Identity of mains transformer used in Grundig como-6 radiogram

[indigo.girl]

I'm working on a Grundig Como-6 radiogram and the mains transformer is not working. Looks like an open circuit on part of the input.

I want to replace it (as I'm assuming mending is not an option) but am having difficulty identifying it.

The Chassis is an RC400 MS - which is common to many Grundigs including the Mandello 6, Bergamo 4, KS850.

There is no circuit diagram on the Service Data CD for this one but I have found one on radiomuseum. The only info I have is a code number 9060.115.02 which is on the diagram and on the transformer itself.

Here is a section of the circuit diagram. This transformer splits the 240V input across two coils with a tap off at 120V to run the GC060/55S record deck.



Can anyone suggest an alternative - or confirm from the diagram the step-down ratio? There are two coils for the output. One is the rectified DC voltage which is 20.5V and the other runs the 7V lamps. Can I assume the step ratio is 120:20 and 120:7 for the transformer or is it more complicated than that?

Many thanks,
Nicola

[TonyDuell]

It's one transformer with 4 windings. There are 2 primary windings, each rated for 120V. They are connected in series for 240V mains and parallel for 120V mains. As you say, there is a 120V tap taken from this (basically these windings act as an autotransformer) to that 4 pin connector. Although not shown on that bit of the diagram, using it for the turntable motor seems reasonable.

Now for the secondaries. The one for the lamps is going to be around 7V. Perhaps a bit lower (6.3V?) to underrun them a bit for longer life. The upper one in the diagram is bridge rectified and smoothed and provides 20.5V at the output of the rectifier. Now that voltage will be close to the peak (not rms) voltage of the transformer secondary, less 2 diode drops (1.4V) in the bridge rectifier. So the rms value of the voltage (which is what is normally quoted for a transformer output) is going to be about (20.5 + 1.4)/sqrt(2) or around 15.5V. Perhaps 16V or so would be fine here.

So you need one transformer with 2 off 120V primaries, a 6.3V and a 16V secondary. I doubt you'll get that 'off the shelf'.

At one time you could get transformer kits with pre-wound primaries (the difficult bit to do in that the insulation has to stand mains voltage), you had to wind the secondaries for the voltages you wanted. But those kits are no longer made as far as I know.

I would have thought the original transformer could be rewound, there are people here (not me) who do that sort of thing, but it would not be cheap.

[Herald1360]

Given it's a radiogram, have you got room for two transformers?


A 120 + 120 to 15 transformer is likely available off the shelf, as are similar to 6V.


If the 6V load isn't great, a suitable 120 + 120 to 15 + 15 TX could be used- just add a suitable dropper or regulator on the dc side of the 6V feed.

[Peter88gate]

No one seems to have mentioned it, have you checked the fuse S:1 on that bit of circuit diagram you attached? It seems to be 125mA for 240V (or 250mA for 120V). If blown, that could account for the transformer not working.

Peter

[Grubhead]

Technically speaking you are supposed to replace Transformers with devices recommended by the manufacture of the device you are repairing. Or at least that's what I have read for several years on ALL service data.
I certainly wouldn't advise to take the advice of someone offering near the specifications of a device even if it did work. Unless they were authorised to do so by the maker of the device.

If the replacement transformer did cause your house to burn down and the insurance company found out that it had been replaced on someone's advice of telling you that it would be OK. Then you would not get the money.

If you can't get a recommended transformer or one from another device that it is not working. I would say you let the device remain dead.

[indigo.girl]

Thanks for these two replies - extremely useful! I have found a toroidal transformer for £15 which looks like it will fit the bill. 2x115V input and 2 x 15V output @30W. Max power consumption on the spec for the radiogram says 25W so this should cover it.

So I can use one secondary for rectifying/smoothing to provide the HT to the circuit.
The other 15VAC I need to reduce down to the 6.3VAC for the lamps. So can I just use a a couple of high wattage (10W) resistors in series to divide the voltage and take the 6.3VAC from across one. Or does this only work with DC?

[ms660]

Just for info.

120v+120v:15v+15v at various ratings, availability times vary:

https://www.rapidonline.com/120-240v...sformers-82716

Also toroidal with 115v+115v primary:

https://www.rapidonline.com/115-230v...-primary-82719

EDIT: Post crossed.

Lawrence.

[Biggles]

Is it possible to wire the lamps in series, if there are two of them? This would give you a requirement of about 12 or 13 volts, so still a bit high though supplying them with 15 volts. Sometimes transformers can also give a bit more voltage out than the spec says, when not on full load.

[john.north]

Grundig transformer.

Hello Nicola,
The two primary windings are different.
The black to red is approx. 100 ohms.
The grey to Blue is approx. 65 0hms.
Remove the white connector which has 240v on one side and 120v on the other. Measure the primaries. Is the fuse ok. Is the mains switch ok.
John.

[indigo.girl]

Hi John
I've removed the primary wires from the chassis to rule out the rest of the circuitry. Looks like one primary is open-circuit (blue-grey). The other primary (red-black) is 100 ohms as quoted.

[indigo.girl]

Originally Posted by Grubhead

Technically speaking you are supposed to replace Transformers with devices recommended by the manufacture of the device you are repairing. If you can't get a recommended transformer or one from another device that it is not working. I would say you let the device remain dead.

This is a good point. Maybe this one should be saved for spares unless I can find an exact replacement.

[TonyDuell]

If you only replace certain components with the manufacturer's spare parts then many things will never be repaired. You just can't get such parts any more (if indeed you could ever get them). In any case, I believe this applies to such parts as the manufacturer classes as 'safety critical'. Is there any such comment in the service manual?

Personally I do not do repairs for other people (I suspect things would be different if I did). I am happy to trust my own judgement (and advice from people on this forum, say) as to what I can use to repair my own units. I never leave things plugged in if I am not using them (never at night or if I am going out) so the changes of a serious problem are vanishingly small. Given that I tend to over-rate replaement parts (I would use a transformer of a higher VA rating than strictly necessary, I suspect my repairs are safer than the original. I also feel that a new (good quality) transformer (say) will be at least as safe as a NOS 50-year-old one.

And I have never had anything I have repaired in this way catch fire, seriously overheat, give me a shock, or... Which is more than I can say for modern devices.

If you are doing repairs for others then I guess you do have to think about idiots who will try to hold you liable. But for myself I just get on with it.

[john.north]

Grundig Transformer.

Hello Nicola,

If you can verify that the transformer from a Mandello 7 is the same then I have one you can have.
John.

[Records]

I get what you are saying Grubhead, but if the job was done properly even with 2 seperate transformers everything I hope would be fused and therefore no chance of a fire. Only reason I can see for not using 2 seperate transformers is if a complete idiot wired it up, to me Indigo girl, does not come across as someone who has no idea what they are doing.

[AC/HL]

We've had this discussion many times, most recently in the last couple of days about a record player. Of course you can do what you want with your own equipment, we all do. No-one could or would intervene. The chances of a fire are minimal if the work is to a competent standard. They aren't zero, and weren't before the repair, but it's your risk. Most of us take it, but...

[Grubhead]

I understand what people are saying. I once phoned up a manufacture of a video recorder for a spare part. They told me that the nearest car boot sale was the best place for it!
The definition of a "Safety Part" must included the transformer. It's sometimes the only part that has 240 volts going into it! A highly dangerous voltage! I have not seen any service manual of the date that makers started putting on the "Safety Part" thing that did not include it.
With devices being sold on auction sites for spares only it's just not true that there are no spares, unless of course it's a rare model. In which case I would advise that as long as you know all the specifications of the part you want to replace you can indeed look for a substitute and replace with something at least of the same quality if not higher.
I wouldn't imagine that parts are at risk of catching fire. Indeed I would have thought that makers of them go out of the way to ensure they don't. However the person fitting them must be aware of the specifications of the original device and that they are certain that the new device meets them.
Nevertheless even if you can't find the original part now, one might turn up in a few months on an auction site etc. It's better to wait than be sorry... as the saying goes!

[julie_m]

As the transformer you've got is dead anyway, there is not much to be lost by attempting to see if the break is near the outside of the winding.

If you have a sensitive capacitance meter, you might be able to measure the capacitance between the intact primary winding and each end in turn of the broken winding. If the break is near the middle, the capacitances should be approximately equal; if it is nearer one end than the other, the capacitance between the shorter section and the other winding will be less than the capacitance between the longer section and the other winding.

[indigo.girl]

Originally Posted by john.north

If you can verify that the transformer from a Mandello 7 is the same then I have one you can have.

Hi John - Well by the looks of it the Mandello 7 uses the same RC400MS AM/FM chassis based on a few photos found on the internet. Would be great if I could buy its transformer from you. I'll message you.

[indigo.girl]

Originally Posted by julie_m

As the transformer you've got is dead anyway, there is not much to be lost by attempting to see if the break is near the outside of the winding.

If you have a sensitive capacitance meter, you might be able to measure the capacitance between the intact primary winding and each end in turn of the broken winding. If the break is near the middle, the capacitances should be approximately equal; if it is nearer one end than the other, the capacitance between the shorter section and the other winding will be less than the capacitance between the longer section and the other winding.

Great idea Julie - I don't have a cap meter but I'll open it up and see if I can find a short near the ends.....

[TonyDuell]

Remember in this case the 2 primaries could be connected in parallel (for 120V mains input) and that therefore they must have the same number of turns. So you can't just remove a few turns if the break is near the outside as you might do with an output transformer or similar. If the transformer primaries are not the same, then connecting them in parallel will result in a large current circulating in the primaries which will cause the transformer to overheat.

You might say that you are never going to use it on 120V mains, but somebody else might in the future, and I feel that this is a much greater hazard than using a substitute transformer.

Yes the mains transformer has to withstand 240V across the ends of the primary and between the primary and earth (it is electrical nonsense to have a voltage 'going into' or 'going through' something). But if I go to RS or Farnell or somewhere and buy something sold as a 'mains transformer' then I would argue it should be able to stand that, as that is the suggested use for it. Provided the ratings are adequate, so it doesn't overheat in use I would have no worries about using such a transformer. In fact unless you are experienced in winding transformers for mains use, I would think a new mains transformer from a known supplier was safer than repairing the old one.

As an aside, many of us have had RIFA mains filter capacitors fail. In most (all) cases, such filter capacitors are listed as a critical safety component (as they are, they have to withstand mains, and fail in safe ways). Now I (and others) just replace them with electically equivalent class X2 or class Y (as appropriate) components. But not indentical RIFA ones. Is that acceptable? If so, then why is replacing a mains transformer with a transformer sold to work on the mains and with adequate ratings not OK?

Incidentally, I assume there's not a thermal fuse hidden in this transformer? Philips were known for doing that, I could believe other manufacturers did the same. Sometimes they could be replaced without replacing the transformer.