SC8560 AM/FM clock radio 50Hz drifts

[manoj.patil.1974]

Hi, My brother gifted me a Duraband CR-502 AM/FM clock radio. Its a low cost item,but its precious for me as its a gift by my brother. This was brought in the USA so obviously it would not work in India as we have 230V, 50Hz. I opened the box, replaced the 110V transformer with a 230v/12V step down transformer. Incidently this item uses a SC8560 digital clock IC which supports both 50Hz/60Hz selected. I have made the jumper settings to select 50Hz. I was so happy that i could make this work.... but i am facing a new problem. The instrument runs perfectly fine now, but it slows down by 4 minutes for every 8 hrs. This means that the line frequency here is NOT accurately 50Hz and I guess it must a bit low (may be 49Hz).
What can be a smart solution ?? Are there so low cost crystal controlled IC's available ? can some suggest me an IC that would just be suitable for this purpose. request you to kindly copy the reply at manoj.patil.1974@gmail.com too.. regards

 

[ljcox]

You could install a 1 MHz crystal followed by a counter to divide it down to 50 Hz. Alternatively, you could use a PIC driven by a crystal and do the division in the PIC.

 

[Hero999]

A good question, but no one's going to email you, we only answer on the open forum.

You seem to know what you're doing.

You need to supply the electronics with an acurate 50Hz supply waveform and this needn't be that difficult.

Remove the transformer and make a simple DC supply out of it, then add crystal controlled a low voltage modified sinewave inverter on the output of the transformer.

Or do away with the inverter idea and hack the clock pin (assuming you can easilly find out where it is), then connect a crystal oscillator plus divider.

 

[manoj.patil.1974]

Apologies for any inconvenience caused by asking a reply copy on my email... u are right.. logging in this forum is a better choice..

firstly thanks for posting reply for my query.

I am looking for one more level details down. I know a crystal and a divider would solve my problem...but i would like to know what frequency and type of crystal should i choose ( i have never worked with crystals) and what IC is the best for the divider job ... i did search google for a standard 50Hz divider, but could not find any standard circuit... the crystal frequencies are denoted to the 4th decimal so i thought to seek expert advise on this forum to chose the right crystal and divider...

Another thought that came to my mind... when i started working on this clock, initially, i did not remover the originial transformer, but used a thyristor based 230V/110V convertor... and after selecting the 50Hz...i noticed that the clock was running twice fast... i guessed that this may be due to the chopping of the sine wave done by thyristor resulting in twice as many pulses for the chip... hence i had to replace the transformer... if i continue on this thought process, will a square wave 50Hz generated by crystal and divider work for me ? or would it result in twice as many pulses for the chip and actually generate 100Hz for timing purposes.. i hope my thoughts are not too wierd !!

 

[ljcox]

I don't have a circuit of your clock, but I expect that a square wave will be better than sine.

You need a crystal whose frequency is either a decimal or binary multiple of 50 Hz.

For example if you had a 1.6384 MHz crystal, then you would need a binary counter that can divide by 32768. See the 4020, 4040 & 4060 ICs.

Or, if you had a 1.000 MHz crystal, then you would need a decimal counter that can divide by 10000 followed by a Flop Flop to divide by 2.

I suggest you check my maths as I've done it in a hurry.

Or if you use a PIC such as the 16F84, then all you need is a 1 MHz crystal some capacitors and a bit of programming.

Did you search this forum for "crystal" or "clock"? There have been plenty of posts on these subjects in the past.

 

[RODALCO]

In your country are you connected to the grid or is your town supplied from a local diesel generator ?
Normally over a 24 hour period the grid generators will keep the total cycles generated as correct as 4,320,000 for a 50 Hz network.
At some periods during high demand the frequency may be a little below 50 Hz, say 49.6 Hz, but at periods of low demand the freuency will usually be increased to 50.4 Hz, to keep all clocks running correctly.
I don't think that thyristor converter idea of yours is too good and you may saturate your transformer with too much dc.
Get a 230 Volts to 12 volts transformer.

 

[Hero999]

I know this isn't your design but I think using the mains frequency as a time base is a bad idea and should be avoided especially in products which need to be exported.

jcox is correct, there are plenty of CMOS ICs capable of performing the dividing, if you can get hold of a 1.6384MHz crystal then great because the CD4060 can perform the oscillator function as well as the dividing function so you'll only need one IC.

 

[Nigel Goodwin]

I know this isn't your design but I think using the mains frequency as a time base is a bad idea and should be avoided especially in products which need to be exported.

But on the other hand, it's an excellent idea, as it keeps perfect time - at least in a country with good mains like the UK.

 

[mvs sarma]

Apologies for any inconvenience caused by asking a reply copy on my email... u are right.. logging in this forum is a better choice..

firstly thanks for posting reply for my query.

I am looking for one more level details down. I know a crystal and a divider would solve my problem...but i would like to know what frequency and type of crystal should i choose ( i have never worked with crystals) and what IC is the best for the divider job ... QUOTE]


Hi patil
I too had a chance of observing this behaviour as the power supply is getting chopped. perhaps you can get a 3.579545MHz and MM5369 from a condemned car clock-- but never try purchaing them-- very costly is the IC now a days. -- with this combination you can derive 60Hz.

 

[Hero999]

But on the other hand, it's an excellent idea, as it keeps perfect time - at least in a country with good mains like the UK.

Until you decide to take your clock on holiday with you.

 

[Nigel Goodwin]

Until you decide to take your clock on holiday with you.

I always find it's very hard to get a grandfather clock in your luggage?

Incidently, I perhaps hadn't better mention the Sony clock radios (only the other year or so) that all lost ten minutes per hour - they had been sent out as 60Hz models instead of 50Hz.

Because they were low cost they fell under the IWX (In Warranty Exchange) scheme, which means Sony won't pay warranty claims on them. However, it was less trouble to take them apart, google for the clock IC datasheet, and configure them for 50Hz - rather than do the paperwork to send them back!.

 

[RODALCO]

Holiday, i take as timeless and don't want to know what time it is while on the beach or in a tent.

Most hotels usually have alarmclocks, or a phone your room service if you need to get somewhere at a particular time.

Cheers

Raymond

 

[RODALCO]

Holiday, i take as timeless and don't want to know what time it is while on the beach or in a tent.

Most hotels usually have alarmclocks, or a phone your room service if you need to get somewhere at a particular time.

New Zealand has also a very accurate mains frequency, same as in the UK or Europe so time errors are minimal.

Cheers

Raymond

 

[Hero999]

Here's an idea.

Use a high stabality capacitor for C and trim R to exactly 120Hz using a frequency counter to monitor the frequency.

The CD4060 produces a 4Hz squarewave, resetting the 7555 every 15 pulses so if it's slightly out it will remain as accurate as the crystal. Providing the astable 555 doesn't go to far out of cal it couldn't effect the clock.

 

[manoj.patil.1974]

SC8560 AM/FM clock radio

Dear All,
Firstly thanks for all the replies. Its wonderful. Thanks for all the opinion.

However, I continue to face some hiccups. I want to generate a very stable 50Hz or a 60Hz signal using a crystal and a single IC. (Thus i dont want to use the Resistor / capacitor based oscillator)

I did some google, but could not get anything helpful. I browsed website of some of the crystal manufactures to enlist all the frequencies of the crystals available. To my surprise, I have not been able to find a single crystal frequency that could be divided by a counter and give a accurate 50Hz or 60Hz signal.

I am attaching an xls sheet for your reference.

Any clues ? Am I doing the right thing ?


Note : 1.6384Mhz is the only frequency that gives 50hz, but i am not able to find here in local stores and also google gives very less references.. looks like its a very uncommon frequency

 

[audioguru]

You are behind the times.
"In the good old days", good clocks were expensive and made in Switzerland.
All of us electronics hobbiests wanted to make our own clocks so National Semiconductor made a fairly inexpensive counter IC that used a cheap common quartz crystal from a colour TV and gave a 60.000Hz output.

Now that Chinese clocks are so cheap, nobody makes their own clock anymore so National Semi discontinued the IC. It was the MM5369.

 

[Hero999]

Can you do frequency division with a PLL?

Perhaps a PLL multiplier followed by a divider.

[latex] 50 = \frac{32768 \times 25}{16384}[/latex]

[latex]60 = \frac{32768 \times 15}{8192}[/latex]
But I don't think it can be done with one IC, unless there's a PLL IC with two counters or a dual PLL I've not heard about.

 

[nura100]

Hello Forum!

Hai Manoj,

this is my first Post on this Forum and what better topic than this to kick off,

my Friend Manoj, if it makes you feel any better ,you are not the only one facing this Freq. problem.

i too am from India and at times the mains are at 49 and even 48.5 cycles.

Recently i have built the Digital Alarm Clock using SC8560/LM8560 and faced the same problem of Freq.

the time is slow by say 4 to 5 mins.

had to do endless Google searches for stable 50/60 HZs but in vain.

So, i decided to use simple Cmos Multivibrator CD4047 tuned at 100Hz split by 2 thru its outputs to get 50Hz.

now for the BIG question how would we be sure of the final Freq. i,e 50Hz ?

i just made a 150W inverter using a pair of Mosfets connected to the 2 Outputs of CD4047 and Connected a Black and White TV to check for the Rolling Bar on the Screen.

finally adjusted the freq. preset till i saw no rolling bar on the screen. all this trouble could be saved if one uses a Frequency Meter.

finally the fine tuned 50 Hz Square wave pulse was fed to the CLock IC, the time keeping drastically Improved when compared to Mains derived Freq.

i know this is not precise when compared to Crystal but better than our grid's mains freq.

hope my experience is of some help

arun

 

[Hero999]

Of course a 3.2768MHz crystal can be divided by 65536 to get 50Hz.

Use a CD4060 followed by another flip-flip or if you can find a counter and oscillator IC with a 16-bit counter then go for it.

Of course your clock might actually use 100Hz because it's the output from the rectifier so the additional flip-flop might not be required.

 

[audioguru]

It is odd that in my part of the world the mains frequency is controlled to be extremely accurate, but in your part of the world the authorities just don't care.