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Old 11th Jul 2017, 6:13 pm   #1
crackle
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Default Restore of KB 428

I have not done many restorations lately, but decided to start on one I have been meaning to do for some time. I bought this fine 1935, KB 428 radio some time ago. It features a neon tuning indicator and "Fototune" dial, which shows one station name at a time through a window.
However it is missing its speaker and power supply shelf.
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more details can be seen here http://www.radiomuseum.org/r/kolster...28_kb_428.html

I also bought this old Alba 815, with the view to using the mains transformer to supply the HT and rectifier heater and OP valve heater. You can see the actual one here in the radio museum, http://www.radiomuseum.org/r/balcombe_815.html .
However I think I failed to realise at the time that the rest of the valves in the KB 428 were 13v heaters. So it looks like I will have to include an additional approximately 12v transformer to run the 13v heater supply.

I will need to make the complete PSU shelf for the transformer, rectifier, capacitor block and speaker transformer, and associated components.
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I will also need to find a 9 pin plug to fit into the socket on the main chassis.

Thinking about it now though, it does seem a shame to use the Alba as a donor set as the cabinet is in very good condition, even if the chassis is a little rusty.
I dont suppose there is anyone who would like to swap the Alba for a suitable mains transformer, energised speaker and OP transformer?

I have removed all the valves, being careful to lever off the top cap clips by prising a thin screwdriver under the clip and between the cap to save stress on the cap and wire in case the cap came loose.
All the 13 volt valves, 9D2, 15D1, 9D2, 2D13C, heaters test good, at around 11 ohms, and the OP valve, a KT41 in this case also tests as heater good. The tuning indicator is a Cossor 3184 and this looks a bit dark on the inside of the glass, but hopefully will still work.

The bad bit found so far (apart from the missing PSU and speaker) is the tuning dial film disc has been rubbing on the metal dial backing and scored a line all the way round it. It is still readable with a bit of imagination but will detract a bit from its looks when working.
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It looks like someone has been in the underside of the radio at some point in the past.
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Mike
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Old 11th Jul 2017, 7:44 pm   #2
Ed_Dinning
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Default Re: Restore of KB 428

Hi Mike, PM/ email me some details of what you are looking for and I'll see what can be found.
It may be possible to modify a standard 6.3v transformer to give 13v as the heater windings are normally on the outside.
I also have some energised speakers, what size are you after?

Ed
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Old 11th Jul 2017, 7:57 pm   #3
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Default Re: Restore of KB 428

That's true enough. ISTR Ed advising that you can add an extra 6V3 voltmeter supply winding to one of his Sussex valve tester transformers with just 19 turns, poked around the bobbin and pulled tight against the core. Even if the laminations in the old core are only half as good as the modern steel alloys in the Sussex laminations, that would only be about an extra 40-odd turns to add in series with an existing 6.3V heater winding, so it's not unfeasible -- assuming you can find enough room.
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Old 11th Jul 2017, 9:47 pm   #4
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Default Re: Restore of KB 428

Hi Ed
Here is a drawing showing the PSU with voltages and currents.
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The PSU shelf should look similar to this one in the KB 540.
So if you had a similar looking transformer which could be adapted that would be great.
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Ill send an email with same details.

thanks
Mike
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Old 12th Jul 2017, 4:22 am   #5
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Default Re: Restore of KB 428

Do you have a close up of the nine pin skt is a B9 also I have several transformers 20v it would be easy to unwind to the req voltage , let me know the total heater current . Mick.
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Old 12th Jul 2017, 6:19 am   #6
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Default Re: Restore of KB 428

Hi Mick

Here is a photo of the 9 pin socket, to me it looks like (and knowing KB) it is possibly American, was there ever a UX9.
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The transformer, apart from the standard 250 0 250 secondary volt @ 55mA windings, needs a 13 volt @ 0.8 amp secondary centre tapped, with 2 additional taps to give 4 volts @ 2 amps. These could be separate windings but in the original they are shown as one winding. See drawing of PSU in post #4.
In addition there is a 4 volt 2.5 amp winding for the rectifier.

thanks

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Old 12th Jul 2017, 5:31 pm   #7
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Default Re: Restore of KB 428

I am really not having an easy time with this radio, there have been some changes to the wiring, which means I will need to thoroughly check all the wiring.
Generally where changes have been made, or replacement components have been fitted the solder joints fall away when moved.
The original volume control had a loudness tap. The control has been changed in the distant past and an attempt had been made to modify a Morganite control by drilling a hole at the edge of the track and fitting a small nut and bolt through to a solder tag on the outside.
This was not working, mainly due to corrosion of the brass screw head. I have cleaned and made good with some silver paint, and now have a good connection to the tap.
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There is a trimmer potentiometer (VR2 25k) in the neon indicator circuit. I am not sure what this does or how the indicator works.
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But the pot was not working, I could not get a connection to the wiper no matter what I tried. So I have fitted a small 50k pot as a replacement.
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Old 12th Jul 2017, 5:52 pm   #8
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Default Re: Restore of KB 428

Hi Mike It looks to me that you need too start anew , Regarding the 13 volt tx is the tap to earth ? if so a 13 volt winding with hum bucking pot will do, You be lucky to find a tx with a 13 volt heater winding . Mick. PS I need to do some research on the nine pin skt

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Old 12th Jul 2017, 6:15 pm   #9
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Default Re: Restore of KB 428

Hi Mick
Yes the centre tap for the heater winding goes to chassis.
The HT transformer from the Alba has two 4 volt secondaries and could probably be adapted by adding extra turns to the 4 volt winding with the centre tap. But would the transformer saturate with the extra loading, the Alba had 5 valves the KB has 6 valves. I guess the addition of one extra RF valve wont make that much difference.

What do others think?

Mike
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Old 12th Jul 2017, 7:31 pm   #10
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Default Re: Restore of KB 428

Hello,
I think that that is a British B9 valve base. http://www.r-type.org/static/baseb9.htm
Yours, Richard
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Old 12th Jul 2017, 8:28 pm   #11
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Default Re: Restore of KB 428

Hi Richard
It looks like you are correct, I didnt know they existed. Now all I have to do is find a British B9 plug.
thanks
Mike
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Old 12th Jul 2017, 8:42 pm   #12
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Default Re: Restore of KB 428

Quote:
Originally Posted by crackle View Post
Yes the centre tap for the heater winding goes to chassis.
The HT transformer from the Alba has two 4 volt secondaries and could probably be adapted by adding extra turns to the 4 volt winding with the centre tap. But would the transformer saturate with the extra loading, the Alba had 5 valves the KB has 6 valves. I guess the addition of one extra RF valve wont make that much difference.
Drawing secondary current will not result in higher flux density so the answer is no. If you try to take more power than the transformer was designed for it will however overheat from resistive losses. If the total power requirements are roughly the same you should be OK.
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Old 13th Jul 2017, 8:19 pm   #13
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Default Re: Restore of KB 428

Hi Mike Let me have your name and address I have a b9 base from a duff valve you can have FOC regards Mick.

PS if you don't have any luck with the heater winding and want a tx let me know .
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Old 13th Jul 2017, 9:16 pm   #14
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Default Re: Restore of KB 428

Hi Mick
Thats a very kind offer, I have sent a PM.
Thanks
Mike
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Old 15th Jul 2017, 6:10 pm   #15
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Default Re: Restore of KB 428

I have been continuing to change/restuff capacitors. Many of the capacitors are contained in metal cans holding 2 or 3 capacitors. They are fairly easy to get into and dig the old contents out.
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There have been about 5 resistors which have needed changing, a couple were open circuit with no visible signs of damage, one was actually blackened where a very leaky capacitor had caused excess current, and 2 were way off spec.
Where possible I have tried to do a sympathetic electrical restore, and have used other vintage dog bone type resistors where I could find ones of correct value. Where the value no longer matched the markings I have repainted a couple.
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The wiring which went to the neon tuning indicator and the dial lamps was rubber and was flaking away so that has been replaced. There is some more but it is buried under a lot of other stuff and is going to be more difficult to extract.
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Anyway nearly there, with the main chassis anyway, just a few more jobs to do. Then I am going to take the power supply shelf out of a KB 540 so I can test this chassis.

It will then be a matter of making a new power supply shelf for this radio from scratch.

Mike

p.s.
For some strange reason KB omitted R24 V5 grid stopper on the under chassis layout, I have drawn it in on this clip from the layout drawing, and updated the service manual on my website.
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Old 15th Jul 2017, 6:37 pm   #16
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Default Re: Restore of KB 428

I like it re painting the dog bone resistors that's what I call dedication to detail, keep it coming Mick.
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Old 15th Jul 2017, 7:51 pm   #17
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Default Re: Restore of KB 428

Quote:
Originally Posted by crackle View Post
Here is a photo of the 9 pin socket, to me it looks like (and knowing KB) it is possibly American, was there ever a UX9.
Attachment 146076
If it's old and British (or American) I'd bet that 25.5mm is really one inch!
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Old 17th Jul 2017, 9:47 pm   #18
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Default Re: Restore of KB 428

There has been a bit of a lull in proceedings whilst waiting on an order for capacitors to turn up.
But I received a B9 valve base today from Mick, (Vinrads) and it fits the 9 pin socket on the chassis perfectly. Thanks Mick.

Tomorrow I will remove the power supply from my KB 540 and give the 428 chassis a bit of a warm up, fingers crossed.

Mike
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Old 18th Jul 2017, 7:46 am   #19
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Default Re: Restore of KB 428

Here is the power supply shelf from the KB 540. It is electrically identical to what I need to make for the 428, but the one I need to make will be 2 cm wider as the cabinet is slightly bigger.

It would be nice if I could find a fuse holder/voltage selector like the one in the photo, a transformer similar to the one seen with rounded corners and screen cover, 265v HT and 2 x 4v heater windings with spare room to wind on a few turns to boost one heater winding to 13v, and also a 4 pin paxolin type valve socket.
I will post an advert in the wanted section..

Thanks
Mike
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Old 18th Jul 2017, 7:57 am   #20
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Default Re: Restore of KB 428

Hi Mike just wondering if a valve socket would do for the selector ? I think you would have to fabricate the fuse holder . Mick.
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