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Re: KB 286 "Kolstar", 1932 superhet radio

PostPosted: Sun Apr 24, 2016 4:48 pm
by crackle
Here is a photo of the damaged pot. the section which adjusts the aerial is 15k, and the section which adjusts the IF gain is 500k.
15k + 500k pot.jpg


I have also attached the circuit, showing where the ganged potentiometers are.
286 cir smaller.png

I would appreciate any advice as to whether the replacement dual pot I have put together is the best choice of values for the application.
22k + 470k pot.jpg


Thanks
Mike

Re: KB 286 "Kolstar", 1932 superhet radio

PostPosted: Sun Apr 24, 2016 5:28 pm
by Cathovisor
That pot should work fine, Mike - the pot's not carrying any current so it'll be okay. Might be worth making sure the body's earthed with a bit of braid to chassis to screen the two halves from each other. I've not seen that particular arrangement before: I've seen pots used to attenuate the aerial in that matter before, often combined with altering the screen grid potential of the first valve (the wiper being earthed) but I've not seen one used in conjunction what what, to all intents and purposes, is an I.F. 'volume control'!!

Re: KB 286 "Kolstar", 1932 superhet radio

PostPosted: Sun Apr 24, 2016 6:11 pm
by crackle
The new dual pot does seem to work OK, there is a good adjustment of volume.
But to my ears it does seem a little bit harsher and slightly distorted compared to the old pot.

Could there a reason for that?

Would a 470k load on the IF transformer cause that to happen more than the 2.5M load, which is about what the faulty pot was.
Or is my ears, or the program content..

Mike

Re: KB 286 "Kolstar", 1932 superhet radio

PostPosted: Sun Apr 24, 2016 6:29 pm
by Cathovisor
In theory, if the detector following the IF pot offered an infinite impedance, then you would be damping the IFT more, which in turn would open out the bandwidth of the system and thus allow an increased amount of higher-frequency audio - assuming all the preceding transformers are of greater bandwidth than this last one. There are however more assumptions there than are good for us.

I assume you've done a line-up?

My instinct says it's more likely the programme material you're listening to. I find I can't listen to any "Gold"-type stations for more than a few seconds because the sheer amount of processing applied to the audio renders it unlistenable to my ears. Now, if only R3 was allowed back on medium wave.... :aaf

Re: KB 286 "Kolstar", 1932 superhet radio

PostPosted: Sun Apr 24, 2016 6:37 pm
by crackle
Hi Mike
No I have not done any alignment, would this set, with a 114kHz IF, be the same sort of procedure as a 50's superhet radio, just peak the IF trimmer caps?

cheers
Mike

Re: KB 286 "Kolstar", 1932 superhet radio

PostPosted: Sun Apr 24, 2016 8:09 pm
by Cathovisor
Given its vintage, yes. It's unlikely to have ventured into the territory of over-coupled transformers and the like requiring damping when adjusting; it'll just be "tune for maximum smoke" at this point.

Re: KB 286 "Kolstar", 1932 superhet radio

PostPosted: Wed Apr 27, 2016 12:03 pm
by crackle
I ordered some "Blackfriar Wood Dye" and it has just arrived. I can confirm it is very similar to the old fashioned Colron wood dye, and it can be thinned with white spirit so it should be compatible to be added to a traditional varnish and applied as a lacquer coat. Available through Amazon, and much better value than the water soluble dyes.
Mike

Re: KB 286 "Kolstar", 1932 superhet radio

PostPosted: Wed May 04, 2016 9:28 am
by crackle
I have been attending to the cabinet over the last couple of weeks. The wood mouldings around the corners of the front and sides of the top, along with the side panels were rather chipped, and as the base wood was a light colour they were too obvious for my liking and impossible to disguise with touchup pens.
So the decision was taken to scrape back to the bare wood.
However I decided to give the decorative veneered curved top and front panels just a light sanding with various grades up to 1000 grit, and then they would have a couple of coats of very thin lacquer so as to retain the original colour.
The sides and mouldings took some time to strip back enough so as the wood stain would take and absorb into the pale wood. The mouldings, particularly the curved one around the front of the top looked like they were made from beach wood, so you can imagine how pale they were.
After going over with about 4 or 5 coats of various shades of wood dye (jacobean oak, indian rosewood, and chestnut) the sides and mouldings were looking about the right shade and colour.
The original finish on the sides was an almost opaque shade of pale beigey-brown. I mixed up some varnish with lots of Jacobean wood dye and small amounts of black, brown and green paint. I find spray can lids very useful for these small quantities of mixing.
The sides and mouldings are now finished and the front and top also. But today I decided to give the top and front one last rub down and fresh coat of thinned varnish/lacquer.
I hope to take some photos in the next couple of days.

In the meantime I need to work out how I am going to fix the speaker I have found, to replace the Rola one which was screwed onto the front panel, onto the original speaker shelf where it should be next to the smoothing cap, and just pushed up to the baffle so it is resting against the front panel instead of screwed to it.
Mike

Re: KB 286 "Kolstar", 1932 superhet radio

PostPosted: Tue Jun 28, 2016 11:59 am
by crackle
It seems to have been a long time since my last post on this restoration. I have been busy making improvements to my KB collection storage and display area.
Although there is still work to be done with wiring and more insulation and finally decoration. It is now damp proof and ready for the winter.

Today I finished putting the KB "Kolstar" back together and thought you might like to see some photos.
I had a suitable speaker which was from a KB radio (a little later model) and had to adapt it so it would bolt onto the speaker shelf and rest against the back of the front panel. Ti do this I had to cut away part of the existing welded on bracket and reshape it to make new flanges to mount it on the wood shelf so the speaker was vertical.

The speaker cloth I have fitted is temporary, it is a bronze colour and the original was a gold/brass colour, I have ordered a new gold cloth specially made to match the original, by Corrien in Holland, but the waiting time is 12 months. :ccb . It will look good when it has the bright gold cloth.

Did I say that the radio works very well, but there is a little distortion on some stations, mainly the pop ones, I have had this on other radios of this age. I think it has something to do with the low IF frequency giving a poor bandwidth not suitable for the high modulation which some stations use now days.


Mike

Re: KB 286 "Kolstar", 1932 superhet radio

PostPosted: Tue Jun 28, 2016 12:24 pm
by Michael Watterson
Totally fantastic!

Re: KB 286 "Kolstar", 1932 superhet radio

PostPosted: Tue Jun 28, 2016 5:52 pm
by peterscott
Great job! Looks beautiful.

Peter

Re: KB 286 "Kolstar", 1932 superhet radio

PostPosted: Tue Jun 28, 2016 6:51 pm
by sideband
Crackle wrote:Did I say that the radio works very well, but there is a little distortion on some stations, mainly the pop ones, I have had this on other radios of this age. I think it has something to do with the low IF frequency giving a poor bandwidth not suitable for the high modulation which some stations use now days. Mike



More likely it's due to detector overload from the excessive compression used. My Philips 470A has a low IF (120Khz if I remember). That doesn't distort on any stations including Gold (uses a PEN4DD). However the Superinductance (TRF) set does. That uses an anode bend detector (994V) and it just doesn't like the high compression.

Re: KB 286 "Kolstar", 1932 superhet radio

PostPosted: Thu Jun 30, 2016 10:01 am
by Briancuff
That's something special. A really fantastic job.
Have you restored the SG valves and labels or are they nos?

Re: KB 286 "Kolstar", 1932 superhet radio

PostPosted: Thu Jun 30, 2016 10:45 am
by crackle
Thanks for your kind comments guys.
Brian, the 3 valves on the left were all suffering from no screen continuity. So I repaired the connections with silver loaded paint, which stood out like a soar thumb against the filthy dirty metal screening. When I peeled of the labels I found the screening material was a dark grey, so I decided to paint the valves. I mixed up a metallic colour paint with little bits of other colours to match, but when it dried it went a light grey/silver colour. The labels are the original Cossor ones. But I am happy with the result.
I think I did a post on this, ah yes here it is, viewtopic.php?f=3&t=12986#p133485

Mike

Re: KB 286 "Kolstar", 1932 superhet radio

PostPosted: Thu Jun 30, 2016 5:07 pm
by Katie Bush
I'm wondering now, if you could use an aluminium loaded exhaust system paint - like the stuff my dad used to use many years ago.

Perhaps even better "VHT" (Very High Temperature) paint, also intended for exhaust systems and manifolds, but importantly, is intended for high temperature applications, spray on and dries to a respectable matte finish.

VHT being the brand name as well as describing the purpose of the paint.

Marion