Ultrasonic cleaner broke working PCB??

[Ian.M]

As I thought:

4. Starting Voltage
Starting voltage refer to the minimum supply voltage at which an oscillation circuit can operate. Starting voltage is affected by all the circuit elements, but it is determined mostly by the characteristics of the IC.

Fig. 5-6 shows an example of an actual measurement for the starting voltage characteristics against the loading capacitance

Fig 5-6 is for a resonator driven by a 74HCU04 unbuffered hex inverter IC.  The IC's specified minimum operating Vcc is 2.0V, so an oscillator built with one is doing quite well to start down around 1.5V.

[Mario87]

Yes, I'll admit, their data is not exactly as per my specific application, but as you say, the min Vcc for their test IC is 2.0V and it starts resonating at about 1.5V and is doing quite well to achieve that. So my resonator sitting at 600-800mV when Vcc is 5.0V is no real surprise that it won't begin to resonate, not to mention the short to ground of the resonator which should not have been there at all.

When I took the resonator off the board my IC Xin & Xout pins both went up to a steady 2.14V which is about the same as what the main MCU pins were sitting at before the 5MHz crystal began to work last night. With the main MCU once it began to resonate the scope was showing min of 200mV and max of Vcc (5.0V).

I got my good board out and this particular resonator is indeed 4.19MHz when probed and just like the resonator of the main MCU it was going between GND & VCC when resonating. That initial 2.14V will just be to get it started, but the 600-800mV and then the resonator breaking into 3-pieces after being removed is a clear indication the resonator was in a bad way after the ultrasonic bath.

The 5MHz resonator has not arrived today for some reason, was sent 24h delivery by RS. Hopefully tomorrow. I've also ordered the 4.19MHz resonator for the M66242, again, hopefully it will be here tomorrow and this will all be sorted.

[Ian.M]

As  I said earlier, the initial bias voltage has *NOTHING* to do with whether or not a Pierce oscillator oscillates, so long as the oscillator's amplifier still provides greater than unity small signal loop gain at that bias point. 600 to 800 mV is suspiciously close to being railed with no gain, but as its CMOS it should drive rail to rail so as its not sitting right on the rail, I wouldn't condemn it yet.   If you've removed the broken resonator completely, you could try patching in a temporary external logic level clock signal at Xin, and check that (a) it works, and (b) you get an inverted drive signal for the crystal or resonator at Xout.

[Mario87]

Well...it looks like I was right in that the resonators had taken a beating and failed. Got the new 5.00MHz and 4.19MHz resonators today, installed them an just like magic the dash works fine.

So lets go over what all went wrong after cleaning this dash....

• Failed 5.00MHz resonator for main MCU
• Failed 4.19MHz resonator for PWM IC
• Corrupt EEPROM (maybe the result of the MCU acting funny from damaged resonator)
• Corroded via washed away
• 560 Ohm current limiting resistor for one bank of LED's failed - In all fairness the one next to it for the other bank of LEDs had already needed replaced before cleaning as it failed previously, so was probably already on its way out.

To be honest, these clocks have not had the easiest life. Corrosion on the board (area with missing via was the worst), damage to PCB traces from bodged repairs, etc, etc...part of me wonders if it had been a brand new board with solid good components, would I have had these issues?? Did all this happen purely because the components were already borderline and the cleaner just shows up the weaknesses??

[SilverSolder]

Obviously cleaning a board that has already had a hard life, is more likely to reveal problems...

On a bike there are a lot of vibrations - who knows, maybe the resonators were already "shaken, not stirred" and ready to go AWOL?

Looking on the bright side, better to reveal all the weaknesses now, on the bench, rather than at 135mph somewhere...

[Ian.M]

You could build a Pierce oscillator on protoboard using a similar resonator, and heat seal it in a little bag of paraffin, with wires coming out for Vcc, Gnd and output, then run it immersed in the operating ultrasonic cleaner and time how long it lasts without going off-frequency, or otherwise failing.   Otherwise, only %DEITY% knows!

[james_s]

I will take it on board and in future I will do a proper rinse, but to put all the issues down to a lack of rinse just sounds like you are trying to prove a point rather than help identify the root cause as to what has happened.

I don't think it has anything to do with the rinse. I have washed dozens and dozens and dozens of boards using plain tap water and a drop of dish soap, gentle scrub with a soft bristled brush followed by a rinse under hot tap water and set it out to dry. If I'm in a hurry I sometimes flush it with IPA as that evaporates much faster. I've never used DI or distilled water.

That said, our tap water here does not have a significant mineral content, if yours does then this might not be adequate.

It never occurred to me that it could happen but I think the most likely thing is that the ultrasonic process somehow damaged the resonator.

[Mario87]

That said, our tap water here does not have a significant mineral content, if yours does then this might not be adequate.

I live in Scotland....I think we are known for some of the best tap water across the globe TBH.

[Ian.M]

With experience and local knowledge, you'll learn what you can get away with rinsing boards.  I gave you the full 'sensitive' high impedance board rinse procedure for when surface leakage must be absolutely minimized.   If your local tap water is soft, with minimal dissolved solids or organics, you can probably get away with skipping the distilled/deionised water rinse.  OTOH if its got so much peat tannin that it looks like a good cup of tea when it comes out the tap, or if you had happened to live in some semi-desert hell-hole relying on heavily mineralized, brackish and sulphurous bore water you may well need to obtain much purer water even to make up the cleaning solution and for the first rinse.