Eddystone 888A Audio Input query

[carnivalpete]

Does anyone have any idea why Eddystone would think it useful to add an audio input option onto a 888A which is after all an amateur comms Rx. There are two pillar terminals below the aerial connectors on the Rx back panel that appear to be connected to the noise limiter V7/6AL5 stage and the input side of the V6/6AT6 pre-amp stage. The feature appears on the wiring diagram but I cannot find any technical details or even a comment in the instruction manual.
Any theories? pete

[G8BBZ]

Hi,
Sidetone from an associated transmitter perhaps? The 730/4 has exactly the same input, which is labelled "Pick Up" - though I can't imagine it saw much use for that purpose in military service.
cheers
Peter G8BBZ

[Herald1360]

Maybe it would appeal to the ex-pat market? Someone with a bit of radio savvy might use the set domestically and still want the same feature as found on ordinary domestic sets. Some of the more consumer oriented Eddystone sets even had push pull audio output.

[Radio Wrangler]

For playing Morse practice records of course!

When I was brushing my Morse speed up, a kind soul made me a load of cassette tapes of decidedly blue jokes from his student rag magazine collection. They wound up in the car radio slot, of course, and some people must have been mystified by the recognisable sound and the laughing driver. in the traffic jams.

David

[Skywave]

If you study the evolution of Eddystone radios of the distinct communications class, you can clearly see how the 'next' model was derived from its predecessor: the S750, the S888 and then the S888A are typical examples. Plus, if you study the cct. diagrams, you can see a similar 'bolt-on' effect. But perhaps the audio input facility was more than just a case of "that bit doesn't change, so we'll retain it", as "our customers are used to certain 'extra facilities': we'd better make sure that they remain there".
So, go back far enough in time and models and Eddystone probably chose to include the audio input as a 'gram input'. Then as the years (and models) rolled by, some 'features' were simply retained.

Al.

[carnivalpete]

Some interesting ideas guys. I certainly remember the 'gram input' as a standard feature on many domestic radios (or do I mean on the wireless?) whilst I was growing up and fiddling with anything electronic in the 1950's.
My suspicion is that it was just easy enough and cheap enough to do at a time when 'brown furniture' wireless would also have had it and Stratton wanted to appeal to the widest possible audience. Nonetheless, I still cant help thinking it was a bit quirky to leave it on when they developed thier only true amateur comms set even if it was developed from a general purpose receiver. Rgds, pete

[Skywave]

Yes, Pete, agreed: it was a bit quirky - but most Eddystone kit had and has distinctive quirky features: one way to stand out from the competition.

Al.

[Silicon]

The model S.670 of 1948 and the S.680/2A both have P.U. audio inputs.

[turretslug]

GEC's BRT400 also has a "PU" input, again connected in the detector/1st audio input circuit, and with no switching. (Anyone familiar with this set will have noticed the designer's apparent liking for construction techniques and components that seem to have come from GEC's cooker parts store, the rear panel terminals would comfortably accomodate 4 or 6 sq. mm cable, or its imperial equivalents!) It had a relatively meaty output valve and an unusually large output transformer- certainly for a communications receiver- so could probably make a good fist of an external source.

Both Eddystone and GEC pitched these sets at the professional user- I wonder if one line of thought was that, in a monitoring situation, something like a single channel priority receiver (possibly squelched) for 500kHz, 2.182MHz etc. could be connected to the PU terminals so as to break through into the main audio channel, or maybe a "now-hear-this" PA line. These sets had muting switches that would quickly quiet the front end but leave the audio chain unaffected- even someone with headphones on would have no excuse for missing a priority source using this mode of operation.

[Skywave]

Originally Posted by Silicon

The model S.670 of 1948 and the S.680/2A both have P.U. audio inputs.

By strange coincidence, I am currently reading some old issues of Wireless World covering the period 1947 - 1950 which I have recently acquired. In one issue, the S.670 is described in detail. I noted with interest that the overall cct. design - including the variable selectivity method used - is very similar to that of subsequent Eddystone receivers: 750, 888 and 888A. Very little change then over a period of about 15 years (except for the tuning dial: half-moon semi-circular to slide-rule style).

Al.

[carnivalpete]

I'm beginning to think that I posed a pretty dumb question here and one that the Stratton boys never even considered asking themselves. They didn't spend any time at all on designing it in as it was already in, as it had been in almost every other previous model. They repeated so much of their previous circuit designs in so many receivers that it was already a fixed feature and it probably didn't even cross their mind that they might be able to take it out in an amateurs Rx. Nonetheless, it would have been quite interesting to see some mention of it in the instruction manual with some hint of the intended input power level and the possible applications they had in mind.

[carnivalpete]

I think I may finally have got a little closer to an answer to this. I have just been reading the Eddystone 750 Instruction Manual, in particular the ''Use of the Standby Switch'' which is operated when transmitting.
All will be aware that the standby switch desentisizes the 750 but leaves oscilators hot and ready for renewed use. The paragraph states: ''....during standby periods ......since the audio stages remain ''alive'' a monitor signal can be fed into the pick-up terminals and become audible on the LS or 'phones. The receiver itself becomes available as a monitor of the outgoing signal.........
There is a little more info but basically there it is. The designers appeared to have found a use for a connection called ''pick-up'' that they already had and originally intended for connection to the record player (ie gramaphone).
Not always a common manly trait, but it's amazing what you find when you reluctantly get around to reading the instruction book.
Well done designers! A Tx monitor to check you are not talking to yourself!
Unless I have come to the wrong conclusion I think thats it guys. Unless anyone has any closing remarks I think this thread is done.
Many thanks for all the responses. Most interesting.
pete

[Skywave]

Pete: I think you've got it: well done! So it started life as a gram. input, and one day someone on the S.888A design team asked "Why is there a gram. input on this AmRad comms. receiver? Who is going to connect a gramophone to a receiver such as this? " And someone replied "So that the transmitting Amateur can monitor his own out-going transmissions."

But isn't that is one of the golden rules of design engineering: where you can, convert a 'problem' into a 'feature' ?

Al.

[Radio Wrangler]

Monitoring is nice for speech transmissions, but sidetone is pretty much essential for Morse transmitting, so even if th receiver is fully muted to antenna inputs, the audio input would be very useful for the tone from a sidetone oscillator.

Aircraft radios routinely sample the output of the transmitter, right at the antenna cable connector, detect it and feed it back to the pilot's headset, so he knows that if he hears himself in the phones, the transmitter is working.

David

[G6Tanuki]

Pretty much every "communications" reciver from the 1940s onwards has provided mechanism for a seriously-desensitized side-channel for monitoring your transmissions - which has been entirely RF.

The traditional approach on superhet radios has been to short the antenna-input to earth [so protecting the front-end from tens/hundreds of volts of RF flying around] along with sticking a seriously-high-value resistor [usually variable] in series with the RF/IF-gain control-to-ground [and/or or grounding the RF/IF stage screen-grids] or to stop-back the RF/IF gain by loads-of-Decibels.

In a military/avionics context: sampling the RF from your transmitter at the antenna, and relaying it back through the [transiently desensitized] receive-chain to recovered-audio proves both that you're feeding a good signal to the antenna, and that your receiver's working. I always called this "local-loop confirmation".

The big thing was to keep the local-oscillator(s) and the BFO running throughout - if you didn't do this you could find that when you switched back to 'receive' you were way, way off-frequency! Keeping the audio-output stages drawing their usual current helped here too, both by providing background heating of the receiver's internal parts and stopping the HT-rail from wandering off high. At least one receiver of my acquaintance had a 5-Watt resistor fitted in the coil-box which was only powered (from the HT rail) when in 'standby' mode so it kept the coil-box warm and also supplied a base-load to the power-supply and stopped the B+ rail wandering off to extremes.

I don't really think a "gram" audio-input would have been a sensible inclusion for communications purposes: I'd class it much more for entertainment purposes.

Look at the "standby" switch on something like a humble Eddystone 840A [which I class as an 'enterteinment' grade radio despite the BFO and up-to-30MHz coverage] - it drops a high-value resistor into the bottom of the RF/IF-gain potential-divider, and typically desenses the receiver by around 60dB.

[carnivalpete]

Thanks for comments guys.

I guess this thread can now be closed

pete