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Old 27th Nov 2020, 2:52 pm   #1
robreddog
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Default Grundig 3028, EM34

hi, coming to the end of my 3028/w/3d restoration and had left the dim tuning indicator till last.
thanks to the posters on this site (great information ) i have the radio working pretty well,someone had worked on the radio before and changed some caps but i changed all the other questionable ones,the rectifier and smoothing capacitors needed changing as well plus a 10watt 120 ohm resistor.
clearly a em34 is not fitted and the base wiring changed for this.
the valve fitted is marked aw6, za/1880 no idea of this can't find any info.
can anyone tell me if this is a replacement for the em34,valve museum gives equivelants,but not this one.
i have seen a previous post about a wiring mod to fit a different valve but cannot find it today, perhaps someone could send me a link.
the em34 in the pic comes from another 3028 in poor condition i bought in case of a major problem.
thanks
Rob
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Old 27th Nov 2020, 3:19 pm   #2
ms660
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Default Re: Grundig 3028, EM34

Some AW6 info here:

http://www.r-type.org/pdfs/cv1077.pdf

Lawrence.
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Old 27th Nov 2020, 3:27 pm   #3
cathoderay57
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Default Re: Grundig 3028, EM34

Hi Rob, the AW6 is equivalent to EM31 which is a single sensitivity magic eye, whereas the EM34 is double sensitivity. The pin connections of the 2 valves are NOT compatible and therefore if you want to fit an EM34 you'll need to rewire it. A Y63 has same connections as an EM31 but requires -22v on its grid to close the eye whereas the EM31 only needs -5v. Cheers, Jerry
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Old 27th Nov 2020, 3:59 pm   #4
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Default Re: Grundig 3028, EM34

As it happens I have a nice bright used EM31. See pics attached. If you are interested then please send me a PM. Cheers, Jerry
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Old 27th Nov 2020, 4:32 pm   #5
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Default Re: Grundig 3028, EM34

hi,thanks for that,i have -3.5v on the grid,if i used em31 how would i increase the volts to -22.
also is the big capacitor on the output transformer bipolar ??.
rob.
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Old 27th Nov 2020, 4:32 pm   #6
cathoderay57
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Default Re: Grundig 3028, EM34

If you want to rewire the valve holder to suit an EM34, the following mod should work. See attached sketch; view is from below the valve holder. Pins 2, 7 and 8 are OK as-is. For the AW6 you should find a 1M or 2.2M resistor soldered between pin 3 (anode) and pin 4 (target). If it is a 1M you can re-use it; if not then find two 1M resistors about 1W rating (tiny ones won't do). Remove the grid wire from pin 5 and swap it with the target wire on pin 4. Leave one end of the 1M resistor soldered to pin 3, remove the other end from pin 4 and solder it to pin 5 along with the target wire. Fit a new 1M resistor between pins 5 and 6. If you find any other components such as capacitors on the valve holder then post details. Once rewired, the snag you might encounter is that the maximum grid deflection voltage of the EM31 is only -5v whereas the EM34 is -16v. Try it first and if the deflection is insufficient then you will need to check the resistors in the magic eye grid circuit below the chassis, and make sure they are reverted back as per circuit diagram. Hope that helps. Jerry
P.S. the only capacitor on an output transformer is usually a whistle filter paper cap, something like 0.01 or 0.02uF. They are not bipolar in the sense used to describe electrolytics, but can be connected either way around.
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Old 27th Nov 2020, 4:43 pm   #7
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Default Re: Grundig 3028, EM34

I see from Radio Museum that some 3028s were fitted with EM35 so you'll need to look carefully at the wiring. Does it look as though the valveholder has been modified?
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Old 27th Nov 2020, 5:01 pm   #8
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Default Re: Grundig 3028, EM34

hi,so looks like i could fit em31 without changing the modified wiring done for the aw6??.
the big capacitor on the output transformer is a 10uf 35volt.
sent jerry a pm.
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Old 27th Nov 2020, 5:28 pm   #9
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Default Re: Grundig 3028, EM34

C63 is the filter capacitor for the tweeters I think. It would be a good idea to use a bipolar electrolytic.
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Old 27th Nov 2020, 5:34 pm   #10
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Default Re: Grundig 3028, EM34

Thanks for that I will check if any other components in the em34 cct have been replaced tomorrow.
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Old 28th Nov 2020, 11:37 am   #11
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Default Re: Grundig 3028, EM34

hi,
so the cct's are ok for the aw6,em31.
think i will stick to these rather rewire back to em34 as i have just seen the
price wanted for them !!
Rob.
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Old 28th Nov 2020, 5:37 pm   #12
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Default Re: Grundig 3028, EM34

So I presume your Dario EM34 shown in the picture is dim? Cheers, Jerry
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Old 29th Nov 2020, 5:51 pm   #13
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Default Re: Grundig 3028, EM34

Hi, well i can't test it, so I'll find out if/when I have a go at my other 3028.
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Old 30th Nov 2020, 1:42 pm   #14
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Default Re: Grundig 3028, EM34

Hi, on another subject I refurbished the cabinet and then finished with danish oil.
Would it be wise to use some sort of polish as a protection or should I leave well alone, when I get my em31 from Jerry I will put back together and post a couple of pics.
Rob
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Old 30th Nov 2020, 3:28 pm   #15
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Default Re: Grundig 3028, EM34

Hi Rob, I guess it depends what condition the existing finish is in, and the level of degradation that you are prepared to tolerate. This thread might help https://www.vintage-radio.net/forum/...t=Grundig+3028 . I had never heard of catalysed lacquer but that seems to be what the typical Grundig high-gloss finish is described as here. If it is badly scratched a complete strip might be the only answer but it sounds like you would need some aggressive stripper. It's a while since I've refinished a cabinet but I used to use Nitromors in a well ventilated garage - never done a Grundig though. Cheers, Jerry
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Old 30th Nov 2020, 3:41 pm   #16
robreddog
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Default Re: Grundig 3028, EM34

Hi, I've already refurbished it, sorry if I didn't make that clear, hell of a job to get the old lacquer off, rubbed down and about 8 applications of danish oil following a YouTube video I found, it looks really good except a minor bit of end grain damage on one side by the speaker surround, going to leave that as it is 60 odd years old, I just wondered if it would be a god idea to use some soft polish on it?.
Rob.
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Old 30th Nov 2020, 4:07 pm   #17
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Default Re: Grundig 3028, EM34

Sorry, it's me - I should learn to read the thread more carefully! AFAIK Danish Oil leaves quite a soft finish that can be damaged. Some recommend top coating with a clear satin varnish or just using several coats of hard wax. Best to experiment on an area that is not easily seen in case it blisters up. Cheers, Jerry
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Old 2nd Dec 2020, 1:59 pm   #18
robreddog
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Default Re: Grundig 3028, EM34

hi, thanks jerry !! got the valve and it's perfect.Posted some pics.
before i stripped down the cabinet i had that slight bass fluffy sound as described in other post's but when all was cleaned,sealed and fitted up, the bass is fine.No idea why.
Rob
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Old 2nd Dec 2020, 10:38 pm   #19
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Default Re: Grundig 3028, EM34

Great stuff - glad it went to a good home, and well done on the resto! Jerry
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Old 3rd Dec 2020, 10:10 am   #20
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Default Re: Grundig 3028, EM34

Wow Rob what a lovely looking set. I've been eyeing some German sets for a while but still find them a bit intimidating electrically.

That's a great job and great outcome, really well done

Gabriel
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