Forums :: Resources :: Features :: Photo Gallery :: Vintage Radio Shows :: Archives :: Books
Support This Site: Contributors :: Advertise


It is currently Sep Mon 24, 2018 12:33 pm


All times are UTC [ DST ]





Post New Topic Post Reply  [ 8 posts ] 
Author Message
 Post subject: Grundig 760 U
PostPosted: Feb Thu 18, 2016 11:45 pm 
New Member

Joined: Feb Thu 18, 2016 11:32 pm
Posts: 3
Hi I have a Grundig 760 U that has two issues.

1) The stereo won't always turn on when I push the buttons (after a while it does end up working). Is this a tube, cap or a connection issue?

2) Once it's on it sounds great. Both speakers nearly perfect. But then after about 2-3 minutes the speakers start to flutter in and out until eventually they're barely audible. There is then a weak static noise heard (though this could just be from the radio signal now being louder than the now less audible speakers).

Would love to help solving these issues. If its a tube or cap issue it would be great if you could guide which ones are most likely the culprits.

Thanks!


Top
 Profile  
 
 Post subject: Re: Grundig 760 U
PostPosted: Feb Fri 19, 2016 1:54 am 
Member

Joined: Jan Mon 16, 2012 4:15 pm
Posts: 4526
Location: Near Brandon, Iowa
dyer0027 wrote:
Hi I have a Grundig 760 U that has two issues.

1) The stereo won't always turn on when I push the buttons (after a while it does end up working). Is this a tube, cap or a connection issue?

2) Once it's on it sounds great. Both speakers nearly perfect. But then after about 2-3 minutes the speakers start to flutter in and out until eventually they're barely audible. There is then a weak static noise heard (though this could just be from the radio signal now being louder than the now less audible speakers).

Would love to help solving these issues. If its a tube or cap issue it would be great if you could guide which ones are most likely the culprits.

Thanks!

1) Probably a deteriorated power switch, or slop/dust buildup in the activating linkage going to it.
2) This is a symptom of the selenium rectifier pack failing. It will pass DC when stone cold but as it heats up in continued operation its internal resistance rises and its DC output voltage drops. Does the tuning eye tube (indicator bar at the left of the dial glass) work? If so, take note of its brightness when the sound is good and also when the sound fades out. If the eye tube brightness fades along with the sound, almost certainly the selenium rectifier bridge needs to be replaced (with a silicon bridge).

If the eye tube brightness stays unchanged as the sound output fades, you have another problem; most likely tube-related.

This is a nice set. To avoid serious problems (possibly leading to a non-economically-repairable failure) down the road it would be advisable to get the all of the caps replaced.


Top
 Profile  
 
 Post subject: Re: Grundig 760 U
PostPosted: Feb Fri 26, 2016 2:19 am 
New Member

Joined: Feb Thu 18, 2016 11:32 pm
Posts: 3
Thanks for the response! I've attached a photo of what I think is the selenium rectifier. Is it the red rectangle at the top? I'm all for replacing it, but starting from 0. Any help or direction would be great. :)

I've also sat and watched the tubes to see if they go out at all and they don't. They all are a glow even when the speakers fade out (not sure if glowing proves the tubes are fine?). I did spray CRC cleaner on all of the tube sockets but that didn't work either. I tapped around in there with a pencil and didn't sound like there were any loose connections. And actually all the caps look perty dern good too.

The tuning eye does flicker with the fade but barely. When the sound fades to its worse the light is still generally in the same area it was before.

I'm up for recapping it but again I'm starting from 0 here. Wish there was a community ed class on this. :)

Attachment:
IMG_2444.JPG
IMG_2444.JPG [ 98.33 KiB | Viewed 1179 times ]


Top
 Profile  
 
 Post subject: Re: Grundig 760 U
PostPosted: Feb Fri 26, 2016 2:55 am 
Member
User avatar

Joined: Jan Tue 31, 2012 1:55 am
Posts: 5063
Location: Mountains of Mourne. Ireland.
Better than Community Education...
http://www.antiqueradios.com/forums/viewtopic.php?p=2455945#p2455945

Welcome aboard :)
Greg.

Edit: Some selenium rectifier information. (Yes, I know, it's a television).
http://www.youtube.com/watch?v=qsoHgtKi5-o&t=7m52s

I assume you have the schematic.


Top
 Profile  
 
 Post subject: Re: Grundig 760 U
PostPosted: Feb Fri 26, 2016 3:31 am 
Member

Joined: Jan Mon 16, 2012 4:15 pm
Posts: 4526
Location: Near Brandon, Iowa
It appears that a previous owner has already replaced the selenium bridge with four silicon diodes (small black cylinders). I can't tell what the red part is that the diodes are sitting atop. The original selenium rectifier would have been fairly large.

You can't tell much about the functionality of a tube by the fact that its heater glows. Same goes for capacitors- they may look fine cosmetically and still have failed internally. To get any further you are going to have to enlist the aid of someone who has some tube troubleshooting skills. That person will be able to show you how to use a digital multimeter to determine if you have correct voltages on the tubes, and how to go about determining if a tube is working or not. I suspect that you have a tube heating up and internally shorting/opening when it reaches a certain temperature. This sort of failure is a hallmark of a bad selenium rectifier but I think that we can rule that out based on your photo of the set showing the silicon diode quad.

You said that the eye tube "flickers" when the sound goes away. The eye tube dark gap should narrow when the set is tuned to a strong station. When the set is tuned in and the gap is at is narrowest, does the gap widen when the set acts up? If not, the RF stages of your radio is probably OK right up to and including the detector tube; and your problematic tube is most likely in the audio amp stage(s). You could try finding some pre-tested replacement audio stage tubes and swapping them into your radio, but that could run into a bit of money with no guarantee that it will resolve the issue.


Top
 Profile  
 
 Post subject: Re: Grundig 760 U
PostPosted: Mar Mon 05, 2018 11:28 pm 
New Member

Joined: Mar Mon 05, 2018 11:11 pm
Posts: 2
I just bought a grundig 760 and it powers up but nothing comes from the fm radio. One of the short wave buttons produces static when pressed but the tuning doesnt seem to work. Can so eone send me the schematic in case im able to find a repair man? Im in chicago and im looking to get this repaired\restored.


Top
 Profile  
 
 Post subject: Re: Grundig 760 U
PostPosted: Mar Thu 08, 2018 6:23 pm 
Member

Joined: Jan Mon 16, 2012 4:15 pm
Posts: 4526
Location: Near Brandon, Iowa
Mikeb437 wrote:
I just bought a grundig 760 and it powers up but nothing comes from the fm radio. One of the short wave buttons produces static when pressed but the tuning doesnt seem to work. Can so eone send me the schematic in case im able to find a repair man? Im in chicago and im looking to get this repaired\restored.

You can download the necessary documentation at no cost to you from this link:
https://www.radiomuseum.org/r/grundig_ks760uks_76.html


Top
 Profile  
 
 Post subject: Re: Grundig 760 U
PostPosted: Mar Thu 08, 2018 6:47 pm 
Member
User avatar

Joined: Jan Tue 31, 2012 1:55 am
Posts: 5063
Location: Mountains of Mourne. Ireland.
viewtopic.php?p=2822766#p2822766

:) Greg.


Top
 Profile  
 
Post New Topic Post Reply  [ 8 posts ]  Moderators: Marcc, Norm Leal

All times are UTC [ DST ]


Who is online

Users browsing this forum: Google [Bot] and 6 guests



Search for:
Jump to:  
























Privacy Policy :: Powered by phpBB