Dead Citizen Crystron

[james_stan]

Hi All

I recently acquired one of these..

http://www.digitalwatchlibrary.com/DWL/1work/citizen-60-7053/

minus it's back and appears lifeless, although the case and band are near mint. The battery compartment was corroded and I have cleaned it as carefully as possible. I have also had most of it apart and it all looks pretty clean with lots of in tact gold contacts BUT with a new battery no display.

Question is if teh qc is nadged would I expect to see anything on the display ?

Any help gratefully accepted.

Cheers

JS

[quietman]

Someone will probably have some specific knowledge of these old Citizens but in general it is very likely that a dead Crystal causes there to be no visible output on its attached LCD.

Apart from clocking the driving logic required to generate a coherent output the actual LC segments require an AC signal to be driven correctly - unlike an LED which of course requires DC and would be toast if reverse biased too far with an AC drive signal. The LCD drive signal is almost always a sharp square wave - generated in the driver circuitry using a ...... clock signal. The clocking on LCD drive signals need to be relatively low frequency (say below 1kHz) and so would almost certainly be divided down from the quartz input. The switching frequency doesn't need to be stable so there are other ways of designing this switching without the QC signal but frankly speaking no one in their right mind would do it that way given that there is a low frequency (QC) clock available.

So no QC, no ac drive signal, no dark segments.

One would hope that an unclocked driver IC from an old device like this doesn't lock its outputs on a DC state. It might though which would be unlikely to destroy the LC segments in the very short term but wouldn't do them much good either.

Nice watch anyhow, Good Luck,

MP.

[james_stan]

Hi

Thanks for the reply. That's interesting. In the Seiko manuals that you can download from DigitalWathLibrary, however it implies you can test the LCD panel with a DMM in resistance mode (i.e. supply a constant voltage 3v across the segments) although I admit I didn't try it yet...

Cheers

James

[quietman]

I'm sure someone here knows more about this than me.

But what I've gathered from working with modern LCDs is that they present an essentially capacitive load. So yes you can put a DC source onto them and they will charge like any capacitive load and generate a static field between the common electrode and the segment electrode. In between of course you have the liquid crystal which is electrochemically twisted in the presence of this field. The problem is that the LC doesn't like to see a permanent (DC) field - this can lead to eventual (electrochemical) breakdown of the carefully aligned LC and so deterioration of the display. As noted in my initial reply, short term DC signals are unlikely to damage the display but neither would I spend a lot of time driving them with DC, particualrly with an aged component. So there's obviously nothing dodgy with the recommendations in the service manuals you cite but you would never design the actual equipment to drive with DC over the long term.

The drive signal to LC segments is usually chosen to be high enough to avoid the static fields that lead to LC degradation, high enough not to have visible flicker and low enough that you don't have trouble driving the capacitive load. In the hundreds of Hz is typical and I've never seen anything over 1kHz. Most LC displays these days incorporate all of the switching drivers into a module so you can just drop DC control signals onto them but if you're designing with glass only (as I think most vintage LC watches) then the ouput control signals will (almost always) be AC/switching.

The problem with not having a QC input to the display driver IC in your watch would be that the outputs to the LC are (almost certainly) designed to be AC/switching waveforms and by removing the clock then the output circuits won't be functioning properly. Which particular voltage they latch to if powered up without a clock is anyones guess but the two most likely states are either high-Z or 0V. Some or all could latch to a non-zero voltage sufficient to switch on a segment but there are many implementations that simply wouldn't. I would be interested to hear from others whether they've had any dark/dim/random segments on dead Xtal vintage LCs - always interesting to work out how they designed these circuits way back when (well for some people anyway )

In an LED watch the output drivers are (most usually) going to be current sources with fixed regulation and a simple on/off input from the logic circuits. This means that when the IC is powered up without a clock and enters whichever mysterious state the logic circuits drop into then a coherent unclocked output is often produced to switch on (latched because the IC logic is unclocked and static) one or more current sources and so LED digits.

I think the answer to your intial question is the same. And the service manuals are also right. Hopefully I've explained why? Or possibly sent you to sleep.

Anythings possible but some things are more likely than others.......

[Old Tom]

However the DMMs in Seiko technical manuals are usually their own dedicated watch DMMs and these actually have an AC LCD output terminal specifically for testing the panel. If I wasn't being lazy I would go & see exactly what voltage & frequency my S-840 delivers (will do it tomorrow as I have a few other jobs that require oscilloscope & frequency meter!).

The real reason you get no display when the oscillator stops is because in practically all pre-1985 LCD watches the oscillator was used to drive a voltage upconverter to supply the voltage needed to operate the LCD panel. In this type of watch if the oscillator stops then there is no voltage on the panel and no electrolytic decay. Very early watches use a tiny ferrite toroidal transformer to perform this upconversion, later watches use a transistor/capacitor pump circuit to double or triple the supply voltage. As panels became thinner and the nematic material more "sensitive" then the need for upconversion disappeared. The downside was as the cell voltage sweeps down towards zero then the output bridge can get "stuck" in one state or another and apply DC to the panel which damages it. The appearance is that of very thick digits with thin spiderweb like strands radiating off. Interesting thing is that if you get a watch like this then fitting a battery and just letting the watch run for 4-6 months will reform the display.

[quietman]

And before someone jumps on my note about toasting LEDs with AC signals......

1/ you're right there probalby isn't high enough voltage present in any vintage watch to toast an LED in reverse bias. I forgot that the world runs on 1.55V or small multiples thereof .....

2/ yes people do routinely provide dimming functionality on current regulated LED drivers with PWM waveforms. Which are a lot like, say the switching waveform used to drive an LCD segment. I was obviously thinking a little too linearly when I wrote the reply last night. It would be interesting to know how dimmed displays were implemented in early LED watches - linear or switched

[james_stan]

Thanks gents - all good stuff.

Will try and scope the crystal with power on and see what we 've got.
Hopefully a duff crystal...

Cheers

JS

[Old Tom]

The Seiko A-840 puts out 4 volts on the LCD test terminal- which suggests that Seiko digital watches have voltage tripler circuits in them.

[james_stan]

Ok all latest update

checked crystal on a scope - nothing ! Swapped crystal - still nothing !

Made test set-up from old promo LCD desk clock then checked both crystals old and new - working great. Hmm..

Not much else to change except trimmer - thought this would be O-C on a DMM resistance range but it read 640 Ohms - kippered maybe ?

Will swap with a fixed cap tomoroow hopefully unles sany other ideas ?

Cheers

JS

[quietman]

Starting by stating the obvious - Xtals can't drive any kind of low impedance load and you've obviously tested your Xtals as OK out of the circuit.

So presumably when in circuit the Xtal output is being killed by a low impedance path somewhere.

1/ Stray path from scraped tracks, leaked conductive salts, etc. etc.

2/ Component (such as the variable cap) presenting a low impedance to one of the rails - quite possible if you've had battery juice all over the place.

3/ The normal clock input to the IC has become low(er) impedance. This could be a catastrophic failure or drift/aging issue.

Assuming that you've cleaned the boards properly then a new cap should eliminate 2/ (I guess there aren't any other components off the unbuffered Xtal nodes - if there are can they fail @kHz short circuit like an open capacitor can?).

If you still have no joy then your best bet is probably to fit a powered oscillator in place of the Xtal. This should be able to drive a kHz clock into most non-catastrophic issues whether a dirty stray that you didn't find or a struggling IC clock input buffer. I think oscillator substitution has been covered elsewhere on this forum in relation to clock issues.with LED watches.

Failing that, prepare the listing - "I couldn't get the back off to check but it might just need a new battery, etc, etc, etc...."

MP.

[james_stan]

Failing that, prepare the listing - "I couldn't get the back off to check but it might just need a new battery, etc, etc, etc...."

MP.

Shame on you, as if..