Kolster Brandes FB10FM

[John G8MWF]

I have just started restoration again after a break due to work.

This set had a faulty solid state rectifier that I replaced with a modern equivalent and a resistor. The triple smoothing capacitor had also failed and this has been temporarily fixed with individual capacitors.

Audio quality is not good at all but I cannot pin down if its just the audio output stage/speaker, the discriminator or just the nature of the beast!

It has had a LOT of work on it by other people in the past and the output stage is not the standard EL84 but a 6BW6 - it has been suggested that this was an experimental set or a small run with this option fitted.

I would really appreciate some help from someone with experience on this model as there has already been a lot of work on it and I don't want to make things unnecessarily worse by trying to make things better.

Cheers for now John

[Sideband]

I don't have one of these although I have been looking for one for some time. What I do know is that their performance is very mediocre. The 6BW6 is correct for the set...it's basically a B9A version of a 6V6. Easy to check if the speaker is at fault...simply try one from another set. I have the standard FB10...AM only set and audio quality is not bad and typical of what you'd expect from a midget set. It uses the same 6BW6 and gives a reasonable quality output. I suspect if considerable work has been done in the past, maybe someone has tried to improve performance not realising it was probably as best as it could be. It's not too difficult to align a discriminator stage as long as you follow the alignment instructions....don't try to guess it.

It certainly wasn't an experimental set but not too many seem to have survived judging by their scarcity on the usual auction sites.

I believe member 'Crackle' has one of these as he collects KB sets so you could try a PM to him for advice.

[Leon Crampin]

I would check the data carefully. The AM FB10 certainly used the 6BW6, but I'm fairly sure the FM version used an EL84. In a set so short of gain, the higher slope of the EL84 over the 6BW6 (derived from the venerable 6V6) would be desirable.

Leon.

[John G8MWF]

All noted.

I took Sidebands advice and tried the loudspeaker on another radio - it is really naff! (why I didn't try this earlier is a mystery to me)
It looks to be in good condition but it sounds like it has been put in an large empty biscuit tin and rattles on every low note.

So that is at least that is one part of the puzzle solved but I expect that getting hold of a loudspeaker for this set is going to be a 'bit' of a challenge.

I will locate another loudspeaker and temporarily connect it to the KB and see what the quality is like and then decide if further action is needed to the electronics.

The 6BW6 certainly seemed to work before and looks like it was a factory installed device and I intend to leave it there if it works satisfactorily.

Thanks for the advice - John

[Nuvistor]

The 6BW6 is a lot of valve in a small space, Pout is capable of over twice an EL84.

Hopefully it is a good one in your radio.
http://www.r-type.org/exhib/aaa0343.htm

[Boater Sam]

The loudspeaker may be fixable, the diaphragm that centres the coil could simply need regluing round the edge.

[John G8MWF]

The speaker 'appears' to be in good physical health, see enclose pictures, with no signs of mechanical damage.

The diameter is 5" and the holes in the rim are used to mount it to the chassis.

[Tractionist]

Hmmmmm ..... I too have the AM-only version and second everything that's been said - don't expect miracles from this tiny set [the enclosure/case looks great but adds nothing to acoustic performance .... rather the opposite].

I seem to recall that mine exhibited speaker rattle during its restoration too ...................

[Sideband]

As per usual with KB there are several versions! There appears to be a very basic set with 4 valves that has a single IF stage....performance of that will be pretty ropy. There appears to be another version that has two IF stages where KB obviously tried to improve performance. Both circuits show an EL84 as output....but don't take that as set in stone! You need to let us know what version (how many valves) your set has. As to the output valve, possibly an EL84 or a 6BW6 could have been fitted...only slight rewiring of the base is required and maybe a change to the cathode resistor. They tended to use what was available so possibly early sets used up remaining stocks of 6BW6.

If you check on Paul's DVD, you'll find three versions of the circuit......

[John G8MWF]

I have had a couple of months break from this project and I have just been looking at my notes from that time.
It is definitely the 5 valve with metal rectifier version and is 'mostly' the same as trader sheet 1263. (almost a hybrid of the data sheets)

There is NO WAY this speaker is any good - I have heard far better transistor radios!

Towards the end of the week I will get a chance to put another speaker on it and see how it performs - I suspect that it will be OK and it will then be a matter of putting it all back together again until I can locate a suitable replacement loudspeaker.

[paulsherwin]

KB mostly designed around American valves because of their close links with Brimar.

[Top Cap]

I found the AM FB10's also suffered from distortion due to the speaker, only after checking everything else of course! Must be the heat of the nearby valves or maybe the angle it sits at? I spent 3 years finding a replacement, that is a proper Goodmans as per the original, which I found at a radio jumble meet. Since then I have had another FB10 with distorted output traced to the speaker BUT I had a wreck of an Ultra Coronation Twin which had the same speaker, so a possible source for you there.

[crackle]

There was a KB FB10FM 4 valve plus rectifier version with a 6AM6 output valve, which is strange as it appears to be equivalent to the EF91. I wonder if someone has substituted that for a 6BW6.
As mentioned earlier, it would be nice to know which version of the FB10FM you have, what is the valve line up?

I have come across speakers in a few different sets where the inner bellows attached to the speech coil have become detached from the frame. The effect is a distortion and rattle in the audio as the diaphragm flaps around and rubs the speech coil against the magnetic gap.
To test this, grasp the diaphragm with both hands, through two opposite holes in the basket between finger and thumb and try to move the diaphragm gently in and out and side to side.

If there is only a faint scraping noise then there may be contaminants in the magnetic gap, this is very difficult to deal with.
But if it moves from side to side as well then the inner bellows may have become partially or totally detached from the housing. This is usually caused by minor corrosion.
I have repaired speakers like this by carefully cutting the dust cap away with a sharp scalpel, wedging 3 plastic shims into the gap between speech coil and the center pole. Then pushing the diaphragm outwards just enough to get an artists paint brush in so you can paint PVA glue around the rim of the inner bellows. (the diaphragm should be held out by the shims).
Gently push the diaphragm back in so the inner bellows are pushed up flush with the frame where you have painted the glue. Leave overnight then gently pull the plastic shims out, the diaphragm should mow move in and out silently.
The dust cap can be carefully re-positioned and a tiny dab of PVA glue can be put on to hold it. When the tiny dab has dried you can paint on more PVA glue to permanently reattach the dust cap to the speech coil.

Points to note.
I dont attempt to remove any rust as the particles will be attracted to the speech coil gap and are impossible to remove.
I use plastic shims cut from the hard clear plastic which comes as the packaging with some of those horrible impossible to get into bubble pack items.

Good luck with it and please update as to how you get on, and confirm the whole valve line up.

Mike

[John G8MWF]

Hello Mike,

The valve line up is ECC81, 6BH6, 6BH6, EABC80 and the 6BW6.

I tried the speaker test but I couldn't hear any noises nor could I get any sideways movement!
Incidentally my speaker looks like an original and it is made by ELAC not Goodmans.
There are two screws and a nut on the rear magnet, see earlier pictures, but so far I have resisted the temptation to twiddle - well not until I have a viable replacement.

[crackle]

So it is the 5 valve version with 2 IF stages.
I would not recommend undoing any screws on the back of the magnet, you will have a devil of a job to get it back together as the magnet tends to stick like glue to the other pole ring.
I wonder if the speech coil has come detached from its former. Not sure what to do about that.
I think it is a new speaker job, shouldn't be too difficult to find a replacement, it is a fairly standard speaker.

Mike