Mains transformer - UK Vintage Radio Repair and Restoration Discussion Forum

WaveyDipole · 3rd Feb 2014, 2:27 pm

I am working on an old and dead transistor radio and have discovered that the transformer is likely blown. The tracks around the fuse and transformer connector had significant scorch marks. About half an inch of track on two of the tracks, between the fuse and the 220v/230v pin of the transformer connector and between the neutral and transformer connector were burned away. The 160mA fuse was still OK (maybe replaced?). I might have suspected that someone had switched to 110v while the equipment was still connected to the mains but the 110v track was intact.

This seemed to suggest that the transformer primary might have overheated and shorted. However a DVM reading showed resistance across both windings, from memory approx 65ohms across the the 100v taps and about 130 ohms across the 220v taps, which seem in reasonable proportion. I'm not sure what one typically might get but this does not seem like its fried or consistent with the level of charring and burned tracks that I saw. Since I have no safe way of testing it, I guess one cannot rule out that at least some windings might be shorted, but I can't see it blowing like that spontaneously though?

Is there any way I can be sure whether this transformer is OK or not?

Looking at a replacement, according to the circuit diagram the secondary has an output voltage of 13.2v and the transformer primary has a rating of 220/230v 'max 16W'. If I remember rightly although the transformer converts voltage the power drawn stays constant across both primary and secondary. So although 16W translates into 0.07A on the primary, at 13.2v this would allow 1.21A to be drawn on the secondary, so am I correct in thinking that I am looking at a transformer with a minimum rating of 1.25A?

The radio can be run form a DC voltage source of between 12 and 15 volts, although the battery is only 9v. The selected power source is regulated by a power transistor and 10v Zenner diode combination to produce between 8.5 and 9.5v output, so on the face of it, it would seem that a transformer with an output of 12v would do?

The only markings on the transformer are Grundig 09058-007.01 and a Google search shows nothing for those numbers. Can anyone suggest what I might use as a replacement?

3rd Feb 2014, 3:15 pm

I would stick it on the mains and see what happens, it will be either bang/no bang or good. This may sound a bit cavalier, done carefully it is a definitive go/no go test.

turretslug · 3rd Feb 2014, 4:51 pm

Is it possible that the radio got damp or had a drink spilled into it and the mains crept between tracks and sizzled them?

The "16W" rating is likely to be a worst case for product registration and also reflects the fact that small mains transformers tend to be woefully inefficient, it's probable that the secondary rating would actually be around 0.5-0.6A or thereabouts. Essentially, if it does turn out to be duff, find a VA rating that physically fits!

WaveyDipole · 3rd Feb 2014, 6:13 pm

Quote:

Originally Posted by merlinmaxwell

I would stick it on the mains and see what happens, it will be either bang/no bang or good. This may sound a bit cavalier, done carefully it is a definitive go/no go test.

Well, erm yes, I agree that would be definitive.
Would prefer to avoid the bang if possible though!

Quote:

Originally Posted by turretslug

Is it possible that the radio got damp or had a drink spilled into it and the mains crept between tracks and sizzled them?

The "16W" rating is likely to be a worst case for product registration and also reflects the fact that small mains transformers tend to be woefully inefficient, it's probable that the secondary rating would actually be around 0.5-0.6A or thereabouts. Essentially, if it does turn out to be duff, find a VA rating that physically fits!

I guess that's not impossible, but I didn't see and evidence of spillage. There was an open 1 ohm thermal resistor in series with a shorted tantalum electrolytic capacitor on the secondary side and connected an LM317 regulator (which I think is only applicable to the mains power source only and being used to regulate a supply for charging an optional manufacturers 'dry battery').

I note your point about the rating. I could measure the current drawn from a DC power supply once I get it fully working to determine how much it actually draws, but ultimately, as you point out, I will need a replacement that actually fits in the case!

TrevorG3VLF · 3rd Feb 2014, 8:13 pm

You could put a small light bulb in series with the mains to make a small bang. A Variac can be used to go gently.

I would disconnect the rectifier and run the transformer on its own. If it fails, then you need another. If it works, you will need to look further into the set.

WaveyDipole · 3rd Feb 2014, 10:16 pm

Trevor, thanks. I have various bulbs so I might give that a go. I don't have a variac, only a spare light dimmer switch but I'm a little concerned about using that. I have already disconnected and removed the transformer from the set so as to get access to the power/audio out board and was intending to test while disconnected. I will let you know how I get on.

Cobaltblue · 3rd Feb 2014, 10:20 pm

Hi WD don,t use a dimmer switch they don,t vary the voltage or limit the current they just change the conduction angle of a Triac to conduct for part of the waveform.

Use a Mains light Bulb in series 25W if its a small transistor Radio.

Cheers

Mike T

WaveyDipole · 3rd Feb 2014, 11:19 pm

Well, so far, so good.

I tried out your suggestion Trevor and wired a standard mains lamp holder in series with transformer and connected to a plug with a 3 amp fuse. I first tried a very low wattage fireplace bulb first. Got a reading of around 6v on the output and no smoke or heat. So I tried a 40w bulb next. Got around 12v output and still cool. Stepped up to 60w and then 100w with no adverse consequences getting 230v on the input and 14.5v on the output. Finally I wired it up directly and held my breath while I pluggged it in. Still no sign of a problem! Phew!

I then left it running for 3 or 4 mins and there was still no appreciable warming. So at least under no load, the transformer seems fine.

I supposed some water or damp getting into the radio might be one explanation as that could presumably evaporate and leave no trace. Someone poking a screwdriver in while it was still plugged in might be another one. Someone has been in here before and broken the ferrite rod among other things.

Thanks for the advice though.

Maarten · 3rd Feb 2014, 11:41 pm

The Grundig partnumber can be very useful to identify an exact replacement, but when searching for specifications or an aftermarket replacement it is more practical to go by the model number of the set and find a service manual.

WaveyDipole · 16th Feb 2014, 11:14 pm

I have now replaced the faulty resistor and capacitor on the power board and replaced all of the electrolytics including the other tantalum one. I also repaired the damaged tracks and connected everything back up to the radio. I then plugged in the transformer and connected the radio to the mains and am happy to report that the radio powered up and worked fine. It seems that the PSU board and audio amp are working as expected.

3rd Feb 2014, 2:27 pm	#1
WaveyDipole Nonode Join Date: Mar 2013 Location: Leicester, UK Posts: 2,535	Mains transformer I am working on an old and dead transistor radio and have discovered that the transformer is likely blown. The tracks around the fuse and transformer connector had significant scorch marks. About half an inch of track on two of the tracks, between the fuse and the 220v/230v pin of the transformer connector and between the neutral and transformer connector were burned away. The 160mA fuse was still OK (maybe replaced?). I might have suspected that someone had switched to 110v while the equipment was still connected to the mains but the 110v track was intact. This seemed to suggest that the transformer primary might have overheated and shorted. However a DVM reading showed resistance across both windings, from memory approx 65ohms across the the 100v taps and about 130 ohms across the 220v taps, which seem in reasonable proportion. I'm not sure what one typically might get but this does not seem like its fried or consistent with the level of charring and burned tracks that I saw. Since I have no safe way of testing it, I guess one cannot rule out that at least some windings might be shorted, but I can't see it blowing like that spontaneously though? Is there any way I can be sure whether this transformer is OK or not? Looking at a replacement, according to the circuit diagram the secondary has an output voltage of 13.2v and the transformer primary has a rating of 220/230v 'max 16W'. If I remember rightly although the transformer converts voltage the power drawn stays constant across both primary and secondary. So although 16W translates into 0.07A on the primary, at 13.2v this would allow 1.21A to be drawn on the secondary, so am I correct in thinking that I am looking at a transformer with a minimum rating of 1.25A? The radio can be run form a DC voltage source of between 12 and 15 volts, although the battery is only 9v. The selected power source is regulated by a power transistor and 10v Zenner diode combination to produce between 8.5 and 9.5v output, so on the face of it, it would seem that a transformer with an output of 12v would do? The only markings on the transformer are Grundig 09058-007.01 and a Google search shows nothing for those numbers. Can anyone suggest what I might use as a replacement?

3rd Feb 2014, 3:15 pm	#2
Guest Guest Posts: n/a	Re: Mains transformer I would stick it on the mains and see what happens, it will be either bang/no bang or good. This may sound a bit cavalier, done carefully it is a definitive go/no go test.

3rd Feb 2014, 4:51 pm	#3
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,400	Re: Mains transformer Is it possible that the radio got damp or had a drink spilled into it and the mains crept between tracks and sizzled them? The "16W" rating is likely to be a worst case for product registration and also reflects the fact that small mains transformers tend to be woefully inefficient, it's probable that the secondary rating would actually be around 0.5-0.6A or thereabouts. Essentially, if it does turn out to be duff, find a VA rating that physically fits!

3rd Feb 2014, 8:13 pm	#5
TrevorG3VLF Rest in Peace Join Date: Apr 2011 Location: Matlock, Derbyshire, UK. Posts: 1,378	Re: Mains transformer You could put a small light bulb in series with the mains to make a small bang. A Variac can be used to go gently. I would disconnect the rectifier and run the transformer on its own. If it fails, then you need another. If it works, you will need to look further into the set.

3rd Feb 2014, 10:16 pm	#6
WaveyDipole Nonode Join Date: Mar 2013 Location: Leicester, UK Posts: 2,535	Re: Mains transformer Trevor, thanks. I have various bulbs so I might give that a go. I don't have a variac, only a spare light dimmer switch but I'm a little concerned about using that. I have already disconnected and removed the transformer from the set so as to get access to the power/audio out board and was intending to test while disconnected. I will let you know how I get on. Last edited by WaveyDipole; 3rd Feb 2014 at 10:30 pm.

3rd Feb 2014, 10:20 pm	#7
Cobaltblue Moderator Join Date: Dec 2010 Location: Exeter, Devon and Poole, Dorset UK. Posts: 6,878	Re: Mains transformer Hi WD don,t use a dimmer switch they don,t vary the voltage or limit the current they just change the conduction angle of a Triac to conduct for part of the waveform. Use a Mains light Bulb in series 25W if its a small transistor Radio. Cheers Mike T __________________ Invisible airwaves crackle with life or at least they used to Mike T BVWS member. www.cossor.co.uk

3rd Feb 2014, 11:19 pm	#8
WaveyDipole Nonode Join Date: Mar 2013 Location: Leicester, UK Posts: 2,535	Re: Mains transformer Well, so far, so good. I tried out your suggestion Trevor and wired a standard mains lamp holder in series with transformer and connected to a plug with a 3 amp fuse. I first tried a very low wattage fireplace bulb first. Got a reading of around 6v on the output and no smoke or heat. So I tried a 40w bulb next. Got around 12v output and still cool. Stepped up to 60w and then 100w with no adverse consequences getting 230v on the input and 14.5v on the output. Finally I wired it up directly and held my breath while I pluggged it in. Still no sign of a problem! Phew! I then left it running for 3 or 4 mins and there was still no appreciable warming. So at least under no load, the transformer seems fine. I supposed some water or damp getting into the radio might be one explanation as that could presumably evaporate and leave no trace. Someone poking a screwdriver in while it was still plugged in might be another one. Someone has been in here before and broken the ferrite rod among other things. Thanks for the advice though.

3rd Feb 2014, 11:41 pm	#9
Maarten Dekatron Join Date: Dec 2007 Location: Haarlem, Netherlands Posts: 4,203	Re: Mains transformer The Grundig partnumber can be very useful to identify an exact replacement, but when searching for specifications or an aftermarket replacement it is more practical to go by the model number of the set and find a service manual.

16th Feb 2014, 11:14 pm	#10
WaveyDipole Nonode Join Date: Mar 2013 Location: Leicester, UK Posts: 2,535	Re: Mains transformer I have now replaced the faulty resistor and capacitor on the power board and replaced all of the electrolytics including the other tantalum one. I also repaired the damaged tracks and connected everything back up to the radio. I then plugged in the transformer and connected the radio to the mains and am happy to report that the radio powered up and worked fine. It seems that the PSU board and audio amp are working as expected.