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Old 4th Nov 2016, 11:15 am   #1
JayBee66
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Default Drifting out from old signal generator

I have a valve/tube RF signal generator but the output drifts in frequency even after warming up.

I replaced all the capacitors. The tubes (I prefer that word) are original. Could they be the problem or is it normal for generators of this vintage not to produce a stable frequency?
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Old 4th Nov 2016, 11:30 am   #2
Peter.N.
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Default Re: Drifting out from old signal generator

They will tend to drift when they warm up but should be reasonably stable at a constant temperature. There may be some temperature sensitive components in the oscillator circuit that are out of spec.

Peter
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Old 4th Nov 2016, 1:19 pm   #3
Skywave
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Arrow Re: Drifting out from old signal generator

Can you provide some figures for the drift? Something like the nominal freq. at switch on, then the freq. produced at intervals of, say, 15, 30 and 45 minutes.
A note of the room temperature when those measurements were taken will also be required.
That data should enable us to assess whether the drift is 'normal' - as in 'to be expected' - or whether it is indicative of a fault.

Al.
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Old 4th Nov 2016, 1:49 pm   #4
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Default Re: Drifting out from old signal generator

Quote:
I replaced all the capacitors
(my italics) Some will be of a specific temperature co-efficient to correct for drift.
 
Old 4th Nov 2016, 2:08 pm   #5
JayBee66
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Default Re: Drifting out from old signal generator

I doubt it. It's just a lowly Tech TE-20D.

I'll probably get a modern one and leave the old one on the boat anchor display shelf.

Thanks all for the help.
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Old 4th Nov 2016, 2:46 pm   #6
Bazz4CQJ
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Default Re: Drifting out from old signal generator

Which direction is it drifting; lo to hi or hi to lo? Are you aware of the difference in temperature coefficients (positive and negative) of different components and how these can be used to minimise drift, especially neg coef ceramic caps? I have a comparable sig gen made by Taylor, and for general workshop, use it's entirely adequate.

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Old 4th Nov 2016, 4:03 pm   #7
G8HQP Dave
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Default Re: Drifting out from old signal generator

There do not appear to be any specific temperature compensation capacitors in the circuit, but that does not mean that one or two of the coupling capacitors were not chosen with this in mind. Did you replace like with like? Swapping an NP0 ceramic for a polystyrene (for example) in either direction would be expected to affect the stability.

Why did you replace all the capacitors? Low value caps are usually stable and only need replacing if faulty.
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Old 5th Dec 2016, 1:55 pm   #8
JayBee66
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Default Re: Drifting out from old signal generator

I replaced old green blobs (some with flakey heat affected paint) with newer green blobs from salvage.

Since writing this request I have had time to relax from the day job and do things properly. I tested the generator against my Heathkit frequency counter with my computer off and there is now just a little drifting in the hundreds of Hz, which I can put up with. I doubt that this device ever gave a rock steady signal.

I have also calibrated the generator dial with a combination of turning the coil slugs and the screws on the tuning capacitor. Within the range I need 400-500 kHz, all is now well. No complaints. Even the MHz bands are pretty close but I doubt I'll be using them. And, unlike other owners of TE-20Ds I am getting signals on all bands.

Thanks for all help.
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