Crystals wont oscillate

Hardware questions and issues with the FV-1

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knutolai
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Joined: Wed Nov 23, 2016 8:43 am
Location: Bergen, Norway

Crystals wont oscillate

Post by knutolai » Tue Jun 06, 2017 12:58 pm

I have a setup where my FV-1 chip reside on a daughter-board together with the clk crystal, crystal cap and the decoupling capacitors. This board connects with header pins to a "main"-board which houses the power regulator (LM1117), more decoupling capacitors and all other necessary circuitry for my project (analog mixer, FV-1 associated pots and switches, etc.). The circuitry on both the main-PCB and daughter-PCB is verified as working. I've built 3 units and they all work as intended without any issue.

However... I built 9 daughter-boards in total of which just 4 are working. The FV-1 chips are sourced from ProFusion while the crystals are sourced from BitsBox and Tayda Electronics. The FV-1 chips were soldered using solder flux. The boards were then cleaned thoroughly using a Q-tip soaked in acetone. I've not been able to detect any broken connections of short-circuits on any of the daughter-boards. The daughter-boards were tested by connecting them to a verified main-board. When any of the non-functioning daughter-boards are connected the clip LED (mounted on the main PCB) is constantly lit, leading me to believe the issue could be related to the clk crystal not starting to oscillate.

Initially only 3 boards worked as intended. For one of the non-working boards I swapped the crystal (Tayda sourced) for another (BitsBox sourced) which solved the problem. However when I tried this fix on another non-functioning board the issue persisted. Two of the daughter-boards with crystals from Tayda initially worked, but when retested would not start up (???). Also the one board with a Tayda sourced crystal that does work takes about 2 seconds to start up, and one of the non-functioning ones sometimes starts up after 10-15 seconds. Generally speaking I feel the boards are acting unpredictably, and I feel very uncertain to what is the definite issue. I'd like to pin the malfunction on the Tayda sourced crystals, but as stated one of the boards with a BitsBox sourced crystal also isn't working.

Could the issue be that I haven't placed the crystal and 22pF cap close enough to the associated legs on the FV-1? There is approximately 5-6mm of copper separating the actual bodies of the crystal and the FV-1. Any suggestions? I'm about ready to pull my hair out..

Daughter-board schematic and PCB:
Image
Image

frank
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Post by frank » Tue Jun 06, 2017 2:45 pm

Crystals are a very finicky device and different ones can have different requirements. Stick with one model, we use Citizen and have had good results. Only get parts from a known good supplier, discount crystals mean big problems.

Try adding a cap to the other pin of the crystal, some need it. You may need a resistor between the legs of the crystal, see the crystal makers datasheet for their recommended circuit.

Flux is OK AS LONG AS IT IS NOT WATER SOLUBLE! Water soluble flux is conductive so just a tiny bit under the chip can cause problems, it also tends to be corrosive and can damage the PCB.
Frank Thomson
Experimental Noize

knutolai
Posts: 33
Joined: Wed Nov 23, 2016 8:43 am
Location: Bergen, Norway

Post by knutolai » Tue Jun 06, 2017 3:53 pm

Flux is OK AS LONG AS IT IS NOT WATER SOLUBLE! Water soluble flux is conductive so just a tiny bit under the chip can cause problems, it also tends to be corrosive and can damage the PCB.
Damn.. Turns out my Flux is water soluble, so this could be the issue. How would I go about removing residue beneath the chip? Oxygen spray?
Crystals are a very finicky device and different ones can have different requirements. Stick with one model, we use Citizen and have had good results. Only get parts from a known good supplier, discount crystals mean big problems.
Good call. I be buying the next batch from ElfaElectronics. Manufactured by Auris. Crystal datasheet:
https://www.elfadistrelec.no/Web/Downlo ... tion%2Fpdf

How do I go about calculating the correct capacitor size when the FV-1 datasheet specifies only one crystal cap? I'd use [CL = (C1 * C2) / (C1 + C2) + Cstray] for a two-capacitor-configuration.

frank
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Post by frank » Tue Jun 06, 2017 4:29 pm

Honestly the best thing is to remove all components and clean everything or start with a new board and abandon these ones. You will not be able to clean under the chip and other components unless you have access to a commercial level defluxer which uses hot deionized water under pressure.

As far as crystal caps start with one and see what happens, only the crystal manufacture can tell you the proper cap size as it is based on the crystal characteristics. In many cases the water soluble flux was the problem and when it went away so did all the problems since it is conductive it could be shorting the crystal, the caps, etc.
Frank Thomson
Experimental Noize

knutolai
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Joined: Wed Nov 23, 2016 8:43 am
Location: Bergen, Norway

Post by knutolai » Wed Jun 07, 2017 9:39 am

Honestly the best thing is to remove all components and clean everything or start with a new board and abandon these ones. You will not be able to clean under the chip and other components unless you have access to a commercial level defluxer which uses hot deionized water under pressure.
Went over two of the non-functioning boards with excess amounts of acetone (because f*&k it...) and managed to get them running after some drying. Their still unpredictable as they wont always start up, but the flux definitely seems to be a major issue. Thank you so much!
As far as crystal caps start with one and see what happens, only the crystal manufacture can tell you the proper cap size as it is based on the crystal characteristics. In many cases the water soluble flux was the problem and when it went away so did all the problems since it is conductive it could be shorting the crystal, the caps, etc.
Could you link to the datasheet from the Citizen crystal you are using? I'd like to compare the specs with the Auris crystals. I've ordered a no-clean flux pen and a complimentary flux removal pen. Looking forward to predictable boards!

smear
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Post by smear » Wed Jun 07, 2017 10:02 am

You seem to have sorted out your problem, but I thought I'd add that I've built over a dozen pedals using Tayda Crystals and they all work fine. Tayda lists the required Load Capacitance as 12.5pF (and the datasheet lists 9pf, 10pf, 12.5pf), and I've used both 10pf and 15pf capacitors which worked fine. It's possible that in addition to your flux problem, the 22pf cap was too large for the particular crystal. Hope that this helps!

http://www.taydaelectronics.com/datasheets/A-1592.pdf

frank
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Post by frank » Wed Jun 07, 2017 1:27 pm

knutolai wrote:Could you link to the datasheet from the Citizen crystal you are using? I'd like to compare the specs with the Auris crystals. I've ordered a no-clean flux pen and a complimentary flux removal pen. Looking forward to predictable boards!
Here it is http://cfd.citizen.co.jp/english/prod-t ... -206_E.pdf
Frank Thomson
Experimental Noize

knutolai
Posts: 33
Joined: Wed Nov 23, 2016 8:43 am
Location: Bergen, Norway

Post by knutolai » Sat Jun 17, 2017 6:27 am

The no-clean flux arrived and it seems to have solved the issue. Thank you thank you thank you!

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