KB 402

[Crackle]

Good idea Terry but the scale is illuminated from behind, it may be possible to fix some lead to the very rim of the scale.

MIke

[Crackle]

I have taken the speaker out of the cabinet to give it a good check over. As I mentioned the energising coil is actually OK which is quite a bonus for a radio of this age.
However the glue on the diaphragm holding the cone joint together has dried and parted and the central spider is only hanging on by about 20%.
       
There was also an awful lot of rubbing when the diaphragm was moved in and out, from all angles of pressure. So I have removed the energising coil assembly to get at the electromagnet poles, which turned out to be very rusty.
   
The worst of the rust was scraped away with a screwdriver then a light rub over with sandpaper. After that I brushed some Krust rust treatment on. I dont plan to do anything else as painting to help preserve from more rust will cause problems if the paint flakes off in future.

There are signs of green spot on the speech coil so I plan to give this a light coat of clear varnish.
   
From the outside the speaker cloth on this radio looks the same as many of my other mid 30's KB's, but on this radio with the speaker baffle removed you can see the original colours of the cloth. I have not seen this before and it must have looked very nice, with the green silk threads giving an overall effect of a shimmering green front with maroon vertical flecks. It must have looked very Art Deco. Has anybody got any original cloth spare.  
   
The rather dull appearance from the outside.
   

Mike

[Crackle]

The radio is working, there are a few things to do yet, the volume control does nothing, it is off or on at full volume. I suspect the track is open at one end, I need to take it out and dismantle to check it. It is odd having the volume control in the aerial circuit, I have seen it in other KB radios.
Although the radio is working, the HT is only 95v so I suspect the rectifier is a bit down, the HT should be 195v. I have to do a load more checks, but I am having a break for the rest of the evening.
The speaker is reassembled and fitted beck in the cabinet.
Here are some photos of the re-working of the last 2 capacitors.
I used a 22uF and a 4.7uF cap together to make about 27uF.
   
   
The capacitor case was cut in half by rolling it under a Stanley knife.
A washer was made out of wire and soldered so it stayed flat, it had a long end which threads through a hole I drilled in the aluminium capacitor base to connect to the negative leads.
   
The case was glued together with superglue and as only one capacitor still had a label that one was fitted in the prominent position.
   
And here they are fitted back in the radio.
   
Mike

[Crackle]

The volume control was wired incorrectly. I have corrected it and it now works and gives a nice smooth adjustment from off to near full, but then the last little bit suddenly doubles the volume. I may still need to check out the resistor.
This volume control measures just over 15k it should be 20K so that may help account for the bad behaviour at the full volume end of the track.
Here are some notes on the volume control and its reason for being in the aerial circuit.
   

Mike

[Pat Pending]

Interesting idea, I suppose it makes the set more user friendly regards its use by the layman owner, it just looks unusual to me because I'm used to more "modern" receivers with what became standard circuitry.
Incidentally, I've been following a thread on another discussion forum about superhet sets with regenerative IF where some claim the idea pointless, it may have become pointless with reduced costs and improvements in valve performance, but was very valid at the time.

Would it be possible to rip off your diagram to illustrate a commercial design of the topology? (Credits would be given).
Andy

[Crackle]

Hi Andy
No problem, please use it, where is the thread, I would be interested to read it.
I am going to remove the volume control to see if it can be improved.
cheers
Mike

[Pat Pending]

Thanks Mike. When I've posted my two cents worth, I'll post a link here. Maybe the old graphite pencil trick would bring the control back to life if its an OC track where the wiper runs?

Andy.

[Crackle]

Oops, I made a mistake measuring the HT I was measuring from the feed to the energising coil to chassis, which is in effect the volt drop across the coil.
The HT is actually a Healthy 205 volts.
The on/off volume control pot that is fitted is an RS 10k one, so it is not correct. I have improved the smoothness of the top end of the volume a little, but I wonder if I can get hold of a vintage 20k switched pot. Do you think linear would be best in this situation. I think I will put a request in the wanted section.
Maybe another reason for putting the volume control on the aerial circuit would be that low ohms value pots were easier to make?
Still having problems with the tuning capacitor slow motion drive tending to slip. It could really do with a new outer bearing housing being made.

Mike

[boater sam]

Tuning cap. How about a weak coiled spring ( like a watch spring) around the shaft to counterbalance the weight?
Any idea what the original pot was like? It could of been the same as any other pot in the set, if there is another!
I have lots of old pots, if you are stuck PM me towards the end of March and I can search. In the mean time a picture of what you have or what you need would help.

[Crackle]

Here is a photo of the pot, the shaft needs to be about 30mm long from the face of the pot mounting flange.
I dont mind what it looks like to much as long as it is not one of those horrid looking square plastic ones.
   
Mike