Eddystone 680X Installation

EDDYSTONE 680X PRODUCT DETECTOR INSTALLATION.

The 680X is, by its design, a very good radio. It was designed for the reception of AM and CW signals. SSB

can be resolved by turning down the RF and turning up the AF control however its not very satisfactory.

The best way is to install a product detector to resolve the SSB and CW signals. The basic idea is that the

original BFO module is removed and rebuilt using a 6BE6/EK90 heptode valve, part of which acts as the

BFO frequency oscillator and the rest as a mixer. There are various circuits available based around the

6BE6/EK90 and the one I used is shown in Fig.1 below and was kindly given to me by M0MLM. (Every

component within the dotted lines goes inside the can).

Start by removing the case. ( obviously!!)

The BFO module is the module behind

the BFO switch with a valve sitting on top

of the can. Looking under the chassis

there are 5 connections to the module.

See Fig.2 below

Remove the BFO valve and unscrew the

two screws between the connections and

unsolder the 5 connections which appear

to be soldered onto wire ended stand

off's. Remove the whole can, then the

two side screws and remove the cover

allowing access to the components. See

Figs 3,4,5 & 6. below

Fig.1 Product Detector

Fig.2.

Fig.5.

Fig.6.

Fig.4.

Fig.3.

Table of Contents

Need help?

Do you have a question about the 680X and is the answer not in the manual?

Page 1 EDDYSTONE 680X PRODUCT DETECTOR INSTALLATION. The 680X is, by its design, a very good radio. It was designed for the reception of AM and CW signals. SSB can be resolved by turning down the RF and turning up the AF control however its not very satisfactory.
Page 2 Now comes the interesting bit. Remove all the components within the can except the heater connec- tions and the two silver mica coil tuning capacitors and coil as shown on Figs 7,8 & 9 below Fig.9. Fig.8. Fig.7. Re-model the components by moving the grid connection from pin 2 to pin 1 and fit the new compo- nents within the dotted line as per the circuit diagram (Fig.1).
Page 3 Fig.12. As you will see in the new circuit there is an audio filter to be made consisting of a LF choke and a couple of capacitors (a Pi network in fact) It asks for a 1 henry choke which I didn’t have so I used the primary of an old transistor radio output trans- former as the choke and adjusted the capacitor to give me a cut-off at about 3 kHz using an audio generator and an oscillo-...
Page 4 As shown on the circuit diagram (Fig.1) bridge the HT contacts with the bleeder resistor 3.3 M ohms on the back of the switch. ( this is there to keep the 8/10 uf cap charged and thus giving an instant voltage onto the product detector on selection to stop frequency blip.) Having completed the audio filter network I mounted mine adjacent to the coil pack and supported it us- ing a bit of 12 gauge wire clamped under the coil box lid.