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  #1  
Old 02-25-2017, 04:40 PM
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Grundig power supply problem

I've got a Grundig RTV-320u receiver that is am/fm/sw from the mid sixties. It's been working fine up until about a month ago when it just went off one day. I found the main fuse in the power supply blown so I put in a new one and blew right away. I can't seem to find out what's going on. Any ideas? Thanks in advance for any help.
I didn't have a picture, so I uploaded one from google for reference.

Last edited by stushug; 02-25-2017 at 04:44 PM.
  #2  
Old 02-25-2017, 11:56 PM
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without any schematics it is hard, but Grundigs have a history of using selenium rectifiers which suddenly blow up.
You can use any silicium ones, provided they fir the voltage and amperage. If it has a tuner, 4 decoupling capacitors may be needed. Check also the big capacitors...
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Old 02-26-2017, 08:03 AM
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schematic

I have the schematic, would that help you direct me to the problem?
Attached Files
File Type: pdf d_grundig_rtv320u_sch1a.pdf (358.2 KB, 4 views)
File Type: pdf d_grundig_rtv320u_sch1b.pdf (391.1 KB, 1 views)
File Type: pdf d_grundig_rtv320u_sch1c.pdf (236.6 KB, 5 views)
  #4  
Old 02-26-2017, 08:54 AM
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pictures

I have included some pictures here. I found that the diodes at D3 and D4 tested bad, so I replaced them. I think the item I circled in the picture may be the problem, but I don't know exactly how to check it, or what to replace it with if its bad. On the schematic it looks like it gets power from the 0.4 trage fuse. Is that correct? The capacitors tested good.
Attached Images
File Type: jpg 320u board.jpg (65.3 KB, 8 views)
File Type: jpg 20170226_103409.jpg (63.6 KB, 9 views)
  #5  
Old 02-26-2017, 09:37 AM
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That's the one. It's a bridge of diodes, each side had to work as a diode. Beware, the legs are not like in the schematics
Anyway, change it even if it works, it will not one good day.
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Old 02-26-2017, 12:06 PM
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You mention the legs are not like the schematics. How do I determine which leg is which? There are no marks on it, except for a red mark on the body near one leg. I put an arrow pointing to that leg in a picture.
Attached Images
File Type: jpg diode arrow.jpg (63.7 KB, 4 views)

Last edited by stushug; 02-26-2017 at 12:30 PM.
  #7  
Old 02-26-2017, 03:05 PM
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Quote:
Originally Posted by stushug View Post
You mention the legs are not like the schematics. How do I determine which leg is which? There are no marks on it, except for a red mark on the body near one leg. I put an arrow pointing to that leg in a picture.
Look close on the PC Board. I say that because I can barely see the sideways S indicating AC input into the bridge/rectifier next to one of the lead holes on the board. The Red dot all the times I have seen it indicates + DC output.

Usually on most (NOT all) bridge rectifier will have cast into it 2 sideways S (indicating the AC input) with + & - symbols indicating the DC output & polarity. But in your case the symbols are silkscreened onto the PC Board next to the component through holes.

If you are not sure please post a clear close up direct top down pic showing the whole silkscreen white box OR 2 clear close up pics of each end of the rectifier showing the silkscreen clear.
  #8  
Old 02-26-2017, 05:01 PM
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I think I have it understood. I attached a picture. The leg with the red arrow is the DC+, the leg with the green arrow is DC-, the leg with the yellow is AC in.
I also attached a picture of the underside of the board with the same markings. The green arrow is on the DC- leg, which is connected to the brown wire on the 9 pin plug, which is pin 3, the ground. The leg with the yellow arrow is attached to one of the wires leading out of the transformer, which I assume is AC in. Please let me know if I am correct.
Attached Images
File Type: jpg boardarrowed.jpg (59.7 KB, 6 views)
File Type: jpg 20170226_182705.jpg (53.2 KB, 6 views)
File Type: jpg underarrow.jpg (73.1 KB, 6 views)
  #9  
Old 02-26-2017, 09:34 PM
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That should be correct. Yes wires coming out of a raw transformer are AC. I don't know the wiring & pin outs on the deck but to verify & confirm the rectifier + & - output just follow each lead to the filter/power supply caps in that circuit. The + from the rectifier should go to the + on the cap & the - on the rectifier should go to the - of the cap.

Once that is confirmed you can replace with a modern bridge rectifier & just bend/align the leads as needed to go to the proper holes/traces on the board.

All you need to do is make sure the replacement rectifier has an equal or higher voltage & current ratings as the original did. I am thinking of like a 1 or 1.5A rated bridge rectifier (just looking at the size of the original) but someone that knows this deck & has serviced them can say for sure IF you can't look up the specs on the old part/part #.

Also with the rectifier out of the circuit it would be a good idea to check & make sure there are no shorted or electrically leaky caps (or shorted other components) in that circuit that could have caused the rectifier to fail. Though rectifiers can fail on their own & is common on selenium rectifiers.

.

Last edited by 390FE; 02-26-2017 at 09:39 PM.
  #10  
Old 02-26-2017, 10:00 PM
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Thank you. I will update when I get the new rectifier installed.
  #11  
Old 02-27-2017, 08:04 AM
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Pay attention to the terminals - the new silicon bridges are different!
Below the two most common selenium bridges, from AEG (Telefunken), used until '80ies by Grundig.

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Old 03-02-2017, 11:32 AM
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I replaced the selenium rectifier with a diode bridge and the 600mA fuse blows as soon as I apply the a/c current, which is exactly what was happening before. The fuse is inline with the a/c current on schematic1c
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Old 03-02-2017, 12:42 PM
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According to the schematic that particular fuse, which you didn't mention in the first place, goes straight to the control unit.
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Old 03-02-2017, 01:55 PM
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I didn't realize about not mentioning the fuse earlier, but that's what started this problem. The fuse blew today without the power supply hooked to the radio chassis, which I assume is the same as the control unit (right?) Sorry for any confusion, guys.
What does that mean as far as sorting out this repair?
  #15  
Old 03-02-2017, 02:02 PM
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The power goes to the turntable section or unit.
Probably a short inside the turntable.
Check anyway the big trafo for continuity and for insulation.
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Old 03-02-2017, 05:13 PM
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I'm not sure what you mean by trafo. This power supply is in its own chassis. It powers the rest of the unit through the 9 pin connector, but I didn't have it connected to the receiver while testing. I've attached a few pictures for reference. One photo shows the receiver chassis inside the cabinet, one shows how the power supply is normally positioned and the last one shows the power supply facing outward so the tan 9 pin wire is visible. This unit is an AM/FM/SW radio. It does have inputs for tape and phono, but I've never attached a tape deck or turntable. The middle picture also shows the location of the 600mA fuse.
Attached Images
File Type: jpg 20170302_191804.jpg (68.2 KB, 8 views)
File Type: jpg 20170302_191842.jpg (64.2 KB, 7 views)
File Type: jpg 20170302_191930.jpg (85.3 KB, 6 views)

Last edited by stushug; 03-02-2017 at 05:29 PM.
  #17  
Old 03-03-2017, 10:49 AM
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Trafo = power transformer
The schematics show the voltage flowing towards that connector - from the schematics it was not clear how that connector looked like.
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Old 03-03-2017, 02:01 PM
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I took the unit to a friend today, he thinks the trafo may be bad. He said the primary impedance measured 10 ohm and it should be more in the 200 to 500 ohm range. Does that sound correct?
He said he couldn't find anything wrong in the DC circuit.
If the transformer is bad, does anyone know how to repair or get a replacement?

Last edited by stushug; 03-03-2017 at 06:22 PM.
  #19  
Old 03-03-2017, 08:55 PM
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I do not know, but 10 ohm would be reasonable for a welding trafo
The impedance may be roughly computed as mains voltage by the amperage (if indicated) or as square voltage by rated power. The real values are more complicated to calculate, but this is simple and efficient enough.
Conservative values, like 50W would yield 250ohm in USA/Japan.
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Old 03-03-2017, 09:22 PM
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How can I find out what the specifications on the transformer are, so I can find a replacement? I see that the original part number is BV 0820-304.97

Last edited by stushug; 03-04-2017 at 08:25 PM.
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