Old digital clock radio display fault
 

I have no experience of repairing clock radios. Can anyone advise what might cause the fault in the attached photo? The buttons to change hour and minute have no effect.

Re: old digital clock radio display fault
 

My first thought was that some segments are dead (so it would/should be showing 18:08) but that does not compute if the time will not alter. Does the display change if you leave it running?

Make absolutely sure you are setting the time correctly (that you haven't a switch in a wrong position) but ultimately either the display is faulty or something more deep seated is going on.

Also make sure any switches such as tact push type are not stuck in a closed position and so inhibiting and confusing the uProcessor.

Re: old digital clock radio display fault
 

My guess would be the usual bad electrolytics. Fortunately they're unlikely to be surface mount in something of that age, so easy to change.

Re: old digital clock radio display fault
 

If you can't find a schematic of the clock/radio online look for a datasheet of the clock chip. There are 2 main types, for a duplexed display (28 pin) and non-duplexed, with an output for each display segment, which will be 40 pin. In addition to the switch and electrolytic checks mentioned above, check for dry joints (the soldering in old clock radio's can be awful). Also check that the 50Hz pulse input to the chip is present and correct as this can stop both any duplexing and the setting/running of the clock. Series resistors can be of poor quality and/or underrated so can fail in time. The power supplies and grounding to clock radios can be a bit confusing as the chips are also designed to drive higher voltages to VFD displays as well as LED, and the voltages higher than you would expect, possibly around 24v. The clock chips can and do fail in these units, and once you've eliminated the other checks, you may need to consider this. Some photographs of the interior might be helpful.

Re: old digital clock radio display fault
 

It probably isn't worth putting a great deal of time and money into fixing this unless it has sentimental value for some reason.

Re: old digital clock radio display fault
 

The other classic problem in old clock radios is the momentary-action push switches becoming intermittent, or even leaky (especially if a cup of tea's found its way inside), the latter leading to all kinds of odd symptoms.

Re: old digital clock radio display fault
 

It looks as if the clock chip isn't running in this case though.

Re: old digital clock radio display fault
 

Fair point, but sometimes "stuck" switches can cause this.

Re: old digital clock radio display fault
 

I remember taking apart my Dad's old clock radio as the microswitches had become intermittent. I was surprised they were held together with what looked like dried out Scotch tape, so simply stuck them back together with insulating tape which worked OK for as long as I kept it. I can't remember what happened to it, but I got the feeling it went in a parentally enforced clearout.

Re: old digital clock radio display fault
 

Have a look inside and see what the clock IC is, it will be a dedicated digital clock chip. Find the datasheet for that IC and the circuit of the clock part of that unit will be nearly enough identical to the application diagram in the datasheet.

Re: old digital clock radio display fault
 

Many thanks for all the advice so far, gentlemen. I've taken a pic of the inside of the clock radio. As you can see the switches seem to be just sprung metal clips to short various pins of the chip to ground. Pressing the clips have no effect at all on the display which has not changed in anyway since I first applied power.
The clock chip is an LM 8560 - so a non-duplexed 40 pin.
I've also included a pic of the LED board.
Might it be worth getting a new LM8560 and soldering it in?

Re: old digital clock radio display fault
 

There are a few data sheets available for this IC, here is one:

http://www.paulanders.com/G5-LED/ver1/datablad.pdf

If you have a scope, try looking at pin 25 which samples the mains frequency and uses that not only as time synchronisation but also as the 'clock' to run all of its internal features.

The display is actually semi-multiplexed, each segment output from the IC is responsible for managing two individual segments on the display. The supply arrangements for these ICs take a bit of getting your head around.

Re: old digital clock radio display fault
 

Scope on pin 23. I’m not an experienced scope user but I took this photo.

Re: old digital clock radio display fault
 

Did you mean pin 25? Can you say what volts/div and time/div settings you made that measurement with?

Re: old digital clock radio display fault
 

Sorry. My lack of concentration. On pin 25 not getting anything

Re: old digital clock radio display fault
 

What are you using as your ground reference point for the scope? As I mentioned, the supply arrangements for these are a bit weird. If possible please compare the circuit around the clock chip in your unit with the sample circuits in the datasheet - as it is a clock-radio (not just a clock) it probably uses something like the 'Positive Supply' arrangement, in which a medium value resistor takes an AC feed from one end of the mains transformer secondary to pin 25, the 50/60HZ input.

Re: Old digital clock radio display fault
 

I used pin 20 on the clock chip as a ground reference. I attached the probe to pin 25 and the attached pic shows what was displayed on the scope. I had trouble capturing a photo of the screen. My scope is quite old. Time/Div was set at 5ms and Volts/Div was set at 0.2
Oddly when I was measuring what DC voltage there might be on pin 25, the clock display suddenly came good. I had also touched up the solder joints around the pins on that side of the chip

Re: Old digital clock radio display fault
 

It's maddening when that happens.

Random theory... I wonder if the frozen display was caused by initially powering up after years of non use and caps (that at the least needed reforming) had leakage and put the chip in a non valid state. Stored charge then kept it in that state. Your probing might have 'spiked' the chip in some way and caused it to reset.

Stranger things have happened.

Re: Old digital clock radio display fault
 

Maddening yes as I don’t know what fixed it. Positive in that I now have a working AM/FM clock radio. Positive too in that it’s made me realise I should learn how to use my ‘scope.
I’ve found some great help in that regard on YouTube.
Many thanks to all you fine gentlemen for your help and advice.

Re: Old digital clock radio display fault
 

Total newby question here, but what's the difference in a clock radio with a red display and one with a green display? Are the displays radically different from one another?