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.4.
Fig.3.
Fig.5.
Fig.2.
Fig.6.
