Graetz Sinfonia Tweeter
 

Hello all,
Some of you might have seen my earlier thread concerning the large elliptical "bass" speaker repair on a Graetz Sinfonia 4R AM/FM radio. Well I changed all the paper capacitors (yes every one) because they were all leaky on the Avo some measuring only a few kilohms. Powered it up and almost everything worked. It is a single-ended EL84 and certainly packs a punch. FM reception is excellent. Nothing needed tweaking apart from one new pilot lamp. However - the tweeter is not making any sound. Assuming I have not totally lost my high tone hearing I'd have thought I'd have picked up something with my ear right next to the tweeter. The "mid-range" is producing a reasonably high note which I can hear clearly. I know these capacitive tweeters (like the Grundigs) do tend to fail with age so I removed it from the baffle board. Externally it appears intact. I put it on my Avo Bridge and measured a capacitance of 45pF. Also when connecting it to the bridge there was some sound from it (a bit of scratching when connections were made and slight background buzz (maybe the smoothing on the Bridge PSU is getting past its best). However, installed in the radio it makes no sound. I have sketched the circuit (see attachment) because it differs from the output stage circuit shown on the Radio Museum schematic. The tweeter coupling capacitors 2.5nF and 50nF) have been replaced and the 5k and 10k resistors measure correctly. I don't have an AF signal generator so even if I put some HT on the tweeter I don't have a suitable signal source to inject into it. I'd have thought if the capacitive film inside had gone south it would measure o/c but it doesn't - as said 45pF. Anybody any ideas? I'm stumped for now. Cheers, Jerry

Re: Graetz Sinfonia Tweeter
 

Have you tried listening for the tweeter with the other speakers disconnected and a dummy load used in their place?

Mike

Re: Graetz Sinfonia Tweeter
 

I've recently restored a Blaupunkt Riviera H2154 with the same kind of tweeter.
It is in fact a piezo-electric speaker made on the basis of sintered metallic oxide powder that will degrade and wither when subj. to moisture.
Much the same problem as with vintage gramophone pick-up's based on Rochelle Salt.
In my Blaupunkt both tweeters were fine but I did some investigation on the matter as I couldn't believe they were of the piezo kind in such an old radio.
Well, I found that piezo-speakers were invented in the 30'ies as I remember.

Rgds,

/Torben

Re: Graetz Sinfonia Tweeter
 

Hi Mike and Torben, thanks for the replies. I tried powering up with dummy load for the LF speakers and there was nothing really audible from the tweeter. So I looked up some U-Tube clips showing work done on similar devices, then decided to open it up. It came apart quite easily by unbending 4 tabs on the copper front grille. Inside it was basically a capacitor: a perforated metal backplane and frontplane separated by a mylar sheet with spayed metal coating on one side. When HT is applied the mylar film is attracted to the positive metal plane and the HF AC signal is superimposed onto the DC which causes modulation of the film creating sound. According to the U-Tube clips the principal issue with this type of device is disintegration of a foam gasket that applies pressure to the mylar and holds it against the backplane, increasing capacitance and audio output. Untouched, mine measured only 45pF and the U-Tube info suggests 300nF is more reasonable. Sure enough, as you can see from the pics, there is the remains of a brown foam ring at the outer circumference that has disintegrated so there was very little pressure on the mylar. Next job is to find something suitable to replace the foam ring. Cheers for now, Jerry

Re: Graetz Sinfonia Tweeter
 

Hi Jerry,

No Piezo, then.
So much the better when repairing.
I've seen pictures of the inside of ruined piezo-tweeters and there seems to be nothing that can be done when the oxide absorbs moisture and disintegrates.

/Torben

Re: Graetz Sinfonia Tweeter
 

Here's a little more info about the Piezo-tweeters.
This has obviously nothing to do with your Graetz, and then again..
When looking at the schematic you've posted it's clear that your speaker is charged with 260VDC, consistent with an electrostatic speaker.
As you will see the Blaupunkt has a blocking capacitor in series with the Piezo-tweeters as they don't need the DC charge.
So this will perhaps be a safe way to spot what variant was used in these vintage sets.

/Torben

Re: Graetz Sinfonia Tweeter
 

Hi Torben, thanks for the additional info - that makes sense. I found some foam and cut out a ring gasket the same size as the tweeter aperture. Moving the pressure around as I reassembled it I got the capacitance only up to about 60pF. Functional test gave some audible sound from the tweeter, but a very puny output. I changed the 2200pF (was 2.5nF) coupling capacitor to 4700pF but it made little difference to the output. I'll have a look on eBay and see if I can find a modern tweeter and see how it sounds. I've attached a couple of seconds clip of the radio playing. Cheers, Jerry

Re: Graetz Sinfonia Tweeter
 

Don't expect too much from the tweeters.I remember the day in the 50's when my dad came home with a brand new Telefunken Opus 7(with 6 loudspeakers)to replace our old faithful Kungsradio only with AM.
I remember that the sound from the electrostatics wasn't very impressing even when new...on the other hand the normal tweeters were quite good.
A quick calculation on my calculator on the net shows 12,7 kHz as crossover frequency.

Re: Graetz Sinfonia Tweeter
 

It sounds quite reasonable imo. but seriously .... PINK ! ? 8-o ;D

The Blaupunkt, with renovating fully completed, probably sounded the best I've ever heard in a vintage toob-radio. The Piezo-tweeters really contributed to the rich and natural FM-sound.

/Torben

Re: Graetz Sinfonia Tweeter
 

Yes the colour is hideous. It's hard to tell if it was originally pink. I suspect it was painted at some point, but if so, the paint has been applied quite well including a strip in the middle of the metal banding......

Re: Graetz Sinfonia Tweeter
 

I am still trying to find a diagram to your radio in my pile of Graetz,but since we still haven't seen a picture of the loudspeaker arrengement it's not easy.
What number(three or four digits)does it say on the back?

To be a genuine 4R('vier Richtungen' I suppose)it has to have openings on all sides ie front,back and left and right side.
On Sinfonias this is clearly seen as a brasscovered opening running all the way around the top of the radio or as extra openings on both sides.

The filter to the 'mid' is also very uncommon as I happened to find it on only one radio,a Grazioso 230!
http://www.graetz-radio.de/sinfonia_serie.htm

Re: Graetz Sinfonia Tweeter
 

Hi Roffe, thanks for your post and for your offer to try to identify the circuit :thumbsup:. I have attached some photos including one of the speaker arrangement. All of the speakers are mounted on the front baffle board; there are no side vents. As you can see from the back cover there is no model number, only Graetz Sinfonia UKW-Spitzensuper. The only number I can find on the chassis is 804158 stamped into the metal. I assumed the set is a Sinfonia 4R because the valve line-up is the same as that shown at the Radio Museum website for the 4R (3354) although there are some differences between my radio and the circuit, particularly the output stage and speaker arrangement. Mine only has a singe-ended EL84 like the 3354 and all the other valves are the same, plus selenium bridge rectifier (now replaced with 1N4007 diodes). Cheers, Jerry

Re: Graetz Sinfonia Tweeter
 

Hello again, I had another look at the Radio Museum website and there is another variant of the Sinfonia with the same valves. :dunce: The circuit is the same as mine - the Sinfonia 3262 from 1954/55. There is a date of 1954 stamped on the large speaker cone. Cheers, Jerry

Re: Graetz Sinfonia Tweeter
 

Ah,now we're getting somewhere!
What you have is a Sinfonia 3262(also called 194W).
If you search for 194w you'll find that the diagram proposed by NVHR(which is a major site)turns out to be wrong!
The diagram for 3262 at RM.org is correct as far as I can see.
https://www.radiomuseum.org/r/graetz_sinfonia_3262.html

You beat me to it by minutes,Jerry!

Re: Graetz Sinfonia Tweeter
 

Thanks! When I get time I'll try to translate some of the German text in the additional sheets and see if there is any mention of a limited edition radio in pink...…. It's a long time since I studied German at school but there's always Google Translate. Not sure how good it is at Technische Deutsch but no doubt better than me.

Re: Graetz Sinfonia Tweeter
 

The capacitance will change when HT is applied across the speaker. Have you checked the film is still conductive and the connections are good.

Re: Graetz Sinfonia Tweeter
 

Hi PJL, yes - as far as poss. There is just a metallised Mylar strip that rests against the conductive Mylar film. The strip and film are held against one another by a fabric mesh. The pressure on the mesh (such as it is) is provided by the foam ring around the edge. There is continuity across the conducting side of the film. Cheers, Jerry

Re: Graetz Sinfonia Tweeter
 

Just remembered this article from Radio-Television Service Dealer, 1955
It gives a good explanation of the theory behind the workings of electro-static tweeters.

Rgds,

/Torben

Re: Graetz Sinfonia Tweeter
 

Thanks Torben, that's a good article giving a clear description. I have now bought a low-cost piezo tweeter and have fitted that with a bipolar 5.6uF cap in series and a 4.7R resistor in parallel with the tweeter, across the output transformer secondary. That seems to work better (you can actually hear the high frequencies now) but I have left the original electrostatic unit temporarily screwed to the inside of the cabinet so that in future somebody can re-fit it if they manage to get it working better than I can. Cheers, Jerry

Re: Graetz Sinfonia Tweeter
 

Mmmmm ..... my experience with Grundig electrostatic tweeters [quite ubiquitous and found in sets like the Bush VHF64/94] is that they do benefit from replacement of the mylar membranes .... and of course the 'pressure foam'. That said - all reports indicate that these are 'quiet' tweeters.:shrug: