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KB FB10 FM

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Anonymous
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Hi
I got this radio on Sunday and I am a little puzzled by the circuit.
There is a switch which is marked "normal" or "adaptor" S1 on the circuit.

In the position shown on the circuit diagram L11 is connected to the volume control and V4 triode anode is connected via C23 to the grid resistor of V5, all fairly "normal".

But in the "adaptor" position L11 is connected direct via C23 to the grid resistor of V5 bypassing a stage of amplification. OK this is going to work but with reduced output.
The odd thing though is in this switch position the speaker winding is short circuited.
The "adaptor" (an external amplifier) is connected to a second winding on the OP transformer.

My Question is;
What reason is there to short the speaker winding on the OP transformer when using an external amplifier, and how would this effect the transformer.?

Thanks
Mike

p.s.
By the way I have put a post on the wanted section for an autotransformer for one of these radios as this one has an open circuit between the 240 & 210 taps and almost short between the 210 tap and ground, (6.8ohms)

 
Posted : 23/09/2013 12:35 pm
sideband
(@sideband)
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By the way I have put a post on the wanted section for an autotransformer for one of these radios as this one has an open circuit between the 240 & 210 taps and almost short between the 210 tap and ground, (6.8ohms)

You luck B.......! I've been after one of these to go with my FB10 (AM). They are not the best of performers but work reasonably well.

OK your request for an autotransformer....is it the same as the standard FB10? If so I have one you are welcome to. As it only has to supply heaters, it will probably do the job. What is the valve line-up?

Rich.

PS. Got the valve line-up. I think it's the same transformer so all I have to do is find it.......! :aaq

 
Posted : 23/09/2013 2:47 pm
Anonymous
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Hi Rich
The FM version does not seem to come up so often on Ebay.
I would be very appreciative if you can find it.

Cheers
Mike

 
Posted : 23/09/2013 3:08 pm
Westcliff
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Mike,

Have you had a look at Trader sheet 1263?

Gus.

 
Posted : 23/09/2013 6:16 pm
Till Eulenspiegel
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Your FB10FM is the much better version. There is another with one IF amplifier stage.
Very rare set.

Till Eulenspiegel.

 
Posted : 23/09/2013 6:27 pm
Anonymous
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Hi Till
Are you saying the 5 valve version is rare or the earlier 4 valve one?

Gus
Yes I have found the Trader version for the 4 valve version, in the 4 valve version the speaker is switched open circuit when the FB10FM is used as an adaptor.
It is still not really making much sense. Why do they short the speaker in one version and open circuit it in the other. Why not simply take an audio feed from the hot end of the volume control and leave the volume turned down in order to silence the FB10FM output valve and speaker.

Mike

 
Posted : 23/09/2013 9:13 pm
Till Eulenspiegel
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I'd reckon all the versions of the FB10FM are rare. There's plenty of the AM only FB10 models about.

Till Eulenspiegel.

 
Posted : 23/09/2013 9:25 pm
Anonymous
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I'd reckon all the versions of the FB10FM are rare. There's plenty of the AM only FB10 models about.

Till Eulenspiegel.

That's my experience of them as well, been after one for ages, only seen about 1 in the last 2 years and I could not afford it.
Mike

 
Posted : 23/09/2013 9:29 pm
Refugee
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I can see the point of the isolated winding on the output transformer. It is due to the set not having an isolated chassis and so there is a risk of short circuiting the power If the external amplifier is not itself isolated.

 
Posted : 23/09/2013 10:31 pm
Anonymous
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Yes that' a good idea but why short the other secondary winding.

 
Posted : 23/09/2013 11:10 pm
Refugee
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Shorting the speaker may be to do with stopping spikes in the audio output transformer. I would have expected to see a resistor in series with the switch though.

 
Posted : 24/09/2013 12:49 pm
Anonymous
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I have changed the one wax and 2 brown hunts and reformed the triple electrolytic can and the 2uf discriminator cap, which seemed to go quite well, hardly any leakage (<500uA for the 3 x 32uf caps and about 100uA for the 2uf cap) I have put it back together for now whilst I try and locate a suitable autotransformer. Here is a photo, I am very pleased with the way it cleaned up.
Mike

 
Posted : 24/09/2013 1:09 pm
Anonymous
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Hi
I am really pleased with the performance of my KB FB10FM now I have fitted the replacement transformer supplied by Rich (Sideband), lovely clear sound with nice tone. Thanks Rich, it arrived today.
The heater voltage is slightly high at 6.4v but it is close enough. The reception is good on the stronger stations with just the 8 inch plate aerial. But it is a bit temperamental to movement around the set. I think it will be better if the mains lead is uncoiled, at the moment it is coiled up ready to go on my display shelf in pride of place.

Mike

P.S. Thanks Gus for letting me have one of these.

 
Posted : 05/10/2013 9:37 pm
Marc
 Marc
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Hi Mike,

Well now, that does look good ! Cosmetically it looks superb, one to be proud of I would say :aad

Well done
Marc.

Marc
BVWS member
RSGB call sign 2E0VTN

 
Posted : 05/10/2013 9:47 pm
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