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Success Stories If you have successfully repaired or restored a piece of equipment, why not write up what you did and post details here. Particularly if it was interesting, unusual or challenging. PLEASE DO NOT POST REQUESTS FOR HELP HERE!

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Old 22nd Apr 2017, 12:20 pm   #1
DigitalNoMore
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Default Marconi 520MHz Universal Coubter Timer 2438

I was given this a few months ago but have only just got round to checking it out. It was last calibrated in 1994 and in a pretty filthy state having been stored for years in a cellar.

After a few cursory checks to make sure the fuses were okay and that it wasn't likely to catch light I powered it up. With all the push buttons out and the Display Time control set to minimum, all eight display segments should be fully lit along with all the decimal points. Instead, the display segments were randomly flickering irrespective of the button settings.

Dismantling didn't show any obvious faults with no sign of any burnt components or dry joints on the PCBs. I therefore sprayed all the push buttons with contact cleaner along with the internal connectors. Same problem as before!

Before getting down to some serious fault finding I decided to unplug and then re-seat the large white socketed IC which I'm guessing is a custom gate array.
Much to my amazement everything sprang to life as a result with the display correctly showing all segments and decimal points fully lit. Upon connecting the Internal Standard 10 MHz output on the rear panel to the Channel B input and selecting Frequency B a correct frequency of 10 MHz was measured and displayed as shown in the photo below. The 2438 has a built in oven controlled 10 MHz crystal oscillator.
Playing with the other functions shows that they all seem to be working correctly. Not all the display segments are exactly the same brightness but the difference isn't enough to be a problem.

I was expecting that getting this up and running would be a long job but was pleasantly surprised. And all down to poor contact(s) on an IC socket!
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Old 22nd Apr 2017, 7:07 pm   #2
mole42uk
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Default Re: Marconi 520MHz Universal Coubter Timer 2438

Well done! That happened to me with a Marconi 2955 radio test set.
Beware the OCXO if it is an original grey box. They are notoriously past their best-before date and can fail. If you have a problem with it, I drew out the circuit which has some notes for test and repair. It's on the forum somewhere, or PM me for a copy.
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Old 23rd Apr 2017, 11:40 am   #3
DigitalNoMore
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Default Re: Marconi 520MHz Universal Coubter Timer 2438

Thanks for the advice Richard. I designed in a couple of OCXOs during my life as a telecomms design engineer but never came across any reliability issues. But, of course, I wasn't dealing with 20+ years old devices!
If I do have any problems I'll look up your notes on the forum.

Regards,

Jerry
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