. That would have put stress on the capacitor the way it is mounted. I will resolder later today and check.Will report back.

author: fzabkar
author_rank: Super Contributor
timestamp: March 17, 2022, 01:08:54 am
content: If you don't measure a voltage on the zener diode, then that would support the hypothesis that the cap is intermittent. The zener would be the DC voltage regulator for the IC's Vcc.

author: amyk
author_rank: Super Contributor
timestamp: March 17, 2022, 03:37:37 am
content: What an odd choice of font for a PCB:https://en.wikipedia.org/wiki/ITC_Zapf_ChanceryLooks like a triac-based speed control?

author: wedgetail
author_rank: Contributor
timestamp: March 17, 2022, 10:57:37 pm
content: Yes the I thought this was a very odd font artistic contribution by the originator perhaps.I re soldered the the cap and set up a test connection and it works. I measured the Zener voltage to 3.1 volt which surprised me. If the IC works quite well at that voltage then I guess the tiny transistor probably does the level translation to the Triac BT134. This is guessing on my part.Suddenly things getting a bit hectic here. I was going to tidy up an posting some more photos and get all the details off the PCB before assembling again.In principle the controller a bit likehttps://www.pinterest.com.au/pin/594756694523457473/A reminder to myself trying to find a schematic a bit closerMore to come

author: wedgetail
author_rank: Contributor
timestamp: March 20, 2022, 12:49:01 am
content: After very careful assembling and testing my wife tested it today. She is very happy though the did not reconised it, she thought it looked odd I think I have worked out a way to put the cap back on, not quite as neat but it will have the nice st st look to it.I have taken quite a number of photos so I have enough info to try and reverse engineer the circuit diagram. When may be a question but I am also trying to use KiCAD version 6 it will be a good exercise.I am going to add a few pictures showing it all finished. Well not that successful, I don't work on the ceiling but for some reason the pictures seem to have there own mind about what should be up or down. Sorry about that must have missed something. The look fine showing orientation up properly.

author: fzabkar
author_rank: Super Contributor
timestamp: March 20, 2022, 02:20:50 am
content: Quote from: wedgetail on March 17, 2022, 10:57:37 pmI see a "3A" at the end of the part number. Could that be a 1N4733A, in which case it would be 5.1V, 1W zener.https://www.jaycar.com.au/medias/sys_master/images/images/9600181665822/ZR1398-dataSheetMain.pdfI would check the resistance of the large resistor in series with the mains capacitor. Also check the capacitor.

author: wedgetail
author_rank: Contributor
timestamp: March 20, 2022, 11:47:18 pm
content: fzabkarYou raise some interesting points, I was torn between taking some more time and getting back to fixing slow leaking water source in the bathroom. Since seems to be working fine I am going to leave for the time being. Must admit if the Zener voltage had been more than 6V I would have been happier,How should that big resistor be read, to me there seem to be too many colour bandscompared with the resistor near Zener.?

author: fzabkar
author_rank: Super Contributor
timestamp: March 21, 2022, 02:41:27 pm
content: What do they measure on your meter?