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Old 15th Apr 2022, 5:22 pm   #1
Pilot Mariner
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Default Grundig 2068 Output Transformer Fault.

A Grundig 2068 I am working on had a random crackle which I managed trace to the output stage before a loud crack, bang and buzz occurred from the speaker followed by the blowing of the mains input fuse.

Probing around the output transformer, I am measuring 22 ohms to ground from the primary centre tap, 37 ohms to ground at the HT end and 800 ohms to ground at the valve end. This is with the centre tap and valve connections isolated.

From this I deduce that the transformer probably has an internal short to ground somewhere down from the centre tap.
Any suggestions on how to proceed, maybe isolate the transformer from chassis?

Cheers

Mark
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Old 15th Apr 2022, 5:32 pm   #2
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Default Re: Grundig 2068 Output Transformer Fault.

I think the OP has done that.

Quote:
Originally Posted by Pilot Mariner View Post
This is with the centre tap and valve connections isolated.
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Old 15th Apr 2022, 5:32 pm   #3
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Default Re: Grundig 2068 Output Transformer Fault.

At the moment I only have the 1.3K connection, but I could disconnect it to be sure.

Last edited by Pilot Mariner; 15th Apr 2022 at 5:39 pm.
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Old 15th Apr 2022, 5:33 pm   #4
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Default Re: Grundig 2068 Output Transformer Fault.

Yes. I realised my mistake and deleted my post, but obviously not before you saw it.
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Old 15th Apr 2022, 5:42 pm   #5
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Default Re: Grundig 2068 Output Transformer Fault.

Ah - a couple of posts have gone?
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Old 15th Apr 2022, 9:54 pm   #6
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Default Re: Grundig 2068 Output Transformer Fault.

Just an update for anyone who's with me on this post.

I lifted the transformer clear of the chassis, but still had the low impedance to ground.
When disconnecting the secondary to ground wire the low impedance went away, so now it looks as though it is a primary to secondary short instead of to ground.

I'm not sure how to get over this problem now, looks like I need another transformer, but they could be as rare as hens teeth.
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Old 15th Apr 2022, 10:14 pm   #7
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Default Re: Grundig 2068 Output Transformer Fault.

If the secondary is on the outside of the bobbin, rewind it after replacing the insulation beneath. A loudspeaker secondary is easily wound by hand as there will be of the order of 60 turns of thickish, easily worked wire. It's likely that the existing wire can be re-used if you're careful.

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Old 15th Apr 2022, 10:30 pm   #8
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Default Re: Grundig 2068 Output Transformer Fault.

Thanks Leon for that suggestion, I've never rewound a transformer before so there's always a first time though.
I was wondering if I could blast it away with some current across the Pri - Sec without damaging the primary wire, but then again the S/C must be fairly stubborn to blow the mains input fuse.
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Old 15th Apr 2022, 11:38 pm   #9
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Default Re: Grundig 2068 Output Transformer Fault.

NO !!! You cant blast it away. You will blast the transformer into oblivion.
If the transformer has flashed over ( which it sounds like to me ) it will need rewinding.
As Leon has said, the secondary is easy.
A little transformer like that ( Especially Grundig ) will be a major PITA !!, for the primary. Trust me I have wound a few. They tended to make very special layering, even for a simple class A single ended output transformer. Yours also has a choke winding to supply the rest of the set before the output stage. Maybe talk to Ed Dinning.

Joe

Last edited by joebog1; 15th Apr 2022 at 11:39 pm. Reason: Punctuation
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Old 16th Apr 2022, 8:57 am   #10
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Default Re: Grundig 2068 Output Transformer Fault.

I'm in Joe's camp. Personally speaking, I would be reluctant to attempt a rewind myself. There are many turns of very fine wire in the primary and if you have never done it before it is likely to be a non-trivial task. There are some pitfalls such as sometimes there is a gap or insulation between the laminations that alters the inductance. If it were me I would ask member Ed Dinning if he would rewind it for you. Cheers, Jerry
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Old 16th Apr 2022, 11:09 am   #11
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Default Re: Grundig 2068 Output Transformer Fault.

The 37 Ohm winding and R39 form part of a hum reduction circuit.

Obtaining an exact equivalent transformer or getting the faulty one rewound are the least complicated options.

It should be possible to use a standard EL84 compatible output transformer if your speaker impedance is one of the typical values, 3 or 8 Ohms.

You may need to add an extra stage of HT smoothing to reduce the hum to its previous level.
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Old 16th Apr 2022, 11:48 am   #12
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Default Re: Grundig 2068 Output Transformer Fault.

There is a number on that transformer, can you post it, please? I might have one somewhere, I think so, I’ll go on the tranny hunt tomorrow morning.
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Old 16th Apr 2022, 12:03 pm   #13
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Default Re: Grundig 2068 Output Transformer Fault.

Reasonable reproduction can be had from a standard single ended tranformer but fingers crossed for a donor transformer for you.
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Old 16th Apr 2022, 12:25 pm   #14
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Default Re: Grundig 2068 Output Transformer Fault.

Thanks everyone for your expert suggestions which I will ponder over.
The sets probably not valuable enough for the cost of a transformer rewind, the cabinet was a bit battered but the chassis looks as though it's just come off the production line, after a clean up.

Valvostef the writing on the transformer is:-

GRUNDIG
Ubertrager
9054-007

Cheers

Mark
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Old 16th Apr 2022, 2:01 pm   #15
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Default Re: Grundig 2068 Output Transformer Fault.

OK I’ll have a look tomorrow morning.
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Old 17th Apr 2022, 8:48 am   #16
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Default Re: Grundig 2068 Output Transformer Fault.

Sorry, couldn’t find it, I suppose I gave it away sometime.
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Old 17th Apr 2022, 8:54 am   #17
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Default Re: Grundig 2068 Output Transformer Fault.

OK thank you very much for looking.

Mark
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Old 17th Apr 2022, 11:45 am   #18
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Default Re: Grundig 2068 Output Transformer Fault.

I have a random 35:1 output transformer with a hum tapped primary, fixing centres 63mm. Interestingly it's part of a stash I bought from ValvoStef a couple of years ago .



If it's of any interest you can have it for the cost of postage.


HTH
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Old 17th Apr 2022, 12:57 pm   #19
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Default Re: Grundig 2068 Output Transformer Fault.

Thanks very much for that kind offer Herald1360, I will let you know later maybe tomorrow. Sadly easter family things are getting in the way today.
A survey of old sets in the garage early this morning has unveiled a rather tatty Grundig 3365. Maybe the TFR from that would be suitable, but I have not decided if it's one to restore in the future yet.
The other complication is that on the 2068 the secondary connections to ground and the tone circuitry are made via inner taps off the secondary not as shown on the circuit.

Mark
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Old 17th Apr 2022, 1:05 pm   #20
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Default Re: Grundig 2068 Output Transformer Fault.

Hello,

I have an output transformer salvaged form Grundig 2043 or 2035 or similar. It may boast a few extra windings, however, I feel it’ll do the job in an EL84 based Grundig radio with a ‘choke’ tap in the primary.

As far as I can remember it’s OK, but when I’m in the workshop later I can get the meter on it to find out.

Your more than welcome to it, if it’s of use that’s of course if it measures OK!

Pictures attached.

Regards
Terry
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