Kolster Brandes FB10FM

[Heatercathodeshort]

Don't expect spectacular results from from the FB10FM, in fact any KB FM receiver. They were very poor performers in London.

The KB factory was almost under the Wrotham transmitter and of course the signal was abundant. Once you moved a bit further down the A2 it dropped off somewhat and the KB models began to show up their short cuts. They were certainly well constructed.

Hopefully your example will give an acceptable result. FM is easier to receive now than it was 30 years ago. John.

[John G8MWF]

Originally Posted by Nuvistor

I would not be keen on leaving the old cap in circuit. What is the 2 watt RS resistor connected to, looks like 47? 470ohm cathode resistor?

Hello Frank,
I am still trying to figure out what is going on with previous work carried out on this set but the are a few differences that need looking at.
The 470 ohm resistor appears to be R11 which feeds the HT to the front end

Why 470 ohm - well I was not happy with the output of the metal rectifier, I cannot remember the figures, but it certainly looked a lot lower than it should be.
Perhaps R11 was changed to compensate for this lower output?

There also appears to be at least one additional electrolytic in place around V1 and the main filter caps are dated around 1992 so there have obviously been 'hum' issues with this set.


The plan is - replace the filter caps and let the set run. Then see if the metal rectifier needs replacing. The total HT current is stated as 64mA so the dropper resistor should not be a Mensa calculation.
If needed I will tidy up 'extra' components and remove them if they are not needed.

Cheers


John

[slidertogrid]

Some of the online information pages quote the case material as Bakelite and because the white DAC90A doesn't use Bakelite then it seemed likely the FB10 didn't - if that makes sense?

My set has a slightly grubby appearance but cleaning it with Polishing Paste No.5 made no difference when used on a test area.

Hi I think the KB case is made of a different formula to the dac90a. the Bush case is translucent and as we all know very prone to heat cracks.
The KB "Bakelite" (if that's what it is) is denser and has a slight mottled effect.
Some sets seem to have more patterns to this effect than others. It can make the case look a bit grubby
I had one that had been badly painted brilliant white. I stripped the case expecting to find painted over damage or repairs but once the grot had been removed the case was fine and cleaned up well. so maybe it was given a makeover because it looked mucky?
A nice find !

Rich.

[John G8MWF]

Originally Posted by crackle

Hope that helps.

That certainly does help. The pictures were useful in pointing out the crimped tags onto the cap - something I had not seen done before.

My decision now is - to tag on parts until the set is electrically sound and then consider stuffing the cap or not.
The can of this Cap seems to be an odd size so if I damage it then finding a replacement could be very difficult and time consuming.

There may be an issue with the current efficiency of the metal rectifier and this may well have to be replaced with a 1N4007 and resistor fix.

I am going to the Audio Jumble at Tonbridge in October and I will see if I can find a donor Cap before taking on this task - it would be nice to have a plan B !


Thanks for you help.

John

[crackle]

The solder tags on the electrolytic can from the MR10 were original. I just drilled through the pins to feed the wires out to the outside to solder to the tags.
Normally on most electrolytic cans I have seen the solder tags are riveted in with aluminium rivets. These are easy to drill so you can push wires through to the solder tags.
Mike

[John G8MWF]

This is a replacement Cap that looks to be from around 1992. There is a picture of the connections in an earlier thread.

It is not what I call a normal base as the pins are round and have crimped tags on them. So for a first time Can Stuffer the job looks a little different!
I would still like to have another Can ready before I start this job.

Like most kit when someone else/others has already worked on it then the soldered connections are more like blobby welding jobs.

So once work has started on one part it will have to be completed on all the other parts that need replacing or those that may get damaged - not a five minute job and I don't want it waiting on the bench any longer than necessary to avoid damage.

[John G8MWF]

To Frank, the 470 ohm resistor WAS the clue!
And then the Eureka moment - someone wired the Adapter switch for the 4 valve on a five valve set. They probably replaced the smoothing cap that did not cure a hum problem, possibly because it was already faulty, then they tacked caps everywhere on the HT rails to try and get rid of it - and - when they noticed the HT rails was low on the front end they then dropped the value of R11 to 470 ohms to try and compensate!

So the fun begins at re-engineering this one back to the way it should be staring by replacing the metal rectifier with a 1N4007, resistor and caps.

[Nuvistor]

When spares are relatively cheap and available it's difficult to understand why someone would go to such lengths.
Anyway you are on the way.
It will be interesting to see how it performs.

Frank

[John G8MWF]

HT power sorted and there is now a healthy 250V DC on the Anode of the 1N4007 with 100 ohm resistor in series - an easy job for a change.

This project is turning into a bit of a monster as the set seems to be a mixture of several set technologies.

All the service date points to the output valve being a EL84 or a 6AM6 and so you can imagine my surprise when I found a 6BW6 instead.

A mate of mine passed me a diagram of a FB10/2 that does indeed use this arrangement. Diagram enclosed. There are no component values on this sheet.

Naturally my set does not quite follow the same idea as the FB10/2 - my set has a Cathode bypass Cap and G2 being pulled up by a 470K that is decoupled to earth by a 2uF cap. Pretty similar to the 6AM6 setup?

So someone has either adapted this set to use a 6BW6, for whatever reason, or this is a hybrid FB10FM/2 that slid out of the factory without being noticed?

As a newbie to valve technology I wonder when EL84s started to get expensive and possibly drove this situation? The work, however, does not appear to be that recent and so I think the chances of that are slim!

Resistor values are close but do not match the diagrams I have values for.

So the decision is to restore the front end values as per the manual and wing it on the output stage. I REALLY do not fancy rebuilding the output stage at this point!

Cup of tea time . . . .

[Heatercathodeshort]

The valve line up for the FB10FM given in R & TV servicing 1957/8 page 142.
12AT7 6BH6 6BH6 EABC80 and EL84.

The 6BW6 is a Brimar valve and could well have been employed in the FB10FM due to supply difficulties. It is the B9A version of the octal 6V6GT. It has a lot less gain compared to the EL84, 4.1ma/v compared to 11.3ma/v for the EL84.

The 6AM6 is an RF pentode equivalent to the EF91, 8D3, Z77 etc. It does give a fair output when used as a audio output valve but I have never seen one in a toaster.

Due to all the problems KB experienced with the 10FM I suspect that they modified the output stage to obtain more audio gain from the EL84.

I repaired a number of these when a colleague closed his long established KB dealership and passed the service work on to me.

Regards, John.

I cannot trace a KB10FM. I presumed it was an FB10FM?

[Nuvistor]

KB at one time had some tie in with STC and Brimar so they used Brimar types quite often.

470k for the output valves g2 supply? That seems more than a tad high.

Frank

[Heatercathodeshort]

Agree Frank. I would have thought 4.7K. J.

[John G8MWF]

Phew! I thought I was cracking up trying to make sense of this!

Sorry - it is a 47K not a 470K. It is not in the picture as it has been tagged onto the adaptor switch along with the cap. You can see a red wire on pin 8 going to them.

With regards to the output 6BW6 stage - it is messy but working - so just leave it alone?

The metal rectifier is shot and I replace it with a 1N4007 resistor and I now get 250V DC on the Anode of the 1N4007 - this seems to be the correct value from what I have seen in the manuals?

R11 (FB10FM service sheet) shows a 820 ohm 1W. My set had a 470 ohm fitted which I assumed was change to take into account the low HT so I changed it back to 820 once the HT was fixed. Is there any reason why it should remain at 470 ohms?

In my opinion this look more like an adaption or a messy repair and it is hard to tell which is which - which do you think is the more likely?

Kind regards

John

[Heatercathodeshort]

The wiring to the 6BW6 looks original to me. The caps have been replaced of course but the valve tags look untouched. J.

[John G8MWF]

Then this set is a bit of an odd ball - perhaps it was a Friday afternoon job!

[crackle]

Hi John
I have taken some more photos of my radio and have added a page for the FB10FM to my website, you may like to compare the photo of the under chassis to what you have there.
It looks to me like a few resistors have are not of the same era and have been changed on your one.
http://kbmuseum.org.uk/kb_images/fb10fm/kb_fb10fm.htm

What is the serial number on your one?

Mike

[crackle]

I wonder if the title of this thread could please be corrected.
The radio under discussion is a Kolster Brandes FB10FM.
Thanks
Mike

[John G8MWF]

Originally Posted by crackle

Hi John
I have taken some more photos of my radio and have added a page for the FB10FM to my website, you may like to compare the photo of the under chassis to what you have there.
It looks to me like a few resistors have are not of the same era and have been changed on your one.
http://kbmuseum.org.uk/kb_images/fb10fm/kb_fb10fm.htm

What is the serial number on your one?

Mike

Hello Mike,

Those pictures are gold dust - thank you. Which O/P valve does your model use?

The serial number is 07866.notably there are some newer RS type resistors and the switch seems to have been altered or unused tags used.

Nothing major but it seems to be either a hybrid of different models or repaired by different engineers through time with at least 3 different versions of the service manual!

Do you know if the can holding the chassis on the electrolytic is 2 or 3 tag?
I can probably get hold of a NOS 32+32 in the same size can size but it has 3 chassis tags.

The notable change was R11 which had a 470 ohm installed. As previously mentioned that may have been because of the low HT?

Cheers

John

[tim.norris]

Some underneath pictures of mine, Has a Brimar EL84, regards, Tim.

[crackle]

John
My one has an EL84 output valve.
The electrolytic can had 3 tags, I cant remember now if it reformed or I re-stuffed it. But I remember the auto-transformer was replaced.

The replacement was larger and would not fit in the same mounting holes, but it fitted vertically bolted to the metal strut which supports the dial. If you look you can see a screw at the top and another at the bottom of the side strut. http://kbmuseum.org.uk/kb_images/fb1...FM_chassis.jpg

Mike