EMUD 920 Restoration / Rejuvination

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EMUD 920 Restoration / Rejuvination

Post by Walston on Fri Mar 15, 2013 7:53 am

I have acquired a EMUD 920. Looks to be pretty similar to some Grundig models. I have not worked on radios for years and I found this one in prestine condition and my father had one of these when I was young, so it has some fond memories attached.

After some inspection it was clear that this unit was completely unmolested and actually pretty clean. It is a ~1960 or 1961. I removed the dust and decided to power it up. FM works but only the left speaker channel, volume pot has some dead spots. AM and SW did no work.

I removed the unit from the cabinet and looked it over good, nothing obvious, no paper capacitors that I can see. It has many many switches, ON Off, SW, AM, FM, Phono, Tape, Ferrit, Stereo, Concert, Speech, Song. I applied some de toxit to the switches the best I could, not easy to get to.

Currently, FM, AM and SW are all working. After cleaning the right speaker channel worked for about 30 minutes and stopped again. While it was working if I pushed the "stereo" switch, the right would go out, turn off the stereo switch and it turned back on.

Any advice on how to clean the switches better would be appreciated. Also do you thing the right channel issue is switch related or should I be looking at something else.

Thanks for the help.

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Re: EMUD 920 Restoration / Rejuvination

Post by Walston on Sun Mar 17, 2013 11:37 am

Hmmm no replies at all, maybe my radio is too new for this forum. Anyway, here is an update, maybe someone has an idea.

Everything is now working except the right channel when I push the "stereo" switch. I am trying this with a strong FM station tuned in. The volume pot has two sections and according to the schematic the one is main and broadcasts on both right and left in Mono. The second is for the right side when in stereo.

I tested the two sections of the volume pot. The front side (this is the "mono" or "main" side) goes from 20 Kohm to 1100 Kohm. The first half goes up the 300, then the second half goes from 300 to 1100 pretty fast. You can see the "dead spots" with the meter. I have used a little de toxit and it is improving. The stereo side goes from 20 Kohm to 266 Kohm, it seems pretty clean. Schematic says each one should be 300 Kohm. There is a way to turn one without the other by uncoupling the knobs, after discovering this I do remember from being a kid, that this is how you balance the stereo, if using "stereo". Makes sense.

With the knobs uncoupled and the radio on, turning the stereo pot does nothing. Even if the "stereo" button is pushed. The stereo circuit is just not working. I have sprayed the switch as best I can several times and worked the snot out of, but no sigh of that having any impact. Pushing the switch in and out with the radio on seems to do nothing except turn the right side off. No scratching or any noise at all on the right side, even if I move the switch in and out quickly.

I am working on the phono now and it will be interesting to see if the phone has any sound on the right side, since it should always be in stereo.

Looking at the components that are near the "stereo knife switch". The only one that looks suspect is R64, it is 1000 ohm 2M. It is actually in the power circuit on the schematic. It tests fine with my volt meter, but it looks so so and it gets plenty hot and smokes a little, but I think the smoke is a little de toxit on the outside.

If anyone has any ideas, please let me know.

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Re: EMUD 920 Restoration / Rejuvination

Post by 75X11 on Sun Mar 17, 2013 5:43 pm

With the selection of positions on your input switch for auxiliary equipment, i would imagine there would be some input jacks to inject audio on either channel, provided they are stereo inputs ( I have no schematic for your console so I can only respond in general.) It might be of value to feed some audio in that manner to confirm it is one channel out.
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Re: EMUD 920 Restoration / Rejuvination

Post by GaryRabbitt on Sun Mar 17, 2013 8:55 pm

Hi, I think the reason for few responses is that many of us aren't familiar with the Euro sets.
I am wondering if you need an extension speaker if the selector is on "Stereo'". I have a Telefunken that supposedly has "stereo". Well, it is not included as you must have an add on unit, plus another speaker. That might be the issue you are having,
You have obviously found that these suffer from dirty switch connections. Bandswitch and otherwise. You should be able to get right into the switch assembly to put a very small amount of Deoxit on them. Then work them up and down a lot.
I know Bill doesn't like to have too many links to other sites, but this time I think I will, to help you out. Post at the UK vintage radio site and post your question. They are very familiar with all those sets from "over there".
Here's the link.
http://www.vintage-radio.net/forum/index.php

Maybe your set is working as it should, I can't say for sure. All I know is that some of those sets do need an adapter for full multiplex stereo.
Good luck!

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Re: EMUD 920 Restoration / Rejuvination

Post by Walston on Mon Mar 18, 2013 7:38 am

Thanks guys. Gary I am leaning towards you way of thinking. I am starting to think that stereo was an option that I don't have. Everything else is working well. I cleaned and lubed the turntable and it now plays good but the same situation with the stereo switch. Push it and the right speaker shuts off. I'll check with the boy across the pond, thanks for the link.

On the input idea, there is a tape plug. I can try to feed a stereo signal into the there and see how we do, but I am not to optimistic since that is basically what the phono feeds in.

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Re: EMUD 920 Restoration / Rejuvination

Post by chrisc on Tue Mar 19, 2013 1:58 pm

Just a thought but I have heard that the term 'stereo' did not always mean stereo as we'd think of it today - it was more a question of surrounding yourself with sound, even if that sound was mono. This is something that seems to have been a major preoccupation with German manufacturers, using multiple speakers at different angles for example, to try and get that effect.

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Re: EMUD 920 Restoration / Rejuvination

Post by Bill Cahill on Tue Mar 19, 2013 2:45 pm

Something else. The German paper caps of that time were cleanr plastic with the ability to see the foill.
The wire ends, I think, were embedded in wax.
They don't seem to get leaky alot, but, I have seen several suddenly short.
Bill Cahill
Also, alot of the German resistors tend to go open.

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Re: EMUD 920 Restoration / Rejuvination

Post by Walston on Fri Mar 22, 2013 7:31 am

Thanks for the help guys. Still searching. I have the schematic if that helps, but I cant figure out how to post it.

Bill,

I have been checking the resistors by putting the lead across the resistor and checking the value on the schematic. I assume if it is open as you say, my meeter would show an open circuit. Correct?

Is there a way I can tel if a cap is shorted using my meter?

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Re: EMUD 920 Restoration / Rejuvination

Post by Bill Cahill on Fri Mar 22, 2013 8:00 am

On the resistor, if your meter shows no movement, it's open.
On caps, not usually. If shorted enough, yes. But, usually not.
They are generally bad.
These German radios usually use a selenium rectifier, and, that is often bad. Electrolytics often are shorted, and, or, wide open.
Bill Cahill

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Re: EMUD 920 Restoration / Rejuvination

Post by chrisc on Fri Mar 22, 2013 7:11 pm

I think it is worth at least giving the selenium rectifier a chance to prove itself - it depends on the sort of life it has lead - but the electrolytics and I'd suggest also the other 'paper' caps should be replaced while you have everything in pieces.

If you do need to change out the selenium, you'll likely need to make a bridge with 4 x 1N4007 diodes and a dropping resistor. You can usually 'stuff' them into the original can - which is invisible and provides a good way to mount them.

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Re: EMUD 920 Restoration / Rejuvination

Post by Bill Cahill on Sat Mar 23, 2013 10:38 am

Actually, it doesn't depend on the life it has led. In 1951 Radio tv electronics was praising selenium rectifiers. In 1957 the same magazine had an article stating that even un used, it had been found that seleniums had a definate shelf life.
Manny years ago I had brand new seleiniums by one of the top rated compannies. I needed them in a Motorola tv. After rebuilding, I plugged it in, and, pops, smoke, and, sparks flew out of new rectifiers.
It was found they had shorted.

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Re: EMUD 920 Restoration / Rejuvination

Post by chrisc on Sat Mar 23, 2013 12:21 pm

That's interesting and I certainly defer to your experience - I started out by routinely swapping them out but I have gradually reached the point where I at least try them first. Don't want to hijack this post but I had thought that the typical failure mode was to progressively increase in resistance rather than to short out so I learned something new there.

Best Regards

Chris

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Re: EMUD 920 Restoration / Rejuvination

Post by Walston on Sun Mar 24, 2013 12:28 am

Back to the original subject. Well I think I have this mystery solved. After some more detailed studying of the schematic, looks like only the phono and tape go to the stereo button hookup. I had to order some parts for the phono so I couldn't test it. The tape is just an input plug, so I rigged up a stereo input (as 75X11 had suggested earlier) and sure enough it played perfect through both speakers with the stereo button pushed! Very Happy Very Happy Very Happy

The FM is not routed to the stereo side at all. Kind of a bummer, but it plays well without the stereo.

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Re: EMUD 920 Restoration / Rejuvination

Post by 75X11 on Sun Mar 24, 2013 1:38 am

Your unit must've came along when stereo audio equipment was available, but before FM multiplex got going.
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Re: EMUD 920 Restoration / Rejuvination

Post by GaryRabbitt on Mon Mar 25, 2013 1:57 am

Walston, I would see if the selenium does function before just replacing it. I have repaired many sets for years, and have only come across one bad selenium. One that developed high resistance and was not performing as it should. I've never seen the blow out ones, although I know that can happen.
As far as checking resistors, you can get a close idea of the part, placing the leads on each side of the part. But if for example a resistor was 'open' you may still get a reading. That's because there may be other parts connected to the same terminals giving you a reading. A good example is when a resistor, and a higher valued capacitor has one lead on one end of the resistor. If the resistor is open, you are really reading thru the capacitor lead, and wherever the other lead goes thru the circuit and back to the other terminal of the resistor. If the resistor is good, the reading still might be thrown off due to the cap.

In those cases, you might have to raise and isolate one lead of the part for a true reading.


Last edited by GaryRabbitt on Sat Mar 30, 2013 3:20 am; edited 1 time in total
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Re: EMUD 920 Restoration / Rejuvination

Post by Walston on Mon Mar 25, 2013 7:41 am

Makes sense, Thanks Gary.

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Re: EMUD 920 Restoration / Rejuvination

Post by Walston on Sat Mar 30, 2013 8:51 am

Well I got the parts for the phono and it is now working like new. Plays perfect stereo, sounds unbelievably good. I also got the 78 RPM needle as I have about 100 or so 78's. Plays perfect. I have an old Victrola, but I don't fool with it much, so it is neat to be able to play 78's on this.

3 weeks ago, nothing worked, now everything works. Very satisfying.

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