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[sevenlayermuddle]

My 25 year old radio/alarm stopped working.  Always resistant to throw stuff out I decided to try and fix it.

I'd expected to find a few obviously dead capacitors, but no… all looked well.   I did however manage to find online data sheets for the long obsolete radio chip, and also the clock chip, and have convinced myself that (surprisingly) the clock chip itself is the culprit; the alarm output pin is meant to be intermittent 900Hz modulated yet it is just DC.   That is consistent with the symptoms, it just 'pops' when it should be 'beeping'.  Most amazing of all, I have found an ebay seller willing to ship the suspect chip, long disappeared from normal supply channels, for under a fiver.  It's on its way. 

Disassembly & diagnosis was non-trivial and included bravely unthreading the cord that connects the tuner 'pointer' from various tiny pulleys, shafts & springs, separating two pcbs that were soldered together, and temporarily desoldering several interconnect wires that were too just too much in the way.  Plus of course countless digital photos to aid reassembly.  Spent about 4 hours on this so far, and budget another 2 or 3 hours to install & reassemble when the new chip arrives.  Not sure how much a new clock would cost, but I'm certain it would be hard to justify my efforts in comparison, and a modern clock would probably a better clock too.   

Plus of course the risk that I may have misdiagnosed, or that the chip failure may have been secondary to some other fault, or that I may damage the pcb swapping the chips, or may fail to reassemble correctly.   Ah well, they say it keeps us out of mischief.   

[roseway]

I admire your perseverance, 7LM.

[burakkucat]

I hope to read of an eventual successful conclusion to this restoration project. 

[sevenlayermuddle]

I hope to read of an eventual successful conclusion to this restoration project. 

OK, I promise to report back, even if it's only to admit I've wasted £5 (and six or seven hours of time).   

[sevenlayermuddle]

The tension mounts… our friendly postman has just popped a jiffy bag through the door containing a near-identically marked 28 pin clock chip, sealed in its anti-static bag.   All that remains is to desolder the old chip without wrecking the PCB, solder in the new, and put it all back together again.  Picture attached, sorry it's naff quality.

It seems to me to be a surprisingly elaborate design, with two separate pcbs, one for the clock components and for the radio/audio.  The clock 'alarm' pin is meant to be capable of driving a piezo sounder directly but instead it is fed through to the second pcb, where it finds its way to an 'audio input' pin on the radio chip, then sounding through the clock's AF stages to the speaker.  A separate bunch of components on the clock pcb seem to recover a DC signal from the 'beep' tone, and use it to enable/disable power to the entire radio PCB.

The small gold disc in the pic is a piezo sounder that I have temporarily wired with a series capacitor across the clock's  alarm output, the fact that it remains silent is the main basis of my diagnosis that the clock chip is defective, wonder if I'm right?   My, what fun.  Or maybe I just need to get out more.   

I suspect my original estimate of its age was high, it is probably nearer 20 years, and maybe towards the tail end of the era when such things were 'repairable', given enough determination?

[kitz]

I admire your perseverance, 7LM.

Yes the words determination and patience also spring to mind

[Berrick]

I'm with you all the way on this one Mr sevenlayermuddle. Git thee bugger fixed.

I cant wait to show this thread to 'er in doors she's convinced i'm the only one that keeps old stuff going rather than buy a new one  

[sevenlayermuddle]

I think I promised to report back.   And, oh dear...

The chip transplant went fine, no major scarring of the pcb, but the behaviour is unaltered.   Let's not say I'm 'giving up', rather I'll just put it all back together and set it aside for when I have more time to figure out how it's meant to work.   I've already spent so much time on it that more pressing matters are getting left behind.

And one of these pressing matters is to go out and buy a radio/alarm clock on Saturday

[4candles]

Awww, that's disappointing.   

Good luck - and keep us in the loop.   

[tonyappuk]

And I was getting ready to cheer. It's the sort of repair that I would attempt causing me to be regarded as a mean b..... in my family. "Why don't you buy a new one?!" comes the cry but it's so satisfying when you succeed. This happens less and less these I'm afraid as eyesight, dexterity and I suppose, abilty wain with age. But I do support you, 7LM
Tony

[sevenlayermuddle]

Well actually, there is a consolation.   I seem to be allowed to get away with endeavours like this on the strict understanding that I know when to quit.   

Being seen to quit may thus earn me a few credits towards the next obsessive repair attempt.  And nobody need know that sometimes, when I appear to be deepy engrossed in TV news or whatever, that I am actually plotting in my minds eye another assault on the sickly clock   

[Ronski]

And one of these pressing matters is to go out and buy a radio/alarm clock on Saturday

I was sat in a cafe last Saturday when two old boys came in, one was really pleased. He just bought a second hand radio alarm clock for £3 that turned out to have never been used when he got it out the box it still looked brand new. I thought of you trying to fix yours, sorry to hear you didn't succeed..........yet.

[sevenlayermuddle]

Update...

I have just cracked it   

The clock can take an optional 9V battery that keeps it in time during power outages, I've never bothered as I know I'd forget it was there and let it corrode.  But the penny dropped - the (missing) 900Hz tone from the chip's alarm pin is derived from the same on-chip oscillator that operates the clock when in battery backup mode.  The frequency of said chip oscillator is determined be an external resistor/capacitor pair and lo', the 150k resister was open circuit.   Normal operation has now been resumed with a shiny new resistor.

I'm surprised to see an unstressed small carbon resistor go O/C like that, it's not a common failure in my experience - not dry jointed -  just O/C.   I'm also embarrassed it took me so long to work out how it all worked, but the only data sheet I could find looked like a scan of a fax which had been written by a person who's first language may not have been English.  It sure wasn't obvious to me that the alarm depended on the backup oscillator!

I haven't yet reassembled the whole, that will be quite fiddly as it involves persuading the two PCBs and various bits of plastic framework to reunite in their original configuration, and then rethreading the radio's tuning cord, and reconnecting various wires I chopped.   Reassembly will probably be attempted tomorrow, once I've recovered from the hangover I'm about to treat myself to by way of celebration… 
 

and PS,  No, I didn't get around to buying a new clock yet.   

[burakkucat]

Excellent news!   

[kitz]

hahahaha..  well done