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Old 19th Dec 2018, 1:55 pm   #1
QQVO6/40
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Default Murphy B40D receiver.

I have on my bench a Murphy B40D receiver.

A previous owner I am certain was a "harness wriggler". Everything I touch has wires falling off it.

Tonight I repaired the "Gain" control and in the process all of the wires to the said control plus the wires to the headphone sockets fell off. Before I had actually removed the small sub-chassis holding them.

The radio sort of worked when I first got it but spiraled down soon after. Very loud buzz from the power transformer. I found half of the centre tapped HT winding was open circuit. Somebody knew that as there was quite a bit of work done there including snipping the wire to the open side of the transformer. I didn't bother to trace what others had done as I just wanted to get it going.

I suppose I have bodged it myself but had to as I do not have a spare power transformer. I have used a solid state bridge rectifier from the good side of the HT secondary and added a little cap to help filter switching transients. Then run the DC into the original power supply circuit also using the 2 rectifier valves to slow down the rate at which the HT is applied to the rest of the radio. The HT rail comes up nice and slowly to around 250V and is nice and clean.
I guess this sort of setup should be able to handle the current draw for the radio which is about 80mA from the book as I haven't measured it yet.

Anyone have thoughts on what I have done so far?

Cheers and all the best for the season.

73s Robert.
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Old 19th Dec 2018, 3:09 pm   #2
turretslug
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Default Re: Murphy B40D receiver.

That certainly sounds like a "best in the circumstances" approach- I (and you!) know that taking all the current from one HT winding half is less than ideal but at least a bridge represents optimum usage of that winding. I suppose if you were really concerned, a little HT current could be saved by throttling the output valve back with a higher value cathode resistor, also using a low-current stabiliser and emitter-following it with a high-voltage transisor will save some standing milli-amps.

I get the impression that the PSU can be an Achilles heel- my B40C had a different mains transformer fitted when I got it (secured with a single short, fat woodscrew into the die-cast chassis- some people should never be allowed near any technology beyond the spoon....). My supposition is that the somewhat height-restricted "double-decker bus" chassis arrangement meant that Murphy couldn't accomodate old stalwart rectifiers like the 5Z4 series, so had used somewhat small, hard-pressed and hot-running rectifiers, i.e. the EZ22/7Z4 initially, then 2x 6X4/EZ90 and that these were prone to internal shorts with bad consequences for the transformer. That height restriction also makes transformer substitution restrictive.

Good luck with it, and cheers in return,

Colin
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Old 19th Dec 2018, 9:13 pm   #3
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Default Re: Murphy B40D receiver.

Good evening Colin and thank you for your reply to my post.

By your comment it seems that the power transformer used in these radios can be a bit of a problem. You are so very correct with your comment about some people not being allowed access to technology past their feeding spoon. I did get a chuckle from that one.
The way I have put this now has both rectifier valves in parallel and they are just floating along just as delay elements to slow the increase in plate voltage on switch on.
Transformer temperature could be a problem so I will have to keep an eye on that. I have numerous ways of measuring the temp but I tend to use the hand method. If I can hold my hand on it continuously then it is OK.
Ambient temperature forecast here today is going to be 38deg C and this is a normal summer temp for this area. It often gets hotter so I will have to watch the transformer temp for a while to see how it copes.
Worth a try as I probably will have an alternate transformer here somewhere but it would take weeks to find one considering my workshop setup.
For tracing out the wiring problems I use an old kit version of the ESR (Equivalent Series Resistance) meter device that was published in the Electronics Australia magazine some years ago.
The ESR meter is my friend! When using an Ohm meter I have to really take note of the value of each reading as some of them are so close but with the ESR meter it will show chokes and other such inductances as open circuit.

I was hoping to use the B40D as my workshop RX as I have been using an Eddystone 1002 but being an amenities receiver it does not have a BFO so SSB is out of the question.
The other receiver on the bench is a 390A and that has become a mammoth task with so many components out of tolerance. That one is a whole new subject on it's own.
Enough of my chatter for now.

Thanks again Colin.
All the best for everybody during this festive season and after of course.

Robert.
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Old 30th Dec 2018, 12:15 am   #4
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Default Re: Murphy B40D receiver.

Hello everyone.
All the very best for the New Year.

I have got my B40D going.
As said in a previous post, the power transformer has half of the HT secondary open circuit. I have spoken to a few people about the B40 receivers and this problem with the transformer is not unheard of. Usual remedy is to replace the transformer however there are space limitations that reduce the simple power transformer replacement options. This radio uses full wave rectification so I thought I would try a different tack. I used a full wave bridge rectifier I had on hand here. It is rated at 6A with a 1kV PIV rating in quite a small package. I used it to rectify the output from the good half of the HT secondary and put a 0.001MFD cap across the DC output of the bridge rectifier to reduce any switching noise. I ran the output of the solid state bridge rectifier through the 2 original rectifier valves to give the HT supply rail a bit of a delay when switching the radio on. This set still has quite a few old paper Hunts capacitors throughout it (I have replaced all of the failed paper caps) and I didn’t want to frighten these old papers by presenting full HT instantly.
Well, it works! I had the radio running for 5 hours straight and the power transformer did get quite hot but I could just keep my hand on the outer casing continuously. Only just! The ambient temp in my workshop was around 35C so that didn’t help.
I have not bothered to do qualitative tests on sensitivity but with a 20 metre long LOGG loop laying on the grass of my back yard I could just pick up WWV on 10MHz around mid day. Considering the current propagation at the moment that was not too bad.

I was listening to some amateur traffic on 40 metres and there was a few static crashes coming through. I looked on the internet and there was no thunder storm strikes anywhere near so I got suspicious. Removed the antenna connector and the static crashes were still there but only faint. My first horrible thought was “silver mica disease”. The B40D has dozens of small mica caps in it. To test them all would be not a nice job! By chance I spotted a very slight dimming of the dial lights in time with the static crashes. Ahah! Light bulb moment! Quite literally. The long bracket that carries the dial lights had a poor rusted earth. Easy fix! Also found the rheostat on the rear deck of the radio that controls the dial lamp brilliance was very touchy and noisy so I replaced it with a 1R8 ohm 10 Watt wire wound resistor and used a “P” clip to hold it against the chassis for cooling. All good now with no static crashes.
Another thing I did notice. The sound output was quite high pitched and scratchy. The radio would not achieve the rated 2.5 watt output the way it was. In the circuit diagram it showed 0.001MFd grid coupling capacitors for the driver and output valves. Too small. I upped them to 0.022MFd. Much better audio response now. Quite loud when turned up.
I had to improve the earthing around the small sub chassis for the gain control and headphone sockets. There was a few intermittent problems there.
One more thing I have noticed is the coax from the antenna terminals to the relevant contacts on the turret fingers. The dielectric has turned quite black and on the one test I have done it is very lossy! With a bit of luck I will replace both of them today.
All up, the B40D works quite well and does pull in the stations with good sound reproduction with a good sized speaker.
I am quite happy with it.
It was 45deg C outside here yesterday and my workshop is not air conditioned so I will see how I go with it. Might have to wait till tonight.

Again, all the very best for the New Year. As “Star Trek’s” Dr. Spock would say, “Live Long and Prosper”.

Cheers, Robert.
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Old 30th Dec 2018, 1:01 am   #5
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Default Re: Murphy B40D receiver.

Congratulations Robert on the Murphy B40 success story,a very nice write up too on chasing the last faults out along with pointers on the grid capacitor changes.
Reassuring that you have no Silver Mica disease issue,you would need a course at the local teaching Hospital on Laparoscopic technique to even get at some if that had been the cause of the crashes heard.I thought that Silver Mica troubles are more an American if transformer thing owing to the type of if transformer in the AA5 domestic radios,think I can only remember a requirement to change a couple of the ww2 round type with riveted wires.
A nice fix on the transformer problem and scale lamp rheostat.
Really must get round to finding a good working one for myself,we had one at home here during my childhood years but was put off by the clunky turret switching and idled my spare time away on a National HRO.

A happy,healthy and prosperous new year for 2019 to you as well.

Mike
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Old 30th Dec 2018, 3:08 am   #6
QQVO6/40
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Default Re: Murphy B40D receiver.

Hello Mike. And of course all of the others on the forum.

Thank you for the nice words Mike. I had a good laugh at your comment on the Laparoscopic training course. Not a bad idea as I have quite a few radios where the methods could be very useful. Also have a HRO5 waiting in the wings.
I do apologize to everybody for being quite verbose with my descriptions. I understand that there are lots of people that are beginners when it comes to radio restoration and as such I try to steer away from jargon and other shorthand as some people will have trouble understanding it.
I have just come back into my house as it is getting too hot to be in my workshop and I will go back when it gets cooler tonight.
I do understand that it is heresy to do it but I am thinking of cutting an opening on the rear of the outer casing of the B40D just behind the power transformer to try to improve the air flow around the transformer. I fully understand that I will get pilloried by the purists. The only other way I could do it is to get a number of Peltior devices and attach them to the outer casing of the transformer and run the heavy cables discretely out of the outer casing. Just dreaming!! Might just use the radio for short periods of around an hour and turn it off before it gets too hot.
I am happy with the way the LOGG loop antenna is working and I will be making it a permanent installation. It seems to cut out most of the man made interference in this area.
Time to box this B40D and put it back on the shelf.
On to the next project as I have an R390A on the bench waiting for some new resistors.

Cheers all.
Robert.
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Old 30th Dec 2018, 7:27 pm   #7
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Default Re: Murphy B40D receiver.

No objection to verbose explanations here! You make a good point re. those who are less conversant with electronics, whether thermionics or generally. Sometimes I've been a bit brief in describing a course of action, perhaps taking another contributor's depth of understanding/familiarity somewhat for granted, other times being a bit wordy and in-depth, and worried that I was being patronising.... A tricky one sometimes.

I think it's fair enough to help something along with ventilation, particularly if it's a bit of a weak spot anyway and more so where circumstances like Australian summers are involved! Indeed, not to take a bit of additional care and allowance where superannuated electronics whose insulation isn't what it once was could be construed as negligence. I wonder if a scavenged small 12V fan would help with circulation, gently and quietly swishing away on bridge-rectified and smoothed 6.3V.

I'm not sure if I've ever had a vintage radio that didn't exhibit some sort of intermittent brightness/contact problem with dial bulbs! Tarnished bases/holders, weak springs, sketchy earthing paths that haven't exactly improved over the years, previous users' bodges/misunderstanding.... All part of the fun,

Colin

PS Good luck with the R390A, they don't often feature on a UK forum like this!
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Old 30th Dec 2018, 9:15 pm   #8
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Default Re: Murphy B40D receiver.

Hi Robert, a good solution to your problems; however be wary of the deadsection of the HT winding as it may have incipient shorted turns on it. It could however just be a poor connection on the leadout.

Note that you will be over running the remaining art of the secondary by about 40%, hence the high temperature.

Ed
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Old 30th Dec 2018, 10:18 pm   #9
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Default Re: Murphy B40D receiver.

Hi Robert. Good result with your B40 nice one . I look forward to to the R390A fun and games .
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Old 31st Dec 2018, 4:45 am   #10
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Default Re: Murphy B40D receiver.

Hello to everybody and a very happy new year to all.

Colin. I spent 30 years as a workplace trainer teaching people how to operate the largest mining equipment in the world. How to do it safely and competently. This has led me to being a bit verbose but it helps people who are not fully up to speed with concepts and terms / abbreviations used for the subject at hand. I will stop there as it is way off topic.
Cooling will, I think, be a problem and Ed has raised it as such in a later post. I might have to put some thought into it later as, for the moment, I intend to put this one back on to the shelving and list it as a go-er. I have many projects to go on with at the moment.
Flickering bulbs are a common issue. You are so right Colin! A funny one. My youngest son was given an old transistor stereo system and brought it home for dad to fix for him. With his participation. I liked the dial lights on this thing as they showed up the failure of the original design because when you cranked up the volume the dial lights varied their brilliance in time with the base of the music. Along with the appropriate distortion of course. Power supply just not big enough.
Hello Ed. Thank you for your very correct and informative response. I could not put a figure on how much strain I was putting on the transformer by hooking it up this way. I have the books that have the theory for this but to tell the truth I took the easy path and just did it and to keep an eye on the transformer temperature. Pretty slack on my part. Then a lovely man such as yourself comes along and tells me how much I am being naughty and over-running the surviving HT winding. Thank you ED. Your expertise is much appreciated as you have saved me all of that reading and struggling with the mathematics.
So far so good! Had it going for around 5 hours last night and I can just hold my hand continuously on the transformer casing.
All noted about the possibility of shorted turns on the dud half of the secondary. Before I went ahead and changed the setup in the radio I did a ring test on all of the windings in the transformer. All passed so I am confident that there are no shorted turns at the moment. And I do stress at the moment!! We never know what the future holds.
I am a keeper of radio stuff so I do not have ethical problems in doing a little bodge such as this. I fully intend to keep an eye out for a replacement transformer. The mods I have done to the radio can be reversed within 5 minutes so no dramas there.
One thing I might do is look around for another fuse holder cap and modify it so I can bring a couple of wires out so I can put the trusted AVO in the HT secondary circuit and measure the current from the working half of the secondary. The Navy book says the HT current should be around 80mA. Maybe I should just run a pair of wires out and just measure it as there is still quite a few Hunts paper caps in this radio. Cumulative leakage from lots of questionable plate bypass caps would not be good.

45deg. C. Outside temperature now. Can’t wait for summer to get here!!

Deliverance and Colin, the R390A is a whole new story!!

I would like to wish all of the members of this forum all of the very best wishes for the coming year. And of course all of the many subsequent years.

Cheers all. Robert.
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