Thread: Eddystone 888A.
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Old 8th Dec 2015, 12:51 am   #74
Skywave
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Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
Arrow Re: Eddystone 888A.

Wendy: two remarks from me.

1. Many moons ago, I had a fault on my 888A on 21 MHz where I could not get the local osc. to track. Like you, I went back and forth until I eventually either ran out of adjustment or gave up, realising that something was seriously wrong and serious intervention was thus required. All of this was such a long time ago that I cannot recall the degree of analysis and proof that I undertook which led me to substantially alter the value of a small value capacitor in the L.O. tuning. As I said, all of that was many years ago - but all these years later, with this receiver having undergone several re-alignment checks since, without any problems making any necessary adjustments (trimmers and coil cores only), that impromptu capacitor is still there. The reason for that substantial change was - and remains - a mystery, especially as all the other caps. in the L.O tuning measured the correct values. The most likely suspect - since it is the one item that wasn't measured nor replaced - is the coil itself, but a much later analysis of the cct. operation seems to rule that component out of the list of suspects.

When I get obscure faults like that, I like to fully understand why - but sometimes with such faults, I simply have to settle for "Ah! Now it's working correctly: I'll settle for that!" Fortunately, that approach doesn't keep me awake at night worrying about it!

One further idea: the tuning coil may have two positions for resonance: one with the core well 'down', one with the core 'up'. You may have had the core in the wrong position: always worth a try.

2. Re the ECC85 modification. There was one little detail I inadvertently overlooked: sorry, but easily remedied. The previous owner had made a module to house a few extra components. It consists of a B7G paxolin valve plug and a B9A chassis-mount socket. The B9A socket has a hand-made cylindrical metal skirt made of tin-plated thin steel sheet which is soldered to the lugs of the B9A socket, thus acting as an earthed screen. Mechanical support between the B7G plug and the B9A socket is accomplished by stout wiring (where no additional components are required - e.g. heater wiring and ground connections). That mod. is thus readily reversible, but the removal of the entire ass'y. from the B7G socket and the subsequent fitment of a 6BA6 does not noticeably affect the set in any readily appreciable manner * - after a few minor compensatory adjustments to the R.F. trimmer in the coil box.
Hopefully, that description will enable you to construct similar - if indeed you so choose.

* One day, I really should perform the 'two signal generators intermodulation test' for that modification. But I have enough things here that aren't working, or not working properly, (plus various design projects, and some of those have nothing to do with electronics & radios) to keep me going for years!

Al.
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