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Old 24th Apr 2019, 9:24 am   #1
crackle
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Default Kolster Brandes (KB) BM20T/1

This thread is about an unusual KB BM20T/1 radio which turned up on Gumtree.
It has the same chassis and cabinet as the normal BM20 but this one has modifications to the front end to give it MW and 2 SW bands, it was obviously designed for export.
The valve line up is 7S7 triode heptode with a B8B base, in place of the normal 6A8GT. The rest of the valves are the same as the BM20, 6B8GT 6V6GT 6X5GT.
This is the first time I recall seeing a valve with a B8B base in a KB radio.
The voltage is marked as 110 and 220 and when it says 220 it means it, as even with the variac reduced to 220 volts the heater supply is 6.4v and rises to 6.9 an full mains. The HT windings are giving 250v AC so the transformer looks OK.
The output transformer has a resistance of about 340 ohms so that may be OK.

The volume and on/off switch are both US so a new one of those will be required, so will a new smoothing can as it is being very slow to respond to a gentle 10mA start up on my reformer.

Mike
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Old 24th Apr 2019, 1:02 pm   #2
crackle
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Default Re: Kolster Brandes (KB) BM20T/1

It would seem at some point the radio was painted (sprayed) white as there is indication of overspray on the inside of the cabinet.

As a bit of a digression, (not wishing to invite discussion on the subject) but when I collected this radio I had an interesting chat over a cup of coffee with the seller, and she told me a bit of history to go with this particular radio. It was was apparently once owned by Zacron, a well respected artist who designed, amongst many other things, the artwork on the covers of the Led Zepplin III album.

Mike
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Last edited by crackle; 24th Apr 2019 at 1:10 pm.
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Old 24th Apr 2019, 9:47 pm   #3
Mr Moose
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Default Re: Kolster Brandes (KB) BM20T/1

Hello,
Does not the KB ER30 and its cousins use a loctal 7R7 and the ER35 a 14R7?
Yours, Richard
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Old 24th Apr 2019, 10:43 pm   #4
crackle
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Default Re: Kolster Brandes (KB) BM20T/1

Oh yes you're correct. When I searched RM I entered 7S7, its a long time now since I looked inside my ER30.
Thanks
Mike
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Old 25th Apr 2019, 7:40 pm   #5
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Default Re: Kolster Brandes (KB) BM20T/1

I am having a bit of a problem with this restoration.
I have got the radio working but it is not going too well, it is very deaf (only one station on MW) but is reasonable when an external long aerial is connected.

I really need a circuit diagram, layout and alignment notes.

So far I have been following the BM20 circuit which seems the same for the PSU, output valve and 2nd IF.
But the RF 7S7 valve is different and the switching is different to allow for the 3 wavebands.
It is quite crowded and difficult to follow and trace the wiring around the valve and switch.

So far I have replaced the audio coupling cap, the smoothing caps and a .1uf cap in the 7S7 circuit which was bubbling and spitting after 20 mins. I also changed the 6V6 cathode resistor as it had dropped from 330 to 260 ohms, I also changed the cathode bypass electrolytic.
HT is up to 250v and the voltage on the screen of the 6V6 is 205v

Please, has anybody got the service information for this BM20T radio.

Thanks
Mike

Last edited by crackle; 25th Apr 2019 at 7:45 pm.
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Old 25th Apr 2019, 8:53 pm   #6
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Default Re: Kolster Brandes (KB) BM20T/1

Excuse me if I'm misunderstanding (not really my field) - but is it not similar to the Lusito BM20T which is mw/sw1/sw2 and uses 7S7?
https://www.radiomuseum.org/r/standa...m20tbm_20.html
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Old 25th Apr 2019, 9:40 pm   #7
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Default Re: Kolster Brandes (KB) BM20T/1

By George I think you have it, please pardon the pun.
That could be very helpful, I will compare to what I have here..

Thanks
Mike
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Old 26th Apr 2019, 10:19 am   #8
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Default Re: Kolster Brandes (KB) BM20T/1

Thanks to George (buggies) I have the Portuguese radio schematic diagram to work with and I can confirm it all looks identical to what I have here in this KB BM20T.

After replacing the wax capacitors and a few of out of spec resistors, including an open circuit 1k smoothing resistor, I had a radio which worked, but was still really deaf.

The problem turns out to be the 1st IF transformer ferrite slugs have cracked away from the brass adjusters.
I noticed this when I carefully turned the radio over whilst it was switched on, it got louder then quieter, and the problem was repeatable.
So to save having to keep the radio at 45 degrees to listen to I have removed and dismantled the 1st IF can.
Here are the bits of ferrite.
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To remove the ferrite slugs I had to unsolder the top paxolin wafer from the 4 wire supports, this freed the top slug, to get the bottom slug out I screwed it fully in from the base so that it could also be removed from the top.

The plan is to rebuild the pieces which have split and crumbled away from the brass with superglue. Then I will grind a flat surface on the brass part and the main portion of the slug prior to glueing it back together.
I guess I am going to have to look at the 2nd IF can as well.
Ill let you know how I get on.

Mike
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Old 26th Apr 2019, 11:35 am   #9
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Default Re: Kolster Brandes (KB) BM20T/1

I have got IFT1 back together and the radio is performing 400% better.
Here are the repaired ferrite slugs.
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I have emailed the 2 guys in Portugal who uploaded the schematics to see if they have the rest of the service manual pages as the component layout one and alignment one would be very useful.
At the moment, apart from MW aerial and oscillator trimmers, I am having difficulty identifying which of the other coils and trimmers is which, the layout plan would help greatly.

Mike
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Old 26th Apr 2019, 11:43 am   #10
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Default Re: Kolster Brandes (KB) BM20T/1

Lovely job on those core Mike.
I have a few sets that have suffered the same fate.
One is an early Cossor Melody Maker that had so much wax poured into the IF to lock the core that it had run down into the former and locked the ferrite in the bore.
The twiddler had forced the adjusters so that the cores were ripped off.
I have trouble getting the core concentric with the brass thread, no lathe, how did you do yours?
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Old 26th Apr 2019, 11:50 am   #11
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Default Re: Kolster Brandes (KB) BM20T/1

Hi Sam
In the end I didnt file the faces flat as the cores fitted quite snugly onto the pieces which I had glued together on the brass shaft. So they sort of self aligned themselves to what was already on the brass shaft.
When I originally tried offering up the broken off end of the core I was trying it on the wrong shaft which is why I couldn't get it to mate correctly as one shaft had a small pip in the and of the brass. But put the correct end on the right shaft and they fitted together nicely.
I have not looked at the other IFT yet but probably will later.
Mike
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Old 27th Apr 2019, 11:09 am   #12
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Default Re: Kolster Brandes (KB) BM20T/1

I am having a break from the electrics and working on the cabinet.
I have given it a good clean and tried to polish out some of the minor gouges left when whoever scraped off the white paint. It is looking reasonable.
There were a couple of cracks which I have fed superglue into to help stabilise them. There is also a chip around one of the screw holes which fix the 2 halves of the case together. I have filled the chip with epoxy filler and waiting for it to set.
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The drill bit is there to help stop the glue running down the hole, I first coated it in wax, and I used a piece of plastic from some packaging to help form a dam to stop the epoxy dripping down. This is a very slow setting epoxy, it takes over 12 hours at room temperature, but it very hard and strong when set.

The dial is made from perspex, probably because it has holes in it which the controls go through. When the paint was stripped from the cabinet I suspect some sort of stripper was used and some got on the dial causing a few areas to get slightly damaged, one area was pitted like it had melted slightly the other bits of damage had just left a cloudy look. Again there were scratches on the dial where someone had been a bit careless.
Here is the dial now, there is still a slight mark on the left where it had been damaged and a couple of areas on the far right where it is still slightly cloudy.
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Mike
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Old 27th Apr 2019, 7:49 pm   #13
Mike. Watterson
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Default Re: Kolster Brandes (KB) BM20T/1

Quote:
Originally Posted by Boater Sam View Post
I have trouble getting the core concentric with the brass thread, no lathe, how did you do yours?
I did my McMichael models (I think three!) by putting slippy plastic tape in the coil tube to centralise the core, then damp it, then superglue & assemble brass and outer.
Then when all set, disassemble and remove plastic spacer.

Indeed if you have all the bits, there is no need to regrind anything. The cores seem to just fall off the McMichael IFTs.
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Old 28th Apr 2019, 9:33 am   #14
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Default Re: Kolster Brandes (KB) BM20T/1

I had a head scratcher with the separate radio unit in an Ekco TS48 combined TV and Radio. The presets had very little effect and it turned out that all the cores in the front end RF and oscillator coils had rusted in the Paxolin formers and broken off! Once discovered it was not too difficult to repair the damage. J.
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Old 30th Apr 2019, 9:11 am   #15
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Default Re: Kolster Brandes (KB) BM20T/1

I have adapted some existing service documents and they are now available in the KB Museum download page.
http://www.kbmuseum.org.uk/kb_servic...KB_BM20T-1.pdf

Mike
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Old 4th May 2019, 3:43 pm   #16
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Default Re: Kolster Brandes (KB) BM20T/1

I have finally fitted a replacement 500k log pot and it works very nicely.
The repair to the chip on the side is finished and I am pleased with the repair even if I do say so myself.
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ID:	182447 Its there in the centre on the right of the black gasket.
The radio is reassembled and has a new home on top of my Brandeset III.
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Mike
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Old 5th May 2019, 2:42 pm   #17
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Default Re: Kolster Brandes (KB) BM20T/1

Very lovely.
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Old 5th Aug 2019, 10:13 pm   #18
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Default Re: Kolster Brandes (KB) BM20T/1

I have now got the official service document for this radio should any one else want it.
http://www.kbmuseum.org.uk/kb_servic...BM20T-1-kb.pdf

Mike
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