UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Radio (domestic)

Notices

Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only.

Closed Thread
 
Thread Tools
Old 13th Feb 2018, 1:32 pm   #1
crackle
Nonode
 
crackle's Avatar
 
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 2,572
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.
Click image for larger version

Name:	KB 381 circuit1.jpg
Views:	105
Size:	92.9 KB
ID:	157296
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.
Click image for larger version

Name:	TCC type M cap.jpg
Views:	93
Size:	60.2 KB
ID:	157295
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
__________________
BVWS Member
The KB Museum

My collection in the Radio Museum
crackle is online now  
Old 13th Feb 2018, 1:52 pm   #2
Boater Sam
Dekatron
 
Boater Sam's Avatar
 
Join Date: Dec 2013
Location: Middlewich, Cheshire, UK. & Winter in the Philippines.
Posts: 3,545
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?
__________________
Boater Sam.
BVWS Member

Last edited by Boater Sam; 13th Feb 2018 at 1:54 pm. Reason: Added
Boater Sam is online now  
Old 14th Feb 2018, 10:11 am   #3
crackle
Nonode
 
crackle's Avatar
 
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 2,572
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.
Click image for larger version

Name:	IMG_2485-s.jpg
Views:	86
Size:	132.1 KB
ID:	157347
Click image for larger version

Name:	IMG_2489-s.jpg
Views:	90
Size:	83.2 KB
ID:	157346
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.
Click image for larger version

Name:	IMG_2487-s.jpg
Views:	114
Size:	99.8 KB
ID:	157348
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
__________________
BVWS Member
The KB Museum

My collection in the Radio Museum
crackle is online now  
Old 14th Feb 2018, 10:20 am   #4
Sideband
Dekatron
 
Sideband's Avatar
 
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 4,938
Default Re: KB 381 5 valve AC/DC superhet

Quote:
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?
__________________
'And, in the end, the love you take is equal to the love you make'
The Beatles.
Sideband is offline  
Old 14th Feb 2018, 12:01 pm   #5
crackle
Nonode
 
crackle's Avatar
 
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 2,572
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.
Click image for larger version

Name:	1 IMG_2490 s.jpg
Views:	72
Size:	103.2 KB
ID:	157353
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.
Click image for larger version

Name:	new circuit.jpg
Views:	66
Size:	49.9 KB
ID:	157352
There are also quite a lot of amendments and corrections to the original KB service manual in another KB corrections document.

Mike
__________________
BVWS Member
The KB Museum

My collection in the Radio Museum
crackle is online now  
Old 14th Feb 2018, 12:06 pm   #6
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 6,915
Default Re: KB 381 5 valve AC/DC superhet

Maybe top end coupling.

Lawrence.
ms660 is online now  
Old 14th Feb 2018, 1:55 pm   #7
crackle
Nonode
 
crackle's Avatar
 
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 2,572
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
__________________
BVWS Member
The KB Museum

My collection in the Radio Museum
crackle is online now  
Old 14th Feb 2018, 2:03 pm   #8
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 6,915
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.

Lawrence.
ms660 is online now  
Old 14th Feb 2018, 2:28 pm   #9
Sideband
Dekatron
 
Sideband's Avatar
 
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 4,938
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.
__________________
'And, in the end, the love you take is equal to the love you make'
The Beatles.
Sideband is offline  
Old 14th Feb 2018, 4:15 pm   #10
crackle
Nonode
 
crackle's Avatar
 
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 2,572
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
__________________
BVWS Member
The KB Museum

My collection in the Radio Museum
crackle is online now  
Old 14th Feb 2018, 4:24 pm   #11
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 6,915
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.

Lawrence.
ms660 is online now  
Old 14th Feb 2018, 4:56 pm   #12
crackle
Nonode
 
crackle's Avatar
 
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 2,572
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
__________________
BVWS Member
The KB Museum

My collection in the Radio Museum

Last edited by crackle; 14th Feb 2018 at 5:07 pm.
crackle is online now  
Old 14th Feb 2018, 5:29 pm   #13
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 6,915
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

Lawrence.
ms660 is online now  
Old 14th Feb 2018, 9:56 pm   #14
crackle
Nonode
 
crackle's Avatar
 
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 2,572
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
__________________
BVWS Member
The KB Museum

My collection in the Radio Museum
crackle is online now  
Old 16th Feb 2018, 7:16 am   #15
crackle
Nonode
 
crackle's Avatar
 
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 2,572
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.
Click image for larger version

Name:	IMG_2492 -s.jpg
Views:	56
Size:	102.7 KB
ID:	157485
Click image for larger version

Name:	KB 381 circuit2.png
Views:	51
Size:	58.0 KB
ID:	157487
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.
Click image for larger version

Name:	IMG_2493 -s.jpg
Views:	82
Size:	95.6 KB
ID:	157483

Mike
__________________
BVWS Member
The KB Museum

My collection in the Radio Museum

Last edited by crackle; 16th Feb 2018 at 7:28 am.
crackle is online now  
Old 17th Feb 2018, 10:54 am   #16
crackle
Nonode
 
crackle's Avatar
 
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 2,572
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.
Click image for larger version

Name:	TCC 25uF 25v Cap.jpg
Views:	55
Size:	90.8 KB
ID:	157560
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
__________________
BVWS Member
The KB Museum

My collection in the Radio Museum

Last edited by crackle; 17th Feb 2018 at 11:14 am.
crackle is online now  
Old 20th Feb 2018, 8:30 am   #17
crackle
Nonode
 
crackle's Avatar
 
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 2,572
Default Re: KB 381 5 valve AC/DC superhet

I cautiously applied some power to this radio yesterday. Monitoring the current of the heater chain and the HT voltage as I bought the mains up via a variac and lamp limiter.
That all went OK and the heater current stabilised out at 200mA on full voltage. The HT though was much higher than it should be so the lamp limiter was switched back in whilst I made a few more checks.
I replaced a few more out of spec resistors. It is a pity that KB changed from the slim "body tip spot" resistors, like this one,
Click image for larger version

Name:	IMG_2504 -s.jpg
Views:	40
Size:	78.7 KB
ID:	157819
to the more standard fatter dogbone resistors.
Click image for larger version

Name:	IMG_2507 -s.jpg
Views:	42
Size:	115.6 KB
ID:	157821
The earlier slim ones that KB used are normally wire wound for the low values and the higher values have a spiral film wound around a ceramic core. they are nearly always still within 10% of their values, as seen in this example, which is probably wirewound.
Click image for larger version

Name:	IMG_2508 -s.jpg
Views:	32
Size:	99.0 KB
ID:	157820
I have only ever had to replace ones which have gone open circuit or burnt out. But on this radio I have now changed all the fatter "newer dogbones", some were double their marked value, and some had apparently already been replaced during past servicing.
Anyway I couldnt see any more obvious physical reasons for the excessive voltage. But on measuring the anode current of the OP valve found it was only 20mA, so I started looking around the rest of my collection for a spare set of valves.
By the way at this stage the radio was working, but very quiet.
I borrowed a set of valves from a KB 405. I knew these were working because when I collected the KB 405 and arrived at the flat where the seller lived, it was sitting on the middle of the floor merrily playing away to itself. "Look it all works" the seller said proudly, "has it been restored" I said, "no", "well lets turn it off then" I said as I pulled the mains plug out of the wall socket imagining it was about to do itself some serious damage.
A replacement double diode triode valved helped a little and a different OP valve also made a small difference but its anode current was excessive so that would require circuit changes to tame it. But the volume was still very much on the quiet side.

I am beginning to wonder if there is a problem in one of the coil cans, I had this problem in the last set I restored, a KB 402, so today I will be measuring the resistance of the coils and comparing to the information in the KB service manual.

Mike
__________________
BVWS Member
The KB Museum

My collection in the Radio Museum
crackle is online now  
Old 20th Feb 2018, 10:57 am   #18
crackle
Nonode
 
crackle's Avatar
 
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 2,572
Default Re: KB 381 5 valve AC/DC superhet

I was wondering if anyone could help me identify the valve number of the valve on the right in this photo.
Click image for larger version

Name:	IMG_2510 -s.jpg
Views:	47
Size:	106.0 KB
ID:	157824
It was in the radio in V3 position used as a replacement for a 11D3. In the photo you can see it next to another 11D3. The valve on the right has what looks like a triode in the upper section and maybe a double diode in the lower section. It does work in place of the 11D3 but not very well and has a strange effect on the volume control. The loudest (but still too quiet) part of the volume control is mid way, if you turn the volume up past midway it starts to go a little quieter.

Thanks
Mike
__________________
BVWS Member
The KB Museum

My collection in the Radio Museum
crackle is online now  
Old 20th Feb 2018, 11:24 am   #19
PJL
Dekatron
 
Join Date: Sep 2005
Location: Seaford, East Sussex, UK.
Posts: 3,854
Default Re: KB 381 5 valve AC/DC superhet

The problem will be that the mystery valve is not biased correctly or is very low emissions (maybe wrong heater specification). The V3 cathode voltage is probably near zero and the incoming signal is rectified on the grid and cuts the valve off as the volume is increased.

Voltage measurements would give you the answer, heater (AC), Anode and Cathode.
PJL is offline  
Old 20th Feb 2018, 2:04 pm   #20
crackle
Nonode
 
crackle's Avatar
 
Join Date: Mar 2011
Location: Basildon, Essex, UK.
Posts: 2,572
Default Re: KB 381 5 valve AC/DC superhet

This radio is beginning to tax me.
I have measured all the coils and they all seem OK apart from the L2 coupling coil, tags C & I which is listed as .5 ohms but is measuring at about .1 ohm.
There is an error in the coil list in the service manual. L1 aerial coupling is listed as A E but in reality and on the circuit it is A D.
The speaker energizing coil is listed as 1.4k but measure about 1k and the output transformer primary is listed as 500 ohms but measures 250 ohms. I have tried another odd OP transformer and it did not improve the output at all so there may not be a fault in the OP transformer. Checking the information I have for the KB 405, which is a similar AC/DC radio, the energizing coil on that is listed at 1k and the OP transformer is 300 ohms, the KB 405 radio uses the same 7D3 OP valve, in fact all the valves are the same except the 405 has an additional 9D2 valve as what looks like a RF amplifier as that radio uses a self contained look aerial.
Anyway, back to the KB 381. As I mentioned all the voltages are a good bit higher than they should be, possibly because of the low emissions of the output valve. To try and counter the high voltage I added a 1k resistor in series with the field coil, the HT on the OP valve anode was still slightly high at 160v, it should be 140v. With this additional dropper resistor in series with the HT most voltages appear to be about right but just a little higher than listed. Except for the g1 voltage on V1. KB have this listed as .5v, I am measuring -12v. Again looking at the service information for the KB 405 they have listed the g1 voltage on the mixer/oscillator as -5v.
The V1 valve by the way is a FC13C, the rest of the valves are; 13VPA 11D3 PP3521 V30.

So why am I getting -12v on g1 of V1, and does this really matter/make any difference.

Thanks
Mike
__________________
BVWS Member
The KB Museum

My collection in the Radio Museum

Last edited by crackle; 20th Feb 2018 at 2:11 pm.
crackle is online now  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 11:19 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2018, vBulletin Solutions, Inc.
Copyright ©2002 - 2018, Paul Stenning.