KB 333 Restoration

[boater sam]

As a little relief from the pot law problem, the plunger switch to activate the pilot lamp sound like an idea to switch a tuning eye on only when required to save them becoming dull, I'll work on that idea.
When needing dog bone resistors of a certain value, I find one a bit lower in value and file a notch in it to increase the value to what I need, if filed on the hidden side, nobody can tell when it is repainted with the new value.
Could the pot be rewound? The wider spacing will cause it to be more susceptible to breaking, a lower value resistance wire with more turns would avoid this. Can you work out the ohms per foot value?

[Terry]

... which is the bottom end of the track and the top end?

Mike, by your own reasoning, fully anticlockwise = minimum reaction = maximum grid bias. As I said, you can prove that because you will know exactly what was originally connected where.

Therefore, the anticlockwise pot connection should be connected to GB and the shorting plate to earth.

You already know that the anti-clockwise section of the pot has the maximum change in resistance per unit of rotation so the fixed resistor should be connected between the slider and the shorting plate to give the results I stated except that I made a mistake!

My calculations were still based on 270° rotation and the figures I stated were for the first 180° of rotation. I need to rejig the spreadsheet to compensate for this.

[Terry]

Let's try again!

180° travel of a 25k linear pot is about 16.5k - assuming that the rest of the track is shorted out, this will now be the maximum value.

The graph shows the laws resulting from shunting the pot with a fixed resistors of four different values from the slider to the clockwise end.

   

Apologies - I seem to have got the trace labels in the reverse order!

[Crackle]

Hi Terry, thanks for that. So the upper mauve trace (less flat at the top) is for the 8.2k resistor, that looks fairly reasonable.
If I use a 50k pot from Blore Edwards the actual effective track will be about 25K, in this case I presume the curve would looks even more logarithmic.

Thanks
Mike

[Terry]

Mike, I keep referring to 270° of rotation but it occurs to me that I'm wrong as that figure allows 30° for the switch operation of a switched pot which, of course, you don't have so 300° is the correct figure.

180° travel will correspond to 30k - to be accurate - but I suppose 25k is close enough under the circumstances!

In that case, the earlier graphs I produced will be suitable - just re-scale the horizontal axis from 270° to 180°.

As we don't know for certain exactly what law the original pot followed, I wonder if it would be a good idea to get the set working again without the extra resistor - just leave a pair of flying leads on the pot is access is awkward - and then try a variety of resistor values to see which one performs the best.

[Crackle]

Hi
Thanks, I am waiting to hear again from the chap at Blore Edwards.
Still have a lot of wiring to replace, the internal wiring is OK, its the rubber and cloth covered type. But the flexible wiring to the batteries is mostly NOT flexible any more and is cracking and crumbling. Its a shame really because apart from that the battery wires looks pretty good. At least the voltages are identified in the service information, quite often these are missing and you end up guessing.
Mike

p.s.
I have just seen KB do a spare part for the combined volume and reaction control, part no. 33015-1  only 10/6p each.
Any one got a time machine?

[Aerodyne]

An interesting restoration story, Mike. I rebuilt one of these a few years back and made it the subject of an article. I stripped the chassis totally: TRFs make this straightforward as long as you take note of what you are UN doing! The cabinet my chassis occupies looks different to yours though it is apparently the same dimensions. I did suspect that the cabinet could have been the wrong one for the chassis - a marriage, if you like - yet the cabinet looked 'commercial', not hand-built.
Tony

[Terry]

   

[Crackle]

Hi Tony
Did your one look like this one which shares the same chassis, and was the deluxe version. http://www.radiomuseum.org/r/kolsterbr_364_2.html

Mike

[Crackle]

That wasn't so bad after all, the only wires that were shedding the rubber insulation were the red and black ones. I have replaced these with rubber wires extracted from an old reel of mains cable dad bought in the 60's. The other rubber wires will live a few more years. I think they may have all been replaced a long time ago, looking at the soldering under the chassis.
Mike