Blaupunkt 9W78 Restoration Project

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Blaupunkt 9W78 Restoration Project

Post by PButros on Thu Nov 22, 2012 5:46 am

Hi all,

first post here.

My winter project for this year is the restoration of a long time family friend, a Blaupunkt 9W78 for the late 1930's

[img][/img]

I hade it fired up a couple of years ago after being stored in the attic at my elderly home for a few decades.

Just some hum from the speakers, nothing else....but to let it back into the house it has to work I don't want to have it as a static 65lbs mantelpiece, nor does the wife Smile

So after reading various articles on the net to following seems a logical order to start:

1. Remove all tubes an have them tested.. Could prove to be a tad difficult to locate someone that has a tube tester, and how reliable are these testers after so many years??

2. Remove & clean the chassis, tuning caps etc.

3. Replace all electrolyte caps

4. Place back all tubes, tested or not....

5. Buy a variac for slow start purposes..

6. Hope it works.....

Now IMHO, the biggest issue I might have is when it proves the 2 AD1 tubes in the amp stage are gone. I did find a carton box with tubes that seem to belong to this radio, including two AD1's but I have no idea about their condition.

I noticed that only EML makes 'new'' ones but at 750$ a pair that would be a bit too much to me....

Now reading thru various places on the web I have learnt that an EL34 or KT88 can also be wired as a triode but, from what I've seen so far, it looks like the filament is not used in that configuration. Is that correct, a bit confused here....

So I hope you understand I need a 'bit' of help.

Can anyone put me on the right track as to if, or how, to make the modification from AD1 to EL34 / KT88 into Triode operation a per the schematic below:

[img][/img]

Any comments / suggestion always welcomed.

Kind regards,

Petrus

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Re: Blaupunkt 9W78 Restoration Project

Post by Guest on Thu Nov 22, 2012 10:27 am

First of all, It would be a good idea to find a old radio group nearby that might have a member with a tester that could check your valves and spares. The testers are an important piece of equipment to a restorer and they will have a functioning one to use. Another resource to look into is Ebay. I did a quick look up and found a sale from a person in Germany for an AD1 that was used and tested good for a lot less than the new price. Finding such items would spare you modifying your radio. Be sure to share some pictures of it with us.

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Re: Blaupunkt 9W78 Restoration Project

Post by Guest on Thu Nov 22, 2012 10:33 am

http://www.radiomuseum.org/r/blaupunkt_9w78.html

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Re: Blaupunkt 9W78 Restoration Project

Post by Bill Cahill on Thu Nov 22, 2012 10:52 am

Don't plug the radio in any more. It needs re building, first.
I'd reccommend that you find an antique radio group in your area that can help you. See above. Good advice.
If you did go that route, first, the tubes you mentionned won't make good triodes. Second, it's a major re wiring, and, you do have to use the filament, or the tubes can't work.
I'd take the radio, and, the tubes to the experts, and, ask for help.
We want you to succeed in this, and, you aren't ready for a major project like this..
Bill Cahill

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Re: Blaupunkt 9W78 Restoration Project

Post by PButros on Thu Nov 22, 2012 11:04 am

Hey Wise Man,

thanks for you advise.

I'm trying lo locate someone with a valve tester 'as we speak' adn may have found one.

I've been tracking some AD1 auctions and most of them go for serious prices when they are any good. Just spotted a pair of NOS ones currently at over 900 Euro's and still some time left.....

Of course I would try to keep it original if I can. I can't remember if it went to the attic as a playing unit or was already defective at that time.

Whatever way I choose to go, I will always keep the original parts so any mods can be undone.

In the meantime, after staring some more at the schematic, I noticed that, if I'm reading correctly, the filament voltage can be likely regulated with a variable resitor. So maybe I can get it up teh 6.3 volts if the EL36, in triode mode, is a low cost replacement.

I think they can handle the 335V anode voltage but do not know what current they would draw. For the AD1 it specs 50mA @ 335V.

If the two above woudl be OK, it will probably mean that I have to figure out what to get to the grid.

Still trying to improve on my schematic reading and almost non existing interpretation skills.


Kind regards,

Piet



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Re: Blaupunkt 9W78 Restoration Project

Post by PButros on Thu Nov 22, 2012 11:09 am

Bill,

I really appreciate your and Wise Man's advice.

Yeah that was pretty dumb to have misread a EL34 tube in Triode mode and not needing a Vf..

Already tried a couple of 'local radio buffs' in a distant past without any solid advice, I may give it a try again.....

Kind regards,

Piet

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Re: Blaupunkt 9W78 Restoration Project

Post by Guest on Thu Nov 22, 2012 12:17 pm

Piet, I believe Wise Man Is my posting level on the site. My user name is 19&41, it is a highway that runs south out of Atlanta. Part of it is known as Tara Boulevard. ( A reference to Gone With The Wind)

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Re: Blaupunkt 9W78 Restoration Project

Post by PButros on Fri Nov 23, 2012 5:07 am

19&41,

sorry for that.

Appologies from a 'not so wise man'.


Kind regards,

Piet


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Re: Blaupunkt 9W78 Restoration Project

Post by Guest on Fri Nov 23, 2012 10:05 am

No need for apologies at all. When I was younger, about 35 years ago, I was in the army, stationed in northwest Germany. I will never forget how decent and friendly folks I met from both Holland and Belgium were. It helped make things just a bit more pleasant for a stranger away from home.

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Re: Blaupunkt 9W78 Restoration Project

Post by jerryhawthorne on Fri Feb 22, 2013 6:24 pm

Piet, I wouldn't jump ahead and look for new expensive tubes without a little more looking at the radio. Expensive output tubes or any can often test weak on a tube tester and work just fine. Dead or shorted no. Get your caps changed out. Your radio sounds promising in that your getting a little noise out of the speaker. If your at that point, time for the simple "touch the center tap of the volume control with it turned up" and see if you get a nice strong hum. It will tell you those expensive output tubes are working. Voltage checks on the tubes should be done if it doesn't give that nice noise. I recently bought a couple of expensive NOS 45 output tubes because the originals tested very weak on my cheap tube tester. Mistake! The old weak tubes performed as well as the NOS that measured substantially better on the tube tester. Give it the old finger on the volume control. Easiest test out there to see if the audio aspect of the radio is working.
Regards, Jerry

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Re: Blaupunkt 9W78 Restoration Project

Post by jerryhawthorne on Fri Feb 22, 2013 6:53 pm

Duplicate sorry.

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