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Ian - G4JQT · 19th Apr 2019, 10:02 am

Thanks for the extra info.

I did realign the IFs (and LO/Ae ccts) again and regarding the motorboating it is much better than it was . For example I couldn't get it to do this at all with the sig gen on max o/p (about 1V RMS!) and when connected to my active dipole or Wellbrook loop (both expecting 50 ohms loads, but anyway...) there is no motorboating at all on any stations and sounds lovely.

But when putting 3ft of wire on the antenna and sitting it in the house with the pantry tx (about 500mW into a short whip) upstairs at the other side of the house, it does still motorboat slightly at max volume.

Another A22 similarly aligned is fine. Valves all swapped with no effect. All voltages the same, and all paper caps on both sets have been replaced.

I even swapped over the grid connections to the o/p valve so set A was driving the audio of set B and vice versa and the 'fault' remained on the original motorboating set.

So it appears to be a PSU issue. I even swapped over the HT leads (both before and after the choke) but that caused them both to motorboat!

An extra 22uF across the smoothing cap completely stops it, but that shouldn't be necessary because it already has a new (and tested) 16+16uF rather than the original 8+16uF. In addition, extra capacitance here might exceed the switch-on surge of the AZ31 rectifier.

In every other way it seems to be operating perfectly, and in normal use no one would ever notice. It's not really a problem as I don't need to have it at maximum volume, but it is intriguing why one A22 does it and the other doesn't. The 'guilty' one has the 220pF in the 2nd IF rather than 100pF, but apart from that they are identical.

Noise-limited sensitivity of both is about 1 to 2 uV on a 30% mod signal. That seems fine to me.

Ian

19th Apr 2019, 10:02 am	#10
Ian - G4JQT Octode Join Date: Jun 2004 Location: Reading/Fakenham, UK. Posts: 1,323	Re: IF core positions Thanks for the extra info. I did realign the IFs (and LO/Ae ccts) again and regarding the motorboating it is much better than it was . For example I couldn't get it to do this at all with the sig gen on max o/p (about 1V RMS!) and when connected to my active dipole or Wellbrook loop (both expecting 50 ohms loads, but anyway...) there is no motorboating at all on any stations and sounds lovely. But when putting 3ft of wire on the antenna and sitting it in the house with the pantry tx (about 500mW into a short whip) upstairs at the other side of the house, it does still motorboat slightly at max volume. Another A22 similarly aligned is fine. Valves all swapped with no effect. All voltages the same, and all paper caps on both sets have been replaced. I even swapped over the grid connections to the o/p valve so set A was driving the audio of set B and vice versa and the 'fault' remained on the original motorboating set. So it appears to be a PSU issue. I even swapped over the HT leads (both before and after the choke) but that caused them both to motorboat! An extra 22uF across the smoothing cap completely stops it, but that shouldn't be necessary because it already has a new (and tested) 16+16uF rather than the original 8+16uF. In addition, extra capacitance here might exceed the switch-on surge of the AZ31 rectifier. In every other way it seems to be operating perfectly, and in normal use no one would ever notice. It's not really a problem as I don't need to have it at maximum volume, but it is intriguing why one A22 does it and the other doesn't. The 'guilty' one has the 220pF in the 2nd IF rather than 100pF, but apart from that they are identical. Noise-limited sensitivity of both is about 1 to 2 uV on a 30% mod signal. That seems fine to me. Ian