Ekco SW86 modification

elorens
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Posts: 247

Ekco SW86 modification Aug 31, 2021 17:37:35 GMT 12

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Post by elorens on Aug 31, 2021 17:37:35 GMT 12

I'm getting to the end of this lockdown project: restoration of an Ekco SW86 and modification in line with the article by Richard Brayne and James Davidson. This has involved a major recapping exercise, loudspeaker repair, and some rust treatment, as well as the mods suggested in the article, which are mainly to provide a more rational approach to AGC.

So far, so good: the radio performs very well, and the new AGC is clearly working. Sensitivity on the medium wave band is very good, but I did notice some heterodyne whistles on this band, where there were none before. I was able to reduce these considerably by changing the wire carrying the AGC voltage across the chassis to a screened lead. I can still notice a few very faint whistles while tuning across the band, but I reckon I can live with these.

It did make me wonder about the original circuit, in which the bias is supposed to change to give reduced sensitivity on medium wave. Maybe Ekco had this problem originally, and came up with the bias change solution? Has anyone else carried out these modifications and noticed these whistles? Any suggestions?

Thanks,

Lawrence
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Richard
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Posts: 294

Ekco SW86 modification Sept 1, 2021 9:11:12 GMT 12

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Post by Richard on Sept 1, 2021 9:11:12 GMT 12

Hi Lawrence,
I don't have these problems you describe with any of the SW86's I have modified as described in the article, however I have never replaced R1 & R2 with a single 470 ohm resistor, R1 being 320 ohm and R2 changed from 1K5 to 470 Ohm. I'm wondering if you have too much gain in the RF amp?
The cathode biasing can be changed in the RF amp, as this variable mu valve is now biased mainly by the AGC voltage (with the modifications suggested), as it should, rather than being fixed biased with the potential to overload on strong signals, which the SW86 did do before modification.

I'm assuming you have all the valve shields in place?

Last Edit: Sept 2, 2021 8:59:47 GMT 12 by Richard

Jon
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Posts: 35

Ekco SW86 modification Sept 1, 2021 12:10:09 GMT 12 via mobile

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Post by Jon on Sept 1, 2021 12:10:09 GMT 12

I have recently finished the restoration of my SW86 including the mods to the avc as per the bulitin of Richard and James, a big thank you to them both for that information.
My set performs very well with none of the whistles you describe, as James suggests sounds like you have to much gain. I opted to omit cathode bypass cap c37 as suggested too.

elorens
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Posts: 247

Ekco SW86 modification Sept 15, 2021 22:10:29 GMT 12 Richard likes this

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Post by elorens on Sept 15, 2021 22:10:29 GMT 12

Thanks for the comments. James has supplied some extra info and I have rearranged the new wiring so as to reduce the amount of IF signal flying around. This has been very successful in cutting down the heterodyne whistles, and I'm happy with it now. Richard, all the valve shields are in place but on checking I found that the one on V1 was not electrically connected to chassis! I fixed this. Jon, I also left out C37 and there is still plenty of AF gain. Thanks again.