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-   -   Kolster-Brandes FB-10 - seems pretty deaf (https://www.vintage-radio.net/forum/showthread.php?t=191325)

6SN7WGTB 20th May 2022 4:22 pm

Kolster-Brandes FB-10 - seems pretty deaf
 
Have been assisting a friend restore one of the above.

Recapped and any seriously out of spec resistors replaced. The 6AT6 (AVC/detector/af preamp) valve replaced.

Voltages close enough to what they should be, not that KB data is very reliable!

IF aligned and loud and clear on the specified 422kc/s, But I'll return to that in a moment.

Basically the set seems very deaf.

Pretty much only TalkSport on MW and R4 on LW, whereas another (modern admittedly) set will tune several stations well. Connecting a long wire adds a little signal but a lot of commensurate noise. (693 and 909 are detectable but not really listenable).

By comparison, my Vidor CN-429 and Bush DAC-90A will receive many stations and I am a mile or so away, with no material obstructions.

My thoughts are:
1. The lower IF cores seem to me to turn far too freely, are a long way out, and do little, suggesting that the screw has detached from the core. If I am right, would this be relevant?
2. AVC not responding correctly - so we will super carefully check the wavechange switch and circuit. Any other thoughts on this?
3. IFT internal capacitor faulty? Could this be a cause? Try bypassing each IFT coil with a 100pF across pins to see?
4. I can't really see what could be duff in the RF end, aside possibly a weak mixer/oscillator valve?

That's pretty much exhausted my thoughts, so all feedback most welcome!

(There is a spare set of valves on the way although 6AT6 done anyway, but not sure I hold that as a solution - unless anyone thinks different?)

ronbryan 20th May 2022 4:48 pm

Re: Kolster-Brandes FB-10 - seems pretty deaf
 
From the notes I made on the middle version of the FB10 (6BE6, 6BA6, 6AT6, 6BW6, 6X5, red lettering, hum bucking output transformer and ferrite rod aerial) that I restored, I improved the front end gain by removing a load of lossy wax around the MW coil on the ferrite rod, changed the aerial trimmer to a 4-40pF compression type (the original was the awkward 'insulated wire wound round a stub of tinned copper' type and adjusted the MW coil position and trimmer to improve the front end alignment.

Ron

6SN7WGTB 20th May 2022 5:02 pm

Re: Kolster-Brandes FB-10 - seems pretty deaf
 
I did wonder about the heavy wax dip and apparent non-adjustability therefore of the aerial coils.

This set has the 'proper' padder VCs not the wire jobbies.

6SN7WGTB 20th May 2022 6:56 pm

Re: Kolster-Brandes FB-10 - seems pretty deaf
 
Just found out it has riveted Silver Mica caps in the IFTs.

And we had a serious bout of seemingly random crackling right across the band.

Will test them but are these worth snipping out and replacing with SM caps external to the can?

sexton_mallard 20th May 2022 9:02 pm

Re: Kolster-Brandes FB-10 - seems pretty deaf
 
Quote:

Originally Posted by 6SN7WGTB (Post 1471885)
Just found out it has riveted Silver Mica caps in the IFTs.

And we had a serious bout of seemingly random crackling right across the band.

Will test them but are these worth snipping out and replacing with SM caps external to the can?

Sounds like 'Silver Mica Disease' that affect a lot of US radios. Maybe KB used US parts or a simular design in the IF cans?

Leon Crampin 20th May 2022 9:46 pm

Re: Kolster-Brandes FB-10 - seems pretty deaf
 
I have one of the last made FB10 receivers fitted with the long thin ferrite rod and 6BJ6 IF stage. It was a basket case with damaged IF transformer cores.

Restoring this set revealed a few weaknesses - the slotted tufnol plate locating the brass
IF core adjusters is not strong enough to prevent accidental movement. This was fixed by aralditing a paxolin piece over it, carrying a decent 6 BA threaded hole. Two cores were broken from the adjusting screws, probably by a previous attempt at a repair. The IF tuning fixed capacitors were all OK.

The worst fault was broken socket forks in the valve holders caused by dezincification of the brass which makes them brittle. Several in the first two stages of the set had to be changed - it's not difficult to fit replacement socket forks from a spare valve holder.

Following careful realignment, this set performs very well and is sensitive for a 4 valve superhet with a pentagrid mixer. I think the IF gain is considerable with the 6BJ6 and I'm surprised it's stable - but it is. The long ferrite rod also helps and the separate coupling coil for an external aerial connection is effective - unlike the stupid bottom-coupled arrangement used on most sets of this vintage.

I suspect when fully restored, your set will work well.

Leon.


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