Mains energised speakers.

[Geoff 555]

Hi I have tried doing a search but came up empty. I am sure that I read some where that to rewind one of these was a simple job. The question comes in 2 parts.
Could some one with no experience or special equipment rewind one?
What kind of price would it be to have a "proper job" done? I believe Ed Dinning ( I have a feeling I have not spelt that right.) would be the chap.
McMicheal Supervox.
While I am here is there a way of checking them anyway.
This was supposed to be a simple question but seems to have expanded a bit, sorry it's me.
Cheers.
Geoff.

[anthonys radios]

Could you say what the fault symptom is ?

[FRANK.C]

Hi Geoff
I assume that it is the Twin Supervox you refer to in which case there are two energized speakers.
The two field coils are different one is connected in series with the HT supply. The other is connected in series with a resistor across the HT and forms a potential divider to supply the screen grid of the valves. The two coils are different.
I was unlucky enough to have to rewind them both due to green spot. The one in series with the HT could probably be rewound with a drill.
I don't think it possible to rewind the one in series with the HT by hand as the wire is quite thin around 0.1mm and there is a lot of turns over 30,000 if I remember right. It is important to get the DC resistance of this one right as it forms part of the potential divider for the screen grids.

Resistance checks will tell you if they are OK

Regards
Frank

[Radio_Dave]

Originally Posted by Geoff 555

Could some one with no experience or special equipment rewind one?

Yes, I've done a couple of 'ordinary' ME speakers with a battery powered drill, clamped in a vice, using threaded rod and a wooden dowel to hold the bobbin central.

It is very important to bolt two discs either side of bobbin, or it will spread as the wire is wound on and you wont be able to refit it. Wind on as much wire as you can, of the correct size then check its resistance against the manufacturers data. Hopefully it'll be high, in which case, unwind some until the required resistance is reached.

David

[Geoff 555]

Hello thank you. Well this was a sort of a toe in the water job just in case it was needed. Symptoms an intermittent but constant 'rustling' sound IE. it does not go away. Also a continuous arcing sound, in the sound. Temperature at start 25 C, 45 minutes running and it has gone up to 47 C and rising, didn't go any further. There is a hum but this may be down to picking up interference, very noisy mains and environment.
This looks as if it may slide off topic if so I will mod the title if allowed.
Cheers.
Geoff.

[FRANK.C]

For what its worth there was sings of ht tracking on one of my bobbins it was the one connected in series across the HT. The bobbin was charred right through to the core.
I suspect that the problem may have originally been caused by a poor connection between the lead in wire and the ECW of the coil.
It may be worth disconnecting the coil an fitting a suitable resistor in its place as a test. There will be some output from the speaker without the coil connected but it will be much reduced.
　
Frank

[anthonys radios]

Originally Posted by Geoff 555

Hello thank you. Well this was a sort of a toe in the water job just in case it was needed. Symptoms an intermittent but constant 'rustling' sound IE. it does not go away. Also a continuous arcing sound, in the sound. Temperature at start 25 C, 45 minutes running and it has gone up to 47 C and rising, didn't go any further. There is a hum but this may be down to picking up interference, very noisy mains and environment.

Ok, dont know if I can help,but will have a go !
Field coil DC resistance, what is it, and most important, is it a constant reading.
Using a testmeter (or If you have a scope), can you see any erratic variations in voltage in and or out.
Ref. tempurature rise, I wonder if the HT current is correct, as a fault such as a leaky grid coupling cap to the output stage might increase the HT current, and cause the field coil temperature to rise.
As an apprentice it was a common fault, in the uncorrected extreme, the voice coil locked solid when the pitch in field coil melted, and the field coil went short circuit turns, just a point to watch if you do end up fixing the speaker, so it does not reoccur.
You could perhaps disconnect the output from the field coil to the set, replace this with a correspondind resistor/lamp load, and see if the fault remains.
Excess HT might cause a hum as well, as would short circuit turns on the field coil (smothing effect of coil reduced)
Thats a bit of a start.

[Zelandeth]

One thing to check which I encountered on the mains energised speaker of a Ferranti 145 recently was a short in the hum cancelling coil (shorting to chassis if I remember right). The voice coil was fine as it turned out. This was resulting in low HT, overheating of the coil and cracklng and rustling in the speaker as soon as the rectifier warmed up.

So first thing I'd do is to find out absolutely for certain in which bit of the speaker has the fault, then have a look to see what we can do about it.

[Geoff 555]

Hello all, thank you for your suggestions and time. It has taken a little time to get back because I had to get some wire wounds, it appears that the local and only supplier round here has stopped doing WW resistors but the chap on the counter said that if I wanted any toys that would not be a problem!! The Royal Mail has gone on strike as well. Back to business, caps are new. I don't know the kind of tolerances involved but here is the stuff I have found so far, for your perusal and thoughts.
1. HT unsmoothed 370V actual 334V. ( see below.)

2. HT + smoothed 252V actual 243V.

3. V4 anode ,to one side of sound output TX. 235V actual 230V.

Speaker coil. 7-5K actual 8-1K.
Speaker coil. 1-5K actual 1-506K.

Sound output TX pri. 716 ohms. 2 different values given on 2 sheets. ' Service engineer' and 'Trader'.

Mains TX. Pri. 36 ohms. actual 39-7.
Heater correct.
HT 74 ohms. actual 70.

The metal rectifier has been replaced with diodes each with a 10n cap in parallel, if I add resistance to drop the voltage to the correct unsmoothed HT of 370V then the smoothed Ht is 274V.(should be252V) So I dropped the smoothed HT to 243V which was the closest I could get under the correct voltage thinking that this would be better than going too high.

Disconnecting the 7-5 K coil and replacing with a resistance makes the hum louder and the arcing /fluttery noise also louder.
I am not at all sure that doing the same with the 1-5 K coil would be very wise, wacking in unsmoothed HT. etc.

That's as far as things have got.
Thank you very much.
Cheers.
Geoff.

[Geoff 555]

Well Hi I think I may have sorted it but I would really like your opinions as to what may have happened.
Replaced the 1-5 K coil with a WW resistor , no difference.
Shorted out the Hum bucking coil , no difference.
Turned it onto gram only, no difference.
So had a bit of inspiration, disconnected the HT from the mains TX leaving the heaters connected. wound up the HT on a variac and the radio came up at 100V and at 240 V sounded fine, no hum or fluttery sound or distortion. Indeed better than it has ever been . Sound like a mains TX to me.
Thanks again chaps.
Cheers.
Geoff.
Hello Ed speakers seem OK but will PM you regards Mains TX rewind.