KB 540, Making a new Power shelf

[crackle]

This is my KB 540, it was a total wreck when I got it.

I restored the cabinet and chassis 2 years ago during the summer.

The restoration write up is in the Vrat archive. http://www.forum.radios-tv.co.uk/vie...hp?f=3&t=11360

But following using the power supply shelf and speaker from the 540 for the repair of my 428, I now need to make a new shelf and find a working speaker.
A transformer arrived today from Ed Dinning, with an extra 4v overwind. The transformer already had a 4v and a 13v winding with the 300 0 300 HT winding. Other parts included some 4 pin valve holders, a screw type voltage adjustment panel, and some coloured wire.
I have dismantled a spare speaker that Unrealdave gave me.
The speech coil winding was a bit loose so I have stabilised that onto the former with superglue.
I have been unwinding the energising coil, to find the break/s.
There were 2 on the first layer of turns (the last layer when unwinding).

Now I have to reverse the bobbins and wind the wire back on. Then put the speaker back together again.

Me with my £10 coil winder.
https://youtu.be/6rBDe14HwpA

Mike

[crackle]

A few days back I started on preparing the chassis to take the transformer and valve holder.

Now here is a strange thing, according to the service manual the power shelf out of the 540 does not have a fused mains adapter, it has a simple wander plug. The one I took out of the 540 had a fused voltage selector just like the one shown in the 428 service manual. I wonder if someone in the past put a 428 power shelf into my old 540.
Anyway it makes it slightly easier for me to make the mains voltage selector.
I have ordered some paxolin sheet, and plan to make the selector sockets using parts from a 4 pin valve socket sandwiched between paxolin sheets.

The 540 schematic does not show a speaker with hum bucking coil, which is OK because the speaker I just repaired did not have one. The speaker mounting holes fit nicely over the 4 mounting bolts fitted to the 540’s baffle, so that’s good news.



I have finished cutting holes in the metal chassis and have painted it to help protect it from rusting, I am just waiting for the paint to fully harden off now.


I ended up painting the metal chassis 3 times.
The first occasion the paint did not adhere to the metal even though it had been sanded and de-waxed. The second time I abraded with larger grit size, it had dried to fairly hard and after 4 hours I placed it on the window sill to continue to harden over night. When I came down in the morning it was evident that a cat had been sitting on it by the paw prints in the paint.
Another light sand and another coat, now its even thicker and taking longer to harden fully.

Mike

[crackle]

The paxolin sheet arrived today, so I have been making the voltage selector.
To make the sockets I cut up a spare 4 pin valve socket leaving a tiny amount of paxolin around each pin.
I guess these photos will be self explanatory, but it basically consists of two sheets of paxolin predrilled and used to sandwich the contacts. I put extra pieces of paxolin in where the mounting holes were so that when bolted to the chassis there was something solid there. The whole lot was glued together with super glue to hold it prior to bolting to the underside of the chassis.



The wander plug will be connected to a wire emerging from the upper common socket hole.

So the speaker is repaired and should fit and work nicely, I have the voltage selector, and the 4 pin valve socket, I was kindly given a 9 pin plug by Mick (Vinrads), I have the mains transformer with all the secondary windings and I even managed to poke a couple more wires between the laminations and the windings to connect up to 3 of the voltage taps.

Just waiting on a tag strip to be delivered, and the paint to harden, then it is assembly time.

Mike

[crackle]

The new power supply shelf is finished, it worked fine first time but there were a few issues along the way and I cant say it all went as planned.

First problem was painting the chassis, whatever paint I used on the steel chassis would not seem to dry. I used 3 types of spray paint, even baked it in the oven for 12 hours at 50 deg.
Then I tried some zinc paint, designed to go onto bare steel. That was still soft after 12 hours so that was baked for a further 4 in the oven.
I must be doing something wrong.

Anyway I decided to go ahead with the assembly.
It went fairly well and on testing the power supply it worked first time.
However the dial bulbs would not work, there was no supply. After scratching my head for a while I realised that this radio uses the centre tapped 13 volt heater supply for the 6v dial bulbs, and on the new transformer there was no centre tap.

I rewired the dial bulbs and their switches onto the 4v heater supply for the output valve. They are just bright enough and should last for ever.

Next it was time to try the completed PSU and speaker in the cabinet, something I should have done a long time ago as I found the dummy capacitor block was fouling the back of the speaker which was about ½ inch deeper magnet than the original.
A slight alteration was made to the layout and the new shelf slid into position nicely.

My son made an arbor for the new dial on his 3D printer. I designed it so that the OHP film was sandwiched between a front bush which slid into the backing disc on the arbour. I drilled and tapped the plastic for two 4 BA screws. After a little reaming of the hole which fits over the hollow tuning drive it all worked nicely and although the new laser printed dial is not as solid black as the original dial it is certainly better now it does not have the nasty scratches all round the station names.

Here are some photos.
new dial arbor prior to painting silver colour

the new dial and arbor fitted after painting the arbor

the new shelf and an original shelf.

the new shelf fitted into the KB 540

the new dial,. the station names (Lyons) are actually in focus on the frosted window of the dial celluloid, this photo does not do them justice.