I've not had an opportunity to spend time in my workshop for a couple of weeks, but I did find time today to have another look at the Radionette Kurer valve portable.
With the faulty set tuned to 1000 kHz on the medium wave band I was able to pick up a strong 1455 kHz signal on another set next to it on the bench. This confirmed that the Kurer's Local Oscillator was running. I tried injecting a modulated 455 kHz test signal again, coupling the output from my signal generator into the set via a few turns of wire around the DK96 valve. Instead of the pure tone that I expected, there was a beat note from the speaker. This usually indicates an unstable and oscillating IF stage.
Turning again to the internet, I found several references to similar faults with battery sets in various vintage radio forums. I had already changed the decoupling capacitors and checked screen resistor values, which left possible problems with poor contacts in the valve bases. I gave both mixer and IF amplifier valve sockets a liberal application of contact cleaner spray, and again following suggestions in a forum post, tightened up some of the contacts in the sockets. The advice turned out to be spot on, because on powering it up again I could immediately hear received signals.
It's currently sitting on the bench playing my local AM station on 1332 kHz. The sensitivity isn't great, and i'ts pretty deaf on long wave as well. I'm assuming that this is because the loop aerials aren't connected, so I'll try it again once its back in the case.
It's been an interesting repair job, but I can't honestly say that there is anything about this set that I like. I'll be quite happy to hand it back to my friend.
Keep listening!
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