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 Post subject: grundig majestic 9078
PostPosted: Oct Tue 08, 2019 7:52 pm 
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Joined: Aug Sat 16, 2008 10:42 pm
Posts: 194
Location: tucson az usa 85745
Hello: I put new filter and bridge in this thing and it was working OK for a few months but then it started making loud sputtering noises in speakers for short bursts, now sound is low weak and anemic. Output tubes are not redplating, no tubes loose in sockets, all the bizarre speaker wiring intact and solid. Wondering what failure modes there are for the output xfmr in this? I had a similar table telefunken with same PP EL84 pair that suddenly developed low output to spkrs with some low hum even though new filter n bridge were in place. I set the thing aside assuming it was the OPT. I have no schematic for this and will not pay the guy in DE to get his. This thing seems to have multiple extra plugs for a second powered speaker cabinet so it can play stereo from the turntable. It has several electrostatic tweeters and dynamic midranges plus a 6x9 and likely a 10 or 12" firing downward. Can a generic 15W EL84 type PP output xfmr be put in here? In some of these it appears the electrostats are polarized by the B+ drawn at the xfmr primary. Wondering if that would complicate the use of a generic device. Client is in love with the thing and cannot be talked out of throwing more and more money at it. Thank you.

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 Post subject: Re: grundig majestic 9078
PostPosted: Oct Tue 08, 2019 9:14 pm 
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Joined: Jan Mon 16, 2012 4:15 pm
Posts: 5743
Location: Near Brandon, Iowa
Can download schematics here (no charge): https://www.radiomuseum.org/r/grundig_m ... _9078.html

You are limited to three sheets per day, total of ten sheets in a 30-day period (choose carefully).

Some German sets have small bypass caps from the output tube plates to ground. When these short, they overheat the output transformer primary and burn it out.

Did you check the cathode resistor? This part frequently overheats and burns out, effectively killing the audio. Usually one resistor serves both EL84 tubes; also, there is an AC bypass electrolytic in parallel with the cathode resistor which can fail shorted and mess up the tube biasing.

This Grundig almost certainly has degenerative feedback, which (depending on how it is implemented) can limit your options for a replacement output transformer. Some models used dedicated secondary windings to support the feedback network. Others just sampled the speaker-drive secondary. The above schematic should tell you what to expect. Loss of degenerative feedback is a big deal in these radios: without it, they just won't sound right.

The electrostatic speakers probably ceased working long ago. They can be replaced with small dynamic speakers wired in parallel (via a nonpolarized coupling cap) with the main speaker. The B+ bias used by the electrostatic speakers is disconnected as it is no longer needed. If the electrostatics still work they should continue to function with a generic replacement output transformer, since they are powered off the primary circuit.


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 Post subject: Re: grundig majestic 9078
PostPosted: Oct Tue 08, 2019 9:29 pm 
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Joined: Jan Tue 31, 2012 1:55 am
Posts: 9846
Location: Mountains of Mourne. Ireland.
American service manual (16-pages) thanks to Bill J.
viewtopic.php?p=2300916#p2300916

Greg.


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 Post subject: Re: grundig majestic 9078
PostPosted: Oct Tue 08, 2019 10:34 pm 
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Joined: Aug Sat 16, 2008 10:42 pm
Posts: 194
Location: tucson az usa 85745
I did not know a person could get any free anything from radiomuseum so thank you for that link. I looked at the last page with power amp and learned a lot. And of course your tips are excellent. Also thank you to second person who provided separate link path for the whole folder free.
The electrostatic tweeters are fed from screens of the EL84 it seems. The negative feed back seems to be tapped off the secondary to all the speakers. But yes typical cathode bypass setup; I am almost certain I replaced that single capacitor on the first go round a couple months back. But all that other jazz they put in between OPT primary and the OP tubes I did not do anything with. There is a strange tap on the primary of OPT I do not understand the meaning of. It looks a bit like a williamson tap for ultralinear but it is not symmetrically present for the other tube. Is that a high pass filter for the electrostats?
Armed with all this I might go back in and look at the voltages to guide me. Again I say thank you to all for the assistance.


Attachments:
grundig schematic[4755].pdf [148.28 KiB]
Downloaded 6 times

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"In an insane society, a sane man would appear insane." --SPOCK to KIRK
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 Post subject: Re: grundig majestic 9078
PostPosted: Oct Wed 09, 2019 7:37 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 717
Location: Merrick,NY,USA
Did you recap the set or just replace the Electros? These Grundigs are also jam-packed with leaky paper caps, usually branded "ERO" as well as paper caps disguised as ceramic caps that are also leaky as hell, some even dead short. The polystyrene caps should be fine. I wouldn't be looking for exotic reasons for your symptoms without addressing this first.


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 Post subject: Re: grundig majestic 9078
PostPosted: Oct Wed 09, 2019 9:29 pm 
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Joined: Dec Wed 25, 2013 7:57 am
Posts: 3842
Location: USA
The radio worked, so there's no reason to question the design and existing wiring. Most likely is a component failed or a connection/solder joint became loose. I agree with replacing paper capacitors. Their reliability long term is about the same as electrolytic capacitors.

Here's a visual guide on identifying capacitors for replacement:

https://www.radiomuseum.org/forum/repla ... itors.html

I have no direct experience but those electrostatic speakers can fail. You can try temporarily disconnecting them to see if that's the source of poor audio.


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 Post subject: Re: grundig majestic 9078
PostPosted: Oct Thu 10, 2019 4:57 am 
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Joined: Aug Sat 16, 2008 10:42 pm
Posts: 194
Location: tucson az usa 85745
The main problem I am confronting with this radio is I am fixing it for a client, not for me. Were it merely my own, I would put some new couplers in it and explore the voltages in the output section. If it works, great but it probably will not yet be perfect. So I continue this process a couple more times and eventually succeed. If I charge the guy for my time spent troubleshooting and for some parts and it still does not work right, he sinks deeper and deeper in to the thing. If I spend his money putting in a bunch of capacitors and the performance does not improve, and believe me this has happened many times to me with complicated units like this, no one is going to be happy. When I worked at a repair shop, we would clip in key parts we suspected would fix the problem THEN call the customer and provide a high confidence estimate. When they approve, we finish it off and everyone is happy. Not having all the resources of a full blown retail shop, I have to do these repairs on a pay as we go basis which works OK if it is a simple situation. If there is ANY chance there is an exotic problem here, and we end up with a bunch of new capacitors in a unit with a bad OPT, it will be a bad situation. That is the only reason I asked if people working on these have had an OPT go bad after new filters and new cathode bypass capacitors.

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"In an insane society, a sane man would appear insane." --SPOCK to KIRK


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