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Old 22nd Apr 2018, 1:06 pm   #1
carnivalpete
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Default Eddystone 888A Receiver

Split from this thread: https://www.vintage-radio.net/forum/...d.php?t=145838

A couple of years back I bought a 888A that had been in someone's garage for a year and I brought it up to speed too quickly. The 3rd IF failed and unfortunately that meant the end for that radio. Probably damp in the winding. I still have the radio but don't hold out any hope of replacing the IF.

Last edited by AC/HL; 24th Jul 2018 at 12:34 pm. Reason: Thread split
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Old 20th Jul 2018, 11:08 pm   #2
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Default Re: Eddystone 770R MK2 Receiver (price?)

Quote:
Probably damp in the winding. I still have the radio but don't hold out any hope of replacing the IF.
An IFT can be re-wound. They are high frequency (5.2MHz) so not many turns. Only the primary is likely to have been damaged, the secondary operates at earth potential.

Might be a bit "fiddly" to get the number of turns right but a scope/wobbulator would show you what is going on in a re-wind.

There is an extra winding on the secondary that broadens the bandwidth on the FM setting. That, like the rest of the secondary, operates at essentially 0 volts but is closer to the primary (which is at over 200 volts). I suppose, if damp, that might have shorted over.

The primary is connected to the HT by a resistor to I'd be surprised if the IFT was damaged at all. More likely shorted or leaky wax/paper capacitors in the IF amplifier.

Last edited by space_charged; 20th Jul 2018 at 11:24 pm.
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Old 21st Jul 2018, 12:21 am   #3
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Default Re: Eddystone 770R MK2 Receiver (price?)

I think carnivalpete is referring to an 888A final IFT, rather than that of a 770R- if so, that operates at 85kHz and has quite a lump of wave-wound inductance on its primary and secondary circuits. That probably would be quite difficult to reproduce (assuming that it's a winding at fault)- though I would happy to be proved wrong in that respect by a forum member with the appropriate skill and facilities, I'm constantly awed by the abilities pool here!

The 750 and its amateur-band-only brethren the 888 and 888A share a common shortcoming/oversight/thrifting- this final IFT is fed directly from the HT line with no decoupling resistor, so that the very fine wire could conceivably be fused by a short/heavy leak within the IFT. Conscious of this, I fitted a small 2k2 film resistor (and associated 330nF polyprop) in series with the anode winding when overhauling so as to at least give the winding (not to mention PSU) a chance. It seemed a strange oversight, if only from the decoupling rather than protection point of view not to have a feed resistor here- but maybe I'm thinking with "thorough", rather than "bean-counter", hat on! I was gratified to subsequently find that the 750's pro-market derivative, the 910, actually does have a 2k2 here.
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Old 21st Jul 2018, 12:31 am   #4
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Default Re: Eddystone 770R MK2 Receiver (price?)

Thanks turretslug for that, yes I was thinking of a 770R. If the IF frequency is 85KHz then the coil would be quite large and more difficult. Probably possible though.

Thanks also for the information on the IF amp. If there is no resistor in the circuit then the coil could well have suffered.

Last edited by space_charged; 21st Jul 2018 at 12:46 am.
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Old 21st Jul 2018, 4:38 am   #5
G4XWDJim
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Default Re: Eddystone 770R MK2 Receiver (price?)

Carnival Pete,

In May 2017 when I had my Grand Garage Giveaway there was a big poly bag of Eddystone coils which had some large LF looking IF coils, maybe 85 k/cs in it taken away by possibly one of the 15 forum members who turned up.

Perhaps a wanted ad in these columns might be useful.

Jim

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Old 23rd Jul 2018, 10:00 pm   #6
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Default Re: Eddystone 770R MK2 Receiver (price?)

Can anyone tell me the value of the resonance capacitor in the 85KHz IF transformer?
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Old 23rd Jul 2018, 10:08 pm   #7
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Default Re: Eddystone 770R MK2 Receiver (price?)

800pF 2% silver mica.
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Old 23rd Jul 2018, 10:11 pm   #8
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Default Re: Eddystone 770R MK2 Receiver (price?)

Thanks turretslug, I'm going to have a go at winding a coil and see if I can get resonance at 85KHz.
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Old 24th Jul 2018, 12:22 am   #9
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Post Re: Eddystone 770R MK2 Receiver (price?)

space-charged: I've read your posts 6, 8, 10, 12 and unless I've misunderstood what you wrote in those, there seems to be some confusion between the IF in the 888A and the 770R.
With that in mind, I'm writing this post so that others reading this thread will not be similarly confused. No disrespect to you is intended.

The 888A is a double-superhet with two fixed-tuned IFs: 1620 kHz and 85 kHz. The tuning capacitors for the 85 kHz IF are 800 pF. That IF has a mechanical arrangement to alter the bandwidth.

The 770R has an IF of 5.2 MHz. All the IF tuning capacitors are 100 pF, except the last one, C55. That is 50 pF.

Aside.
5.2 MHz seems a strange choice for the IF: I would have thought 10.7 MHz would have been a better choice, or, better still, a double-conversion scheme.
With a 5.2 MHz single IF, image problems on range 2 will be serious.
http://www.radiomuseum.co.uk/eddy770r.html

Al.

Last edited by Skywave; 24th Jul 2018 at 12:31 am.
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Old 24th Jul 2018, 1:09 am   #10
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Default Re: Eddystone 770R MK2 Receiver (price?)

Nop, not confused. The thread was about the (financial) value of a 770R if I'm not mistaken. Someone then added something about his Eddystone having had a fry up and lost an IFT as a result. That turned out not to be in a 770R but an 888A which has a much lower frequency IFT. I had originally said it would be relatively easy to rewind an IFT (thinking it was for a 770R). In that I agree I made a wee mistake, but the thread was about 770Rs so perhaps understandable.

A rewind of a much larger coil would be less easy, but having thought about it, not impossible so I might have a go at doing a 85KHz coil.

As to the choice of IF frequency for a 770R, well that is of course down to Eddystone and 5.2MHz it is. Of course you are quite right that these days 10.7MHz is the "norm".
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Old 24th Jul 2018, 4:21 pm   #11
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Default Re: Eddystone 888A Receiver

I started the original Eddystone 770R value thread as I was looking to buy one.
I actually paid £75 for it unchecked as I needed a mains lead which I sourced eventually .
Works well after having an expert look it over for me .

Last edited by electronicskip; 24th Jul 2018 at 4:32 pm.
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Old 24th Jul 2018, 4:56 pm   #12
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Default Re: Eddystone 888A Receiver

Worth taking the IF apart just in case it has 'fused' around the solder joints.
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Old 27th Jul 2018, 8:04 pm   #13
carnivalpete
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Default Re: Eddystone 888A Receiver

With ref to the above ''Eddystone 888A Receiver''. Many thanks for the suggestions and please accept my apologies if my note has caused any unintended confusion.

My intention was not to bemoan the lost IF, although it was hardly welcome. I was simply offering my sad example as a warning and suggesting to the contributor (Electronicskip: Eddystone 770R MK2 Receiver) that if the Rx had indeed not been used for 5yrs it should be brought back to life very, very gently, with the supply voltage being raised only very slowly in order to ensure that everything was properly dried out. Otherwise, it was possible that something would go phut (technical expression) as occurred to my 888A when it's 3rd IF failed.

As to the sad fate of that 888A, it was replaced in due course by another 888A which I am happy to report is still working very well thanks to the tender, technical touch of a good friend on these boards.

I still have the defunct 888A intact, but currrently have no plans to attempt a repair. If somebody else would like to take a run at it, we could discuss it.

Moderator:
Your decision of course but as far as I am concerned this thread could be closed.
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