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Old 30th Nov 2015, 11:47 pm   #21
poppydog
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Default Re: Grundig Concert Boy 1100

Tested the rectifier this evening and by the look of the readings it is bad. Will order the new fuses and rectifier this evening. What would be the cause of the rectifier packing up? Would this indicate that I will find something else wrong? When I solder the new rectifier in if there is anything else wrong will that damage the new rectifier?? Thank you
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Old 30th Nov 2015, 11:51 pm   #22
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Default Re: Grundig Concert Boy 1100

Well as long as you can not read a short circuit after the rectifier all should be ok. I would think it is just a case of that rectifier coming up to around 40 years old.
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Old 30th Nov 2015, 11:56 pm   #23
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Default Re: Grundig Concert Boy 1100

OK, how would I go about reading a short circuit after the rectifier? Sorry to be so thick!
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Old 1st Dec 2015, 12:16 am   #24
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Default Re: Grundig Concert Boy 1100

I don't think you're being over-cautious. The old rectifier could just have died of old age, but it also could have been finished off by a fault in the radio.

You could temporarily connect a 12 V, 5 W car bulb in place of the 400 mA fuse. If there is a short-circuit, it will light up brightly but the current will be limited to about 400 mA. (Actually 416.7 mA.) If all is well with the radio, it should only glow dimly if at all. The bulb will glow brighter and the sound will distort if turned up loud, as though the set were running on spent batteries. When replaced with the correct fuse, the set should be fine. If the bulb glows brightly, that would indicate a problem, but try it first. Sets of spare car bulbs can be bought in pound stores. You won't need a holder, just solder wires to the bulb contacts. 16/0.2 will be adequate.
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Old 1st Dec 2015, 12:47 am   #25
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Default Re: Grundig Concert Boy 1100

thank you for the replies I will let you know how I get on
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Old 3rd Dec 2015, 8:15 pm   #26
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Default Re: Grundig Concert Boy 1100

I have soldered the new bridge rectifier in and connected a bulb as suggested by Julie_m. I plugged the radio in and switched it in at the mains and the bulb is glowing quite brightly without switching the radio on. Should it glow like this with the radio switched off? I am not comfortable switching the radio on incase it ruins something else.
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Old 3rd Dec 2015, 8:28 pm   #27
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Default Re: Grundig Concert Boy 1100

OK, well with the switch at BATT the lamp should not glow. Now Switch to AC and then
switch the radio ON at low volume, and observe the lamp ; if bright and/or you hear
distorted sound possibly with a loud hum, switch off immediately and check around
the power supply circuit. However I hope you will instead be rewarded by a working set.
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Old 3rd Dec 2015, 8:43 pm   #28
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OK switched it all on. Bulb still glowing bright, no sound nothing no pops or crackles

The bulb is glowing with the switch switched to BATT. Any advice
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Old 3rd Dec 2015, 9:03 pm   #29
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Default Re: Grundig Concert Boy 1100

You've wired the new bridge rectifier in incorrectly.

OR

You have the wrong wattage of bulb in place of the fuse. I assume you have wired it in place of the fuse?

OR

You have a short-circuit on the output of the rectifier.
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Old 3rd Dec 2015, 9:17 pm   #30
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Default Re: Grundig Concert Boy 1100

I have soldered the new bridge rectifier so that the + matches where the + was on the original rectifier (which is where the + sign is on the printed circuit board) then followed suit. The bulb is a 12v, 5w (as advised by Julie_m). How do I test if I have a short circuit on the output of the rectifier?
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Old 3rd Dec 2015, 9:33 pm   #31
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Default Re: Grundig Concert Boy 1100

You also have to get the negative connection in the right place. If it has been swapped with one of the AC input pins you may have a diode in the bridge shorting the AC from the transformer.

The two AC inputs of the bridge may be interchanged with each other.

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Old 3rd Dec 2015, 9:36 pm   #32
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Default Re: Grundig Concert Boy 1100

A bridge rectifier has 4 connections, labelled '+', '-' and two '~' (AC symbol). The two '~' ones are equivalent.

Often on a modern bridge rectifier the connections are in a square. The '+' one is labelled. The '-' one is diagonally opposited it. The '~' ones are on the other diagonal.

I don't know how the connections on the original rectifier were arranged. You may need to trace the PCB tracks to find out. This is not hard. From the schematic posted earlier we see that :

'+' goes to a contact on the battery/mains switch.

The 2 '~' connections go to the transformer (secondary winding)

'-' goes to the battery pack -ve terminal (and lots of other places).

So check that out. Identify the 4 holes in the PCB and connect up the new rectifier.

If the old rectifier had 4 connections in a line it is likely the order is '+', '~', '~', '-'. You might have got -ve swapped with one of the AC terminals in that case, which would end up (with the test bulb wired across the fuseholder) with a diode (part of the rectifier) and the bulb across the transformer secondary winding. In that case the bulb would glow brightly. Check this and correct if necessary.

But until the bulb is dark (particulary with the switch in the 'battery' position) do not fit the fuse. It will just blow.
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Old 3rd Dec 2015, 9:40 pm   #33
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Default Re: Grundig Concert Boy 1100

The rectifier has 4 pins. The symptom you describe could be caused by the other pins not being in the same positions as on the original rectifier. If - and one of the ac pins are crossed you will get a half wave short across the transformer secondary.
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Old 3rd Dec 2015, 9:44 pm   #34
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Default Re: Grundig Concert Boy 1100

Thank you for all of the advise but it is getting a bit too technical for us! Is the configuration on the rectifiers always the same i.e. - opposite + and the 2 AC either side
+
ac ac
-
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Old 3rd Dec 2015, 10:03 pm   #35
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Default Re: Grundig Concert Boy 1100

Normally it is on modern-ish rectifiers. I am not so sure about the original one though.

What rectifier did you buy? One of us will know it or be able to track down the data sheet which will show how it is connected.

On the little PCB that holds the rectifier (shown in a much earlier photo) there are 2 yellow wires at one corner.

I think one of those will go to one of the clips that holds the fuse. The other fuse clip will go to one of the rectifier connections. That is one of the '~' ones.

The other yellow wire will go directly to a rectifier connection. That is the other '~' one,

'+' is marked on the board.

Which leaves '-' as the other connection.

Maybe post a clear photo of each side of that little PCB.
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Old 3rd Dec 2015, 10:06 pm   #36
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Default Re: Grundig Concert Boy 1100

Our + on the rectifier matches the + sign on the printed circuit board and goes to the AC switch, our - on the rectifier is going to the transformer (yellow wire) which is diagonally opposite the + on the printed circuit board. As for the 2 AC pins on the rectifier, one is going to one side of the fuse holder and the other is going to the transformer and to the blue wire (please see my first picture in original post). If I have soldered this in incorrectly does this mean I have to twist the pins round on the rectifier to solder them in to the correct positions?
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Old 3rd Dec 2015, 10:11 pm   #37
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Default Re: Grundig Concert Boy 1100

Yes, that appears to be incorrectly wired.

The '+' is right. The AC one that goes to the fuse holder is right. The other 2 wires are the wrong way round. Swap them over so that the other AC goes to the transformer yellow wire and '-' goes to the blue wire.

Yes, you will have to twist the wires round. If there is a danger of them touching you should put insulating sleeving over the wires. If you don't have any then the insulation stripped from one core of a bit of scrap mains flex will do.
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Old 3rd Dec 2015, 10:14 pm   #38
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Default Re: Grundig Concert Boy 1100

You've got it wired incorrectly.

The negative terminal shouldn't go to the transformer.

One '~' terminal goes to the transformer.
The other '~' terminal goes to the fuse.
Positive terminal goes to the Battery/Mains switch.
Negative terminal goes to the capacitor.

As per the diagram in post #8
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Old 3rd Dec 2015, 10:22 pm   #39
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Default Re: Grundig Concert Boy 1100

I have taken some pictures.....
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Old 3rd Dec 2015, 10:43 pm   #40
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Default Re: Grundig Concert Boy 1100

The picture is handy, shows the error. The correct connections are to the four holes ;
Top Left = - Top Right = AC
Bottom Left = + Bottom Right = AC
As mentioned, sleeve the wires where they cross and could touch.
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