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Old 7th Apr 2021, 3:51 pm   #9
turretslug
Dekatron
 
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 3,728
Default Re: MF/HF Receiver type R/75/WRDV/3550

The tuning gang is constructed as a 4-gang, but the two wider-spaced sections are strapped in parallel to form the oscillator section, reminiscent of the way that the R1155 and TCS12 etc. use a wider-spaced oscillator gang to help improve stability. One point of interest (OK, to me anyway....) is the way that the whole of the earthy side of the aerial input circuitry up to the 6BA6 RF stage grid is carefully isolated from the main chassis, only connected at a single point to reduce circulating currents that could increase oscillator radiation. The aerial coilbox sits on several (hidden) Paxolin bushes and strips, the rear section of the bandswitch shaft is also made of Paxolin with a clamp coupling to the conventional steel mixer/oscillator front section and the ceramic gang capacitor shaft completes the isolation. There's even an anti-circulating-current slot stamped into the chassis below the aerial section of the tuning gang and the other two. In these respects, it's reminiscent of the pioneering Scott SLR (Scott Low Radiation) receivers of WW2, also intended for ship-board use with the need for minimal oscillator radiation.

I found a suitable 6400kHz crystal in a rummage box at a rally, just for the sake of 5MHz time-signal reception. It must have one of the nicest tuning drives on commonly-seen receivers, light, smooth and precise with no clever-dick bevel gears or worm drives in the way to wear and cause lash and, unlike the AR88, they bothered to fit a positive end-stop mechanism.

The wiring loom is lovely neat stuff but being early single-strand PVC stuff is horribly prone to shrink-back with the (inevitable) capacitor replacement, I used a small croc-clip loaded with damp blotting-paper clipped to the wire insulation immediately adjacent to each terminal spine before applying the iron- time-consuming but it worked better thanI expected.
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