Nat Semi Xtal replacement

[quietman]

Hi,

I've just resurrected my Nat Semi module with a new 'QC' as they seem to be referred to here. Just thought I'd add a note that I seemed to have some problems with stray capacitance between the Xtal case and the Vbat+ trace that is nearby. Don't forget to put some solid dielectric (like paper tape / paper label) in the gap between the Xtal and PCB. Even with a small air gap there was enough stray to put the module back into dead Xtal mode in some orientations.



I also did some experimenting with my dead dots issue (thats dead dots not dead segments.....) and discovered a few things about my Nat Semi module but didn't fix the issue. I will enjoy the watch as it is now working minus two quarter segments and one eighth of a segment. But when I conformally coated the module I did leave the faulty areas clear of coat so perhaps I'll try one or two other ideas when I don't mind risking this module....





Rgds,

MP

[james_stan]

Good post and tips. I've attempted to repair a few LEDs with new crystals
etc.. and acheived very little. You have encouraged me to try again..

JS

[quietman]

JS,

I think the biggest danger with replacing these xtals is that you fry the new xtal with heat as you fit it (try soldering a scrap one to something whilst holding it between your fingers....ouch). Using as little heat as possible should be stating the obvious but you should also use a heatsink to hold the Xtal during soldering. Ratnose pliers would work but they would need a smooth gripping surface to get decent heat transfer. You can also get little Aluminium Heat Shunts designed for the purpose. You can pick one up at Maplin Retail Store for £3 (the British Radio Shack.....). Whether you use a proper shunt or pliers, place the sink on the compnent leads if possible, if not then put it on the case.

Rgds,

Mark.

[retroleds]

Uh, some of those tips will lead straight to failure. The crystals cannot take heat very well at all. Most have plastic holding the leads and more importantly the crystal itself is changed dramatically from the heat, if not totally destroyed. This article(I know, got to get in there and fix that site up) will give you some good tips on that whole deal, including the heat sink issue. Those big aluminum heat-sinks sold with soldering irons are generally too big for this job. Try some smaller, flat-nosed clips (MApins or Radio Shack).

http://www.thedigitalwatch.com/Archives/QuickTip_002.htm

[quietman]

Hi,

Sorry but I'm not sure where the bad advice is? I was just making a point about heat to the guy who said he was going to have another bash at new Xtals.....

- Aluminium shunt clamped onto leads works....

- Ratnose pliers clamped onto leads just about works.....

- Where there are no leads to clamp onto (SM Xtal for example?) then sinking on the QC casing can keep the insides cool enough to not melt the potting or fry the quartz.

The article you referenced was interesting. I was just trying to share some of my experiences and offer some encouragement! Having read through lots of posts on this forum and got lots of tips myself before starting I, Um, thought that was what the forum was for.

Rgds,

MP.

[quietman]

There are as many ways of doing this as there are of getting it wrong. Equipment supports technique, it doesn't substitute for it.

The bottom line is that heat kills Quartz Crystals and frys board mounted ICs faster than you can say "maybe I should have practised on something worthless before destroying my vintage LED module".

Rgds,

MP

[james_stan]

AHA !

Just repaired (I think) a similar Nat-semi module with a new crystal,
maybe I was lucky with the heat - I was holding the crystal can ( a biggish one too compared to quietmans - may have helped) with the wife's metal eyebrow tweezers.

Ran it for > 1 hour last night with AA batteries and croc clips to provide the power. So far so good.

Luckily for me, since I'm so ham-fisted the display side of mine is silicone encapsulated so wire bonds not so delicate.

Love those dots !

Cheers

JS

PS I already had one of those Maplin ally heatsink thingies but found it too bendy and a rather unwieldy..

[retroleds]

Hi,

Sorry but I'm not sure where the bad advice is? I was just making a point about heat to the guy who said he was going to have another bash at new Xtals.....
- Where there are no leads to clamp onto (SM Xtal for example?) then sinking on the QC casing can keep the insides cool enough to not melt the potting or fry the quartz.

The can is at the farthest point from the heat source, and in fact, the heat sink is going to conduct heat that direction. So your idea of subjecting the leads, on through the fine crystal mounts(metal), through the actual crystal, to the can with all that heat is totally non-advisable. IMO.

A home hobbyiest can't safely remount a SM (surface mount) crystal, unless they want to give it a go with silver epoxy, which can work. I have never seen a quartz oscillator with potting medium inside - I have chopped open a very large number for my edification. I've also successfully replaced thousands....for what that's worth.

I only suggested a flatter jawed clamp as it has better surface area, physically holds the part whilst doing heat sinking duty and doesn't get in the way like some larger holding tools.

The forum IS for exchanging ideas and learning - some of my suggestions are good, some are bad. Same for all others who strive to do something. This is a relatively anonymous medium for most of you (unlike me), so we must not take our differences of opinion on technique too seriously. But consider their merit. In the Holy Bible it is suggested that "one man sharpens another, as steel sharpens steel". So true.
Peace. ~

[retroleds]

A little followup, since we're on "technique". I use the tool with the slot (comes in those basic solder tool sets) to bend the leads. As with all good metal bending technique, turn the tool to bend the material, don't turn/bend the material. I'd suggest you don't try to remove solder unless it is blocking holes or otherwise stopping you from making your connection..... the additional heat is just asking for trouble.




Quietman and I had gone around a bit PM...we are agreed to cool our jets!
QM - you wouldn't fit in if you didn't have some fight in you. Carry on!
Ed