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Old 1st Aug 2018, 1:16 am   #55
Aubade65
Tetrode
 
Join Date: May 2012
Location: Oxfordshire, UK.
Posts: 69
Default Re: IF fault on Eddystone 770R

There seems to be some critical comments regarding the performance of the 770R in this thread but it should be remembered where this design is coming from.
The 770R was designed to meet a requirement for VHF surveillance in 1951 for use in the Korean War. The set originally produced for this role (770M) had coverage up to 220 MHz and was apparently a dcouble conversion set but was found to have stability issues so a redesign was necessary.
The most common VHF set at this time was probably the Hallicrafters S27 which covered a similar frequency range, was single conversion and had an IF of 5.25 MHz. As the S27 had been made in large numbers for use during the war it may have seen to Eddystone designers that the using a fairly simple single conversion design with a lowish IF would provide the desired performance and would enable them to concentrate on the front end. This seem to have been a possibility as the specification was issued in 1951 and the 770R was released in 1953 - 2 years that included the false start of the 770M.
I doubt if the Eddystone designers would have been too worried about image response as there would have fewer signals at these frequencies during this period - they would probably have been more interested in good stable gain.
Also Eddystone have always seemed to have been in the 'if it ain't broke don't fix it' camp with conservative designs that once proved are reused in future designs. They have always relied upon high quality components (particularly wound components) and a sturdy construction that often uses die castings that provide excellent mechanical stability. As has been noted these qualities are often placed above maintainability - do you really want to have to take the front panel off (not a trivial task) just to clean the dial glass?
In these days when a set covering these frequencies can be produced with vast improvements in performance in a almost trivial manner it should be remembered that the 770R is a 70 year old design and represented the cutting edge in design and build at the time. These sets were good enough to be used by the Americans and Jodrell Bank to track Sputnik - (but they do have a big aerial).
I have a 770R Mk2 that has more modern styling and a 3W push-pull output stage that is fairly flat to 10kHz - no idea why but it does sound nice. I find it is delightful to use with a concise set of controls and a silky smooth tuning that oozes quality. If I need more gain then I will either put up a larger aerial or a masthead amp (or maybe both)
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