Alright, just received the parts from Farnell, and went straight to the bench
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 !
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 !
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 !
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/182185834926They 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
- 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 !