Author Topic: Board repair : HOBART " hood " type dish washer  (Read 2804 times)

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Offline coromonadalix

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Re: Board repair : HOBART " hood " type dish washer
« Reply #25 on: September 22, 2018, 02:05:40 am »
maybe this diode was an tvs/tds protection device, it could have been triggered by an surge in the signal line ???

We use this at my job, theses puppy can short pretty bad, an big zero ohms, burn /fuse pcb traces etc ...

Happy you got it working
 

Online tautech

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Re: Board repair : HOBART " hood " type dish washer
« Reply #26 on: September 22, 2018, 02:09:06 am »
The MELF diode could be a 1N4148 and if so you can just bend the leads 'gull-wing' style and use a leaded one.

Congrats Vince.  :clap:
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Offline Vince

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Re: Board repair : HOBART " hood " type dish washer
« Reply #27 on: September 22, 2018, 02:22:17 am »
Thanks guys !  :)

I think I will just buy "proper" SMD replacement components, since I needed to place an order anyhow, for some other stuff I need, like a few UHF connectors to start putting my Tek 317 scope back together, at loooong last.

Will keep you updated once it's all fitted back into the dish-washer... hopefully that will work just fine  8)

 

Online james_s

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Re: Board repair : HOBART " hood " type dish washer
« Reply #28 on: September 22, 2018, 04:52:43 am »
Almost !  ;)  Not 0.8mV but 80mV (60mV typical) on the 12V rail, which we don't care too much about as it only powers the relay coils.

Apparently I either can't read, can't type, or was trying to do too many things at once.

Looks like you got it working anyway though, nice job!
 

Offline drussell

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Re: Board repair : HOBART " hood " type dish washer
« Reply #29 on: September 22, 2018, 01:33:48 pm »
Yeah, it is amazing how often a "€1500" board can be repaired with a few pennies worth of parts.  The problem is spending the hours required to find the problem. :)

I, like you, though, find it a fun and challenging exercise to things like that.  :)

Congrats on finding the issue!

I'm currently fixing a domestic dishwasher control board for a friend.  It is "only" a $330 board (which is still ridiculous) that had the area around the heater relays blown to smithereens, obviously due to quite the arc festival since there was black soot blasted everywhere and severely charred PCB, melted plastic relay housings and board cover all over the place. 

First guess was shorted heating element blowing the traces, then arcing out, but the heater measures fine, even tried powering it for a while to be sure it wasn't a heat related issue that would cause it to short out while hot.  It looks like it must have just randomly arced over in there into a plasma ball, despite having isolation slots between the traces and connector pins.  LOL

I have a feeling if I just cut off that corner of the board and mount a good heavy duty relay (or 2) off-board and wire it all in (even add a fuse for the heater, wouldn't that be novel) that the control board will work just fine.  It is interesting that they used two relays, one on the hot side, one on the neutral as a redundancy in case a relay welds itself on and leaves the heating element running.  They must have added that in this newer generation since the previous generation Whirlpool models, which I have worked on many, (the ones with the tiny TRIACs that always blow up from things like the defective soap dispenser solenoids shorting out and the capacitors that puff up and quit filtering) only used one relay on the heater.  They must have had issues and added that for a reason.  At least those ones had a main fuse instead of relying only on the breaker upstream to limit damage.  This newer one has a tiny SMT fuse for the motor and a tiny SMT fuse for the board itself (including all the TRIAC and small relay loads direct from the board) but that is it.  Nothing for the heater except the circuit's breaker in the panel.

At least you're working on a Hobart, something more industrial.  This modern consumer stuff (especially since Whirlpool bought all the good ol' brands and gutted the quality to the level of "steaming pile") is junk.  This dishwasher is only 5 years old for goodness sake.  </rant>  :)
 

Offline cvanc

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Re: Board repair : HOBART " hood " type dish washer
« Reply #30 on: September 22, 2018, 01:39:03 pm »
Good job Vince!  Here ya go
 

Offline Vince

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Re: Board repair : HOBART " hood " type dish washer
« Reply #31 on: September 22, 2018, 02:05:24 pm »
Yeah something like that, I must admit  !  :-DD

 

Offline Vince

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Re: Board repair : HOBART " hood " type dish washer
« Reply #32 on: September 22, 2018, 02:23:05 pm »
I'm currently fixing a domestic dishwasher control board for a friend. 

Ah, I am not alone fixing dish-washers, thanks for the head up !  ;D

Looks like it is in good hands, you have it all figured out, should be back to working condition in no time now  8)

Quote
At least you're working on a Hobart, something more industrial.  This modern consumer stuff (especially since Whirlpool bought all the good ol' brands and gutted the quality to the level of "steaming pile") is junk.  This dishwasher is only 5 years old for goodness sake.  </rant>  :)

Yes this Hobart is much more pleasing/interesting to work on, than a consumer grade dish-washer, for sure... and a lot more durable, and worth spending time on it because it is so mega expensive to buy !  :o
It's already 13 years old, and with new diodes and new quality electrolytic filter caps, for 2 bucks worth of parts it will be good for another 10 years or more !  8)

Time, yes. I think I must have spent maybe 24 hours or so, on the bench, 80% of that was spent reverse engineering the thing though ! So if schematics and a half decent service manual were available, it would have taken an hour or two, not 24 !   :--

I don't regret any of these 24 hours though, since all this reverse engineering was 80% of the fun , most interesting. Managing to actually fix the board was just the icing on the cake !  8)
 

Offline drussell

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Re: Board repair : HOBART " hood " type dish washer
« Reply #33 on: September 22, 2018, 02:46:01 pm »
I don't regret any of these 24 hours though, since all this reverse engineering was 80% of the fun , most interesting. Managing to actually fix the board was just the icing on the cake !  8)

Indeed.  I couldn't have said it better myself.  :)

Most people don't understand why I would "waste my time" fixing something like that (especially since it's certainly not going to make any money for the time invested) but they just don't understand:)
 

Online james_s

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Re: Board repair : HOBART " hood " type dish washer
« Reply #34 on: September 23, 2018, 04:56:43 am »
Yeah, it is amazing how often a "€1500" board can be repaired with a few pennies worth of parts.  The problem is spending the hours required to find the problem. :)

I got my Maytag Neptune washing machine for free because it needed a new $450 motor control board. I touched up some solder joints on the board and had it working within 30 minutes of bringing it into the house for an investment of $0. That was 10 years ago and I'm still using it.
 

Offline cvanc

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Re: Board repair : HOBART " hood " type dish washer
« Reply #35 on: September 23, 2018, 08:31:15 am »
Yeah, it is amazing how often a "€1500" board can be repaired with a few pennies worth of parts.  The problem is spending the hours required to find the problem. :)

I got my Maytag Neptune washing machine for free because it needed a new $450 motor control board. I touched up some solder joints on the board and had it working within 30 minutes of bringing it into the house for an investment of $0. That was 10 years ago and I'm still using it.

I have on no less than 4 occasions taken in really nice flat screen TVs (1080p, WiFi, just a couple years old) that were given away free because they had stopped working.  In every case it involved replacing a couple electrolytics from my existing stock or reseating the little ribbon cables that connect to the panel.  With one exception they are all still with me and still working.  I love it when that happens  :-+
 

Offline Vince

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Re: Board repair : HOBART " hood " type dish washer
« Reply #36 on: September 23, 2018, 11:50:40 am »
Yeah, always feels good to rescue stuff, especially recent stuff that didn't deserve to be scraped at a young age...

I rescued my washing machine last year. I bought it new, just 9 years ago, for only 350 Euros. Entry level model (I don't need fancy stuff), but decent brand (Indesit), in a hope that it would last me more than a couple years. So last year when it started driving me nuts because it would stop its washing cycle in mid-air randomly... which is not fun, I was pissed to say the least.
The thing was 8 year old at the time then, so out of warranty of course, and too cheap to be worth paying a service tech to fix it for me.
Since it was still in excellent overall condition and worked just fine, other than it randomly stopping, I just could not get myself to scrap it.
So I had a go at it, nothing to lose after all.  It was fixed in 10 minutes. I pulled the front panel board out, and oh boy not much on it...

It was just the ON/OFF  SMD push button that was at fault. Well, not even the switch itself, but just the solder joints. The board is very thin/flimsy, and that push button is tiny too, and not much solder to hold it on the board. So after 8 years of actuation/flexing/bending, eventually the solder joints cracked. Some fresh solder and away I go, problem fixed. Been a year now, and still going strong !  :)


« Last Edit: September 23, 2018, 11:52:35 am by Vince »
 

Offline Vince

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Re: Board repair : HOBART " hood " type dish washer
« Reply #37 on: September 29, 2018, 11:58:04 am »
Hmmm.... trying to order new components to repair this thing is not as straight forward as  I thought !  >:(



SMD Electrolytic caps

Meant to replace all 3 electrolytic caps on the power supplies, as preventative maintenance since they are 13 years old now.
The big through-hole one on the 12V rail is straight forward as it's clearly labeled. However the two small SMD caps on the 5V rail, I can't decipher what's printed on them, hence don't know what to order :-/

See pictures of their tops, below, if anyone can help ?!  I pulled a datasheet for SMD electrolytic for Nichicon and Panasonic caps, and both have different coding systems, and neither corresponds to these caps. So, if every manufacturer makes their own system.... you basically are screwed, unless you know what brand these caps are... but how the hell would you figure this out just by looking at these caps....  :o

So I am in the dark....

One is marked "220" and the other "220C". Depending on manufucturer, "220" can either mean 220uF or 22uF. In my case both caps measure at 220uF so at least this bit is figured out.   The ' C ' suffix apparently often refers to a 16V voltage rating, which would make sense for 5 V rail with ample derating. However this is just a guess, and the other cap does not have any suffix to it.

Also, one of the caps is actually at least twice the size/volume as the other, so either for some reason tehy have a much different voltage rating and/or they don't have the specs... maybe because they have different roles ? The biggest one of the two is the one located on the main board, used in the DC-DC converter.
The other one is 6 meters worth of cable away, on the remote front panel board. So this is just local decoupling.

So what to do ? Just use a couple low ESR and high-quality long life puppies, with at least 16V voltage rating, and hope for the best ?
Would probably do it, but I still want to know how, if at all possible, one is supposed to decipher SMD electrolytic markings in general !  ???




MLCC caps

Now, as for the tiny MLCC cap on the input of the comparator... bad luck : I went to measure the value of it's twin brother on the other comparator (data line), and what do you know : this one too is faulty, open circuit ! So now I can'"t know what value to buy ! Argh !   :--

So what to do ?! I think I will need to buy a few different values (since I guess these tiny things must be dirt cheap, I can afford to buy lots of them) and experiment.  If anyone has a better course of action please do tell. My best shot is :

I think these tiny cap at the input of the comparator must be to reject HF crap to help clean the signal and get more reliable operation/switching. So You would want it to be as large as possible so as to reject as much stuff as possible. On the other hand, this caps does form a  low-pass filter, combined with the 10K resistor that's in-line with the signal, that's located far away from the comparator, by the header connector. So this means the cap needs to be small enough so that it does not distort the incoming signal too much. If it slows down the rising edge too much, I guess it could upset the comparator, cause jitter on its output... well worse case scenario I mean.
So my plan is to start with a small value, say 1nF, and work my way up progressively, until I start noticing signal degradation.

I guess we can do a quick back of the envelope calculation to get a ballpark figure ?!  The in-line resistor is 10K.
The shortest pulse we have to deal with, it of course the clock pulses, not the data bits. Data bits are 100us wide, clock pulse is half that (50% duty cycle), so only 50us.  So the RC time constant must be such that this still 50us pulse still looks about square and not like a sine...  ;D

So let's say we want the rising edge to settle within an amount of time "much less" than the pulse width, so as always the empirical order of magnitude, therefore 5us max. So three time constants must last no more than 5us say. So one RC is no more than 1,5us or so. 
R is 10K so C must be no more than around 150pF ?! That's not much....
But of course this does not take into account the effect of the feedback resistor, and I am not sure how it would jeopardize my little theory above...

OK. So it's clear I will have to just experiment with a few small values between 1nF or a bit small if MLCC are available in the sub nF range, will see... and up to say 10nF. Luckily I can observe the shape of the data and clock line so I can easily see what effect such or such value cap has, and go based on that...



MELF Diode


Then there is the diode ! Still can't believe it, but although Farnell lists like 50 different 1N4148, NONE of them are available in MELF package ! Eh ?!  :o
So I searched for ANY MELF diode... and surprise there too : a VERY limited number of diodes available and all "special" purpose : Zeners mostly.
I thought these MELF diodes were common place... not so much.

But that's alright, I can just buy a 1N4148 in whatever "normal" SMD package with gull wings, that should do.

Still, I am curious to know the reason for this MELF package existing at all ? What are its specific advantages over "regular" SMD packages, that command its use here and there ?!......





Thanks in advance for any help/comment/suggestions on any of the above !  ;D



« Last Edit: September 29, 2018, 12:05:41 pm by Vince »
 

Online tautech

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Re: Board repair : HOBART " hood " type dish washer
« Reply #38 on: September 29, 2018, 12:11:29 pm »
In the early days of SMD, IN4148 MELF diodes were common, not so much now....or even MELF resistors with their good wattage ratings....well better than similar sized TH size equivalents.

Such is the fun of repairing old equipment.  :rant:
My pile of old PCB's get raided often looking for components that are difficult to source today.


Smart guy like you Vince will work through these little inconveniences.  ;)
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Online tautech

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Re: Board repair : HOBART " hood " type dish washer
« Reply #39 on: September 29, 2018, 12:33:48 pm »
Oh and the caps, close to the DC-DC converter one will be low ESR while the other is best thought of as remote bulk capacitance so an ordinary cap is best there with any MLCC's close to IC's doing the decoupling duties.

You come across this capacitor configuration in test equipment too when the power rails travel onto various PCB's and each will have its own bulk cap on the same rail. Mainly used in conjunction with linear PSU's whereas modern design has even multiple DC-DC converters on a single PCB.
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Offline Vince

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Re: Board repair : HOBART " hood " type dish washer
« Reply #40 on: September 29, 2018, 05:46:16 pm »
Thanks for your input Tautech.

Other point I needed to clarify in order to make an informed choice about the MLCC caps, is all this "C0G" / NP0 / XYZ / YHT / KSH / LFO stuff they are categorized into. From some video on YT, EEVBLOG maybe, a while ago, I seem to remember that these MLCC caps came with significant issues, mostly the capacitance value that could change drastically, influenced by temperature and DC bias. And they also were very microphonic.  But  these vague recollections were not quite accurate enough to let me choose a particular MLCC type....

... but I found this nice page on Kemet's website, precisely meant to synthesize and clarify all this stuff.
Others might find it helpful too, so here it is :

https://ec.kemet.com/mlcc-dielectric-differences

So now I can make an informed decision...

So basically all these NPO/C0G etc stuff, is just a code to describe how much the capacitance varies with temperature, the main flaw of these caps it seems.
And this property is also tightly related to the dielectric material that's used.

If I understand their article and their graph, basically the NP0/C0G are basically extremely good, super stable vs temp, top notch.
Next best choice would be X7R, very flat as well. X5R if high temperature is not of concern.
Then all the others, Y5V, Z5U have an extremely poor response so best to just not buy them at all I think... unless you wanted to engineer an thermometer using these caps as the sensor ! LOL Hey why not, that would be "cool" in some weird twisted way !  ;D

So now it's now just a matter of price and availability of the NP0/C0OG versus the X7R / X5R .... let's see !

I like Kemet, lots of good info on their site... I also found from them, a 314 page long capacitor selection guide !  ;D
Then there is also this famous video on YT, that was pretty interesting too.  As always, best info comes from the people who actually know what they are talking about !  ;D 


« Last Edit: September 29, 2018, 05:49:20 pm by Vince »
 

Offline drussell

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Re: Board repair : HOBART " hood " type dish washer
« Reply #41 on: September 29, 2018, 05:59:54 pm »
o basically all these NPO/C0G etc stuff, is just a code to describe how much the capacitance varies with temperature, the main flaw of these caps it seems.

Yes, ceramics have always been available with a variety of temperature coefficients and it is a while it can be major "gotcha" to the designer if you don't know about it at all, you can use NP0/C0G to avoid that isue.  A larger problem (especially with the modern MLCC designs) is the voltage coefficient.  Many of the recent ones can end up with only a fraction of their original capacitance when there is a DC bias on them.  Be aware of that when comparing capacitors to choose for replacements.
 

Offline Vince

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Re: Board repair : HOBART " hood " type dish washer
« Reply #42 on: September 29, 2018, 06:47:54 pm »
Yeah the article speaks about DC bias too... apparently the X7R / X5R are susceptible to this, while C0G is not. So I guess this is the main thing to consider when choosing between the two. As always it all boils down to price I guess... need to check their respective prices.

All this info allows to select the dielectric type. But now what about choosing voltage rating ? From what I gather, for ceramics "in general", not MLCC particularly, a 250V cap can be applied 250V safely ? So zero "derating" required ?

Still on that subject : for non- NP0/C0G where DC bias is a major concern... then how does the voltage rating influence the magnitude of this problem ?
For example : on the boards I need to repair here, the caps see 5V digital signals. The clock has a 50% duty cycle, so DC bias is 2.5V.. well more like 1.2 V since the clock is gated and half of it's time it is not running.

So. voltage levels is 5Volts, so if I use a 6.3V MLCC, then :

1) With this 1.2 DC bias, how much will the capacitance decrease and

2) If I increase the voltage rating of the cap, will the problem be tamed ? In what proportions ? So for example if I use a 10V or 16V MLCC instead of 6.3V, will the capacitance be less affected by this 1.2V bias ? IOW is DC bias to be taken relative to the voltage rating, or as an absolute value ?

I need to find info on that....   :P

... in the interest of future repairs and design work...

...however, as far as my dish-washer repair is concerned, the value of the cap is hardly critical I think... given that both were open-circuit yet the machine worked just fine ! It's only the leaky diodes that caused it to stop working.

So I feel any low value cap, between 1nF and 10nF, will work just fine. Will take X7R, or C0G if price is not too silly.

« Last Edit: September 29, 2018, 06:52:37 pm by Vince »
 

Offline Vince

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Re: Board repair : HOBART " hood " type dish washer
« Reply #43 on: September 29, 2018, 06:54:40 pm »
Replying to myself... stupid me : the Kemet article does link to a simulator they designed, which precisely allows to simulate the capacitance change versus applied voltage !  Need to play with this a bit !  ;D
 

Offline Vince

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Re: Board repair : HOBART " hood " type dish washer
« Reply #44 on: September 29, 2018, 07:39:41 pm »
OK, just ran a parametric search on Farnell's site.

35,000 MLCC cpas to choose from !

Gee...

So I restricted the search to just Kemet caps.

Then looked at the various dielectric options, and boy in an eye blink you can see which dielectrics are most popular... surprise surprise : the two I was considering earlier : C0G/NP0, and X7R.  pretty much equally.   All other dielectric are down in the noise really !

Pulled a few datasheet for various package sizes, soI can see actual dimensions as I am not familiar with them just yet.

All MLCC and resistors on these two board are the same size. I measured them best I could using calipers. Looks to be something like 1.5mm long by 0.8mm wide.   I see that 0603 (imperial units), is 1.6x0.8 .... so this is it. Now I now what package size I need. 1.6x0.8mm... sounds like nice round numbers, corresponds to 1/16" long by 1/32" wide. So "6" means 1/16", and so on, OK...

So I narrowed the search to Kemet, 0603, C0G/NP0, X7R. Voltage rating I picked 25V to be extremely safe and use that as "maximum price, lower voltzage ratings ought to be cheaper", reference point....

So, if I buy in 10 quantitites, which is the minimum allowed, then yeah, there is a big price difference between COG and X7R , on to two !  :-\
However for some reason I don't understand.... it's the X7R that's twice the price of the C0G, not the other way around !

Don't understand... C0G is supposed to be better... there must be more than meets the eye... would like to understand...

Anyway, these aren't cheap (given how tiny they are !) : 15 cents for C0G, 30 cents for the X7R, and you have to buy 10 of them... and I need to buy a few different capacitance values to experiment with...

Yeah if I select a lower voltage rating, and a cheaper brand... maybe I can halve those prices...

Just did some googling, indeed there is no voltage derating required... however from what I understand, the horrible DC bias problem and microphonic problems are mostly due to the extremely thin dielectric layer. So by derating a lot, you get a thicker dielectric layer hence less problems.... well that's my intuition at least ! :-//

Anyway, I know enough now, to be able to place an order... and get this dish-washer board fixed for good ! 8)

 

Offline Vince

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Re: Board repair : HOBART " hood " type dish washer
« Reply #45 on: September 29, 2018, 08:08:25 pm »
OK just filled my Farnell shopping kart.

All X7R , 16V, Kemet, 8 different values, 10 of each, ranging from 470pF up to 100nF, for 8.50 Euros including tax, that's OK...

I should be covered and be able to experiment and figure out what value suits this board best... and all  the remains of course will be handy to have for future repairs or design work/prototyping... you can never have too many components in stock, can you...

 

Offline Vince

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Re: Board repair : HOBART " hood " type dish washer
« Reply #46 on: September 29, 2018, 09:28:21 pm »
Hell, just checked Kemet's  cap simulator and ... WOW ! That's one hell of a tool, very comprehensive indeed, a little gem  :D

For those who don't already know it, well worth a look !

ksim.kemet.com

You can get graphs for all sorts of parameters for all their caps not just MLCC.

You basically select the exact Kemet cap you are interested in, and then you have the option of displaying a bunch of interesting graphs to se how they behave : ESR and impedance vs frequency for example, and of course capacitance change vs DC bias !

So I  checked the X7R caps I just bought, and well... might not be C0G of course, but they are still much better than what I feared !
The capacitance drops very little, and very "slowly" over the voltage range. You really have to apply the full operating voltage to the cap to get a "significant" capacitance drop, and even then it's much less than 10% !  7.5% or so in my case. Far from disastrous... plus, since you have a graph, you can see how it changes over the entire voltage range, so you can take that into account in your design anyway.

So it's not all that bad.... at least for X7R caps. No wonder why it's the second best selling dielectric, behind C0G/NP0.

« Last Edit: October 02, 2018, 06:49:41 pm by Vince »
 

Offline Vince

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Re: Board repair : HOBART " hood " type dish washer
« Reply #47 on: October 02, 2018, 07:44:18 pm »
Alright, just received the parts from Farnell, and went straight to the bench  ;D

Installed the diodes first, and was a bit pissed that their package/body did not match what was illustrated on Farnell's web site... and that the package is a tad on the short side : the tip of the gull-wings barely made contact with the pads on the PCB. Luckily the solder joint was able to form properly, so I am not worried about reliability.

Then I looked at the comparator input signal on the scope, to see what the edge looked like, and measure rise time, so I get a reference point before I started experimenting with various MLCC cap values.

I attached waveforms every step of the process.

With the new diode and no cap, the edge displays a funny "bump" which I can't explain, and the rise time is about 1us.

Then I added a cap, progressing from the highest value I had ordered (100nF) down to the smallest one (220pF).

- At 100nF, of course I expected nothing but laughs : the rising edge is so long that it spans the entire idle time of the data line, and the falling edge spans the entire length of the data packet ! LOL  you can guess/ figure this out by the looks of the waveform : the "clean" portion of the RC shape, must be the idle time of the frame I guess, and the downhill part of the waveform, where "saw teeth"  are superimposed on it, must be the bits in the data packet ! LOL

- At 10nF now : board still doesn't work, but at least we can now clearly make out where the idle part is, and where the data packet is.. though they look like spikes rather than clean pulses, of course...

- At 1nF : now the board works !  ;D  The data bits still look far from square, with the rising edge being about 20us when the bit time is 100us... though it's good enough for the comparator to square that up and get the thing working just fine.  Of course I was not going to tolerate that much damping... so I lowered the cap value again.

- At 470pF : rise time is halved of course, so now about 10us. Edge is looking better, almost acceptable...

- So halved that again and tried with 220pF : as expected, now 5us rise time. Looks good enough to me so I stopped there. Making it smaller would not make it work more reliably I guess, and the smaller I make the cap, the less efficient it will be at reject HF crap. So, the point was to find a good balance... not too big, not too small...
Plus, 220pF was the last / smallest value cap I had ordered anyway, so couldn't go lower than this ! :P


Note : ahem... how to say that... ashamed... I now think that the original caps may well have been just fine  and I didn't need to replace them ! Why ? I measured all my new caps, before fitting them. DMM was reading just fine with the 100nF cap, the 10nF and 1nF caps... but... when I tried measuring the 470pF and 220pF caps, the DMM would read open circuit ! Did not register anything at all... just like it did when I measured the original caps ! Eh ?!  DMM has a 10pF resolution, should have been alright I would have thought !  >:(
Then noticed the "Low Battery" indicator on the DMM screen, oops... replaced the battery and now oh miracle, it does manage to read something ! So the old caps where not shot, grrr !!! However the reading is highly unstable and varies over a wild range : keeps jumping all over the place, from as low as 16pF up to 2nF or so, in the blink of an eye ! Even though I was making good contact with my just arrived gold plated sharp test probes !  Tried with my "regular" 2mm test leads, DMM behaved exactly the same.

So, from this we can deduct :

1) The old caps were probably just fine, they didn't need replacing ! >:(
2) Their value we can now guess, was less than 1nF.
3) My DMM does have a 10pF resolution BUT is reliable only down to 1nF, not good for sub nF caps... so I need a better instrument for caps in the pF range...


Power supply filter caps : I replaced the big 2200uF through hole electrolytic with a 35V rater one, since the 12V rail puts out almost 15V and the original cap was rated at only 16V !

As for the two SMD caps on the 5V rail... I ended up NOT replacing them, sadly, because of high thermal mass. My old 50W "magnatstat" Weller iron is not powerful enough to melt the solder on those pads... so I gave up on replacing these caps, before heating them too much and damaging them. They were just fine anyway. I was only trying to replace them as preventative maintenance..    So I leave them like this and will replace them (destructively this time...) when/if they do fail. By that time hopefully I will have better soldering gear, a 90 or 100+ Watts iron with adjustable temperature...

Final repair is due on Friday afternoon, where we will go back on site to refit these boards to the dish washer, cross fingers and see what it does ! ;D

Also bought a little magnifying lens with LED lighting, still from Farnell, 10 bucks... needed to add something to the shopping cart in order to reach the minimum amount to get free shipping...
Looked crappy on the website, but I am pleasantly surprised. Magnification is ... I don't know, maybe only x2, but it makes all the difference compared with naked eyes. Base is stable, heavy metallic. Mast is bendable, metallic too. Len is quite large and with a wide field of view : most of the lens area is in focus, you can see a good chunk of the board. The LED lighting ( 2 little crappy LEDs) works actually quite well : even lighting across the lens, no glare, it works really well.
Even has a tiny secondary lens embedded at the front of the main lens, just large enough to inspect one component at a time, which provides a higher magnification. I don't know, maybe x3 at a guess. Quite useful too.

As for the sharp "SMD" test leads, I bought this model :

https://www.ebay.co.uk/itm/182185834926

They are actually very decent overall quality, nothing to complain about other than maybe the length of the leads, only 90cm is a bit on the short side, but still it does the job, no worries. The tips are sharp and seems like the metal they used is durable, doesn't look/feel like they will loose their sharpness in under 5 minutes of use. Gold plated also. They do the job just fine : I can now easily probe the fine pitched chips on these boards, without risking a short. Their sharp tip also "bites" into the pins easily, so the risk of slipping the tip over adjacent pins is much reduced compared to standard leads. I also found that the probe tips are a great tool while soldering too : they were great to help me hold those tiny caps firmly in place, onto the board, while I was trying to solder them !

So quite happy with this repair, it was an excuse to get a first experience at working on SMD stuff, and get my first "SMD" oriented gear... it's great, it's like I now have access to a whole new world of electronics ! LOL   Now all I can think of is getting better tools to view better and solder better  ;D

- A decent microscope along with good lighting

- A decent set of tweezers : have only one pair, they are too big for these small 0603 packages (never mind smaller packages of course !), and the metal is too soft, they bent out of shape way too easily.

- A better more modern soldering iron, more powerful, and temperature controlled.

- Some way of storing all these tiny components I just bought, in a compact/efficient way !



« Last Edit: October 02, 2018, 08:47:16 pm by Vince »
 

Online tautech

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Re: Board repair : HOBART " hood " type dish washer
« Reply #48 on: October 02, 2018, 07:58:25 pm »
Nice detective work Vince.  :)

- Some way of storing all these tiny components I just bought, in a compact/efficient way !
Sorry about the loooong link but get one or two of these:
https://www.aliexpress.com/item/Free-shipping-Resistor-Capacitor-Inductor-Blank-SMD-Components-Empty-Sample-Book-For-0402-0603-0805-1206/32478009014.html?spm=2114.search0104.3.15.60507fb5YRerXN&ws_ab_test=searchweb0_0,searchweb201602_1_10065_10068_204_318_10547_10059_5727211_10884_10548_10887_10696_100031_10084_10083_10103_10618_452_10307_532_5727311,searchweb201603_60,ppcSwitch_0&algo_expid=f77ba03d-dd3b-4e1a-85ac-a1d54f9e2999-1&algo_pvid=f77ba03d-dd3b-4e1a-85ac-a1d54f9e2999&transAbTest=ae803_4&priceBeautifyAB=0

Then over time stuff them with the SMD you use the most. One book holds a hell of a lot.....I only hold 0805 and the full E96 resistor selection and a good MLCC SMD cap selection comes nowhere near to filling one book.
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Offline Vince

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Re: Board repair : HOBART " hood " type dish washer
« Reply #49 on: October 02, 2018, 08:58:56 pm »
Yeah I was thinking of something like this ! Thanks for the link, looks like it will hold quite a lot indeed, and is quite affordable as well, and free shipping to Frog land... going to click the buy button me think !  ;D

 


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