[SOLVED]KitchenAid blender repair - Page 1
Forum section: electronics / repair

author: flywheelz
author_rank: Regular Contributor
timestamp: July 20, 2016, 02:17:50 am
content: Hi,I have a KitchenAid blender that stopped working. When plugged in, the pair of resistors get very hot to touch, so I figured there must be a short somewhere. I pulled the smd melf diodes that are part of the bridge and the one connected to the resistors showing 1.6v in reverse with .580v forward voltage drop. The other three diodes are good. They are 5mm by 2.5mm size with printed "V928 (diode symbol) 7". I can't find what spec they are. Looking at component side, there is 24v DC relay so is that minimum voltage diodes should be? They are using pic16f676 for control on the input board.What replacement diode should I get?Thank you.

author: singapol
author_rank: Frequent Contributor
timestamp: July 20, 2016, 05:19:49 am
content: You should trace what each of these diodes are connected to so as to have an idea. I tried searching V,V928 and 7 but no luck but there is a listing on page 52 of this smd databook. It's over 1000 pages! It list as a 13V zener diode 3 watt rating but the case style does not tally at the end of the book drawings. Part of a bridge?1060 pages:smd databookhttp://caxapa.ru/thumbs/588912/smd-codes.pdfSee page 52: For header info of each column. Good luck.Marking Mfg. Part928A 1SMB5928A Z-diode 13V±10%, Izt=28,8mA, Zzt=7?, 3W DO-214AA 1d 1a Eic928B 1PMT5928B Z-diode 13V±5%, 115mA, 2,5W DO-216AA 4d Mot928B 1SMB5928 Z-diode 13V±5%, Izt=28,8mA, Zzt=7?, 1.5W DO-214AA 1d 1a Pjt928B 1SMB5928B Z-diode 13V±5%, Izt=28,8mA, Zzt=7?, 3W 403A-03 3d 1s Ons

author: flywheelz
author_rank: Regular Contributor
timestamp: August 01, 2016, 07:33:27 pm
content: singapol, thank you for your research. After studying the circuit as you recommended, I think the bad diode is 1N400x, I went ahead and replaced it with 1N4007 1000v diode. I also found a bad "W2A 96" PNP transistor that activates the relay, I have not found a replacement yet.But I have another question.Is it normal for the R1 and R2 pair to get too hot to keep your finger on them only when zener attached?At the C1 I get 86v. Across the ZD1 zener diode I get 5.2v, if I remove the zener then the resistors are slightly warm.Can anyone help figure this out.Thanks.Update: The resistors measured 3.4k so they are actually 6.8k.

author: flywheelz
author_rank: Regular Contributor
timestamp: August 02, 2016, 05:04:37 am
content: I tried my best to trace the main pcb and here is what I came up with.I get 30v across ZD1 and C9, 5.1v across ZD2, 0.6v across C2. D8 was bad which I replaced with 1N4007 but unit still not responding.I am trying to figure out WHY R12 and R23 get super hotCould someone please give me some pointers.Thank you.edit: schematic below

author: bitwelder
author_rank: Super Contributor
timestamp: August 02, 2016, 05:24:18 am
content: Did you measure the voltage across R12//R13 ? Through Ohm's law you can then find the current going through them (and also the power dissipated).Probably taking voltage measurements also at other points (e.g. across the electrolytic) could help you find which sections of the circuit are likely not running according to the design.

author: flywheelz
author_rank: Regular Contributor
timestamp: August 02, 2016, 05:36:04 pm
content: I get 40v DC across those pair of resistors r12/r23 that get hot. The resistance of both is 3.4k ohms. So is that 40/3400=0.012amps? And about 0.470 watts (40*40/3400)? On paper to me these numbers look small but I can't keep my finger on these things. In about 5 minutes they get to 154 degrees FAcross C9 I get 30v. Across C2 I get 0.6v.I am stumped

author: oldway
author_rank: Super Contributor
timestamp: August 02, 2016, 08:09:17 pm
content: Use a true rms ac+dc multimeter if you want to measure the right value.

author: helius
author_rank: Super Contributor
timestamp: August 02, 2016, 08:12:27 pm
content: It looks like the voltage supplies are negative polarity, half-wave rectified with voltage set by zeners.The drawing of Q3 doesn't make sense, though: Besides being upside down to the conventional PNP orientation, it's drawn with the collector on neutral. The -30V supply on A2 is always negative with respect to neutral, so current can't flow into the transistor this way.For excessive current to flow through R12//R23, the reservoir C9 has to get drained faster than designed. What are the possibilities?C9 is leaking charge.C9 capacitance is reduced.Reverse leakage on D10, ZD1 or Q3.R29 too low value.

author: flywheelz
author_rank: Regular Contributor
timestamp: August 03, 2016, 05:15:59 am
content: helius, this is my first attempt on drawing a schematic. I think this is how Q3 is placed on PCB, pic in #1. Also the mains input was swapped wrong. I have updated the schematic, does it look correct now?I will try to do more testing on parts you listed and report back what I find.error in Q6, corrected schematic few posts down.

author: helius
author_rank: Super Contributor
timestamp: August 03, 2016, 06:44:31 am
content: Are you sure your changes are correct? They mean that nearly every part of the unit is live when turned off, including the motor. While that's possible, it's not normal to design something that way. I think it would also be undesirable for RFI reasons to have the whole assembly tied to line as a common point.

author: flywheelz
author_rank: Regular Contributor
timestamp: August 03, 2016, 03:57:05 pm
content: The Hot/Neutral are correct now. Here is a pic of PCB as mains come in. The unit is Live as soon as you plug it in. I think they use Q3 to close relay which supplies neutral to motor and then they PWM Q2 that activates the TRIAC that supplies the HOT.helius, could you look at C2 470uF cap on schem above, how/when is it charged? I am getting about 0.6 volts. Should it be charged to 5v as soon as power applied or only when motor running? I do get 5.1v across ZD2. Then there is ZD3 with 5231B stamped on it which also seems like 5.1v zener, whats its funnction?Here is the beast I am working on, KitchenAid Artisan Blender KSB560.

author: helius
author_rank: Super Contributor
timestamp: August 03, 2016, 06:35:12 pm
content: C2 only gets charged when the relay is closed. Whichever way the line and neutral come in, there is a common node that is effectively 0V (on the + side of C9 and C2, and the emitter of Q3, and a lot of other things), and a switched AC that is +/- 170 Vp relative to that (at R12//R23). Only the negative-going half cycles charge up C9, with ZD1 limiting the peak to -30V, and R12//R23 limiting the current into that node.When the relay closes, the switched AC enters R1. The network of C11, C2, D11, and D9 form a single-stage Cockroft-Walton multiplier, generating a negative voltage on C2 that is limited by ZD3. The multiplier can generate up to -340 VDC, but its voltage is clamped when it reaches the Zener voltage of ZD3.

author: flywheelz
author_rank: Regular Contributor
timestamp: August 03, 2016, 08:22:04 pm
content: Thank you for the in-dept explanationSo far I replaced D8, D10 but R12 and R23 still get hot too touch. Maybe its designed lto reach 155F. While unsoldering Q3, I shorted Base and Emitter with the tip of the iron which caused a spark. I forgot to discharge the capSo now I need to find a replacement PNP transistor as this one shorted on all pins.Here is something interesting. I used a screw driver and shorted the E and C of Q3, then pressed and held in one of the speed buttons on the keypad, applied power and the blender spins and the indicator light comes on. All 5 speeds work. So the Pic16f676 chip powers up and least its doing PWM fine.Maybe that ZD2 is bad and when sending neutral through the relay for ZD3, the C2 charges up and powers the micro.Update:It also works like this. Plug in the power, press and let go one of speed selection buttons, then short out C and E of Q3 and the blender comes on. I do see 5.16v on pins 2 & 3 on keypad connector as soon as I plug in power cord.

author: helius
author_rank: Super Contributor
timestamp: August 03, 2016, 09:07:14 pm
content: Quote from: flywheelz on August 03, 2016, 08:22:04 pmMaybe. From the schematic, it looks like the pinout of the panel connector is:1&2: standby -5.1V3: Ground4: AC frequency input?5: temperature input (bimetallic strip?)6: active -5V7: PWM out8: relay controlOne question is how bootstrapping the relay is supposed to work. The base of Q3 needs to be pulled negative to allow current through the relay coil. This is done by Q6, but before the relay closes, there is no power on pin 6. The emitter of Q6 also connects to pin 8, but the base will only turn on if pin 8 is positive with respect to... pin 8. Maybe there is an internal connection on the panel board that makes it possible.

author: flywheelz
author_rank: Regular Contributor
timestamp: August 05, 2016, 11:24:46 pm
content: Quote from: helius on August 03, 2016, 09:07:14 pmFirst of all thank you so much for your participationI have learned so much about electronics in past few weeks.It did not make sense to you because of me making errors in tracing out the pcb...there might be more.The pinout I believe is like this. I have verified 1, 2, and 3.1&2: Ground3: Standby 5.1V4: AC frequency input?5: Hall Effect sensor6: Active Ground from Main pcb??7: PWM out8: relay controlI have removed ZD2 and put ZD3 in its place. At pin 3 read +5v and pin 2 as ground. But R12/R23 still get to 250F. The only thing left is is C9 and ZD1. However, a while back in post #3, I removed everything except those pair of resistors, a diode, the 220uF main cap, the 4.6k current limiting resistor and the 5v zener. The R12/R23 still got hot. This only leaves the cap that I have not replaced. I checked its capacitance using UT210E and it reads .265mF. If I remove the zener then the R12/R23 are barely warm.Question, could it be normal for R12/R23 to run hot at 150F in this circuit with 30v zener and then 5v zener clamping down? Perhaps with original D8 that showed 1.6v in reverse caused the R12/R23 to get super hot and that's what cause discoloration to PCB? Now with new D8 its ok?Next I need to probe pin 8 and see if its being pulled high, then check if Q6 switches on.I put together the panel schematic now. If something don't make sense, plz tell me, I will retrace.Does this thinking sound valid about the workings of the Panel circuit?The PIC powers up and sends +5v to all the function switches and all the LEDs.Pressing a function sends +5v to pin 8 that drives Q6 on main pcb, also Q4 that grounds pin 13 of Micro.PIC pulls pin 3 high which switches Q5 that gives Active ground from main PCB for LEDs??Pressing OFF shorts out +5v and Active ground from main PCB which causes PIC to resetPanelMain

author: helius
author_rank: Super Contributor
timestamp: August 05, 2016, 11:41:03 pm
content: It's hard to test capacitors for leakage with the capacitance range of general DMMs. Using a lab power supply works better, since you can hook the cap up to the power supply through the milliamp range of your DMM, and see whether the current quickly goes to zero. If C9 is leaky, it would explain why too much current is flowing through those hot resistors.I don't think they're designed to be boiling hot, that would present a fire hazard.

author: flywheelz
author_rank: Regular Contributor
timestamp: August 06, 2016, 05:21:03 am
content: Tested for leakage using Actron analog meter on 10mA scale. Applied 28 volts. Current overshoots to about 12mA.I also used mySM8124on the cap and its reading 644 milliohms.Safe to say I need a new cap?https://youtu.be/VncSk1B83t8

author: flywheelz
author_rank: Regular Contributor
timestamp: October 26, 2016, 12:09:19 am
content: I have finally got the blender to function.I've highlighted in red on the schematic the four things that were bad which I've replaced:1N4007 DiodePNP transistorNPN transistor56resistorI've highlighted in green on the schematic the four things which measured good but I've replaced them anyway.30v zener diode5v zener diode220 uF cap470 uF capThe blender runs good and goes through all the speeds. There is one thing that still bothers me is the fact that the pair of 6.8k resistor get too hot to keep my finger on them. They are around 57C. Also the 30v zener gets hot but maybe around 45C.I put my UT210E in ACA mode around 110VAC Live wire and its showing 0.013A in Standby. Is that 110V * 0.013A = 1.43 Watts being burned up with it plugged in? Not very efficient. With motor unplugged, selecting a speed causes relay to close and at this state UT210E showing 0.097A being used. I don't know what to make off these numbers.Perhaps someone with skills can do some math of what it takes to take 110vAC and convert to 30vDC and 5vDC, using: 1 1N4007 diode, 3.4k resistor, 30v zener, 4.6k resistor, and 5v zener. Could a simulator do it such as LTSpice?I will be plugging in the blender only when I use it and cautiously at that.Thanks to everyone that responded and shared your advice.