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Old 27th Apr 2023, 4:46 pm   #1
E93AFAN
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Default MBFB or BBFM

Some years ago I built an Octal superhet 3 x EF39, ECH35, EBC33, 6V6 loosely based on an article in RC of September 1948. The original design used hand made coils for which I substituted a set of Wearite P3 series coils (14 - 47 metres) as I was only really interested in this range, it acquits itself pretty well, is stable (even with EF39s!) and boringly reliable so has seen much use.

I have a shed load of Wearites covering 12 - 2000 metres and before either I shuffle off or am radio dies completely I thought it would be nice to install a switch and add a few more ranges. As I am sure you all know suitable, shafted switches with separate wafers offering sufficient shafting to install screens are like hens teeth, so I planned to bite the bullet and order some shafting and wafers from RS or Farnell. That is when my Catch 22 moment arose, I have always assumed RF switching needs to be MBFB, but the NFS Controls wafers offered by RS and Farnell are (at least in the range I need) BBFM.

Put simply does it matter whether I use MBFB or BBFM, I can't think of anything likely to be an issue, but I am not a technically qualified person as many of you are, just an ancient bodger, so your thoughts would be very welcome before I shell out my cash.

As always, many thanks in anticipation of your help.
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Old 27th Apr 2023, 4:55 pm   #2
G6Tanuki
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Default Re: MBFB or BBFM

I wouldn't worry. Have used both options in the past.

My bigger issue has been the one of wanting to short-circuit the unused coils in a coilpack to stop stray resonances of the unused coils causing 'suck out' of the wanted signals in the active coils.

Finding wafer-switches with the necessary shorting-sectors is nowadays not easy.
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Old 27th Apr 2023, 7:35 pm   #3
Ed_Dinning
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Default Re: MBFB or BBFM

Hi Gents, keep a look out for the Plessey Switch Kit, occasionally seen on ebay and invaluable for this as a custom shorting wafer can be fitted to the rear of each rotor.

It may be possible to find suitable wafers if you have a good spares box of stuff salvaged from old coilpacks
The switch mech should not be a problem and 6BA studding is easily bought for the wafer supports

Ed
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Old 28th Apr 2023, 12:01 am   #4
Radio Wrangler
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Default Re: MBFB or BBFM

Quote:
Originally Posted by G6Tanuki View Post
I wouldn't worry. Have used both options in the past.

My bigger issue has been the one of wanting to short-circuit the unused coils in a coilpack to stop stray resonances of the unused coils causing 'suck out' of the wanted signals in the active coils.

Finding wafer-switches with the necessary shorting-sectors is nowadays not easy.
A colleague while a student had built an RSGB handbook (60's era) shortwave receiver design. It used Electroniques (STC) front end and 1.6MHz IFTs. It was never stable. Many years later he found a screening plate that should have been built into the switch between the RF amp in and out wafers was missing and making one fixed its ills.

Getting shorting discs is pretty much impossible, inserted screens may also be needed but are more within reach of DIY.

David
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Old 30th Apr 2023, 2:01 pm   #5
E93AFAN
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Default Re: MBFB or BBFM

Thank you for your replies, order placed with RS for wafers, spacers, inter wafer screens and a switch assembly, had to put the dog on the streets with a begging bowl to pay for it, but looking forward to the project nonetheless.

Best wishes to you and thanks again.
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