[R-390] RA17 receivers

[bavarianradio at comcast.net]

Hello Bernard, Thanks so much for the "off topic" info on the RA 17. I have 2 of these, an "L" model and a MKII/C3. I appreciate your experience and knowledge. I will reply "off List" Happy New Year, Ross W1EKG
----- "Bernard nicholson" <vk2abn at bigpond.net.au> wrote:

> 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