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Old 18th Jul 2018, 12:03 am   #41
Skywave
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Arrow Re: IF fault on Eddystone 770R

space charged: you have a PM.

Al.
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Old 18th Jul 2018, 1:30 am   #42
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Default Re: IF fault on Eddystone 770R

Consider small NPO ceramics, each one tested before use for pfd, leakage, and temperature effects.
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Old 18th Jul 2018, 1:41 am   #43
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Default Re: IF fault on Eddystone 770R

OK, thanks Al. I've emailed you.
C
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Old 18th Jul 2018, 1:47 am   #44
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Default Re: IF fault on Eddystone 770R

Quote:
Originally Posted by Radio1950 View Post
Consider small NPO ceramics
Thanks for the suggestion. I knew some caps were temperature compensated. The range you suggest seems to be ideal. I guess that apart from stability, caps for IFTs have to have low leakage and also be very low inductance. Also small, but any 100pF cap is going to be intrinsically small!
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Old 18th Jul 2018, 7:16 am   #45
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Default Re: IF fault on Eddystone 770R

NP0 isn't actually the name of ceramic material. It is the code for zero temperature coefficient. It has an allowed range up to +/- 30 ppm per degree C. It follows the coding of things like N750 capacitors often used for temperature compensation.... nominally -750 ppm/C in this case.

It turns out that the ceramics used to get NP0 tempco are pretty well-behaved with high Q and low dielectric absorption, so the NP0 code might as well apply to one material. They are an extremely good choice for RF tuning applications.

Sometimes, though, wily designers pair-up magnetic core materials and capacitor types to cancel each other's tempco. Mica gives about +50ppm/C so a core material with a negative tempco is helpful, and games can be played with the amount of field path length spent in the core versus in air to dilute the core tempco to make -50ppm overall for the coil. Such foxiness in design is pretty much invisible unless you start digging.

Generally, it's safest to replace with the same materials, because the ones where it really matters aren't obvious, even if they're not that common.

For a 465kHz IFT, 50ppm/C in C is going to give 25ppm/C in frequency. 11.625Hz/C so a 50C change will give a 581Hz shift. Unlikely to be noticed!

In a 5MHz VFO, then 6.25kHz is significant, and in a well-designed circuit swapping from mica to NP0 or C0G may spoil the temperature compensation.

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Old 18th Jul 2018, 7:55 am   #46
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Default Re: IF fault on Eddystone 770R

The lower sensitivity of range 1 is a 'feature'.

This is the story of my oddyssey, frightening to think it was 20 years ago:

http://www.schimmel.talktalk.net/radios/770r.htm

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Old 18th Jul 2018, 11:22 am   #47
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Default Re: IF fault on Eddystone 770R

Thanks Radio Wrangler for your insights into capacitors for resonant circuits. I will order some NPO caps and have already ordered some silver/mica ones. That was on the basis that "all sorts of magic" go on in the design of an IFT and the fewer changes made to them the better. I'll try both types and see what sort of results I get.

Thanks wd40addict, I thought is might be a "feature"! Thanks for the link to your article. Turns out yours is one of the ones I found in a search for stuff about 770R. Hope you have still got yours.
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Old 18th Jul 2018, 12:26 pm   #48
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Default Re: IF fault on Eddystone 770R

Very interesting thread. These receivers have a reputation for being deaf - I wonder how many others out there have been dismissed due to these obscure capacitors? I have a 770U which I've never bothered to work on for much the same reason, athough a more fundamental problem with mine is that some of the turret biscuits are missing.
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Old 18th Jul 2018, 2:03 pm   #49
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Default Re: IF fault on Eddystone 770R

I think the 770R was "deaf" by modern standards even with a fully working aligned IF strip. I'm sure mine will improve once I've replaced the caps.

I only found the fault when using a wobbulator/scope and working back from the last IF. I found one IFT that didn't have a resonant peak at all! I could screw the core from fully in to actually removing it and the peak didn't change at all! Then when poking round with a soldering iron (for dry joints) it started working again. Seemed to be the heat that provoked it to work - for a while.

Interesting Keith that you have a 770U. As a matter of fact, so do I. When I collected my 770R from the seller, he offered me the 770U for a small additional sum. Its a bit of a "basket case" and I haven't even started restoration. Its main problem is the biscuits. I'm not sure they are all there and any that are there are in poor/rubbish condition. Seems like they fall to bits. Added that, if you think valves are a bit "deaf" at VHF you should hear them (not) at UHF. Once had a valve UHF tuner that was part of a UHF converter for a TV. The tuner was seriously deaf. When I replaced it with a transistor tuner it all sprang to life.

From what others here have said, some versions of the 770R had IFTs with discrete capacitors fitted. The problem I'm seeing may not apply to all 770Rs.

Last edited by space_charged; 18th Jul 2018 at 2:09 pm.
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Old 18th Jul 2018, 9:08 pm   #50
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Default Re: IF fault on Eddystone 770R

Hi Space, yes I've still got mine, however a house move means I'm lacking my discone and the new loft is too shallow for it to fit :-(

Eddystone drop hints the range 1 isn't that marvellous with the sensitivity figures, see attached. It's unlikely that the other ranges are all a uniform 2uv, but they had to admit range 1 was a little worse!

I also have a 770U which needs a full service, however it did pick up analogue TV sound before the digital switch over. I'm not sure there's anything left now for it to tune in to!
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