[Premium-Rx] TenTec 340

[Michael O'Beirne]

Hi Gents,

I am going to upset some of you with this - but it is the evidence of my eyes.  

I tried out a 340 briefly at a ham rally in London a few years ago.  I thought the panel was well laid out and highly logical, but the tuning was utterly dead (no silky heavy flywheel tuning) and the knobs and buttons seemed to be made of fairly cheap plastic, at least in comparison with what I have on my Racals.  I was also upset that the meter does not sit in line above the grey panel legend, but perched over the edge as if it had had too much beer.  These radios are also very expensive over here.  Thanks to import duty and VAT, we pay in £s what you pay in $s, and that's a hell of a difference.  The price was £3,995.  It's all so unfair.

The set was reviewed in considerable technical depth in RadCom March 2002 by Peter Hart G3SJX.  He was suitably impressed with the audio quality, particularly in the AM synchro mode, but he found problems of close-in phase noise.  He was unable to make any blocking or IP3 measurements closer in to the carrier.  This is from a skilled professional with a good array of HP test gear.

He also identified a problem at MW and below.  I quote:
"The third order intercept was in excess of +30dB above 1.8MHz, corresponding to a dynamic range of of over 100dB in SSB bandwidths.  Below 1.8MHz, where the front-end filter switches to a low-pass design, the intercept and dynamic range reduced noticeably. The dynamic range figures hold well at close spacings down to the bandwidth of of the IF roofing filters. However, when a strong signal falls inside the roofing filter bandwidth, ie within some 10kHz of the receive frequency, but outside the main DSP filter bandwidth, some AGC action occurs, reducing gain and sensitivity.  This prevented meaningful closein measurements from being made" unquote.

I don't know what the night time signal strengths at LW MW are in the USA but here in Europe they are HUGE.  The BBC Radio 5 Live on 909kHz from Brookman's Park, just north of London, is about 95dBuV on the RA1792's meter using an indoor Wellbrook Loop!  The only way to cope with this sort of strength is old-fashioned premixer selectivity, as we had with the HRO, AR88, R390 etc.  In fact I reckon that for MW dxing, that sort of set could well knock the socks off most most synthesised receivers provided that the set is in proper alignment and with decent valves (sorry "tubes" for you!) and aided with a good external synchro or precision AM detector.  

Last weekend I dug out my Eddystone 730/4 (used by the Army in the late 1950s before they took the RA17s).  These were of better quality than domestic Eddystones and were very expensive in their days.   It's not in brilliant condition now and needs quite a lot of TLC, but I was so surprised at the audio quality via phones and how it coped so well with the big MW signals.  The big ones just came up and dropped off the filter edges with no squeaks or nasty synthesiser sproggies, though of course the selectivity could not match that of a modern crystal or mechanical filter.  It has the classic two RF stages with three high Q tuned circuits before the only mixer and then dropping to 450kHz.  The image rejection is way above 100dB at the top end of MW.

A good test of a receiver in the UK is to listen to Radio 5 and then tune 9kHz lower to 900kHz and see if you can pick up a very weak Italian station without being swamped by Radio 5.  There is another weak foreign one at night on 918kHz.  It's a lot cheaper than two 8640B sig genies and hybrid combiner!  I am sure that in N America there will be similar strong stations alongside a weak one.

A further thought for LW and MW addicts is to build a traditional indoor rotatable tuned loop about a metre square.  You should get 30dB attenuation off the ends which can be handy, and you get some very cheap selectivity right up front.

Some of you have mentioned smartening up the front-end with a narower roofing filter.  Alas it is not so easy.  There is a complicated process of matching.  All the components are in a simbiotic sort of relationship with each other. Some crystal filters have a lower IP3 rating than the mixers of their own receivers, even though they are passive devices.  For example, when the RA1772 was being developed back in the early 1970s, the roofing filter had to be redesigned omitting some ferrites which were causing all sorts of problems.  I know of a UK ham who had a special replacement roofing filter made for his RA1792 (40.455MHz) only 6kHz wide, and it cost a fortune, but they can be made if you are prepared to wait and to pay.

Au revoir for now
Michael
G8MOB
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