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Old 6th Feb 2019, 9:07 pm   #1
cathoderay57
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Default Graetz Sinfonia 4R O/C bass speaker

Evening all,

I have started work on a recently acquired Graetz Sinfonia 4R radio. It is the one with a single ended EL84, and 3 speakers: the tweeter measures open circuit but I would expect that because it is probably electrostatic. There is a small circular mid-range speaker that is OK and has a DC resistance of 3.5R. However, the biggie is a 12"x8" bass elliptical and the speech coil is open circuit. See pics. I have measured the continuity of the braided flexi cables from the rear solder tags to the solder blobs on the front centre of the cone and they are OK. Putting the meter probes across the 2 blobs (cleaned of varnish) reads open circuit so the break must either be in the coil itself or the micro wires stuck to the back of the cone going down to the coil. The coil suspension is held in place by 4 small screws but I am loathe to undo them. The edges of the cone are firmly glued to the metal frame so I am afraid that if I try to remove the cone I will trash it. Anybody got any ideas? I guess a replacement speaker of some sort would be the obvious answer.... Cheers, Jerry
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Old 6th Feb 2019, 9:23 pm   #2
G0HFCFrank
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Default Re: Graetz Sinfonia 4R O/C bass speaker

Jerry,
You appear to have three options. First look at the repair services offered and wanted section of this site for someone to fix it. Second, if no one comes forward you may as well tackle it yourself; it may look very nice but it's no earthly use in its present condition so you have nothing to lose.
Third option is to put it in the bin. (This is also a possible first option but that depends on how much you enjoy taking things apart).
Good luck,
Frank C.
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Old 6th Feb 2019, 9:32 pm   #3
cathoderay57
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Default Re: Graetz Sinfonia 4R O/C bass speaker

Hi Frank, fair game, thanks for the reply. I am happy to have a go but I was really after advice and tips from anyone who has done a similar repair. If nobody comes up with any better ideas then I would propose to slacken the coil suspension screws (there are 3, not 4) and gently prise the coil suspension forward to see if I can access the speech coil wires from behind nearer to the coil. The next step would be to use a scalpel to try to lift the stuck down edges of the cone away from the frame and remove the cone entirely causing as little damage as poss. Rewinding the speech coil is beyond my skill level. However, if I could fix the break I would be able to re-centre the cone using the paper shim technique having removed the centre dust cover first. Before I launch on such a potentially destructive exercise I will take your tip and post a request for repair. I have not seen any correspondence here from people who repair speakers but if you don't ask..... Cheers, Jerry
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Old 6th Feb 2019, 10:15 pm   #4
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Default Re: Graetz Sinfonia 4R O/C bass speaker

If all else fails and you have enough money there is someone in Germany selling a "Graetz 21 x 32cm" loudspeaker on Ebay. $89 plus postage. The price also includes an "Isophon P1826".
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Old 6th Feb 2019, 10:22 pm   #5
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Default Re: Graetz Sinfonia 4R O/C bass speaker

Rewinding a speech coil is not that difficult they are often only a couple of layers of turns.
Rather than using paper shims to help align the speech coil I recommend using the hard clear plastic. The sort which comes with bubble pack items, and is murder to remove. Its nice and stiff and is easy to push in the gap and slide around to space evenly. You can often find a piece which is curved to fit in the gap around the magnet centre pole.
I dont know the construction of this speaker, but if a permanent magnet housing is dismantled so the poles become loose you will have a devil of a job separating the centre pole from the outer ring.
Hopefully that wont need to happen if you are just removing the diaphragm.



Good luck
Mike
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Old 6th Feb 2019, 10:52 pm   #6
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Default Re: Graetz Sinfonia 4R O/C bass speaker

Thanks for the replies. I have been busy checking the AF output transformer and thankfully it appears to be OK, as does the mains transformer. The German replacement on eBay is beyond my budget unfortunately. I have got a modern 8 inch round 3 Ohm unit somewhere but it needs the cone surround refoaming, if I can get a repair kit. If I don't get any affordable replies to the repair request I will have a shot at the elliptical unit myself in a few days time and let you know how I get on. Cheers for now Jerry
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Old 6th Feb 2019, 11:20 pm   #7
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Default Re: Graetz Sinfonia 4R O/C bass speaker

could you fit an EMI 13x8 unit?
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Old 7th Feb 2019, 10:08 am   #8
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Default Re: Graetz Sinfonia 4R O/C bass speaker

If you decide to "bin" it. I have found it is worthwhile extracting the magnet. These are usually very powerful and are handy for magnetising screwdrivers etc. They also make a useful magnetic drill bit holder for the den or garage.
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Old 7th Feb 2019, 12:14 pm   #9
Edward Huggins
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Default Re: Graetz Sinfonia 4R O/C bass speaker

As you have called this a "Bass" speaker, this suggests that there is an LC network that allows just the low frequencies to pass. Any different type of replacement may need to take account of this e.g. most of the EMI 13" x 8" units come with a concentric tweeter.
In terms of other options, a full range 10" x 6" unit is likely to be more readily found.
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Old 7th Feb 2019, 1:50 pm   #10
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Default Re: Graetz Sinfonia 4R O/C bass speaker

If you decide to remove the magnet then make sure you have screwed the loudspeaker onto a non-movable surface first. This will reduce the chances of the magnet, which will be attracted to the surrounding iron frame, pulling towards it and in the process moving the loudspeaker frame (if it is not screwed down); and possibly crushing the voice coil in the process. You will need steady and strong hands!
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Old 7th Feb 2019, 5:26 pm   #11
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Default Re: Graetz Sinfonia 4R O/C bass speaker

Hi folks, thanks again for the ideas. Yes, a 13x8 EMI unit would fit physically but maybe those are 8 Ohm impedance, whereas I am guessing the original is supposed to be 3 Ohm since the other round speaker is 3.5 Ohm? The cabinet is vast, one of the biggest table top sets I have ever seen. I will put up a wants thread for a replacement similar sized elliptical 3 Ohm unit. I used the term "bass speaker" liberally. There are no inductors in the circuit (apart from the output transformer). The transformer appears to be original and is marked Graetz-Radio Bv 8566 but I couldn't find anything on it from Mr Google. The output transformer does not seem to match the one shown in the circuit that I downloaded from the Radio Museum which shows 2 speakers each powered from separate secondaries. On mine the large elliptical is connected directly to the single secondary winding which is also connected to the smaller circular unit via a 5uF and 10R resistor in parallel with each other so a crude crossover I guess. There does appear to be a negative feedback secondary winding as per circuit diagram. The tweeter is wired as per circuit with a resistor-capacitor network between the EL84 anode and via a switch to the secondary HT (downstream of a 500R dropper) and the second 50uF HT capacitor. It looks like the tweeter gets HT volts only when switched to FM. Cheers, Jerry
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Old 7th Feb 2019, 10:32 pm   #12
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Default Re: Graetz Sinfonia 4R O/C bass speaker

Have you tried "re-flowing" the two solder joints on the cone?

If it is a 3 ohm unit you wouldn't expect the coil to go o/c unless its been abused, and there's no sign of that?

John
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Old 7th Feb 2019, 11:33 pm   #13
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Default Re: Graetz Sinfonia 4R O/C bass speaker

Quote:
Originally Posted by John_BS View Post
Have you tried "re-flowing" the two solder joints on the cone?

If it is a 3 ohm unit you wouldn't expect the coil to go o/c unless its been abused, and there's no sign of that?

John
The circuit diagram on radiomuseum shows the HF units are electrostatic and taken from the anode of the output valve and the mid and bass have separate secondary windings.

I believe a few German radios use high impedance speakers and they often go open circuit so more information is needed before deciding on a solution.
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Old 7th Feb 2019, 11:56 pm   #14
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Default Re: Graetz Sinfonia 4R O/C bass speaker

Good evening all, I decided to bite the bullet and attempt a repair. First of all I scraped the adhesive off the fine wires going down the inside back of the cone from the solder blobs towards the speech coil. There was no continuity between them but there was continuity from the frame solder tags to those same bared wires at the centre back of the cone. Therefore I knew the break was either in the coil itself or the wires behind the cone suspension diaphragm. I released the 3 screws securing the cone suspension diaphragm but could not hold it forward far enough to access anything. Therefore reluctantly I had to remove the entire cone assembly. First I unsoldered the braided flexi wires from the frame solder tags and released the rubber grommets where these wires entered 2 holes in the frame. With a sharp knife I cut under and removed the anti-vibration gasket around the edge of the frame (it was like thin rope and can hopefully be re-used). I then slid a narrow blade underneath the edges of the cone to cut through the glue holding it to the frame. It was fiddly and inevitably I have damaged the edges of the cone. Once released I removed the cone complete with coil. See pics. I scraped the enamel off the wires going to the edge of the speech coil. There was continuity and so the speech coil was good. The break was in one of the thin wires going from the coil to the other side of the suspension diaphragm. I punched a tiny hole through the diaphragm with a needle and inserted and soldered a replacement wire and coated it in clear enamel varnish. On the edge of the centre pole piece is a burn mark at the 9 o'clock position where it looks like the coil shorted to frame. I shall have to check the output transformer again very carefully to figure out what might have cause this. If I manage to reassemble the speaker I definitely don't want to do this repair a second time...……….
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Old 10th Feb 2019, 3:30 pm   #15
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Default Re: Graetz Sinfonia 4R O/C bass speaker

Hello again. I've completed the speaker repair. I used Evostik to glue the remains of the lip of the ellipse back onto the frame and cut an elliptical strip of blotting paper which was just the right stiffness for the job. I glued it on top of the edge of the cone with enough overlap to add strength where the original cone edge had been destroyed by my previous hacking. I cut out the centre dust shield with a sharp knife and inserted plastic shims as suggested earlier (much better than paper, thanks for the tip). Cone alignment was then locked with the 3 diaphragm fixing screws at the rear. Speech coil dc resistance now measures 4R as it should. I've slaved it onto a Mullard MAS 281/15 as extension speaker and it works well. Now back to replacing a multitude of dodgy paper capacitors on the Graetz. I did test the valves on the VCM163. The valves, all manufactured by Valvo, appear to be the originals. All are useable except perhaps the ECH81 which was well down on emission and the ECC85 half of which was OK and the other wasn't; replacement ECC85 ordered and a less-bad ECH81 fitted. Cheers, Jerry
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Old 10th Feb 2019, 3:35 pm   #16
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Default Re: Graetz Sinfonia 4R O/C bass speaker

Hats off to you, Jerry, a fiddly job done well, and much nicer than scrapping the driver and fitting a non-original one.
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Old 11th Feb 2019, 6:26 pm   #17
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Default Re: Graetz Sinfonia 4R O/C bass speaker

I'm really impressed with that fiddly job - well done! It was a good job the 'spider' was un-bolt-able - I don't think I've ever seen one like that, usually they are also glued on like the outer surround, and perhaps even more difficult to extract without destroying them.
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Old 11th Feb 2019, 7:12 pm   #18
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Default Re: Graetz Sinfonia 4R O/C bass speaker

Nice Job, and good write up, well done.

Mike
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Old 12th Feb 2019, 10:16 am   #19
cathoderay57
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Default Re: Graetz Sinfonia 4R O/C bass speaker

Thanks for the compliments folks - much appreciated. I'm still not sure what caused the coil wire to rupture. The small burn mark indicates it must have suffered a fairly high current. I've gone over the circuit several times and as far as I can see there is no path for HT to get through the negative feedback capacitor network which only goes back to the volume control in any case. There's no possibility of HT leakage onto the vol cont itself because it comes before the EABC80 triode not after. The only thing I can think of is someone tried to use it as an extension speaker by plugging wires into the extension speaker sockets and overloaded it somehow. I've checked the output transformer and there is no measurable leakage from primary to secondary. I think when I first power up I will use a dummy load just in case. Cheers, Jerry
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