Clock crystal: heatsink or grounding required?

Hi,

I have been having intermittent problems with the clock on my video recorder. When on standby the clock fails (displays ----) and hence will not autostart or record from standby. On opening the box it appeared that the clock crystal had been secured to a copper pad on the pcb with a white silicon-type substance (heat sink? But not sticky). The crystal had 'popped-up' and was no longer in contact with the pcb. Since re-securing the crystal (by pushing it down) the clock has behaved itself. I was not aware that a clock crystal needed either heat dissipation or the 'can' grounding? Is this a red herring?

The PVR is a Humax PVR9200T and the crystal is ...

Manufacturer: AEL CRYSTALS Order Code: SCC4065 WATCH CRYSTAL 32.768KHZ Frequency, output:32.768kHz Accuracy, frequency +@:20ppm Accuracy, frequency -@:20ppm Temp, op. min:-10=B0C Temp, op. max:60=B0C Capacitance, load:12.5pF Temperature stability +@:50ppm Crystal case type:Watch A Temperature stability -@:50ppm Thanks, Ian Perthshire

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ianw
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I have been having intermittent problems with the clock on my video recorder. When on standby the clock fails (displays ----) and hence will not autostart or record from standby. On opening the box it appeared that the clock crystal had been secured to a copper pad on the pcb with a white silicon-type substance (heat sink? But not sticky). The crystal had 'popped-up' and was no longer in contact with the pcb. Since re-securing the crystal (by pushing it down) the clock has behaved itself. I was not aware that a clock crystal needed either heat dissipation or the 'can' grounding? Is this a red herring?

The PVR is a Humax PVR9200T and the crystal is ...

Manufacturer: AEL CRYSTALS Order Code: SCC4065 WATCH CRYSTAL 32.768KHZ Frequency, output:32.768kHz Accuracy, frequency +@:20ppm Accuracy, frequency -@:20ppm Temp, op. min:-10°C Temp, op. max:60°C Capacitance, load:12.5pF Temperature stability +@:50ppm Crystal case type:Watch A Temperature stability -@:50ppm Thanks, Ian Perthshire

Crystals are often 'fixed' to the board to improve vibration resistance, although they should, of course, be firmly soldered as well :-) Anything that helps to keep the can temperature stable, is also no bad thing, as they are quite temperature-sensitive for long term frequency stability. You will also quite often see a crystal can soldered around its skirt to a topside groundplane, or with a wire strap soldered from the can to some other nearby piece of metal, or in the case of an HCxU package, layed on its side and soldered down to ground. In the case of a VCR, the accuracy of the clock crystal for the RTC generator, is neither here nor there - within reason - and I don't think that you should bother yourself too much about the gunge blob that was originally there, as long as you now have good soldered connections to it.

Arfa

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Arfa Daily

"ianw" schreef in bericht news: snipped-for-privacy@57g2000hsv.googlegroups.com... Hi,

I have been having intermittent problems with the clock on my video recorder. When on standby the clock fails (displays ----) and hence will not autostart or record from standby. On opening the box it appeared that the clock crystal had been secured to a copper pad on the pcb with a white silicon-type substance (heat sink? But not sticky). The crystal had 'popped-up' and was no longer in contact with the pcb. Since re-securing the crystal (by pushing it down) the clock has behaved itself. I was not aware that a clock crystal needed either heat dissipation or the 'can' grounding? Is this a red herring?

The PVR is a Humax PVR9200T and the crystal is ...

Manufacturer: AEL CRYSTALS Order Code: SCC4065 WATCH CRYSTAL 32.768KHZ Frequency, output:32.768kHz Accuracy, frequency +@:20ppm Accuracy, frequency -@:20ppm Temp, op. min:-10°C Temp, op. max:60°C Capacitance, load:12.5pF Temperature stability +@:50ppm Crystal case type:Watch A Temperature stability -@:50ppm Thanks, Ian Perthshire

The construction of crystal oscillators is often underestimated. A not well designed oscillator may look like good in a prototype and in 90% or more of the production. The problems appear in the 10% or less. Ever had a similar problem in a keyboard in which the crystal was not fixed to the pcb and had no correct groundplane. It also was placed too far from the processor. In that case the problem was solved by soldering a short, thick (stranded) wire to the metal can and the nearest GND. (In that days keyboards were still pretty expensive so retrofit on failure was an option.) In your particular case, the best option is the one that works. So press the crystal to the pcb using some (hot melt) glue or kit to keep it firmly fixed in place. Don't solder if you don't need to.

petrus bitbyter

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petrus bitbyter

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