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Old 18th Oct 2017, 3:49 pm   #1
bigfathairyvika
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Default Grundig 3025 LW MW fault.

Hi All,

I've been going through faults on a grundig 3025 one at a time.
1, Seized volume control and ferrite direction control = fixed
2. Scratchy tuning / popping = fixed dirty air cap and bent vane.

The lw and mw reception were great until I replaced the fixed air cap.

Kw reception was also now marginal.
This was fixed by reseating the ech81 valve.

But still no lw or mw. I can pick up hum/ buzz from a fluorescent light.

UKW ( FM ) is great.

I have a schematic but as the entire image is on one sheet of a4 it is really hard to make out.

The set has had all the important caps replaced previously.

I now suspect either the band switches ( multi contact slide ) or possibly the ech81 valve as it does have two sections and one maybe just for lw/mw. ( havent got a spare valve to try )

Could anyone offer any help as I am working blind as far as a schematic is concerned.?

Many thanks

Mark
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Old 18th Oct 2017, 4:29 pm   #2
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Default Re: Grundig 3025 LW MW fault.

Why did you replace fixed air gap on the tuning cap, perhaps you have put wrong value in or got it partially short circuit. Have you tried realigning IF cans and there may be another cap which is u/s.

Ken
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Old 18th Oct 2017, 10:52 pm   #3
bigfathairyvika
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Default Re: Grundig 3025 LW MW fault.

I didnt replace anything. Maybe i worded it wrong.
I cleaned the air variable capacitor and straightened the shorting vanes.

I dont want to realign the i.f. cans as it was working with the current settings.

It could well be a coincidence that a capacitor has failed.
All I have done since the mw/lw section was working is clean the air variable capacitor.
That just involved desoldering three wires to it, unscrewing it from the chassis, cleaning then putting it back again.

I think the confusion was when I said "replace" when I meant "put back".
But then again "to replace" doesnt mean "change".



Mark
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Old 18th Oct 2017, 11:25 pm   #4
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Default Re: Grundig 3025 LW MW fault.

I wonder if the clue lies in having to re- seat the ECH81. In removing the tuning capacitor I suggest that when you knocked the valve you may also have inadvertantly damaged/broken the ferrite rod connection. The connection to look out for is on the metal support which the ferrite rod sits and is rotated. Look carefully at where the litz wires are routed through the bearing and then to the fixed connection.

Best of luck

Chris
PS The Grundig 2035 is the same chassis as the 3028 and the You Tube renovation shown may help you.
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Old 19th Oct 2017, 12:12 am   #5
bigfathairyvika
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Default Re: Grundig 3025 LW MW fault.

Woo hoo!
I hear the glorious sound of mw valve reception.
This one really does sound nice. Much better than my ER30.
It is also the first valve radio I have had apart that uses a more "refined" component choice.


It was the connection between the two coils on the ferrite!
How did I miss it? It just didn't cross my mind, I've probably been so cautious trying not to break the rod that I didn't look closely at it.

Just need to remove the yellow nicotine varnish and put it back together.

Thank you very much everyone.

Oh well, live and learn...

Btw: the black bulbs for front panel... replaceable with ordinary bulb with a bit of paint?

Mark

Last edited by bigfathairyvika; 19th Oct 2017 at 12:26 am.
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Old 19th Oct 2017, 9:01 am   #6
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Default Re: Grundig 3025 LW MW fault.

I wouldn't have thought they were meant to be black, Mark.
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Old 19th Oct 2017, 9:41 am   #7
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Default Re: Grundig 3025 LW MW fault.

Yes, that sounds like the bulbs have had a long life and the filament has gradually been vapourised and deposited on the inside of the glass envelope, turning it a shiny grey-black colour.

Replace with "normal" MES bulbs, probably 6.3V at 0.3A, but see what's printed/stamped on them.
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Old 19th Oct 2017, 9:42 am   #8
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Default Re: Grundig 3025 LW MW fault.

The bulbs started out clear! Its filament deposited on the inside that turns them into black pearls.

Crossed post

Last edited by Boater Sam; 19th Oct 2017 at 9:43 am. Reason: crossed
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Old 19th Oct 2017, 11:31 am   #9
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Default Re: Grundig 3025 LW MW fault.

If this is a 220 volt radio used on 240 volts then 6.3 volt bulbs will be over run and go black inside the glass. Original bulbs were 7 volt which may still be over run if there is no 240 volt setting.
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Old 19th Oct 2017, 6:23 pm   #10
bigfathairyvika
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Default Re: Grundig 3025 LW MW fault.

Yes it is a 220v volt max one.
The lamps are 7v 0.3amp Osram.
I did at first think of the filament deposits but they both looked too perfectly coated in matt black.
But after looking at them through a lens it is obvious.

I will measure the voltage again, but I seem to remember it being 6.8v a/c.
I suppose the valve heaters will be getting a hard time as well.
I can run it from a variac and isolation transformer I have.
I may well set up a permanent 220v and 110v supply just for my radios.

This particular Grundig hasnt been powered on since the 70's.
I was surprised how well it has faired.

Mark
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Old 19th Oct 2017, 6:46 pm   #11
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Default Re: Grundig 3025 LW MW fault.

I have thought about this overvoltage problem in the past when I had a few sets that had no 240v/250v settings. The mains here can exceed 256v regularly, its an overhead supply with a dedicated transformer set high because the load of the farm can be very heavy at certain times.

The thought I had was; could the excess voltage be dropped with a capacitor before the transformer primary? It would be a large capacitor to drop say 30v and would be straight on the mains.

Comment please.
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Old 19th Oct 2017, 7:14 pm   #12
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Default Re: Grundig 3025 LW MW fault.

You mean a series RLC circuit across the mains

Lawrence.
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Old 19th Oct 2017, 7:27 pm   #13
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Default Re: Grundig 3025 LW MW fault.

A Bucking Transformer might be better: http://sound.whsites.net/articles/buck-xfmr.htm
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Old 19th Oct 2017, 7:59 pm   #14
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Default Re: Grundig 3025 LW MW fault.

A transformer would be the only feasable option due to the unfeasable size of capacitive reactance needed for house hold currents.
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Old 19th Oct 2017, 8:12 pm   #15
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Default Re: Grundig 3025 LW MW fault.

I'd be more inspired to fit a couple of resistors in series with the bulbs, and live with it.

What we consider 'significant variations' of mains voltage today were quite normal in the immediate pre/post-WWII days when even in quite major conurbations your mains came from a local generator whose voltage fluctuated with load.

6.3V-heater valves are usually specced to work within-spec for a 10% variation of heater voltage, for example. That's around 5.5 to 7v. Given that '6.3V' was arrived-at in the US pre-WWII originally focussed on valves for 6-volt-car-battery power [and realising that mid-1930s cars had truly primitive generator-voltage-control] I honestly don't get too uptight about voltage variations.
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Old 19th Oct 2017, 8:16 pm   #16
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Default Re: Grundig 3025 LW MW fault.

I agree, I neither have any worries about the extra voltage.
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Old 19th Oct 2017, 9:41 pm   #17
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Default Re: Grundig 3025 LW MW fault.

If you check out the 3028 circuit diagrams, there are several, and early UK versions had a resistor in series with the transformer 220 volt primary. Think it was 68 ohms at about 10 Watts. It was wired above the fuse tag strip.
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