Kb 321 - UK Vintage Radio Repair and Restoration Discussion Forum

crackle · 8th Jun 2012, 7:29 am

Hi
I have bought a KB 321 and have been going through the circuit slowly testing each part.
The diagnosis so far is the loudspeaker field winding is open circuit and VI screen resistor is also open circuit.
The electrolytics are all US the other capacitors seem reasonable. I have yet to test the valve heaters.
The resistors are incredibly close to the marked value and are easily within 10% remarkable for the age. Most of the components are held in spring clips so are easy to remove and test, I am cleaning the contacts as I refit them.

The 12k HF screen resistor is fairly large at approx. 30 x 10mm and would appear to be about 5 watt. The 10k 2 watt resistor is there just to help get a scale.
Am I correct in thinking that if the HT is 250v and the screen voltage is 75v then the power in this resistor is about 2.5 watt. I was hoping to fit a 3 watt ww resistor inside the hollow ceramic body of the old resistor.

I have stripped down the speaker field winding in the remote chance of finding the break external, but no chance, it must be somewhere in the centre of the mass of coils.
I don’t suppose there is any body who is going to Harpenden who has a good 8 inch speaker spare which they want to sell.

Thanks
Mike

TrevorG3VLF · 8th Jun 2012, 10:28 am

I agree with your calculation of the resistor power however, if you put it inside a sleeve, it will reduce the permissible dissipation. It might get hot and bothered.

crackle · 8th Jun 2012, 12:39 pm

Hi
I was wondering if any one could help me understand why the 2 resistors acting as a potential divider for the screen have to draw as much 14mA when the screen only needs 1.5mA.
Would it be possible to increase the proportional resistance’s of the screen resistance’s by say a factor of 2 or 3 to reduce the heat and current draw.
Thanks
Mike

Herald1360 · 8th Jun 2012, 12:47 pm

That sounds like the old rule of thumb that for a reasonably stable voltage output from a potential divider, you need a chain current around 10X the load current, especially if the load varies somewhat.

At risk of inviting trouble, you could replace the bottom resistor by a zener chain pulling about 3mA off load

crackle · 10th Jun 2012, 9:16 pm

Hi Chris
I think I will replace with same value in that case.
Thanks
Mike

8th Jun 2012, 7:29 am	#1
crackle Rest in Peace Join Date: Mar 2011 Location: Basildon, Essex, UK. Posts: 4,100	Kb 321 Hi I have bought a KB 321 and have been going through the circuit slowly testing each part. The diagnosis so far is the loudspeaker field winding is open circuit and VI screen resistor is also open circuit. The electrolytics are all US the other capacitors seem reasonable. I have yet to test the valve heaters. The resistors are incredibly close to the marked value and are easily within 10% remarkable for the age. Most of the components are held in spring clips so are easy to remove and test, I am cleaning the contacts as I refit them. The 12k HF screen resistor is fairly large at approx. 30 x 10mm and would appear to be about 5 watt. The 10k 2 watt resistor is there just to help get a scale. Am I correct in thinking that if the HT is 250v and the screen voltage is 75v then the power in this resistor is about 2.5 watt. I was hoping to fit a 3 watt ww resistor inside the hollow ceramic body of the old resistor. I have stripped down the speaker field winding in the remote chance of finding the break external, but no chance, it must be somewhere in the centre of the mass of coils. I don’t suppose there is any body who is going to Harpenden who has a good 8 inch speaker spare which they want to sell. Thanks Mike Attached Thumbnails

8th Jun 2012, 10:28 am	#2
TrevorG3VLF Rest in Peace Join Date: Apr 2011 Location: Matlock, Derbyshire, UK. Posts: 1,378	Re: Kb 321 I agree with your calculation of the resistor power however, if you put it inside a sleeve, it will reduce the permissible dissipation. It might get hot and bothered.

8th Jun 2012, 12:39 pm	#3
crackle Rest in Peace Join Date: Mar 2011 Location: Basildon, Essex, UK. Posts: 4,100	Re: Kb 321 Hi I was wondering if any one could help me understand why the 2 resistors acting as a potential divider for the screen have to draw as much 14mA when the screen only needs 1.5mA. Would it be possible to increase the proportional resistance’s of the screen resistance’s by say a factor of 2 or 3 to reduce the heat and current draw. Thanks Mike

8th Jun 2012, 12:47 pm	#4
Herald1360 Dekatron Join Date: Feb 2007 Location: Leominster, Herefordshire, UK. Posts: 16,536	Re: Kb 321 That sounds like the old rule of thumb that for a reasonably stable voltage output from a potential divider, you need a chain current around 10X the load current, especially if the load varies somewhat. At risk of inviting trouble, you could replace the bottom resistor by a zener chain pulling about 3mA off load __________________ ....__________ ....\|____\|\|__\|__\_____ .=.\| _---\__\|__\|_---_\|. .........O..Chris....O

10th Jun 2012, 9:16 pm	#5
crackle Rest in Peace Join Date: Mar 2011 Location: Basildon, Essex, UK. Posts: 4,100	Re: Kb 321 Hi Chris I think I will replace with same value in that case. Thanks Mike