I'm in the process of restoring a quite lovely AWA portable radiogram. The chassis is labelled as a 539MA but it is in a portable wooden cabinet with vinyl covering.
Uses a Collaro turntable which seems to be OK apart from needing a new stylus.
However, the speaker is pretty old and the cone appears on the verge of falling apart - it also distorts at anything over moderate volume.
I'm wondering if I need the original speaker or can I upgrade? The wiring to the speaker is a little strange with some wiring coming up from the chassis and some from the speaker transformer.
Any ideas???

The schematic for model 539MA shows a permanent magnet speaker with two wires from the output transformer (coloured blue and brown) and a tinned copper wire going to chassis earth. These 3 wires go though a 5 pin plug, with 2 pins unused. The notes give the speaker as:

12 inch, type AU60, impedance 6.5 ohms at 400 Hz, 5 watts.

The associated output transformer is type TU201.

On that basis, I'd say you could get away with using practically any 8 ohm 12 inch speaker.

Otherwise, get it re-coned. You could ask VIC company Total Recoil for a quote:

http://www.totalrecoil.websyte.com.au/site.cfm?/totalrecoil/.

Thanks GTC.

Just to let you know, this is a portable radiogram so is quite small and does not have a 12 inch speaker, only a small speaker mounted behind the tuner dial.

I have posted a couple of pics on Photobucket and you can see them at:
http://s1186.photobucket.com/albums/z371/Nucky11/Vintage%20Radio%20Restoration/ This may help.

Incidentally. the same page has some pics of my Breville Precedent which has what appears to be an unusual valve, a 6AC7 which is shown in detail in the pics.

Ooops, my circuit look-up index is faulty and I inadvertently gave the speaker info for 538GA.

Yes, 539MA has a 5 inch permanent magnet speaker and, as with the 538GA, the schematic shows an earth wire as well as the two wires from the output transformer.

The impedance of the speaker isn't given, however a reasonable rule of thumb is to measure the DC resistance of your existing speaker and multiply that by 1.25 to get an idea of the impedance.

Other than re-coning, any 5 inch speaker of the correct impedance should do the job, although mounting hole placement may also be a consideration in that particular radiogram.

I should add that the output transformer may also be dodgy. If it were on my bench I'd test speaker and transformer independently by substitution.

GTC..

Thanks for that. As a relative newbie, can you tell me where I should measure the DC resistance of the speaker?

Also, when wiring in the replacement which will have only 2 terminals, which wiring from the old speaker should be connected where?

As you can see by the pics, there is quite a bit of wiring going into the current speaker on the set so I'd appreciate some advice here.

The schematic I have for the 539MA doesn't show wire colours, so going by your photo I'd say the blue and brown wires would be going to the primary side of the output transformer. Check under the chassis to see if the blue/brown pair goes to pin 5 of the 6AQ5 and pin 7 of the 6X4.

The circuit incorporates a feedback arrangement coming from the secondary side of the output transformer, so I'm guessing that's the red and white pair. The connection to the speaker's voice coil ought to be paralleled with the red and white pair.

The green would thus be chassis earth.

It would help if you could carefully unbolt the transformer from the speaker and take a good photo so that we can try to confirm what's what on the currently hidden side of that.

As for measuring DC resistance in that arrangement, once we confirm the wiring, you'll need to remove all connections to at least one side of the the voice coil and then put your ohmmeter across the isolated voice coil.

PS: I don't much like the look of that power transformer. It appears to have been subject to quite a lot of moisture, which is the bane of coil windings. Personally, I'd get it "Meggered" (high voltage tested) to check for insulation breakdown.

Thanks again GTC...

Have added a detailed picture of the speaker wiring in photobucket at http://s1186.photobucket.com/albums/z371/Nucky11/Vintage%20Radio%20Restoration/.

Re the power tranny... are replacements for this easily available as the set has a pronounced hum which could be to do with the transformer?

Your help is very much appreciated. BTW, can you post/email a copy of the schematic of this to me? My email is nick.boslypaskgmail.com

Re the power transformer: I would say that, in general, no replacement tranny for any old set is easily available these days. The usual source is another hobbyist via a similar chassis acting as a parts donor.

You might be able to find something close, but first we need to know the secondary's specs. The circuit does not provide voltages, but the maximum plate voltage for the 6X4 rectifier is given in my book as 325 volts per plate. I suggest that you measure the voltage between each plate of the 6X4 and chassis and let us know what you get.

*** Be very careful doing that. These are lethal voltages.

The hum is almost certainly caused by tired old filter capacitors C17 and C18 given as 16 μF, 525 peak voltage.

As a general rule, a resto job would replace all electros and paper capacitors anyway. They will be way out of spec by now and present a danger of short circuit.

Note that the filter caps must be replaced by elecros designed for the purpose: they will be high ripple current types and these days you'll usually find them rated at 630 volts.

Note also that electrolytics have polarity that must be respected.

Now for the speaker: The voice coil will be connected to the underneath of that solder panel. You can just make out the output transformer's secondary wires in the dark area, paralleled with the red and white wires and soldered to the voice coil terminal strip.

I note that there is an additional earth in the form of a bit of naked wire on the (as pictured) right hand solder joint. As the other terminal is presumably also earthed via the green wire, it appears that the speaker frame must be insulated from the main chassis -- otherwise those earth connections would short out the speaker and output transformer. Strikes me as weird at this distance.

Please let us know where the green wire is terminated under the chassis.

BTW: I've just found the technical notes associated with model 539MA and they give the speaker's impedance as 3 ohms ( 400Hz) and power capability as 3 watts. So, a 4 ohm 5 inch speaker will suit your purposes, mounting issues notwithstanding.

Finally, I'll email you the schematic and notes, as requested.

With 6X5 that is a heater rectifier and will not present a surge anything like 5Y3 and similar, non cathode filament rectifiers.

The old filters will likely have a voltage rating on them I doubt the need to go above 450V.

The transformer HT AC output voltage can be measured with the rectifier removed.

Be very careful, many modern meters cannot withstand the voltages in Valve radio's.

Pins 3 & 5 of a 6X5 are the plates, measuring across them can result in voltages of around 700VAC. Measure between a plate & chassis as directed.

Marc

Marcc, schematic gives the rectifier as a 6X4, and the chassis photo seems to support that.

As above, the parts list shows the filter electros as 525 peak voltage.

Misread that I have not got circuit however, samer deal.

525 surge volts was the most common of the Ducon caps and tended to get used for nearly everything.

Marc

Inexpensive Five inch speakers with 4 ohmn voice coil are available from Wiltronics.com.au also Jaycar and other suppliers.
Mounting (or re-mounting) the output transformer may be the more difficult part, a little ingenuity will help to make a new bracket in needed.
Keep the phasing of the (green) feedback wire the same as it is in relation to the primary (blue & brown) wires conected to the 6AQ5 plate and 6X4 (B+) , otherwise the feedback polarity may be inadvertently reversed.

Hi Nucky
Just had a look at your pics.
The speaker looks (physically) good.
The Red and White wires are non original, and may have been added to connect to another speaker ? In any case you need to find out where they go, if shorted out it may be your "speaker" problem.
The Power transformer looks very dirty / corroded and lacks the top cover and the holding down bolts are much smaller than originally used, or could even be a non original transformer fitted, this could result in transformer lamination rattle (not quite the same as the usual "hum" in the speaker associated with poor filtering).
I have some of the original transformers for these if that is the problem.
hope this helps and give me an email if need be.

Thanks to all.. to GTC for the schematics (received via email and very useful!)
and to radiola49 - your observation is spot on.. a little further investigation reveals that someone in the past has tapped off the red and white wires for an extension speaker so I will remove them (even though they are not shorted).
By the look of the power tranny it could do with replacing so if you have one of the originals I can buy, please email me at nick.boslypaskgmail.com
Next job is to replace the caps and clean it up as well as replace the speaker now that I know what's involved.
All power to the forum - what a great resource!

Looks can be deceiving:

There are ways of "prooving" the transformer. I find failures of Australian valve radio transformers to be rare.

Normally they are caused to "smoke" by being loaded & cooked when the set should never have been switched on without checking, or even rarer, being left on with a fault.

Normally they wil "growl" if there is a severe short.

Marc