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Old 17th Oct 2012, 8:51 pm   #1
alanworland
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Default Murphy A574

I have started work on my transport damaged radio cabinet, and have a couple of questions!
While I have the chassis out I am proposing to replace any suspect or potentially suspect components prior to powering it up again (it seemed to work ok but with noisy switches and pots - although no voltages checked)
I have a photo of C57/58 (same can) which I think looks a bit suspect and was going to replace - good idea? Is this what is called 'that capacitor'?
My second question concerns a variety of Hunts capacitors, in various values, black plastic axials with a seam down their length - should I change all of these?

Alan
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Old 17th Oct 2012, 8:59 pm   #2
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Default Re: Murphy A574

The big capacitor is the smoothing electrolytic. If the radio works without humming then it's fine and you should leave it alone.

You should change all the waxies and Mouldseals with values between 0.1µF and 0.001µF. Values smaller than that will be mica or ceramic and should be OK.
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Old 17th Oct 2012, 9:04 pm   #3
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Default Re: Murphy A574

edit, Paul got in just before me

Hi Alan
The most critical capacitors are the audio coupling ones, “that capacitor” is the one which connects from V4 to V6.
This is the one that is confusingly called "that capacitor". I dont know which schematic you are looking at but on the manufactures one it is C63 0.005uf.
The aluminium can with 2 capacitors is the Ht smoothing electrolytics. You will know if this needs changing if there is excessive hum and if the can gets warm.
If you start up the radio slowly on a lamp limiter it may “reform” OK and not need changing.
The black hunts capacitors are notorious and do often need replacing.
Mike
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Old 17th Oct 2012, 9:06 pm   #4
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Default Re: Murphy A574

Thanks for your swift reply!
I thought that the bulge in the end seal was an indication of possible failure? No hum.
I have found 2 'waxies'? are the Hunts known as Mouldseals?

Alan
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Old 17th Oct 2012, 9:10 pm   #5
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Default Re: Murphy A574

The black caps are Hunts Mouldseals. Some have 'Mouldseal' printed on the side and some don't. They were a type of capacitor made by Hunts in the 1950s and used by most British manufacturers.

It's normal for the base of an old smoothing electrolytic to buige slightly. As Mike says, check it isn't getting hot after 10 minutes of operation. If it's cool or only slightly warm, and there's little or no hum, then it's probably OK.
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Old 17th Oct 2012, 9:29 pm   #6
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Default Re: Murphy A574

I have recently restored 2 of these sets, there are some capacitors that look like resistors in the RF and IF section, these should also be replaced. Mine were all faulty causing all sorts of strange symptoms.

Mark
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Old 18th Oct 2012, 8:09 am   #7
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Default Re: Murphy A574

Hi Alan
I to have had problems with the capacitors Mark mentions.
I have marked some (there may be more) that look like the type on the attached photo.
Try the radio first before you replace them, if it all works fine, then they should be OK. I spent hours looking for the cause of a fault on a set once and was missing these types of capacitor because they looked like resistors.

Mike
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Old 18th Oct 2012, 3:47 pm   #8
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Default Re: Murphy A574

Thanks Mike, who would have thought they were capacitors?
The set works, so I think I shall finish the cabinet repair, clean the chassis, re assemble and check some voltages before delving in.
Theres a reflective panel behind the dial lamp which I had assumed was painted white, but now I am not so sure - looks like it may have started off as a cream colour (and then gone yellowy?
Any idea what the original colour might have been? The rear of the panel is the same as the front, but less dirty!

Alan
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Old 19th Oct 2012, 7:55 am   #9
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Default Re: Murphy A574

Hello Alan
I have painted the reflector on a radio white and it does not look quite right. I think it depends on the brightness of the bulb. I have found that most bulbs I have put in all seem to be under run and hence there is more orange in the light.
I think it is personal taste on what colour to use. I think I would use a pale cream next.
Mike
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Old 21st Oct 2012, 5:39 pm   #10
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Default Re: Murphy A574

Thanks Mike, when I emoved the lampholder to get at the panel it was quite white underneath! So thats what I have sprayed it!
I found a cracked case on a Hunts cap, so I have replaced it and while I was having a look around decided to replace the two wax types.
One is fairly large physically and .005uF 1000v, I have found some replacement ceramic disc types but they are much smaller although to spec - would it be ok to use these?
Another question! Would it seem a good idea to put the chassis to ground for safety reasons?

Alan
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Old 21st Oct 2012, 6:27 pm   #11
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Default Re: Murphy A574

Most people will fit a 3 core cable with the chassis connected to earth when restoring an AC radio. There are drawbacks to this - the mains transformer will be a bit more stressed for example, so there's a remote chance of provoking a transformer failure. However, on balance the safety benefits outweigh the potential drawbacks.

The Hunts caps will need replacing. The cases disintegrate when disturbed, but even if left alone they tend to leak.

Is the 0.005µF the tone correction capacitor? If so, any 1kV 0.0047µF cap will be fine, regardless of physical size. Anything across the mains should be X2 rated now.
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Old 21st Oct 2012, 6:46 pm   #12
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Default Re: Murphy A574

Thanks, the .005 1000v cap is from pin 7 of V5 (also one side of output primary) and earth. Not near the tone control?
V5 is a 6P15 valve

Alan
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Old 21st Oct 2012, 7:36 pm   #13
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Default Re: Murphy A574

That's the tone correction capacitor- in this case from O/P valve anode to deck so if it fails S/C you may say goodbye to the O/P transformer and maybe the rectifier and even the mains transformer ;

Definitely a good idea to replace with a nice high quality component. Some sets have them from anode to HT+ which is less fraught since a failure would just kill the sound, not other components, but here there is a humbucking arrangement involving a tapped primary on the O/P TX which would make that impractical.
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Old 21st Oct 2012, 8:10 pm   #14
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Default Re: Murphy A574

You can simply remove the tone correction capacitor. People in the 50s liked a nice muddy sound so practically all radios were fitted with this cap. Today we tend to like a brighter sound so you may prefer it with the cap removed. It will certainly do no harm to run the radio without it.

If replacing this cap I would prefer to use a class Y component, which should never fail short circuit.
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Old 21st Oct 2012, 8:51 pm   #15
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Default Re: Murphy A574

Brilliant! Learning all the time.
could this be why 'frequency respones' are never seen? - because it was destroyed at the final stage!
I shall remove it once I have it up and running and listen for the improvement (after attacking the Hunts)

Alan
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Old 21st Oct 2012, 10:35 pm   #16
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Default Re: Murphy A574

Roughly speaking, the 5n tone corrector will give a response about 3dB down at 4-5kHz. This is certainly woolly for an FM set, though probably acceptable on AM. Dropping it to 1n will push this up beyond 10kHz whilst still providing some spike suppression for the O/P TX primary. Also a 1n Y-type capacitor should be fairly easy to find.
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Old 23rd Oct 2012, 9:50 pm   #17
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Default Re: Murphy A574

I have made a list of all the 'Hunts' capacitors in the set and am attempting to make an order for replacements but am confused by the different types there are.
The originals are an axial component and I would like the replacement also to be of that form but I am not sure what type I should be looking for, other than non electrolytic?

Alan
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Old 23rd Oct 2012, 10:17 pm   #18
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Default Re: Murphy A574

Axial components are harder to find than radials. There is no reason not to use radials other than cosmetic appearance as the leads are usually long enough to do the job.

Any modern types with appropriate ratings and values will be fine. You can select on price, appearance, or a combination of the two. I mostly use polyester film caps but it doesn't really matter. Many forum members like to order from Justradios in Canada who supply components that look the part, though they're by no means the cheapest source of caps.
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Old 23rd Oct 2012, 11:42 pm   #19
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Default Re: Murphy A574

Thanks Paul, that's got me looking in the right direction.

Alan
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Old 26th Oct 2012, 8:16 pm   #20
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Default Re: Murphy A574

I have replaced all the Hunts and a couple of other 'suspects'
Upon removal of the parts I put a meter on the legs to see the 'kick' in both directions and noted that several of them seem to measure down to about 500k resistance!
Well worth changing I thought.
Is this normal?

Alan
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