Grundig Concert Boy 1100
 

I am hoping someone can help walk me through a problem I am having (I am a complete novice).
I have a concert boy 1100 that doesn't work at all. I took the back off, got 240volts AC going into the transformer, got no voltage out. I changed the 400mh fuse, I haven't got a new 400mh fuse so replaced it with a 2 amp fuse. I plugged it in but haven't switched the radio on. I have now got 15 volts dc coming out of the other side of the transformer going in to the main unit (the red and blue wires - please see picture). I am worried about pressing the on/off button on the radio as I thought the transformer would be kicking out 9 volts and not 15 volts and also if there is a problem will the 2 amp fuse blow in time so as not to ruin anything else? Please help.

Re: Grundig Concert Boy 1100
 

You really should replace with the correct rated fuse as soon as possible to prevent damage to the set. You will be seeing a higher voltage as without the radio switched on there is no load on the transformer

Re: Grundig Concert Boy 1100
 

OK, thank you very much. I might be back to ask more questions later!!

Re: Grundig Concert Boy 1100
 

I found a 300mh fuse out of another grundig radio, turned the radio on and I am now getting 2.3volts going into the red and blue wires. Also in the pictures is a blue what looks like a rectifier - it is very hot after only being on for a few seconds.

Re: Grundig Concert Boy 1100
 

The 400mA fuse protects the mains transformer and the bridge rectifier. It isn't in circuit when the set is used on battery power.

You'd be far safer to investigate the fault (assuming there is one) using battery power rather than mains, both from the point of view of the set and yourself!

Cheers,

Frank

Re: Grundig Concert Boy 1100
 

Do you have access to a Multimeter? If the rectifier is heating up drasticaly within seconds of power being applied you possibly have short circuit downstream from it. It could also be that the rectifier itself has failed.

Re: Grundig Concert Boy 1100
 

The blue thing has a '+' sign marked on the board, and the output side of the transformer has only a single winding (two yellow wires) so the blue thing should be a bridge rectifier with four connections to the board.

Due to the proximity of the fuse and the rectifier and the need for safety clearances on the board, I suspect that the fuse on the board is connected on the secondary side. If so, the mains side of the transformer may have a thermal fuse embedded in the primary winding.

If my guesses are correct, then using too high a value fuse in the board will risk the thermal fuse in the transformer blowing. If that happens, the transformer becomes scrap.

The mains cable can be seen in the photos coming into the radio and connecting straight to the transformer without benefit of a proper switch. This is quite common, but it leaves the transformer rectifier and reservoir capacitor running all the time the radio is plugged in whether the radio is turned on or off. THis wastes a small amount of power and it leaves the transformer running warm. Parts are left under stress all the time. These sorts of things should be unplugged when not in use - or switched off at the wall.

David

Re: Grundig Concert Boy 1100
 

You're quite right in your conjectures, David - here's the PS board.

Poppydog - note that the main power for the radio is 9V, either from the battery or from the bridge rectifier. You may see more than that at the rectifier, because the DC supply is clamped down to 9V by a 9.1V zener diode downstream. Note as well that the full circuit diagram for this radio is available from the Forum as well- see top RHS of page - for a couple of quid. Invaluable if you need to fault find.

Cheers,

Frank

Re: Grundig Concert Boy 1100
 

Thank you for all of the replies, yes I have a multimeter but I am going to need some help in knowing what and how to test. I do have a schematic but it just as well be in a foreign language as I don't understand it all (can read the capacitor values). I have required several of these grundig radios and most of them have the same problem. I would love to be able to know how to resolve the problem myself.

Re: Grundig Concert Boy 1100
 

Only part of the power feed gets clamped by the zener. there's a line from before R802 sneaking off lower right, presumably to the audio output stage.

Possibilities:

Failed bridge rectifier
C805 gone very leaky or shorted
Fault in audio output stage

David

Re: Grundig Concert Boy 1100
 

These sets stabilise the supply for the front end with the 9V zener diode, and use the unstabilised output from the rectifier bridge to feed the audio stages.

Thus, when running on mains (netz) the output stage sees a higher voltage than is the case during battery operation. The transformers are generally wound for 220V mains, so 12 or 13V applied to the audio stages is not unusual on 240V (actual) UK mains.

These sets give good audio results when mains operated - mine needed some felt inside the cabinet to stop it rattling - too few screws.

If your blue (selenium) rectifier is running hot it will be scrap. Investigate the cause, replace with a silicon bridge and don't fit a heavier fuse than is intended.

Leon.

Re: Grundig Concert Boy 1100
 

That PSU circuit manages to be upside down and inside out in my head. It's positive ground, with a capacitor-resistor-capacitor filter in the ground side and the rectifier positive is at the top with negative at the bottom. Bleeaargh!

A switched bleed resistor on the main smoothing cap is just the cherry on the icing on the cake.

Re: Grundig Concert Boy 1100
 

Bad circuit drawing - but a good design.

On one of the ERT sheets, this Grundig circuit fooled them completely - and they drew it wrongly with the 9V supply shorted. The discharge resistor ensures that the set shuts down gracefully; it has a big reservoir capacitor.

Leon.

Re: Grundig Concert Boy 1100
 

They may have found that it switched off with a squawk as the voltage faded, so they added the bleeder to pull it down quicker.

There were jokes that Max Grundig designed a nice radio and then started removing parts until it stopped working, then he put the last bit back in and put that version into production. So the bleeder must be needed.

David

Re: Grundig Concert Boy 1100
 

Yup, they usually do that as a warning that it's full of PNP transistors.

Re: Grundig Concert Boy 1100
 

Not always true, this radio uses NPN silicons with the collector loads going to + ground.

Re: Grundig Concert Boy 1100
 

Thanks again for all your responses. Next question is - how do I test the bridge rectifier? Can I test it in situ or do I have to de-solder it?

Re: Grundig Concert Boy 1100
 

In situ, remove fuse, switch to BATT. Using meter on lowest resistance range, check
across each diode, depending on the probes polarity you will get infinity in one direction,
with 20-100 ohms when the probes are reversed. All four diodes should read the same,
if not replace with a modern 1A silicon bridge, e.g. W01, or W005.

Re: Grundig Concert Boy 1100
 

Thank you for the reply. [I didn't think anyone was going to bother]. I will test it later. Another quick question slow blow or fast blow fuses? I'm ordering new ones today.

Re: Grundig Concert Boy 1100
 

The service manual stipulates anti-surge (slow blow).

Cheers,

Frank