[greenkit]

I did an RF test similar to Sean’s: I don’t have an AF power meter, so again monitored the voltage across the set’s internal 620 Ohm load. A similar result to that found previously, with early AF clipping and quite severe distortion with the AF gain at maximum. At this stage I pulled the ‘original’ PSU/AF chassis and set it up on the bench alongside the one from my ‘spare’ B40D (this hadn’t seen AC mains in 15 years, so I wound it up slowly). I had 620 Ohm loads connected directly across the output transformers, plus the preamp’s ground lines and 2.2M input bias resistors (R301) taken to earth: just needed to switch the sig-gen and ‘scope connections to compare the amplifiers. The HT rails do run a bit high (320V) with only the 6BA6 & 6BW6 taking current, but so far everything has survived. The good news is that I got a nice 110V p-p sine wave on the spare’s output (near enough 2.5W into the load), while the original amplifier started to clip at about 45V p-p (0.4W). The corresponding p-p anode swings were 400V and 180V before non-linearity set in. Quiescent DC levels across both amplifiers were the same, within 5% or so. The only suspect item left was the output transformer TR301 and it turned out that’s the problem. I connected my Hygrade output tranny with a 30:1 ratio feeding a resistive load, and was rewarded with a 400V anode swing and full output on the secondary. Transplanting TR301 from the ‘spare’ receiver also worked well. There may be shorted turns on the poor transformer, though a winding-resistance comparison didn’t reveal any vast difference between it and the good one. The transformers are built slightly differently, so some mystery remains. I’ll either fit the Hygrade permanently or look out for a genuine replacement from a scrap receiver, if only to preserve originality. Thanks to all for your interest and comments.