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Old 13th Feb 2018, 12:32 pm   #1
crackle
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Default KB 381 5 valve AC/DC superhet

I am restoring one of these radios at the moment. (not the one seen in the Museum) https://www.radiomuseum.org/r/kolste...iversal_3.html
This one was wired up in a different way to the circuit diagram, the red mains wire went to chassis after the filter choke and the heater dropper resistor was not wired as per the diagram.
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I am in the process of returning it to how it was drawn on the circuit diagram.
There are 2 TCC type M .01uf mica caps, C4 for aerial earth to chassis and C3 across the mains, after the 2 series filter chokes.
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Both caps test OK for capacitance and show zero leakage current at 370v DC test voltage and with the AVO switched to 50uA range.

What is the feeling about returning these original capacitors to the circuit to carry on what they were intended to do.

thanks
MIke
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Old 13th Feb 2018, 12:52 pm   #2
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Default Re: KB 381 5 valve AC/DC superhet

To be in the modern idiom, most would say a Y and an X types respectively should be fitted.
But if the radio was for my collection, not to be sold on or given away, I would keep the originality with the TCC Ms. They have lasted this long and not killed anyone or blown up.
But then I don't mind if one of my radios goes wrong and I have to repair it at a later date. I also know its a non isolated chassis and treat it with respect.
Its your radio, your judgement call.
It may be an idea though to put a new extra Y type in the aerial lead to prevent any possibility of the long wire becoming live, there is no cap in that position. If the one across the mains shorts it may take out the chokes/switch, does it really need to be there at all?
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Old 14th Feb 2018, 9:11 am   #3
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Default Re: KB 381 5 valve AC/DC superhet

I wonder what "M" stands for on the capacitor, could it just be mica or could it mean these are spacial for mains.

I have replaced all the rotten rubber covered wires that were crumbling and shedding their insulation. I also removed the tuning cap in order to get at the connections for the rubber wires which went to the bang sections.
Whilst out I made some new foam washers for the mountings as the old ones had also crumbled and broken up.
I used adhesive backed foam for the new washers and stuck them to the metal washer. I then glued the metal washers to the feet of the tuning cap body. This should make refitting much easier as it is almost impossible to get fingers in to refit the washers.
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The foam washers look a bit untidy because it gives as it is cut but they wont be seen.

I am puzzled by a strange looking item which is not shown on the circuit or the layout drawings. It is a paxolin former, similar to what I have seen used for small coils or wire wound resistors. It connects between terminals D on L1 and B on L2, so in other words between the aerial and G4 of V1. I cant think why KB would go to the bother of fitting this if it was not to be used for anything.
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I have made repairs to the speaker speech coil which was coming adrift from the cone and causing rubbing when the cone moved in ans out. To get at this I had to remove the spider, which came away fairly easily. I then put short shims in between the speech coil and the magnet centre pole to ensure the coil was positioned and aligned accurately, then used PVA to glue down each of the little tabs. A C cell battery was just the right size to sit on the coil tabs whilst the glue dried. Once dry I pulled the short shims out and then used PVA to again glue the spider back in position. Once the spider was glued I inserted longer shims which would protrude through the spider whilst I tightened up the centre nut. When done the cone moved in and out silently.

The radio chassis has had repairs done in the past, the volume on/off has been replaced and various resistors have been replaced with the banded ceramic Erie types.

Also C6 appears to be missing, it looks like it may never have been fitted, it was only rated at 9pF and was just a pair of twisted enamelled wires.

I Have another of these sets I have had for some while and not done anything with, I am going to check under the chassis to see if things are the same.

Next to do are the metal can multi capacitors, waxies, and the card covered electrolytic block.

Mike
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Old 14th Feb 2018, 9:20 am   #4
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Default Re: KB 381 5 valve AC/DC superhet

Quote:
Originally Posted by crackle View Post
I am puzzled by a strange looking item which is not shown on the circuit or the layout drawings. It is a paxolin former, similar to what I have seen used for small coils or wire wound resistors. It connects between terminals D on L1 and B on L2, so in other words between the aerial and G4 of V1. I cant think why KB would go to the bother of fitting this if it was not to be used for anything.
Hi Mike,

As a complete guess, perhaps they are test points for easy access to G4 and the aerial circuit whilst aligning?
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Old 14th Feb 2018, 11:01 am   #5
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Default Re: KB 381 5 valve AC/DC superhet

Hi Rich
That is an good suggestion but I dont think it is for that, I have read more of the service manual now, (23 pages) and that paxolin former type device is not mentioned.
I have taken a look in the other KB 381 radio I have and the paxolin former is not there.
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But, there is a stiff tinned copper wire in a brown sticky sleeve connected to L1 terminal D, it bends round towards the tuning capacitor body and is fitted in a small clamp, the other end of the wire is not terminated, just floating.
In addition the wire from L2 terminal B which goes to the centre section of the 3 gang tuning cap, is excessively longer and loops through the sticky wire from L1 D.
I am not sure what the purpose is for, any ideas are welcome.

It helps when you read the instructions, at the end of the service manual on page 23 is another circuit diagram and it shows the different way the dial lamps are wired to avoid excessive over voltage on them when the heaters are warming up causing premature bulb failure.
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There are also quite a lot of amendments and corrections to the original KB service manual in another KB corrections document.

Mike
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Old 14th Feb 2018, 11:06 am   #6
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Default Re: KB 381 5 valve AC/DC superhet

Maybe top end coupling.

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Old 14th Feb 2018, 12:55 pm   #7
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Default Re: KB 381 5 valve AC/DC superhet

Please can you explain "top end coupling", I have never come across that term.
thanks
Mike
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Old 14th Feb 2018, 1:03 pm   #8
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Default Re: KB 381 5 valve AC/DC superhet

Coupling between the top end of the coils for the top end (HF end) of the band, quite common in twin gang band pass circuits.

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Old 14th Feb 2018, 1:28 pm   #9
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Default Re: KB 381 5 valve AC/DC superhet

That seems to be a likely explanation. Also called 'loose' coupling. I've not come across it in ordinary AM radio's but in SW sets fitted with a BFO, the output of the BFO can be loosely coupled into the last IF stage by using just a piece of wire placed near, but not connected to, the tags on the IF transformer.
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Old 14th Feb 2018, 3:15 pm   #10
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Default Re: KB 381 5 valve AC/DC superhet

Thanks, funny it does not seem to be mentioned in the service pages, maybe I missed it. I think I will give the Trader pages a read see if they mention it.
To be honest as I didnt know about this I could easily have cut it out thinking it was not needed.

Mike
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Old 14th Feb 2018, 3:24 pm   #11
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Default Re: KB 381 5 valve AC/DC superhet

As far as I remember, low value capacitance top coupling was part of the mix to obtain as near as possible an even bandwidth across the tuning range.

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Old 14th Feb 2018, 3:56 pm   #12
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Default Re: KB 381 5 valve AC/DC superhet

So I take it that the positioning of the wires involved in this are fairly critical and should not be disturbed.
The Trader service sheet no. 533 does a good job of explaining the circuit operation but still no mention of the "top end coupling".

My other 381 radio which has been sitting on display for a few years now has a couple of valves missing and the mains safety isolator is missing, probably it is still attached to the card back which is also missing. But it does have the card which slots into the bottom of the cabinet to stop fingers being pushed inside the slots left for trimming the RF.
So it looks like this latest radio I am working on at the moment will end up as the "number one" when finished.
KB went to a fair bit of trouble to make this AC/DC set as safe as they thought possible.

Mike
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Old 14th Feb 2018, 4:29 pm   #13
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Default Re: KB 381 5 valve AC/DC superhet

There's an article on mag page 2 in the link below about band pass coupling if that's any help with the basics (I realize that on yours it's from the top of the antenna coil):

http://www.americanradiohistory.com/...ld-1932-01.pdf

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Old 14th Feb 2018, 8:56 pm   #14
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Default Re: KB 381 5 valve AC/DC superhet

Hi Lawrence
Thanks for finding and posting that link, I will give it a read.

Mike
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Old 16th Feb 2018, 6:16 am   #15
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Default Re: KB 381 5 valve AC/DC superhet

I think I have found the reason for the missing C6, the twisted wire 9pF capacitor. There is an additional coil connected in series with the connection from L3 terminal H to T5 trimmer. I assume this additional coil in series would have an equivalent effect as the extra capacitance in parallel.
This modification is not mentioned in any of the documents I have.
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Below are the new contents of the multi capacitor block. The block contains 2 x 8uF and 2 x 4uF capacitors, 2 x 4.7uF caps were used for each 8uF section.
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Mike
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Old Yesterday, 9:54 am   #16
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Default Re: KB 381 5 valve AC/DC superhet

I have scanned and made an image of the label from a TCC type AW 25uF 25v electrolytic capacitor,
This is the original label after cleaning up a little.
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The lettering has been tidied up a little but the image still retains its "vintage patina".
Being a PDF file the image is sized correctly at 113mm high and 50mm wide so it should print out the right size. The label wraps twice around the capacitor.
Here it is if anybody wants to use it.
http://www.kbmuseum.org.uk/documents..._cap_label.pdf

Mike
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