[David Dunlop]

With the checks on the Supply Unit in the Carriers No. 4 out of the way, I returned to the Main Set Receiver, which was still idling away, on the workbench. Several interesting things have been going on with it.

First, upon warm up, the only audio output I have had is white noise static. I typically have the receiver set on Band 2, tuned to 5.0 KC. This is a convenient frequency because it means Bands 1 and 3 are already set at 2.5 KC and 10 KC respectively and there are nearly always good SW reference signals to be found at, or very near those frequencies one can tune the receiver to quickly. It also makes checking the Crystal Calibrator on each of the three Bands relatively quick as well.

Up until now, this receiver does not detect any signals from anywhere. Just constant static. The odd thing is that usually, on these receivers, if you touch the aerial terminal when the set is on, you introduce a bias to the incoming signal that changes the audio output somewhat. If you are wearing shoes, your body acts like a giant aerial and shifts the signal somewhat. If you are in bare feet on a concrete floor, your body becomes a ground for the signal and the output changes a bit as well. On this receiver, none of that happens at all.

My approach is to start at the very back end of the circuits, right up behind the aerial tuning at Valve V1A, the RF Amplifier and progressively work my way to the front end of the receiver. The first three stages are all ARP-3 valves and all run sequentially from right to left in the upper right corner of the chassis. These are V1A the RF Amplifier, V1B, the Conversion Oscillator and V1C, the Mixer.

First thing I wanted to do with these three valves was pull the shield caps and then touch the end caps of each valve to see if that might impart a bias to the audio output. No changes at all for all three. Next step was to pull the three valves and test them again. I have two NOS ARP-3 valves that both test in the upper 80’s on my valve tester. All three of these valves checked out in the upper 70’s, very close to one another. Good news, but it didn’t shed too much light on the issues at hand. Then for some odd reason, with the set nicely warmed up, I decided to check for Crystal Calibrator signals once more.

Nothing detected at 5.0 KC on Band 2 with the calibrator set at 1000. So I switched to Band 1 and backed the dial up from 2.5 KC to 2.0. And there was the calibrator tone, clear as a bell. I switched the calibrator to 100 and once again, there was the tone, just slightly less loud. When I switched the calibrator to 10, I had to really listen to the loudspeaker, but could make out the tone in the background. That made my evening!

A quick check on Band 1 at 2.0, 3.0 and 4.0 found the calibrator signals with similar intensities. So I switched back to Band 2 at 5.0 KC. No calibrator. I backed up to 4.0 KC and there it was again. But at 5.0, 6.0 7.0 and 8.0 nothing detected at all. One last look at Band 3 was now required. Nothing detected anywhere at all on that band. Overall good news, but I now need to do some thinking about why the Crystal Calibrator signal drops off the dial above 4.0 KC.

David