[R-390] RA17 receivers

Mon Dec 29 23:36:21 EST 2008

 Gentlmen please excuse the off topic post   But I did see someone making enquiries about RA17  Rx s  A few months ago  and I am aware that there is interest in these  British Wadley Loop  Receivers  , this is a potted Compendium of my experience with RA17 Rxs , Most problems I have encountered have been due to the disc ceramic caps that were used  in these receivers that  were manufactured from 1954 -1974, over theyears I have repaired around 50  of these Rxs .
 These are the  caps that  go  faulty, They are very poorly made  in comparison with modern Capacitors ,    the failure mode is the plating onto the ceramic , the caps that cause  the trouble are in the shunt tuning in the 37.5mhz & 40mHhz  filters ,they are used also in the  Anode tuning in the  1MHZ crystal osc, And Harmonic gen/low pass Filter ,    when used as coupling caps between tuned circuits they also  cause ,problems , I replace with High stability ceramic or plastic/polystyrene .   The caps I have had to change [From my RF workbook  Are:-  C21 , C33,C44,C54,C62,C71,C80,C89,C75 C25,C36, C46,C56,C64,C73,C82,C91,C121,C124,C150,C51,C68,C50,C67,C42,C9, C3 C113,C143 ,C107, Also  the  Image Rejection , on all these RX s  CAN BE IMPROVED   Its  is a function of the 400Khz bandwidth of the 2nd If filter, with the caps they had they put up with this limitation , But  with more modern High stability  caps  you  can make this filter Bandwidth at the 40Db down the slope +&- 100Khz so the total bandwidth  is 200Khz  instead of 400 Khz  As you are aware  signal to noise  ratio is proportional to bandwidth, and the performance is  improved and the image ratio is   improved over the original settings BUT you need to use good quality  HI Stability Caps for it to stay aligned,  I did my RX about 4 years ago and I checked the alignment and performance a few weeks ago and it was Spot on , I couldn't make it any better By TWEAKING   , I used  the spectrum analyser.  and tracking Gen  and its  a cinch to do ,To allign these filters  takes a couple of minutes , The other re-occuring  Problem is  in the 100Khz IF. is the Litz wire used in the  Final 100Khz IF.  inside the IF transformers its terminated under tension and after being cooled and heated over the years some strands have "let go"  And the band pass characteristics in the service manual cant be achieved When Re-alligning , I simply Run Solder  into the teminations burning off the varnish , problem Fixed !   Looking at the Block diagram of the RA17   you will notice MOST gain in the RX  is after the   Mixer V11 , So any SLIGHT LOSS ahead of this stage causes a large signal to noise  ratio problem , also the selection of first and second mixer tubes E180F/6688 s can also pep up the gain and give a discernable improvement in Sig to Noise+noise, if you have 10 of these tubes you would be surprised at the improvement  with HOT Tubes ,  I hope this hasn't annoyed too many R390A Aficionados ,as an inveterate Tinkerer I find  the RA17 s   Problems even more challenging  sometimes than the 390As   and the 51J4s that I own  Regards to everyone Bernie  VK2ABN  , Anyone wanting to discuss RA17 RX s  Please  Email me off list 

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