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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

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Old 18th May 2015, 10:23 pm   #1
g4aaw pete
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Default B40 PSU question.

I'm hoping someone can explain the reasoning behind the design on the Murphy B40 high voltage power supply circuit.

I've re-drawn it from the B.R.1617 manual.

My question is:
what is the advantage of this configuration over more conventional designs using a smoothing choke and a swinging choke on the +h.t. side?

Choke L302 is described as being in the + h.t. line, whilst L304 is in the -h.t. return line.

This, I understand, but cannot see why its done this way.

Regard
Pete
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Old 18th May 2015, 11:58 pm   #2
turretslug
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Default Re: B40 PSU question.

The question could be approached from the other direction!- i.e. why put the choke(s) in the positive side of the supply, exposing them to greater insulation stress between winding and core and eschewing a potential source of "free" negative bias. I suppose it made the construction of multi-section electrolytics a little more straightforward in that the can could be made a common negative clamped and inherently electrically connected to chassis but the B40 uses separate paper block types with two insulated terminals for reservoir, smoothing and subsequent filter. So, no real necessity for the first choke to be in the positive side. As well as the standing DC HT, there'll be a considerable ripple amplitude, so perhaps the designer wished to reduce insulation stress. All the same, it's hardly a make-or-break choice- though I would have done the same under the circumstances. I've long been puzzled that the AR88, which does use several negative bias voltages derived by HT current flow, doesn't have its first filter choke between transformer HT winding centre tap and chassis, improving efficiency and reducing insulation stress- it also has a filter capacitor block with separated negatives
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Old 19th May 2015, 10:57 pm   #3
BobGreen
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Default Re: B40 PSU question.

It does looks rather odd these days but I think Mr Slug has the answer.
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Old 20th May 2015, 9:53 am   #4
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Default Re: B40 PSU question.

It might also reduce the amount of mains borne interference going to the chassis/ground.
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Old 20th May 2015, 11:05 am   #5
G8HQP Dave
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Default Re: B40 PSU question.

L304 appears to be a tuned choke i.e. it resonates (presumably at 100Hz) with the parallel caps C316 and C317. Given the tolerance of components back then, it is likely that the smaller value cap had to be selected on test - or maybe included or omitted as necessary. This would be easier, quicker and safer on the production line if the caps were grounded, as it would not be necessary to wait for the HT rail voltage to decay after switching off before the soldering iron could be used. On switch off, both ends of the caps would be at ground voltage.
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Old 24th May 2015, 3:06 pm   #6
Leon Crampin
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Default Re: B40 PSU question.

I wonder if anyone remembered that HM ships had a 60Hz supply?

Leon.
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