I think that theese "echo" units are built like a normal reverb tank for use with electric guitars and Hammond organs. There are 1-3 springs with pickup coils in both ends.

-Søren

Yes, you are both right. This one is made that way too - we'll
get to that.
And Peter, if that's true go sit in the corner !
And you too, Lee ! - A different corner !
We can't have you two together...

Martin

The EK4 is an echo-unit that is more or less self-contained
and will work with almost any speaker-level output and line-level
input but has been physically constructed so cleverly as to
mount at the rear side of the Grand Prix 609 (and other similar
contemporary sets) where most of its somewhat "industrial"
design will be out of normal view.
It has been fitted with two brackets that grip into two of the
slots at the top of the rear cover frame and will then hang
down there with its control knob facing up.
On this unit, the two holding brackets are bent in a slightly
wrong angle, probably happened during years of storage. Should be
an easy fix anyway.
The control knob adjusts the echo level and has a switch function
with a distinct click when rotated to the extreme
minimum setting.
A cable comes out of the unit, ending in a valve-type
(nine-pin Noval) plug that fits the socket at the rear of
the Grand Prix 609.
No other connections or controls are provided.

Martin

Off with the lid and let's have a look inside.
First thing I noticed was a tiny ball of solder that came out
when the covers were pried apart.
So that was the little object, fine !
Inside we find a printed circuit board with the transistor-based
amplifier, the potentiometer with switch, the "Hammond" spring
based reverberation element with its two tranducers and two
pickups (all coils) and a folded post-it note.
The latter not being part of the original construction.
Wait a minute, a post-it note ?  That means that someone
has been in here recently.
Well, recently may not be the right word. A quick wiki lookup
gave the period for the beginning of nationwide use of post-its
as the early 1980's (learning every day), so we will have to
extend the limits of "recently" back to around that time, I suppose.
The note merely showed the plug connections, drawn and handwritten.
I have all that info in the service manual already but it's very nice
to see that someone kept info in there.
It could just as well have come in very useful and it's
not uncommon to find little notes and drawings like this inside
the older pieces. Sometimes even hidden keys for whatever, other
well-kept secrets and/or notes of more private character show up
in old radios.

The potentiometer looks fine, rotates with a nice slow-moving
friction feel and switches with a nice click sound.

The circuit board looks fine too but a closer look reveals
the electrolytic capacitors to having more or less dried
out and/or having cracked end caps and the larger one has
the typical bulge outwards near its positive lead indicating
overpressure.
The need for a complete re-cap comes as no surprise.

The spring element has been mounted "floating", suspended by
glueing its metal chassis to two pieces of soft foam
material, again glued to the inside of the EK 4 casing.
Yes, you guessed it, the foam has rotted away, letting go of the
whole spring element, allowing it to bump against the sides
of the unit when tipped.
So that was the larger rattling object.

All in all a couple of evenings work lined up here.
Wonderful !

Martin

A couple of evenings were spent in the repairshop but to make
a long story short, the spring element had as much as
possible of the remains of the old foam removed and was glued
back into place using two new strips of foam.
The circuit board had a good handful of new capacitors fitted,
the angle of the two brackets was corrected and the metal
casing received a surface treatment.
Much better now.

Martin

A little difficult to photograph but it is hammer-effect lacquer, quite
similar to the original.
Cabinet screws were cleaned from oxidation and painted a silky black in
perfect following of the idea of keeping as much as possible of the original
and repair and restore rather than replace.

Martin

Dillen:

A little difficult to photograph but it is hammer-effect lacquer, quite
similar to the original.

Wow, this is simply amazing. The cabinet looks like new! Are you going to restore the text around the knob, too?

Yes. 

A little decal job later :

Martin

And the center one, deliberately background toned a bit :

Martin

WOW!

Double WOW +1

The sharp-eyed reader will notice that the decals and brackets
are now upside down compared to what they were originally.
The reason is simple and why it was made differently puzzles me;
You see, the echo unit was meant to sit vertically at the rear
of the Grand Prix with its two brackets gripping into the frame
woodwork, in other words with a certain side facing forward.
The original layout of the decals was then facing the wrong way,
readable from behind the set.

"Why not just rotate the two brackets ?", I hear you ask.
Well, that would solve the lettering problem but it would also put the
cable exit from the echo unit at the other end and the cable is not long
enough for its plug to reach the socket on the rear of the
Grand Prix set, about 5cm short, and the positions of the sets connectors
at the rear, including the one for the echo unit itself, does not allow the
echo unit to go any farther left (as seen from the front).
So, the decals and brackets were rotated to suit the fitting
on the set and to be readable from the front.

And thanks for the comments, yes - compared to the first photo, it does look nicer now.

Martin

Maybe one or two of the readers wonder about the technical side of things (I would):
The two speaker outputs from the Grand Prix is fed directly
to the speakers as it would normally be, but also to the echo unit where it
drives the Hammond element's transducer or "sender" coil.
The sender coil introduces longitudinal vibrations to the long springs according to
the programme material (as if the springs were attached to the cone of a
loudspeaker) and this "information" arrives, slightly delayed and now
with a certain "roomy" effect at the other end of the springs where the receiver
coil (microphone or pickup) converts it back into an electric signal.
The longitudinal reflections from the ends of the springs also adds to the effect, with
the signal echo'ing or reverb'ing as the waves travel back and forth the length of the spring
and fade out as they go.
The signal from the receiver coil is then looped through a pre-amplifier inside
the echo unit and returned back to the Grand Prix stereo amplifier where it is fed
to the left and right inputs in opposite phases.
There's only one channel through the EK4. This means that the programme signal
played by the speakers can be stereo but the echo will remain mono, though with
reverse polarity. This will give what we today would call an "echo" effect
plus some kind of "hall" effect at the same time.

Making it this way means that the best effects of using the echo unit is
achieved when playing mono material which will have a certain amount of roomy
feel (artificial stereo) applied.
Stereo material will still be stereo but the echo/hall effect will be mono - in
reverse phases.
At a time where most available records and indeed most records in your own collection would
still have been mono, this unit would have brought a bit of modern sound to the old recordings.
I suppose that was the big idea with this unit. A DIY-stereo conversion. Or at least something that sounds
a bit less flat.
- I can't wait to try this.

What may also be obvious to electronic nerds is that the position
of the balance control on the set needs to be rather exact.
Any offset here can cause instability (think microphone/speaker
feedback) and that should of course be avoided.
For this particular reason, B&O provided a setting of the Echo-knob
on the front of the Grand Prix to do a momentarily coupling of the
two channels in anti-phase while you adjust the balance control for
minimum sound output to get the exact point of equal balance.

The choice and amount of echo can then be set on that same knob on the
front of the Grand Prix while leaving the control on the echo unit
set at max.
The EK4 will also work with the Dirigent 609K and 610K where
the same control and plug is provided.
Owners of the (slightly cheaper little brother) Master Deluxe 609
will not have the control on the front of the set but will
instead be using the control provided on the echo unit itself.

Martin

Very good... but HOW DOES IT SOUND LIKE? ...sound like... sound like... sound like...

Well Mika, nothing like that - that - that..
More of a hall effect than actual echo'inginginginging..... 

A couple of sound recordings were made to demonstrate the echo unit in action.
Both are from FM radio source and recorded "live" using the same method,
microphones and setup as earlier in this thread.

First recording is Elvis Presley, originally mono but play the file
back in stereo and note the roomy "artificial stereo" hall-like sound
as the effect from the echo unit is gradually applied and finally switched
off again :
http://users.cybercity.dk/~dsl42943/Lyd02.mp3

Note also that the Grand Prix makes a short thumb noise when the echo
unit is switched completely out.

Martin

Brilliant! A much better effect than I would've expected

Great job - Martin!

I love that valve sound!

Menahem

Next recording is danish speech.
Danes will recognize this as part of the traditional
saturday "Giro 413" programme though with varying amounts of
hall effect.
http://users.cybercity.dk/~dsl42943/Lyd03.mp3

Martin

Martin.  As usual, a fantastic thread, I have learned so much. Your use of plain English makes it so easy to follow.
BTW, I think I was there when Peter and Lee commited the crime.

Now, how can I listen to your sound clips? Left clicking just opens a blank screen.

Kindest regards, Andy.

Andy,

The above links are to mp3-files. (The links are in the text - it's not the photos).
You need to have your operating system set up to associate mp3 files
with a program capable of playing them and it will automatically launch
that program and play the file when you click on its link.
Alternatively, download the file to your hard-drive by right-clicking
on the link and then choose "save target as". Choose a place where you
can locate the file again and click "save".
Launch your favourite sound programme (media player or similar), open
the file you just saved and play it.

If you have a slow internet connection, the sound files may take a moment to download.

Thanks for the language comment. I try my best.

Martin

Cheers Martin. Used "Save target as" and played with itunes.
Interesting hearing how the echo is generated through such a simple device.  Thanks for sharing.

Andy T.

Yes, it would have been interesting to see the looks on peoples faces when some engineer first
presented his idea of making a hall effect by sending sound through long springs.
A bit cartoon-like, it must have felt at first I suppose, and it's still a bit ackward but it works wonderfully none-the-less.
One thing that beats me though is that the whole echo unit is quite unsensitive to touching. Even tapping it using
finger-force equalling that normally used on a stubborn PC keyboard has no influence on it's functionality
and causes no new sounds to be introduced.
All down to good filtering.

Martin

I've been asked:

The signal from the amplifiers sent to the speakers now contain both the original signal and the echo effect signals.
Note that all this is sent to the echo-unit - which for the produced echo signals means "again" - so what is
preventing this whole thing from going into a howl with the echo effects travelling round and round and never escaping ?

Good question !
Well, since the echo effect is mono in reverse phases, it will equal itself out as the left and right channels are
mixed together inside the EK4, leaving only signals that are not in opposite phases = the original signal to enter
the echo circuit.
So the original signal will only be allowed that one trip through the EK4. As soon as it has been there, the resulting
signals are in opposite phases and are automatically eliminated when it comes around again.
Clever and simple.

Martin

Wonderful job you have done with that old "609" Martin.

Oh, I remember my parents had one of these when I was a kid. It was probably this receiver, that started my affection for B&O. Years after, when I was about 13-14 years old I bought my first B&O -a Beomaster 901. A few years after it was replaced with a BM 2400 -which was replaced by a BM 3300 and so on... Today I have a nice little collection of B&O sets from the 70's, 80's and 90's.

By the way - my parents speakers were in the same design as the cabinet of the radio (not like the speakers you used -though they were very nice  ). I also remember, my sister once had a receiver from another danish company -an "LL -Apollo". It had the same knobs as this B&O... Standard knobs fom the 60's..?

Well -it's always nice to read your posts. Keep up the good work.

Steffen