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Old 14th May 2018, 2:40 pm   #1
Ancient Geek
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Default Ferguson 208UL - capacitive dropper or not?

Hello all,

I've started work on a lovely little Ferguson 208UL short superhet in a cream-coloured bakelite case. The inside of the case above the dropper resistor was greatly discoloured so, despite there being no signs of heat damage on the top of the case, I was considering converting it to use a capacitive dropper.

However, a very light application of T-Cut on the discoloured area revealed the inside of the the case to be pristine once the grime was removed, with not a trace of heat damage. Given that the set is not of any great value and AM only, what is the collective opinion here? Capacitive dropper or not?
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Old 14th May 2018, 2:47 pm   #2
paulsherwin
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Default Re: Ferguson 208UL - capacitive dropper or not?

There's no reason to fit a capacitive dropper if the original resistive dropper is OK (unless you want to do it as an exercise). The only exception would be if the radio is likely to see heavy use, but that's unlikely if it's part of a collection.

Personally I've never been keen on capacitive droppers, as there's no (easy) way to make them fail safe. If I wanted to reduce the heat somewhat I'd use a diode dropper plus a resistor, which can be made to fail safe. Everyone has their own opinion though.
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Old 14th May 2018, 2:59 pm   #3
julie_m
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Default Re: Ferguson 208UL - capacitive dropper or not?

Capacitive droppers work well when done right. Do not use a class X rated capacitor -- use a fusible resistor in series with the capacitor instead. The self-healing property of class X capacitors is not desirable in this application.

A very common mistake with capacitive droppers is not providing a discharge path for the dropper capacitor and then wondering why it does not work. Either fit a diode in inverse parallel with the set (so it will conduct when the set's rectifier is not conducting) or (perhaps slightly better as it will give less power hum) wrap a bridge rectifier around the set.
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Old 14th May 2018, 4:04 pm   #4
Vicboduk
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Default Re: Ferguson 208UL - capacitive dropper or not?

On my set I decided to leave the dropper in place and divert the heat out the back using a heat-shield, see here.
There is plenty of room for this and the back cover has a significant gap at the top to allow the heat out.

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Old 14th May 2018, 4:54 pm   #5
Ancient Geek
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Default Re: Ferguson 208UL - capacitive dropper or not?

Nice work, Vic. I'm inclined to leave it as it is, depending on the owner's intentions (restoring it for a friend) with respect to usage. I doubt if I'll get it looking as good as yours though.
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Old 14th May 2018, 6:39 pm   #6
kalee20
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Default Re: Ferguson 208UL - capacitive dropper or not?

It's a good radio! Ferguson took a lot of care over the RF and Oscillator tracking... there are lots of adjustments to make, but the alignment is much less of a compromise than many sets.

I never found heat to be an issue. One precaution though is not to pile up books and magazines on top - else the cabinet will be insulated and naturally get hotter. And don't block the rear vents.

As you have already found it to be OK with no heat damage, you might as well consider that the thermal management aspects of the design are satisfactory, so why complicate something that doesn't need it? Leave as-is! And enjoy this little radio!
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Old 14th May 2018, 6:45 pm   #7
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Default Re: Ferguson 208UL - capacitive dropper or not?

Quote:
As you have already found it to be OK with no heat damage, you might as well consider that the thermal management aspects of the design are satisfactory, so why complicate something that doesn't need it?
That's the conclusion I've come to. Thanks to all for the replies.
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