Replacing a linecord resistor. - UK Vintage Radio Repair and Restoration Discussion Forum

matthewhouse · 14th Mar 2007, 10:13 pm

Hello,
I may be getting an Alba C112 midget radio to repair soon, I have seen a photograph, and I have got the service information. The main concern I have is the resistance line cord, the photograph shows what looks to be normal flat twin pvc flex, so the radio may have been modified before, (or just had the old flex replaced with a normal one

). The main question I have is can a capacitive dropper be fitted in the usual way? Usually the dropper runs the heater chain and HT comes direct from the mains. Looking at the diagram it shows the dropper runs the whole radio. I used Paul's dropper calculator to get the required capacitor and surge limiting resistor values (2.03uF 100 Ohms) The result in the resistance part of the calculator came to the exact value of the line cord (680 Ohms) (This I assume proves that the values I entered were correct?) I used 100mA as the heater current but wouldn't this be more supplying the whole radio? (100mA heaters + everything else?), requiring a bigger capacitor? Or should I just stop worrying and just fit it, running it up slowly, with a voltmeter connected to the heaters to check the voltage is correct, and then adjust the capacitor(s) / resistor accordingly? Any other points to note gratefully received. Fitting a resistor is out of the question due to the tiny cabinet.

Thanks,
Matty

Ed_Dinning · 14th Mar 2007, 10:33 pm

Hi Matty, in theory you cannot use the cap dropper to supply the HT as it is DC.
In practice the HT current is much less than the heater current and it works!

I have done a few sets this way, I normally calculate for the total HT and heater current and add balancing resistors of a few 10's of Ohms in both circuits as necessary to get close to the exact figure.

Ed

matthewhouse · 16th Mar 2007, 7:16 pm

Thanks for your reply Ed, I will see how it goes when the set arrives, I am told that the flex is standard oval black PVC.

Cheers
Matty

paul dawson · 17th Mar 2007, 9:39 am

Hello Matthew.
I have an Alba C112 and use a 2.2uF Capacitor to sort the supply out. The radio runs fine and at the correct voltages.
PLEASE DO NOT USE A RESISTIVE DROPPER WITH THIS SET!!!
Firstly there is 'little to no room' to place such a component and secondly the extra heat generated would almost certainly damage that rather thin, fragile looking plastic case.
You can hide the capacitor underneath quite easily and thus maintain the original appearence of the chassis.
These little sets, when correctly restored, perform very well for their size and are a bonus to anybodys collection.
I hope this may be of help to you.
Regards, Paul Dawson.

matthewhouse · 17th Mar 2007, 1:17 pm

Thanks for the information Paul, now I know it can easily be done. No I definitely wouldn't even think of using a resistive dropper inside this tiny radio!! I should have it by next weekend. Somebody may have already done this mod as the old flex has been changed.

Many thanks,
Matty

Paul Stenning · 17th Mar 2007, 6:30 pm

Quote:

Originally Posted by paul dawson

Hello Matthew.
I have an Alba C112 and use a 2.2uF Capacitor to sort the supply out.

Just for clarification, this is a motor capacitor or similar AC rated capacitor - not an electrolytic!

paul dawson · 17th Mar 2007, 10:18 pm

Hello again Matthew.

Just to clarify, this capacitor is certainly not an electrolytic !!!
The required value (including voltage grade) is easily obtainable.
If you have problems finding one, let me know and I will sort you one out.
However, what you must remember is to use a suitable resistor in series to protect this beautiful little midget from any undue harm.
This is incase, in the unlikely event, the dropper capacitor short circuits and dies.
Let the resistor take the strain and not your valves!!!
I'm sure you knew this already...
Kind regards and please let me know how you get on...

Paul Dawson.

matthewhouse · 17th Mar 2007, 11:06 pm

Thanks for the warnings Paul and Paul, I am aware of the capacitor type and to fit an appropriate series resistor. This was well explained in Paul Stennings useful guide to capacitive droppers. I will let you know how I get on, or if I get stuck.

Many thanks for your help,
Matty

matthewhouse · 26th Mar 2007, 9:46 pm

It arrived today, the flex is PVC which replaced the line dropper, a rheostat style dropper has been fitted above chassis on a crudely cut metal plate, where the smoothing can should be, this dropper is so tall it scrapes the top of the cabinet when the chassis is fitted.

Another resistor is hanging off one of the dropper terminals. The wiring near these resistors is scorched, it is amazing that the cabinet has survived this heat. The quality of the 'repair' is poor at best, yet a fair bit of effort has been put into it. (Just no common sense) One of the smoothing caps is positioned right next to the dropper too.

I have ordered replacement caps, and a dropper cap and I'll see how it goes. I need to source a replacement speaker before I go too far tho.

Matty

14th Mar 2007, 10:13 pm	#1
matthewhouse Octode Join Date: Jul 2006 Location: Willand, Devon, UK. Posts: 1,023	Replacing a linecord resistor. Hello, I may be getting an Alba C112 midget radio to repair soon, I have seen a photograph, and I have got the service information. The main concern I have is the resistance line cord, the photograph shows what looks to be normal flat twin pvc flex, so the radio may have been modified before, (or just had the old flex replaced with a normal one ). The main question I have is can a capacitive dropper be fitted in the usual way? Usually the dropper runs the heater chain and HT comes direct from the mains. Looking at the diagram it shows the dropper runs the whole radio. I used Paul's dropper calculator to get the required capacitor and surge limiting resistor values (2.03uF 100 Ohms) The result in the resistance part of the calculator came to the exact value of the line cord (680 Ohms) (This I assume proves that the values I entered were correct?) I used 100mA as the heater current but wouldn't this be more supplying the whole radio? (100mA heaters + everything else?), requiring a bigger capacitor? Or should I just stop worrying and just fit it, running it up slowly, with a voltmeter connected to the heaters to check the voltage is correct, and then adjust the capacitor(s) / resistor accordingly? Any other points to note gratefully received. Fitting a resistor is out of the question due to the tiny cabinet. Thanks, Matty Attached Thumbnails Last edited by Darren-UK; 15th Jul 2011 at 1:54 pm.

14th Mar 2007, 10:33 pm	#2
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: Alba C112 Resistance line cord Hi Matty, in theory you cannot use the cap dropper to supply the HT as it is DC. In practice the HT current is much less than the heater current and it works! I have done a few sets this way, I normally calculate for the total HT and heater current and add balancing resistors of a few 10's of Ohms in both circuits as necessary to get close to the exact figure. Ed

16th Mar 2007, 7:16 pm	#3
matthewhouse Octode Join Date: Jul 2006 Location: Willand, Devon, UK. Posts: 1,023	Re: Alba C112 Resistance line cord Thanks for your reply Ed, I will see how it goes when the set arrives, I am told that the flex is standard oval black PVC. Cheers Matty Last edited by matthewhouse; 16th Mar 2007 at 7:16 pm. Reason: spelling

17th Mar 2007, 9:39 am	#4
paul dawson Diode Join Date: Dec 2006 Location: Guisborough Posts: 3	Re: Alba C112 Resistance line cord Hello Matthew. I have an Alba C112 and use a 2.2uF Capacitor to sort the supply out. The radio runs fine and at the correct voltages. PLEASE DO NOT USE A RESISTIVE DROPPER WITH THIS SET!!! Firstly there is 'little to no room' to place such a component and secondly the extra heat generated would almost certainly damage that rather thin, fragile looking plastic case. You can hide the capacitor underneath quite easily and thus maintain the original appearence of the chassis. These little sets, when correctly restored, perform very well for their size and are a bonus to anybodys collection. I hope this may be of help to you. Regards, Paul Dawson. __________________ Radio Ga Ga

17th Mar 2007, 1:17 pm	#5
matthewhouse Octode Join Date: Jul 2006 Location: Willand, Devon, UK. Posts: 1,023	Re: Alba C112 Resistance line cord Thanks for the information Paul, now I know it can easily be done. No I definitely wouldn't even think of using a resistive dropper inside this tiny radio!! I should have it by next weekend. Somebody may have already done this mod as the old flex has been changed. Many thanks, Matty

17th Mar 2007, 10:18 pm	#7
paul dawson Diode Join Date: Dec 2006 Location: Guisborough Posts: 3	Re: Alba C112 Resistance line cord Hello again Matthew. Just to clarify, this capacitor is certainly not an electrolytic !!! The required value (including voltage grade) is easily obtainable. If you have problems finding one, let me know and I will sort you one out. However, what you must remember is to use a suitable resistor in series to protect this beautiful little midget from any undue harm. This is incase, in the unlikely event, the dropper capacitor short circuits and dies. Let the resistor take the strain and not your valves!!! I'm sure you knew this already... Kind regards and please let me know how you get on... Paul Dawson. __________________ Radio Ga Ga Last edited by paul dawson; 17th Mar 2007 at 10:39 pm.

17th Mar 2007, 11:06 pm	#8
matthewhouse Octode Join Date: Jul 2006 Location: Willand, Devon, UK. Posts: 1,023	Re: Alba C112 Resistance line cord Thanks for the warnings Paul and Paul, I am aware of the capacitor type and to fit an appropriate series resistor. This was well explained in Paul Stennings useful guide to capacitive droppers. I will let you know how I get on, or if I get stuck. Many thanks for your help, Matty

26th Mar 2007, 9:46 pm	#9
matthewhouse Octode Join Date: Jul 2006 Location: Willand, Devon, UK. Posts: 1,023	Re: Alba C112 Resistance line cord It arrived today, the flex is PVC which replaced the line dropper, a rheostat style dropper has been fitted above chassis on a crudely cut metal plate, where the smoothing can should be, this dropper is so tall it scrapes the top of the cabinet when the chassis is fitted. Another resistor is hanging off one of the dropper terminals. The wiring near these resistors is scorched, it is amazing that the cabinet has survived this heat. The quality of the 'repair' is poor at best, yet a fair bit of effort has been put into it. (Just no common sense) One of the smoothing caps is positioned right next to the dropper too. I have ordered replacement caps, and a dropper cap and I'll see how it goes. I need to source a replacement speaker before I go too far tho. Matty