EKCO RS3, discovered in gran's loft. - Page 3 - UK Vintage Radio Repair and Restoration Discussion Forum

Aerodyne · 8th Aug 2007, 10:33 am

Has anyone suggested that an imbalance in the mains transformer split filament winding supply could be the cause? Typically for the period, the output valve has no cathode. Hum will be worse when the valve is driven, amplifying a station - the M23 used a hum-cancelling variable resistor across the filament supply. The value of this isn't given in the circuitry I have. It is in the form of a flat wirewound strip with a slider to chassis and can be set to minimise hum levels. With a set such as the RS3, which uses, I recall, a cone type vibrating reed LS, the hum should be unobtrusive when the balance is set properly. If no such component is fitted to the RS3, you might consider fitting something yourself, just to see if in fact it is the transformer imbalance or some other cause that is producing the hum. A wirewound pot would be needed.
-Tony

dark_waters · 8th Aug 2007, 1:31 pm

Ok - after running the set again, i've noticed the humming is drastically less depending which way around you plug the mains lead in.....and now the the MW / LW switch which used to work now just seems to pick up MW regardless of its position......

So - anybody want to work on the set for me? I'll drop it off personally and obviously pay for your time. Its so close to working right I dont want to risk stuffing it up!

PJL · 8th Aug 2007, 4:09 pm

Not the time to give up as you are very close...

To measure leakage current you need your meter set to AC (uA). It is wise to include a series resistor of say 10K to prevent damage to the meter if you have a problem. It should be measured from chassis to earth but will be pretty meaningless with C22 in place.

The problem with the waveband switch is simply poor contacts. Plenty of switch cleaner and a small artists paint brush should do the trick. If this fails you may have to strip it out to clean properly.

The hum is an earthing problem. C22 (0.001uF) connects the chassis to one side of the mains transformer primary. When you swap the mains connections around you are either RF coupling the chassis to live or neutral. This may have been OK in the 30's but for modern domestic mains I would use a 3 wire mains cable and connect the chassis to earth and connect C22 across the mains input. C22 needs to be a Y2 rated capacitor if left to chassis and X2 if connected across the mains...you could try disconnecting C22 altogether...Peter

dark_waters · 8th Aug 2007, 5:11 pm

I have changed C22 ....its definately making a good contact...

PJL · 8th Aug 2007, 5:21 pm

Hi Rob, try removing C22 and connecting the chassis to the earth pin on the mains plug...Peter

As C20 (0.05uF) and C21 (0.05uF) connect the HT secondary to earth I would recommend these are Y2 rated capacitors as well as C22.

Mersey_Swimmer · 8th Aug 2007, 8:53 pm

Quote:

Originally Posted by PJL

C22 needs to be a Y2 rated capacitor if left to chassis and X2 if connected across the mains...you could try disconnecting C22 altogether...Peter

C22 in Rob's set is now two 2,200pF Class Ys in series, so it's a real belt-and-braces job. Reactance of 1,100pF @ 50Hz = 2.9Mohm which will explain the few volts with respect to ground when measured with a (presumably 10Mohm i/p) digital meter.

Again C20 and C21 are both "double" capacitors, in Rob's set each one is now two 100n 1kV LCR polypropylene caps wired in series - an extremely conservative rating. I didn't supply Class X as these caps are not wired directly to mains (if they were it would be a legal requirement to supply Class X or Y as appropriate), and what is effectively a 2kVDC capacitor actually exceeds the voltage rating of Class X.

Looking at the Trader service sheet an earth (rather than just an earth terminal) is shown on the circuit diagram. To me this suggests the receiver is specifically intended to be run with an earth connection.

Pete

PJL · 8th Aug 2007, 9:56 pm

Hi Pete, didn't realise you had sorted the capacitors for Rob and thought he could have made an easy mistake and used standard ones....

From what Rob is saying it appears C22 is injecting noise onto the chassis which I guess isn't that surprising looking at the circuit.

Sets of this period always seem to have odd RF by-pass arrangements and I am sure you are right in saying that it is designed to be connected to THE EARTH. I suspect two pin mains supplies in the 30's were not tied tightly to earth anyway.

For safety reasons I normally connect the chassis to domestic earth which pretty much kills the need to connect it to THE EARTH. It is possible this alone might fix the hum problem. C22 would then help with common mode noise but a Y2 capacitor from the other side of the primary to chassis would help sort out differential mode as well...probably better than my suggestion above...Peter

dark_waters · 9th Aug 2007, 5:47 pm

Right guys - I think the end is in sight......

Changed C16 as per Petes instructions.

Right switch it on - no humming - happy days

After 5 mins I get humming which leads me to believe its a faulty valve that has some sort of leakage when it warms up. The rectifier valve has been changed. So the PM24B valve has a screw on the side of it to attach a wire ....there is no wire "hanging loose" which does make me wonder!

Anyway I'm feeling a lot happier now. Plus i've replaced the missing speaker cloth (supplied by Sid Chaplin) and its looking good now

8th Aug 2007, 10:33 am	#41
Aerodyne Octode Join Date: Jun 2004 Location: Hampton Vale, Peterborough, UK. Posts: 1,698	Re: Ekco Rs3 Has anyone suggested that an imbalance in the mains transformer split filament winding supply could be the cause? Typically for the period, the output valve has no cathode. Hum will be worse when the valve is driven, amplifying a station - the M23 used a hum-cancelling variable resistor across the filament supply. The value of this isn't given in the circuitry I have. It is in the form of a flat wirewound strip with a slider to chassis and can be set to minimise hum levels. With a set such as the RS3, which uses, I recall, a cone type vibrating reed LS, the hum should be unobtrusive when the balance is set properly. If no such component is fitted to the RS3, you might consider fitting something yourself, just to see if in fact it is the transformer imbalance or some other cause that is producing the hum. A wirewound pot would be needed. -Tony Last edited by Aerodyne; 8th Aug 2007 at 10:39 am. Reason: Stated 'RS2' instead of 'RS3'

8th Aug 2007, 1:31 pm	#42
dark_waters Retired Dormant Member Join Date: Jul 2007 Location: Bedfordshire Posts: 17	Re: Ekco Rs3 Ok - after running the set again, i've noticed the humming is drastically less depending which way around you plug the mains lead in.....and now the the MW / LW switch which used to work now just seems to pick up MW regardless of its position...... So - anybody want to work on the set for me? I'll drop it off personally and obviously pay for your time. Its so close to working right I dont want to risk stuffing it up!

8th Aug 2007, 4:09 pm	#43
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Ekco Rs3 Not the time to give up as you are very close... To measure leakage current you need your meter set to AC (uA). It is wise to include a series resistor of say 10K to prevent damage to the meter if you have a problem. It should be measured from chassis to earth but will be pretty meaningless with C22 in place. The problem with the waveband switch is simply poor contacts. Plenty of switch cleaner and a small artists paint brush should do the trick. If this fails you may have to strip it out to clean properly. The hum is an earthing problem. C22 (0.001uF) connects the chassis to one side of the mains transformer primary. When you swap the mains connections around you are either RF coupling the chassis to live or neutral. This may have been OK in the 30's but for modern domestic mains I would use a 3 wire mains cable and connect the chassis to earth and connect C22 across the mains input. C22 needs to be a Y2 rated capacitor if left to chassis and X2 if connected across the mains...you could try disconnecting C22 altogether...Peter Last edited by PJL; 8th Aug 2007 at 4:32 pm.

8th Aug 2007, 5:11 pm	#44
dark_waters Retired Dormant Member Join Date: Jul 2007 Location: Bedfordshire Posts: 17	Re: Ekco Rs3 I have changed C22 ....its definately making a good contact...

8th Aug 2007, 5:21 pm	#45
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Ekco Rs3 Hi Rob, try removing C22 and connecting the chassis to the earth pin on the mains plug...Peter As C20 (0.05uF) and C21 (0.05uF) connect the HT secondary to earth I would recommend these are Y2 rated capacitors as well as C22. Last edited by PJL; 8th Aug 2007 at 5:28 pm.

8th Aug 2007, 9:56 pm	#47
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: Ekco Rs3 Hi Pete, didn't realise you had sorted the capacitors for Rob and thought he could have made an easy mistake and used standard ones.... From what Rob is saying it appears C22 is injecting noise onto the chassis which I guess isn't that surprising looking at the circuit. Sets of this period always seem to have odd RF by-pass arrangements and I am sure you are right in saying that it is designed to be connected to THE EARTH. I suspect two pin mains supplies in the 30's were not tied tightly to earth anyway. For safety reasons I normally connect the chassis to domestic earth which pretty much kills the need to connect it to THE EARTH. It is possible this alone might fix the hum problem. C22 would then help with common mode noise but a Y2 capacitor from the other side of the primary to chassis would help sort out differential mode as well...probably better than my suggestion above...Peter

9th Aug 2007, 5:47 pm	#48
dark_waters Retired Dormant Member Join Date: Jul 2007 Location: Bedfordshire Posts: 17	Re: Ekco Rs3 Right guys - I think the end is in sight...... Changed C16 as per Petes instructions. Right switch it on - no humming - happy days After 5 mins I get humming which leads me to believe its a faulty valve that has some sort of leakage when it warms up. The rectifier valve has been changed. So the PM24B valve has a screw on the side of it to attach a wire ....there is no wire "hanging loose" which does make me wonder! Anyway I'm feeling a lot happier now. Plus i've replaced the missing speaker cloth (supplied by Sid Chaplin) and its looking good now Last edited by Darren-UK; 20th Apr 2008 at 10:13 pm. Reason: To conclude thread.