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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.
At last I can sleep at night, knowing where the blighters were hiding!
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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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Old 31st Jul 2018, 4:11 am   #51
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Default Re: IF fault on Eddystone 770R

Been away for a while but back now and been fitting the 100pf silver mica caps to all the IFTs. I've devised a way to remove the built in caps in the top paxolin plate. The IF gain is dramatically improved. I'll post some photos of what I did shortly.

My 770R had a long standing problem of frequency drift which I thought could be drift in the IF strip but much more likely in the local oscillator. I had already replaced all the components in the oscillator and the valve, with no improvement. I did notice that slightly moving the valve would produce big changes in frequency so I was seriously suspecting the valve base. The base has a mixture of normal pins and special ones that go directly to the tuning capacitor. One of these sets of pins goes to the variable capacitor via what Eddystone call a "built in" 40pf capacitor. That translates to a mica plate and some insulating washers between the two parts. I removed that in order to get the valve base out. The mica turned out to be in poor condition, possibly having been sprayed by someone in the past with switch cleaner. Not me I hasten to add.

I used an ultrasonic bath to clean as best I could the valve base. The pins were tightened up including the "special" ones and checked to make sure they now made good contact. After re-assembly with new mica the oscillator functioned correctly and showed no sign of the irritating drift.
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Old 31st Jul 2018, 5:44 am   #52
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Default Re: IF fault on Eddystone 770R

Good work!

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Old 31st Jul 2018, 11:51 am   #53
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Default Re: IF fault on Eddystone 770R

Thumbs up for persevering- TBH, I might have thought that a '50s bandswitched wide-span valved VHF comms receiver was on a bit of a hiding to nothing as regards sensitivity and stability anyway, but sounds like some real positive results coming on there,

Colin
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Old 31st Jul 2018, 11:51 pm   #54
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Default Re: IF fault on Eddystone 770R

The 100pg caps are built in to the paxolin squares (photos attached). They are attached to the top of the four wire pillars running up the IF transformer. To remove the built in caps the square has to be removed. Then the built in caps are dealt with by carefully slotting with a small saw. After all the conducting pieces have been removed, the paxolin square is re-fitted. It is needed to make the IF transformer mechanically stable. The modern 100pf caps are soldered to the pilers just below the square.
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Last edited by space_charged; 31st Jul 2018 at 11:57 pm.
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Old 1st Aug 2018, 1:16 am   #55
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Default Re: IF fault on Eddystone 770R

There seems to be some critical comments regarding the performance of the 770R in this thread but it should be remembered where this design is coming from.
The 770R was designed to meet a requirement for VHF surveillance in 1951 for use in the Korean War. The set originally produced for this role (770M) had coverage up to 220 MHz and was apparently a dcouble conversion set but was found to have stability issues so a redesign was necessary.
The most common VHF set at this time was probably the Hallicrafters S27 which covered a similar frequency range, was single conversion and had an IF of 5.25 MHz. As the S27 had been made in large numbers for use during the war it may have seen to Eddystone designers that the using a fairly simple single conversion design with a lowish IF would provide the desired performance and would enable them to concentrate on the front end. This seem to have been a possibility as the specification was issued in 1951 and the 770R was released in 1953 - 2 years that included the false start of the 770M.
I doubt if the Eddystone designers would have been too worried about image response as there would have fewer signals at these frequencies during this period - they would probably have been more interested in good stable gain.
Also Eddystone have always seemed to have been in the 'if it ain't broke don't fix it' camp with conservative designs that once proved are reused in future designs. They have always relied upon high quality components (particularly wound components) and a sturdy construction that often uses die castings that provide excellent mechanical stability. As has been noted these qualities are often placed above maintainability - do you really want to have to take the front panel off (not a trivial task) just to clean the dial glass?
In these days when a set covering these frequencies can be produced with vast improvements in performance in a almost trivial manner it should be remembered that the 770R is a 70 year old design and represented the cutting edge in design and build at the time. These sets were good enough to be used by the Americans and Jodrell Bank to track Sputnik - (but they do have a big aerial).
I have a 770R Mk2 that has more modern styling and a 3W push-pull output stage that is fairly flat to 10kHz - no idea why but it does sound nice. I find it is delightful to use with a concise set of controls and a silky smooth tuning that oozes quality. If I need more gain then I will either put up a larger aerial or a masthead amp (or maybe both)
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Old 1st Aug 2018, 2:06 am   #56
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Default Re: IF fault on Eddystone 770R

Well done Space Charged.
And thanks for the follow up aspects.

Looks like the original caps were TCC specials.
I think we all would be interested in the RX specs when you are finished.
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Old 1st Aug 2018, 2:09 am   #57
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Default Re: IF fault on Eddystone 770R

Well said Aubade65.

The reason I'm restoring my 770R is because I like Eddystones. I also have a 680X which has much the same look as the 770R. The 770R also has push-pull output and wide band FM (in addition to narrow band) and it does sound nice on radio 4! The tuning on Eddystones of its vintage have a wonderful "feel" to them. You have to get your hands on one to know what I mean. There is real "magic" moving up and down the bands on an analogue wireless - I love it. True there is more to listen to on the 680X and I regularly do that but I will use the 770R.

Thanks for the history and the political environment at the time the 770R was designed. My particular 770R was ex BBC and was modified to be used to drive monitoring equipment for early FM stereo (19Kc/s pilot tone) multiplex broadcast signals. Apart from being modified, which I have had to restore, it is in very good physical condition.

Last edited by space_charged; 1st Aug 2018 at 2:18 am.
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Old 10th Aug 2018, 6:09 pm   #58
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Default Re: IF fault on Eddystone 770R

Quote:
Originally Posted by Nuvistor View Post
Similar types were used in American production, they did give problems.
Your statement is an understatement, at best! Did they give trouble!
It wasn't as bad ages ago, as replacements were readily available, but now you have to take the time to replace the bad base caps. It's referred to as "Silver Mica Disease".
You can't even depend on new-old stock, as they can be bad too.
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Old 10th Aug 2018, 7:26 pm   #59
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Default Re: IF fault on Eddystone 770R

Quote:
Originally Posted by wd40addict View Post
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
Thanks for this link - very interesting
NB: TalkTalk are closing their Webspace service - screenshot herewith.
Best wishes
Guy
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Old 10th Aug 2018, 7:31 pm   #60
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Default Re: IF fault on Eddystone 770R

Re. TalkTalk Webspace closure ... the 10th is today - ! - so here's the PDF equivalent.
[wd40addict, I hope that's OK with you]
Best wishes
Guy
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