[R-390] DIY R-725

[Tom M.]

Ken,
I originally though the new transformer in the R-725 was for the IF filaments, but the late Wally Chambers had the right answer; it was for the PTO only to provide hum bucking.
All the other filaments in the new IF deck come from the R-390A PS and the modification I wrote up is faithful to the SERIES 500 IF deck as built by Motorola, Arvin and Servo. 

73 Tom 


      From: Ken Harpur <kenharpur at startmail.com>
 To: "r-390 at mailman.qth.net" <r-390 at mailman.qth.net> 
 Sent: Friday, December 9, 2016 2:18 AM
 Subject: [R-390] DIY R-725
   
Hi everyone,

About a year ago I made my own R-725 by rewiring a 390 deck following Tom's guide. While daunting it is well worth the effort IMO. I paced myself and did it over a few nights, budgeting for rewiring two sockets per evening. This ensured I didn't become restless and impatient thereby eliminating the possibility of me making a mistake. 

The challenge for me was keeping the soldering iron away from other components and wiring! The results were better than expected...it seemed to transform the radio. The RF gain action was much better, carrier level meter deflection was more lively and signals seemed to jump out of the radio. The interesting thing is the 390A deck it replaced was 3/4 rebuilt and otherwise functioning normally so I put it down to loss in the mechanical filters. My ears aren’t the best anymore so I didn’t notice much of a difference in audio although a better quality speaker might yield more dramatic results. The main transformation for me was the overall performance of the radio…said receiver is currently packed away while I re-establish my ham shack after moving back to my hometown but I clearly remember tuning around 40 meters and signals were everywhere. Tuning around the local broadcast band and many stations would have the carrier meter almost at full deflection. Being curious, I swapped back the original 390A deck and despite both decks having the I.F Gain pot set up identically (150mV @ -7V at the diode load) the receiver went back to being relatively deaf by comparison.

Would I do it again? Yes and no…While my own experience was extremely positive, if I am ever lucky enough to acquire another 390 deck I wouldn’t re-wire the tube sockets again but I would install an additional 26v filament transformer similar to the real R-725. I am curious though…I wonder if the factory fitted 26v transformer is for the PTO and the series 500 deck gets it’s 26v from the power transformer, or does the additional 26v transformer supply both the I.F Deck and the PTO? Hum bucking supply for the PTO suggests the former.

Kind regards,
Ken 
ZL3AA


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