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Old 17th Jan 2023, 7:48 pm   #1
Philips210
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Default Racal-Dana 9914 VHF Frequency Counter

Hello again.

My Racal-Dana 9914 frequency counter has become more and more unstable and is now really unusable. Even with TTL levels it often struggles to lock to the correct frequency. It's as if the signal level presented is of insufficient amplitude. I've been having to turn the sensitivity control up to maximum and even then it appears to display the frequency very erratically. A slight reduction on the control alters the displayed frequency so the reading is not to be relied upon.

I don't have a circuit diagram for this counter and was wondering if anyone can assist with any service information?

Sometime back, I read on this forum about problems with the input circuit, in particular the JFET transistor. Perhaps my fault may be associated with the input stages.
Please see attached pics.

Any help and suggestions appreciated. Thanks.

Regards,
Symon
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Old 17th Jan 2023, 8:51 pm   #2
AdrianH
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Default Re: Racal-Dana 9914 VHF Frequency Counter

I can not help with the 9914 manual, but I recently repaired my 9915 that was doing very daft things, it was the Tantalum capacitors starting to go leaky/short. I basically replaced as many as I could , they are all over the place on the main logic supply rails so drag them down. All being on the same rail meant it was difficult to fault find, hence rI just replaced a big batch of them.

I would suggest you have a look at the power rails within the set, be it 5 Volt or 10 Volts etc. see what you find. You can check the specs of the IC's in there and see what rails they should have.

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Old 17th Jan 2023, 10:33 pm   #3
Philips210
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Default Re: Racal-Dana 9914 VHF Frequency Counter

Hello Adrian.

That's interesting to read about your experience with the tantalum bead caps and it gives me more encouragement to tackle my frequency counter.

Recently I've become very suspicious of tants and assumed that they're either serviceable or obviously defective due to catastrophic failure. The worst offenders seem to be those across power supply rails especially if they're operating near to their voltage rating. I suppose they're generally OK in timing circuit applications but who knows if they can really be trusted...

When they begin to fail, the gradual increase in leakage is not apparently obvious. We used to get various problems with tants in some colour TV receivers. They must have been suffering excess leakage. I used to put it down to a poor batch but it seems various makes are prone to this failure particularly after several decades of service.

I'm therefore going to check and replace all of the tants with good quality low leakage electrolytic caps as a first step to eliminate this potential/actual problem. I will probably try to fit either Panasonic or Rubycon types.
I've become wary about working on live equipment with tants as there's always the danger of a sudden failure. Not worth the risk of harm or damage. It seems it's a failed technology.

Thanks for your helpful reply.

Regards,
Symon.
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