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Old 11th Dec 2020, 4:16 pm   #1
_Cosmic_
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Default Grundig Electrostatic Tweeter Repair Tutorial

The Grundig Electrostatic Tweeter.

I am writing this to hopefully guide someone, just like me, who would like to try and repair their electrostatic tweeter.
This unit was commonly utilised by many manufacturers of radio, radiograms and record players during the 1960's. They sounded fantastic. Quad even made electrostatic speakers at one time, but these were huge free-standing units that I resisted the temptation to buy, due to the astronomical cost of them and their size, when I bought my Quad 33/303. Size is often a problem, as seen today with the amount of radiograms that are discarded and that remain unsold on the well known auction site.

Often new acquisitions such as radios and record players sound pretty good when first heard but sooner or later an enthusiast, for various reasons, will come to investigate the sound and discover that the small speaker, situated separately but close-by the main speaker isn't working.

Although I'm not an electronics or electrical engineer, I have owned a great many record players over the years since the 1960's so have a great deal of experience using them and keeping them maintained with new styli and cartridges. Just lately, I have been renovating a couple of Bush SRP 31D's. This was because I was forced into it after buying the first one on the well-known auction site, and it arriving damaged and parts of it not working, then another for the parts to fix the first one. It's usually because I have been conned so to speak by at least 2 sellers to date who said that the players were "working". They certainly were not damaged in the post.

One part in common that didn't work at all in both players was the Grundig electrostatic tweeter.

Immediately I did a bit of research on them and discovered you can no longer buy them, either new or second-hand. I discovered a video on the well-known tube site but it wasn't very clear exactly how to go about repairing them. I also discovered a thread of an owner making enquiries about how to go about repairing one and actually breaking it beyond repair in the process which was very sad to hear.

So after being prompted by one of our vintage-radio fantastic members I am going to attempt to go through the process of reviving your tweeter and not doing as some have suggested, discarding the Grundig and replacing it with a modern tweeter unit.

You will know when you have got it right because when they are working correctly they make a real difference to the frequency range and the spatial projection, in that the sound is spread a little more to the LHS of the speaker grill.

So here we go..just bear with me as it might take a little time to post with the photos and thinking about how I did it, and I might have to edit it a few times..
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Old 11th Dec 2020, 4:42 pm   #2
_Cosmic_
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Default Re: Grundig Electrostatic Tweeter Repair Tutorial

The first thing to do, of course is to unplug it. I always like to place the plug next to where I'm working so I definitely know beyond a shadow of a doubt that its disconnected from the mains.
To access the speakers you dont go in through the earthed metal grill access panel on the inside of the record player but rather you have to remove the front panel that has the bush logo on.

Secure the deck using the transit screws and make sure the tonearm is secured and the stylus definitely can't be damaged.

Turn the player onto its side to access the three large screws that secure the black plastic strip that covers the tone controls. Carefully remove the strip and don't lose the screws as the two at the ends have a special spacer on them and might well be difficult to replace.

After removing the black strip the speaker grill will drop down.
You can put the player right way up now. The front grill might just drop down or you might have to wiggle it to free it and pull it downwards. Its held on by simple spring-clips and it will push back on the same way.
When it drops down you will now be able to see the location of the Celestion speaker and the troublesome Grundig tweeter that is situated to the left.

Remove the four retaining wood screws and it will easily detach. Take a photo so you know which coloured wire goes to each speaker terminal. One is orange and the other, purple.

Be aware there might be residual current in the circuitry, the electrostatic speakers run on high voltage I believe so leave time for the capacitors to discharge.

Cut the two wires to detach the unit leaving a small amount of insulation so you know which colour will get re-soldered to the terminal as a further precaution. You can now work on the tweeter.
nb. The sturdy grill retaining clips are at the bottom of the image.
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Old 11th Dec 2020, 5:09 pm   #3
_Cosmic_
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Default Re: Grundig Electrostatic Tweeter Repair Tutorial

You must go slowly and delicately at all stages of the repair as you will not be able to get these parts again.

The next task (the most difficult) is to gain access to the inside of the brown plastic casing whilst inflicting the least amount of damage possible. The aim is to split it into two halves. They are held together by very small round ball-like clips and also on one side by two round plastic posts that are held in place by heat welding the surrounding plastic of the casing to the posts.

There are two of them on one side, where the ring terminals are, and there are plastic clips on the opposite side. On the first one I repaired I used heat from a soldering iron to try and release the post but ended up plunging the iron bit right through the casing and unintentionally making an unnecessarily large hole in it. I wasn't expecting the iron to go through so easily, it was like a hot knife through butter. So be very careful if you try to un-weld the casing this way. You can see the hole in one of the images. Luckily it didn't damage anything. My second repair was much better!
You need to try and prize the two halves apart. If you are lucky they might just prize apart from the clips simply using a small flat head screwdriver. Go slowly and cautiously and you should be able to take it apart. It will probably ping but be very careful because the two metal ring connections are anchored by the posts. Try very hard not to damage them.

When you have the unit in halves don't try to remove anything straight away.
inspect the terminals, one is soldered to the metal grill and the other goes to the copper film strip. Take photos so you can reassemble it again.
If you have got this far this is a great success and you have done very well...
Here's some photos so you know what to expect.

My next post will go through what you will encounter when you are inside, and why the units always eventually fail given time.
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Old 11th Dec 2020, 5:47 pm   #4
_Cosmic_
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Default Re: Grundig Electrostatic Tweeter Repair Tutorial

Go slowly and cautiously all the time.

We should now be able to inspect the inside of the unit.
With the grill facing downwards towards the bench, take off the brown plastic half of the case. Hopefully you will be faced with a layer of fibre wadding or if you are unlucky, a mass of decomposing foam that will turn into powder as soon as you touch it or disturb it. Both mine were wadding.

carefully feel off the white wadding with a pair of tweezers and turn it over and place it out of the way. Yu are now confronted with a shiny surface. This is the mylon sheet. It is rather special in that the upper surface is conductive. Attached to the conductive (upper) surface is the very thin copper-coloured conductive film which brings the signal from the output transformer to the mylon sheet surface.

Do not try to remove the sheet from the metal grill in case you damage it. It will be probably be fastened down anyhow.

At this stage, if it hasn't already moved, with your tweezers, carefully remove the coper-coloured film strip and inspect it. It will be tarnished. I used a small amount of Brasso on a cotton bud to gently (stroking gently one way) clean the strip to ensure it will make a nice clean contact when you reassemble it. Rub one way in case you damage the film on the back stroke and it rucks up. Perhaps try a little isopropyl alcohol if you have any in place of the Brasso. I use it for everything!

We have now hopefully eliminated one possible cause of the tweeter to fail.

Take photos of everything as you go along. If case you don't, you can always come back to look at mine.
You have done well so far. We are now going to inspect the wadding so you know what to expect and discuss what causes these units fail.

I posted some images so you can refer to them again.
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Old 11th Dec 2020, 6:12 pm   #5
_Cosmic_
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Default Re: Grundig Electrostatic Tweeter Repair Tutorial

Remember to keep taking photos, and above all, be gentle with it and you will be successful..

Looking at the white wadding. You can see where the copper-coloured film has stained the wadding and also permanent indentations in the wadding. The idea of the wadding is to gently but firmly press the copper-coloured film strip so it makes a secure contact with the mylar sheet and also to ensure the mylar sheet makes an evenly distributed pressure so the mylar sheet contacts the metal grill evenly.

Simply relocate the wadding by turning it so you are using a fresh section to hold down the film strip.

Now inspect the inside of the brown casing. You will see the remnants of where there was once a small strip of foam. If its still there it will immediately disintegrate as soon as you touch it. Remove it and suck out the dust with a vacuum or tip it out. Locate a similar sized piece of thin foam. This might be hard to find but I used a piece that was in a stereo cartridge box and cut it down with a Stanley knife. It's going to be trial end error here but when you get it right, stick it down to the brown casing in exactly the same place as the previous decomposed one.

You can see what it does; when reassembled the foam will press onto the copper film strip to secure it to the mylar.

We have just, hopefully again, rectified another major cause of the tweeter to fail.

Just about done. Here are some photos. One shows the position of the decomposed foam in the brown casing, the other shows a section of new foam in position.

The next post is reassembly.. not quite as simple as you might imagine.
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Old 11th Dec 2020, 6:46 pm   #6
_Cosmic_
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Default Re: Grundig Electrostatic Tweeter Repair Tutorial

This is the final stage; reassembly.

It's a little fiddly getting everything back together again but if you refer to your photos you can do it. Just make sure that your new foam installation presses onto the copper strip.

The strip lays down in position and the hole in the end makes contact with the lead terminal contact-ring along the top edge of the casing. I had to add a small metal washer to ensure it presses down hard enough. The strip often slips out of position when you are trying to close to two halves of the casing.

Closing up depends on how easy it was to prize open the casing in the first place. It might well now be loose, but rather than gluing it or re-melting the plastic (you might need to take it apart again) just screw the tweeter back in firmly to the board and it should be OK.

It's quite possible that after all this it still doesn't work! If it does you are lucky but the chances are that it wont.

DO NOT GIVE UP AT THIS STAGE.

Even after all the care and trouble I took, I had to take mine apart and reassemble a couple of times before the components seated in and began to perform perfectly. But it was well worth it. I now have two perfectly performing tweeters, and they really do make such a difference. I marvel at the clarity I am receiving when I play my old 45rpm singles. I have 4000 of them, and they badly needed an airing.

If you are following this thread and it helped you bring your radio or record player back to it's sparkling acoustical glory then I'm happy.

Have fun.. Cosmic Chris
Than treat yourself, you deserve it.

nb. It might well be that you don't have the white wadding but instead encounter decomposed foam inside the casing. Just replace it with fibre wadding or fresh foam. It might take a bit of trial and error and experimentation to source the material.
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Old 12th Dec 2020, 12:53 pm   #7
MurphyNut
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Default Re: Grundig Electrostatic Tweeter Repair Tutorial

Thanks for this Chris, since repairing my SRD 31D I've been using it almost daily but with the tweeter not working. I've been reading this with great interest and it's now given me the confidence to have a go at fixing it.
What fantastic record players they are and perhaps even more so with the tweeter working!
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Old 13th Dec 2020, 12:08 am   #8
_Cosmic_
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Default Re: Grundig Electrostatic Tweeter Repair Tutorial

You're welcome mate!! Have a crack at it, a great deal is knowing what to expect along the way...
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Old 19th Dec 2020, 10:06 pm   #9
_Cosmic_
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Default Re: Grundig Electrostatic Tweeter Repair Tutorial

Clive, let me know if you decided to have a go repairing your tweeter. If it all goes wrong or you haven't the time, feel free to send it up to me and I will try to repair it for you. They are valueless if they arn't working so you can just post it, well packaged of course. I can't absolutely guarantee it will work (I've only done three) but mine are all still sounding nice. If you get in touch after Christmas there's less chance of it going AWOL in the utterly broken Royal Mail!
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Old 27th Apr 2021, 7:29 am   #10
Spencervs
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Default Re: Grundig Electrostatic Tweeter Repair Tutorial

I have had my Bush player for years and someone attempted this for me but I still think it's dead... I might have a go myself as he already opened it and replaced the rivets with easy access nuts and bolts! Thank you for taking the time to write this amazing account!
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Old 27th Apr 2021, 7:46 am   #11
vinrads
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Default Re: Grundig Electrostatic Tweeter Repair Tutorial

Thanks Chris. I haven't had a faulty one yet, but if I come across one, I now know how to proceed

Mick.
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