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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 16th Oct 2019, 11:32 pm   #1
Jolly 7
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Default Replacing a 5000 pF capacitor with 4700 pF

Having accidentally damaged my component tester, I cannot check if this 5000 pF disc capacitor in my signal generator is within spec. This is one of the caps that might prevent ECF80 valve oscillation as per an earlier thread on this forum.
Would replacing it with a new 4700 pf mustard capacitor be ok ? I don't have a spare 5000 pF cap.
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Old 16th Oct 2019, 11:50 pm   #2
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Default Re: Replacing a 5000 pF capacitor with 4700 pF

Its very easy to tack it into the circuit and see!. I suspect that 5000 puff was the standard value when the generator was made. You may need recalibrate the dial at worst.

Just my thoughts

Joe
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Old 17th Oct 2019, 3:31 am   #3
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Default Re: Replacing a 5000 pF capacitor with 4700 pF

I think you'll find the capacitor in your picture is 470pF unless that last colour is red instead of brown (picture is a bit too fuzzy to tell properly), in which case it will be 4700pF.
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Old 17th Oct 2019, 7:50 am   #4
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Default Re: Replacing a 5000 pF capacitor with 4700 pF

Looks like yel, pur, red Terry. Try it and see, it's only 300p difference which in terms of frequency isn't much. If not use two 10n in parallel.

Andy.
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Old 17th Oct 2019, 9:35 am   #5
David G4EBT
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Default Re: Replacing a 5000 pF capacitor with 4700 pF

If itís C13, connected via the red wire to T2 - the modulation transformer - itís stated as 5,000 pf on the circuit. However, 4,700 pf is only 5% below and the original ceramic disc cap wonít have been a precision component - +/- 10% at best. Apart from that, the audio frequency used for modulation of the RF is unimportant and unlikely to be as specified in the manual anyway, due to component variations. If the oscillator section was working before, a 4,700pf cap wonít stop it, and if it wasnít working before, unless the original cap was faulty (probably not) itís unlikely to make it burst into life.

If you wanted to retain originality, ceramic disc caps are cheap and plentiful, but if youíre not bothered about that, a tubular mustard cap should be OK.

Hope that helps a bit.
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Old 17th Oct 2019, 11:21 am   #6
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Default Re: Replacing a 5000 pF capacitor with 4700 pF

Quote:
Originally Posted by Diabolical Artificer View Post
Looks like yel, pur, red Terry. Try it and see, it's only 300p difference which in terms of frequency isn't much. If not use two 10n in parallel.

Andy.
In series, surely?
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Old 17th Oct 2019, 1:39 pm   #7
Jolly 7
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Default Re: Replacing a 5000 pF capacitor with 4700 pF

Thanks for all the responses. Apologies for the poor picture quality, it is yellow, purple, red.
@David: this is indeed C13 of the RF-1U signal generator. C13 to 15 were described in a thread from last year as being important for proper oscillation of the triode section of the ECF80 valve so I thought of trying a swap. It made no difference, as you have stated.
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Old 18th Oct 2019, 7:50 am   #8
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Default Re: Replacing a 5000 pF capacitor with 4700 pF

"In series, surely? " Your right tony, senior moment, realised after I'd typed it whilst playing back my reply in my head and meant to comeback and edit, then forgot, another triumph for brain power.

Glad you got it sorted. A.
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