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Old 28th Feb 2023, 11:35 am   #1
ToonMurphyB40
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Default Question Murphy B40 Power Supply

I have a technical question that has been bothering me for a while. Perhaps someone can help me?
I wonder why the 100 Hz resonant ripple smoothing filter (L304 / C316 / C317) in the Murphy B40 receiver Power Supply Unit is positioned in the ‘zero’ return line.
In my opinion the same effect of ripple smoothing is obtained with this ripple filter in the ‘plus’ line.
Does anyone have an idea why this choice was made?

Best regards,
Toon. PE1NHP
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Old 28th Feb 2023, 12:28 pm   #2
turretslug
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Default Re: Question Murphy B40 Power Supply

It's one of those questions that surfaces from time to time, but I don't think I've seen a defintive answer! I suppose that it means less stress on the winding-to-core insulation, as it only has to withstand peak ripple, rather than (peak ripple + HT). As the set uses separate paper block capacitors with both terminals isolated for reservoir and smoothing rather than the double- or multiple-section electrolytics used in consumer equipment, maybe the designer felt that it was convenient to take advantage of this feature for the sake of the first choke's insulation.

In some power supplies, a choke or resistor(s) in the negative supply side gets used to derives bias voltage for various purposes but not in this case.

Colin
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Old 1st Mar 2023, 5:11 pm   #3
ToonMurphyB40
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Default Re: Question Murphy B40 Power Supply

Thank you Colin for your comment.
Whe have to speculate what the Murphy Engineers wanted to achieve with the 100 Hz resonant choke in the ‘zero’ return line.
Your approach is interesting.
The insulation stress between the choke windings and the core material is certainly leds when placed in the zero return line.
I wonder of there are any other suggestions.
The Murphy Engineers have made remarkable choices with the combination of a ‘swinging choke’ and the ‘resonant choke’.
Best regards,
Toon
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Old 3rd Mar 2023, 1:29 am   #4
Silicon
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Default Re: Question Murphy B40 Power Supply

The Racal RA17 used a resistor in the negative supply line to derive a 25V negative bias which was used for the AGC circuit.

Perhaps the designers of the B40 were considering something similar.
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Old 4th Mar 2023, 10:24 pm   #5
ToonMurphyB40
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Default Re: Question Murphy B40 Power Supply

This thought has also occurred to me before.
My feeling was that the design of the Power Supply of the B40 Receiver might also be intended for external equipment.
Above-average efforts have been made in the design.
A negative bias voltage could possibly be intended for external accessoiries.
However it is a speculative thought.
Anyway. I’m going to continue my research om the power supply.
Thanks for the support,
73’s Toon. PE1NHP
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Old 5th Mar 2023, 9:38 am   #6
Leon Crampin
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Default Re: Question Murphy B40 Power Supply

I wonder if any thought was given to the fact that most post-WW2 British ships had 60 Hz supplies?

Leon.
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Old 5th Mar 2023, 12:40 pm   #7
ToonMurphyB40
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Default Re: Question Murphy B40 Power Supply

The 60 Hz ships frequency is correct for the ‘old days’.
However if we calculate with L304 = 20H and capacitors C316 // C317 = 120nF, then the filter resonance frequency = 102 Hz.
So it seems that the design is based on two times the 50 Hz mains frequency = 100 Hz.
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Old 5th Mar 2023, 4:13 pm   #8
Leon Crampin
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Default Re: Question Murphy B40 Power Supply

When I worked for the UK Admiralty in the '70s, all naval vessels were 115V 60 Hz and had been since the war. To the best of my knowledge, this still applies to current build UK ships - someone may have accurate information.

Out of interest, my Murphy A34 radio (1934 vintage) also uses a resonant smoothing choke. The 25 Hz version of the set marketed at the time had an appropriately changed tuning capacitor. I wonder what the downside of this arrangement is - it's seldom used.

Leon.
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Old 6th Mar 2023, 1:27 pm   #9
ToonMurphyB40
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Default Re: Question Murphy B40 Power Supply

Hey Leon,
You put me on the trail of the ‘table model’ Murphy radios. I found a beautiful overview on “The Murpy diagram dungeon” from 1930. In the very first A3 (1931) model of Murphy, a resonant filter choke is used in the positive high tension line. Two capacitor values are listed for the resonant filter. Apparently had radio models for two mains frequencies…?
The transformer centertap is grounded.
The Murpy A8 model (1932) no longer uses a resonant filter choke. In the Murphy A4 model (1933) the resonant filter choke reappears. Again with two capacitance values for the resonant filter!
A 25 Hz and 50 Hz option is mentioned here.
In 1934 the Murpy A24 model appears.
Then it gets interesting again.
Here a resonant filter is placed in the positive (+) line and another choke in the minus return line to the center tap.
This is already close to the design in the B40 receiver power supply.However one value is stated for the capacitor in the resonant filter….?
In 1935 we see the Murphy A26 model.
Remarkable that here is only a choke filter present in the ‘negative’ return line.
In 1936 we see again a choke in the ‘negative’ center tap in the Murpy A30 model.
In 1937 there is a choke in the transformer center tap in the models A34 and A36.
At that time, the center tap choke disappears in model 38C and it becomes a traditional filtering in the ‘positive’ HT-line.
The 1937 A40 model shows two double diode rectifier Calvé’s in parallel (as in the B40 receiver). The filter chokes are in the plus line. The loudspeaker field Coils is also regularly used as filter choke.
From 1938 the models certainly no longer have the resonant filter chokes and also no longer placed in the minus return line. The speaker field coil is more often used as a filter choke.
All in all, we see in ‘the thirties’ that Murphy continuously optimizes the radio Power supply circuit.
The critical and expensive resonant filters disappear and a choke in the return line is also abandoned.
It seems that the ‘old Murphy knowledge’ of ripple filter design from ‘the thirties’ had been applied in the Murphy B40 receivers.
A resonant filter choke has disadvantages such as ‘costly’ and only effective for one ripple frequency. The harmonics of the ripple frequency are not efficiently suppressed.
I fond the reasons in the literature about ‘disappearence’ of ripple filter chokes.
So I am legt with my questions about the reason for the filter choke in the transformer center tap….!!
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Old 7th Mar 2023, 9:21 am   #10
Leon Crampin
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Default Re: Question Murphy B40 Power Supply

Thanks Toon, an interesting analysis of Murphy's designs of the period. It would be useful to compare the performance of a resonant choke system with a standard unturned choke. There's not a huge cost penalty - one capacitor.

I realise that I mistakenly described my Murphy radio as an A34. It is of course an A24 as you describe in your text.

The point made regarding harmonics in the supply frequency is well made - ship supplies are anything but "clean". I wonder if the Murphy engineers ever evaluated the power supply performance on board. If so, the small matter of a 60Hz supply would surely have arisen.

Leon.
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Old 7th Mar 2023, 2:22 pm   #11
ToonMurphyB40
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Default Re: Question Murphy B40 Power Supply

Hi Leon,
Thank you for your response. I am currently investigating the power supply Unit of the B40D receiver. I have made a special measuring setup for this. By measuring I could determe that the ripple suppression of the PS-smoothing filter is 62dB@100 Hz. In a PSpice simulation the calculated value is 98 dB@100 Hz. That difference is substantial.
If I switch off the capacitors over the resonant filter in my setup, the ripple suppression is 61 dB@100 Hz.
The added value of the resonant filter is therefore negligibly small.
My next step is to investigate what the impuls current to the transformer center tap looks like.
The reservoir capacitor C314 is charged with a high pulsed current during the conducting time of the rectifiers.
I will also investigate the harmonic distortion of this pulse current.
For the higher harmonics of 100 Hz, the resonant filter is ineffective.
The Murphy B40 receiver fascinates me.
Choices have made in the design that are remarkable and that I would like to understand.
Best regards,
Toon. PE1NHP
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Old 7th Mar 2023, 3:32 pm   #12
ms660
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Default Re: Question Murphy B40 Power Supply

In an online copy of a manual I'm looking at, the choke in the HT-ve rail is 10H in the B40A/B/C and 20H in the B40D according to the components list.

Lawrence.
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Old 7th Mar 2023, 4:58 pm   #13
ms660
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Default Re: Question Murphy B40 Power Supply

Re: Post#12, I forgot the link:

http://www.portabletubes.co.uk/boats...yB40Manual.pdf

Also note that there's no 0.12uF fitted across the choke in the A.P. 57140/A power supply schematic.

Lawrence.
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Old 8th Mar 2023, 10:53 am   #14
ToonMurphyB40
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Default Re: Question Murphy B40 Power Supply

Hoi Lawrence,
Thank you for your response.
It is correct that the Murphy engineers have made changes to the ripple smoothing components in the power supply of the different patterns. A transformer center tap choke is used in all patterns.
In patterns A, B and C this is 10H.
In the D-pattern this becomes 20H.
The choke in the B+ line is 18H in all patterns, however in pattern D this becomes a swinging choke (18H - 25H).
In the A-pattern, no resonant choke is used in the center tap.
But this is the case in the B, C and D patterns.
It is striking that the resonance choke frequency of the B and C patterns is near 145 Hz (10H //120nF) and that the D-pattern the resonance frequency is near 100 Hz (20H // 120nF).
The B40 Receiver power supply is specified in the B.R.1617 as 40/60 c/s A.C.
So we see that the design evaluated in time.
Best regards, 73’s
Toon. PE1NHP
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Old 8th Mar 2023, 11:10 am   #15
ms660
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Default Re: Question Murphy B40 Power Supply

Buried away somewhere I've an original manual for the B40 series which came with a B40 series receiver from HMS Ark Royal, if it ever surfaces I'll check it against the online copy for any extra info.

Lawrence.
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Old 8th Mar 2023, 12:00 pm   #16
ms660
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Default Re: Question Murphy B40 Power Supply

There's also at least one discrepancy in that part of the online copy of the manual, in the components list for the AF/Power unit for the B/C versions the choke in question is listed as L303, 10H, but in the top view drawing of the AF/Power unit for the B/C versions (Fig. 20) shows the choke to be 20H with a component ref. L304, the schematic for the same also shows it as L304.

Lawrence.
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Old 8th Mar 2023, 1:21 pm   #17
ToonMurphyB40
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Default Re: Question Murphy B40 Power Supply

Lawrence,
Thank you for your valuable responses. I am happy to meet people here who also look under the ‘hood’ of the B40 receiver.
I also noticed that there are a number of discrepancies in the online copy of the BR1617.
Unfortunately I do not have An original manual of the B40 receiver.
I regularly appeal to the B40 enthusiasts who have united on Facebook at the “Murphy B40 B41 62B Museum”.
Especially the moderator of this museum, Sander Leunissen, has a lot of knowledge of details in the pattern version differences.
I regularly consult Jim for advice and information.
The mentioned “Museum” is always and actively looking for original documents of the Murpy B40. For the members of the “Museum”, the Murpy B40 receiver is an important historical radio heritage.
I also noticed the comments you made about the coding of the power supply chokes.
Chapter 4 is about “Pattern Differences”.
In paragraph 4 (g) to (h) the text provides more clarity about the inductance of the mentioned chokes.
“Increased smoothing is provided by replacing the 10H choke L303, by a 20H choke L304……..”.
Best regards,
Toon
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Old 8th Mar 2023, 2:26 pm   #18
ms660
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Default Re: Question Murphy B40 Power Supply

Yes, also mentioned in chapter 3-15(a)

Here's a photo of the last one I owned (B40D as received) The power plug and socket had been replaced with Bulgin types.

Apart from an incurable tuning backlash in the top gear box it worked very well.

Lawrence.
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Last edited by ms660; 8th Mar 2023 at 2:38 pm. Reason: extra info
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Old 8th Mar 2023, 2:39 pm   #19
ToonMurphyB40
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Default Re: Question Murphy B40 Power Supply

This is my setup for investigate the properties of the ripple smoothing filters of the B40D power supply.
The switches slow me to turn the inductors L302, L304 and capacitors C316 // C317 in are out of the circuit.
The BNC-cables go to the analyzer.
Click image for larger version

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ID:	274588
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Old 8th Mar 2023, 2:42 pm   #20
ToonMurphyB40
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Default Re: Question Murphy B40 Power Supply

A beautiful receiver Lawrence.
The picture makes me happy!
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