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Thread: Kenwood Food Mixer - No Power - Mint Cond - Dead.

  1. #121
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    I should add that MCLR resets the pic when it's at Vss (logic 0) but it's also VPP (programming voltage) when raised to about 12V above Vss. It's does nothing when held close to Vdd (5V). Not sure what sort of damage may occur if an alternating voltage was applied to MCLR/VPP relative to Vss, maybe the high impedance of the DMM saved it.

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  • #122
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    Quote Originally Posted by Skepticist View Post
    Best to not reference anything on the PIC board to actual earth. Neutral and earth are connected together at your switch board and the only components getting that 'true' neutral (=earth) are the motor and the Triac when the triac is 'off'. Neutral has been 'fiddled' to create the -5V which is Vss and the common reference (ground) for the electronics but it's not the same as the mains earth which is connected to the metal case and motor housing.
    The 'earth' of the case is at a different voltage to Vss (I'm thinking it's at least 5V higher than Vss, probably a lot more than that).

    Hope that makes sense - all DC voltages on the PIC board must be referenced to Vss (the yellow wire from the power supply).
    Higher than 5V would raise red flags with me in respect to the house wiring. Even while using two 2kW portable electric heaters on one powerpoint circuit the difference between earth and neutral within a standard sized house should not exceed 5V which would relate to an overall voltage drop of 10V or roughly 200W power loss.
    I am currently measuring 0.3V
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  • #123
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    Quote Originally Posted by GT250 View Post
    Nomeat: #117. As mentioned, have replaced R103. But will test the Ω to Vdd and pin 4 tomorrow. Nomeat, I really appreciate your time in this.
    I'll do a more in depth look at your reply to your post and yes, it's possible that I have mixed things up. Just have lot going on at the moment in my life and sometimes I try and get away by 'fixing up' silly things.
    Don't forget the wire bridge that connects the 10k to Vdd, re-solder that too.

    If that all gives no result, it would be good if you could repeat that test where your probe got the motor running smooth. I can't see no harm as the DMM in voltage mode is like a 10MΩ resistor to anything you probe on the circuit and therefore should not matter if it is against earth or neutral. Maybe you did get it mixed up and held it against pin 2 or pin 3 instead of pin 4.
    In fact pin 2 could make actually sense. I would not try pin 3 though. It is unused and has no resistor to protect it, although PICs should have internal protection.

    However if you held it at pin 13 I could expect a major difference. Maybe that cap that Skep marked as C4 is faulty and the capacity of your DMM leads simulated it (a wild guess but makes a bit more sense right now).
    Last edited by Uncle Fester; 25-02-19 at 10:27 PM.
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  • #124
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    Maybe I'm confusing myself but the reasoning is that Vss (-5V DC) is generated from neutral in relation to Active (0V DC) so the entire circuit is floating at or very close to 240V RMS above true neutral which is connected to mains earth. Connecting any part of that PIC circuit to mains earth would apply alternating voltage to it EG a 10MOhm impedance (via DMM) might only introduce microamps but -ve voltages on MCLR can cause the PIC to lock up according to the datasheet.

    Could be that microamps is not enough to cause much harm
    I could be wrong too

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  • #125
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    The confusion is that it runs when he connects it to earth so it is not in a latched up state at that time. I was already thinking about a low voltage programming mode that could be mistakenly enabled and pin 13 or pin 10(when not selected) are high Z inputs which could cause latch up.
    There are no pull ups for the speed selector and an internal pull up in the PIC could be faulty but his probe could have substituted it.
    ATM we have no clue where he might have really probed it until he can repeat this.
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  • #126
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    The speed selector pullups are that resistor array next to the 7 pin plug
    I did wonder why internal pullups weren't used though.

    Momentary contact between pins 4 & 5 should cause a reset if all is well

    Edit: the reason for the external pullups is RB0 & RB7 (AC and tacho signals) - internal pullups can only be applied to all of port B, not selectively
    Last edited by Skepticist; 25-02-19 at 11:05 PM.

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  • #127
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    skepto and no meat both are trying to remove earth volts from the possibility of getting at the pic ????? correct to make the fault finding easier on several occasions i have suggested you use a isolating transformer but seems to be ignored this is the only way that this fault can be found , look at the theory .No earth no problem just 240v ac earth floating to where it wants to be ( at nothing) no danger of bang or zap , Surely some one must have one gt could borrow to this end , can be found in just about every junkshop etc plumbers builders electricians painters so it goes on rcd have taken over but cannot replace the one to one ratio of a transformer cheers don

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    Ok. Sorry for no real info in this post. Just got back from the oldies. Been working since 5:00am

    nomeat: #123 I'm pretty sure I had it on pin 4. So tomorrow I'll pop a soldered post to it and do a real power slide test.

    I have refreshed the JP106 link.

    What is the value of this SMD C104? I can't recall if it has been mentioned. I'll pick up a 'normal' valued Cap from Jaycar if I can tomorrow, or if they do sell SMD's.

    Skepticist #126 Should I reset the PIC by crossing pins 4&5 - I do this with 240vac disconnected or with a 5vdc supply to 5 & 14?

    I'll do the PIC testing with the power slider with the DMM (turned off), on pin 4. And I'll replace C104.

    After that, unless something else stands out, I might just get the replacement board for $38. I understand that you guys out there, you're geniuses compared to me and both these boards have probably little components (in the greater scheme of things), to make a motor work with a 'variable' power selector.

  • #129
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    That C104 value is not critical but probably around 0.1uF - all it does is absorb rapid rising noise pulses on the power supply so a ceramic would do there as long as its diameter is less than 0.3" (the space between the rows of PIC pins) - standard 50V one or even a 25V would be ok as it's across the 5V supply.

    Reset requires power on (240V) - all it does is restart the PIC exactly the same as a power-on start so you'd be looking at how it affects the erratic running when reset is triggered. Just use an insulated screw driver and touch between pins 4 & 5 and you'll cause a reset (it's very fast so just a momentary touch is all you need).

    Not familiar with that fluke DMM but do make sure you've selected the highest voltage range before touching anything live with it even though it's switched off to be certain it's impedance is maxed (for the DMM and your own well being)

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  • #130
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    I have been following this thread with a mixture of interest and (some) trepidation.

    Hinekadon has a very VALID point when he refers to the use of an isolating transformer.

    Let me explain...

    A novice is attempting, under direction, to fault find unearthed equipment, which is at MAINS POTENTIAL!

    In my view, and I have over fifty years experience in the electronics industry, including management of a service department and the supervision of novices, it is absolutely mandatory, when attempting to service or fault-find unearthed, transformerless equipment operating at mains potential, that an appropriate mains isolating transformer be used for the protection of servicing personnel.

    Imagine how members would feel if the OP was to be electrocuted in the course of following instruction from members here, regardless of how well intentioned. (And I haven't even mentioned the legal ramifications!).

    I commend Skepticist for his post #6 in which he adds the warning, "Troubleshooting this thing live is extremely hazardous with mains voltage all over the place so no amount of precautions is too much."

    PLEASE USE A MAINS-ISOLATING TRANSFORMER!

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  • #131
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    As Skepticist pointed out and I fully agree:

    Quote Originally Posted by Skepticist View Post
    An isolating transformer won't stop you getting across the mains between active and neutral with the added disadvantage that an RCD wouldn't be protecting you when the earth is isolated (any A-N fault will not create an imbalance in that case so short circuit protection is about all you'll have)
    I also suggested in an earlier post to test the function of the RCD which may have gone unnoticed.

    In any case I suggest to abstain from any further measurements and either order a replacement board and hope for the best or place it thoughtfully in the bin.

    This will be my last post in this thread.
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  • #132
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    Is it not obvious that GT250 has no access to an isolation transformer !
    If he had he would be using it.
    I have helped him in the past and he very capable and careful.
    Concentrate on the fault.

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    This device is known as a dog by owners. Usually rebuild is triac &caps maybe res. I think motor is. suss as spark on comm. Check brushes,comm, short turns
    motor windings.

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    Quote Originally Posted by Tom Mix View Post
    This device is known as a dog by owners. Usually rebuild is triac &caps maybe res. I think motor is. suss as spark on comm. Check brushes,comm, short turns
    motor windings.
    Read the thread. The motor runs perfectly when the Triac is forced on.
    The initial problem of the failed capacitor has been dealt with.
    He is probably looking at something that has occurred during fault finding.

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  • #135
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    Quote Originally Posted by Tom Mix View Post
    This device is known as a dog by owners................
    I own one, and it is certainly NOT a dog. For certain you could mix concrete in one and it wouldn't bat an eyelid, but they are still the best mixer you can buy, and despite a minor problem, are as solid and reliable as they come.

    Our Mixer exhibited the same symptoms many years back and it was, (then) a relatively cheap and easy fix. The only difference here is the fact that GT250 is attempting to repair it HIMSELF! WITHOUT the backup of a good circuit diagram or someone intimately familiar with the machine. Something and authorised repairer would have! The fact that there is a paucity of information of this issue on the 'net is indicative of the quality of the product. i.e. It is not as common as a sunbeam or tiffany.
    I'm out of my mind, but feel free to leave a message...

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    I found site on web by putting km270_tells about probs with this model from owners. Yes other models are great.

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    Quote Originally Posted by Tom Mix View Post
    I found site on web by putting km270_tells about probs with this model from owners. Yes other models are great.
    I see what you mean. The problem might well be just that model. Mine will be 40 years old this Christmas. Ours is the A901.
    I'm out of my mind, but feel free to leave a message...

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    Quote Originally Posted by lsemmens View Post
    I see what you mean. The problem might well be just that model. Mine will be 40 years old this Christmas. Ours is the A901.
    I searched it too. I notice there's more that one shape in that model so I suppose they are sub models or revisions. I saw nothing with these symptoms though.

  • #139
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    Hi People,
    Jeeze, I can't keep clicking on all the thanks, I'd be here all day. So my sincere thankyou's to all

    Ok. New board has arrived:

    SO BEFORE I POP IT IN!!!! - Are there any measurements I can do with the old board out (isolated), and test various things comparing side by side to see what is different between the 'old' and the 'new' with a my Fluke 289, to see what component might have been at fault?

    Or do you want me to just stick it in and see if it works?

    As nomeat has inferred, how much more testing can I do at my level? Some of the terminology has gone way over my head. However, I try and learn a bit here and there from you guys, and it is very much appreciated

    The 'New' board looks to be identical to the old, from my brief looking. Bit more info on the PIC, still says V0.31 though.




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  • #140
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    just kick it in the guts and youre away any more testing is just a waste of time as we think its the pic cheers don

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