How to replace a metal rectifier on a pre-war radio? - UK Vintage Radio Repair and Restoration Discussion Forum

DoctorWho · 25th Aug 2007, 5:29 pm

I have a few pre-war radios with metal rectifiers, some of which are not performing as well as they should.

I know that they are generally replaced with a 1N4007 diode, but what value resistor do you use with it generally, and does it go before or after the diode? A general value would be fine.

Thank you.

Peter.

Dr-Watts · 25th Aug 2007, 6:37 pm

100 Ohms

ppppenguin · 25th Aug 2007, 7:01 pm

It's in series so it doesn't matter which side of the rectifier. Some might say 150R or even 220R, especially for a small set with not much HT current. Use a 2 watt modern film resistor or a small wirewound.

Neil Purling · 25th Aug 2007, 7:18 pm

Well, I used 100R. I think it is rated 10W, in a 3 valve kit radio. Maybe the resistance and power rating are off a little. I also put a capacitor in parallel with the diode.
The resistor has to limit the switch-on surge yet not have any adverse effect on the HT voltage. 50-250 ohms I was told. As for the parallel capacitor? It is there to clean up any hash the diode could produce. Value between 0.01 and 0.1uf, X2 rated of course.

Station X · 25th Aug 2007, 7:51 pm

Why do you need to reduce switch on surge? I can understand that this might be a problem when replacing a valve rectifier where the current increases slowly as the heater warms up, but don't metal rectifiers and silicon diodes/rectifiers both conduct the full current instantly? This will rush into the smoothing capacitors, but the valves will need to warm up before they start pulling HT current.

I think that silicon diodes have a lower forward volt drop than metal rectifiers, so the HT voltage wiil be higher unless a dropper resistor is used.

Ed_Dinning · 25th Aug 2007, 8:44 pm

Hi Gents, the easly rectifiers were usually copper oxide. These slowly went higher resistance and did not die with the release of toxic gasses like selenium rectifiers.
They did have an higher forward resistance than modern diodes, hence the series resistor.
The parallel capacitor (ideally a C/R combination) is there to reduce the effects of incoming mains spikes, there will be no switching hash from a "slow" 1N4004 diode on 50Hz supplies. An X cap is not the ideal component here. These are usually rated at 250V AC. This cap will "see" the full Peak mains voltage (when the supply swings negative), plus the peak rectified DC on the smoothing caps. A 1000V or 1500v component would be best here, with a value of between 1000pF and 10nF max. Too high a value may increase hum levels.
Having said the above, an X cap will probably work perfectly well and has the advantage of dying quietly when they do expire.

Ed

Mersey_Swimmer · 25th Aug 2007, 9:13 pm

Quote:

Originally Posted by Ed_Dinning

An X cap is not the ideal component here. These are usually rated at 250V AC. This cap will "see" the full Peak mains voltage (when the supply swings negative), plus the peak rectified DC on the smoothing caps. A 1000V or 1500v component would be best here, with a value of between 1000pF and 10nF max.

Ed

But a "Y" ceramic part should be OK. I'm sure these are rated at something like 3kV DC.

Pete

paulsherwin · 25th Aug 2007, 9:32 pm

Strictly speaking you don't need a surge limiting resistor if the radio has been fitted with modern caps. The idea is to mimic the electrical characteristics of the metal rectifier being replaced, which as others have said would have a significant forward resistance even when new. A resistor makes life easier for the other components. The value of the resistor certainly isn't critical. Personally I think it's not worth losing sleep over. I fit something like a 100-220 ohm resistor, whatever I have in the spares box, and check the HT voltage to make sure it sin't too far out (it usually isn't).

Paul

PJL · 26th Aug 2007, 1:01 am

I usually choose a resistor that sets the HT to the trader sheet value.

I think the resistor is a good idea - there is otherwise the potential of a massive surge current when the mains switch is turned on and I suspect this is not good for the rectifier, reservoir capacitor or the mains switch and is likely to generate plenty of EMI.

Ed_Dinning · 26th Aug 2007, 9:50 am

Hi Gents, agreed with Mesery Swimmer that the Y cap is probably the best, and generally available in the correct sort of values.

Ed

DoctorWho · 26th Aug 2007, 11:19 am

Thanks guys for the advice, I'll experiment with something in the region of 100-220 ohms to get the advised HT voltage, where available.

All of my sets have all had their condensers replaced (with the exception of mica ones) so hopefully they should all be alright.

Peter.

25th Aug 2007, 5:29 pm	#1
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	How to replace a metal rectifier on a pre-war radio? I have a few pre-war radios with metal rectifiers, some of which are not performing as well as they should. I know that they are generally replaced with a 1N4007 diode, but what value resistor do you use with it generally, and does it go before or after the diode? A general value would be fine. Thank you. Peter.

25th Aug 2007, 6:37 pm	#2
Dr-Watts Heptode Join Date: Dec 2006 Location: Milton Keynes, Buckinghamshire, UK. Posts: 696	Re: How to replace a metal rectifier on a pre-war radio? 100 Ohms __________________ Do you have the scope to be a TV repair man

25th Aug 2007, 7:01 pm	#3
ppppenguin Retired Dormant Member Join Date: Dec 2003 Location: North London, UK. Posts: 6,168	Re: How to replace a metal rectifier on a pre-war radio? It's in series so it doesn't matter which side of the rectifier. Some might say 150R or even 220R, especially for a small set with not much HT current. Use a 2 watt modern film resistor or a small wirewound.

25th Aug 2007, 7:18 pm	#4
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,085	Re: How to replace a metal rectifier on a pre-war radio? Well, I used 100R. I think it is rated 10W, in a 3 valve kit radio. Maybe the resistance and power rating are off a little. I also put a capacitor in parallel with the diode. The resistor has to limit the switch-on surge yet not have any adverse effect on the HT voltage. 50-250 ohms I was told. As for the parallel capacitor? It is there to clean up any hash the diode could produce. Value between 0.01 and 0.1uf, X2 rated of course.

25th Aug 2007, 7:51 pm	#5
Station X Moderator Join Date: Jan 2003 Location: Ipswich, Suffolk, IP4, UK. Posts: 21,287	Re: How to replace a metal rectifier on a pre-war radio? Why do you need to reduce switch on surge? I can understand that this might be a problem when replacing a valve rectifier where the current increases slowly as the heater warms up, but don't metal rectifiers and silicon diodes/rectifiers both conduct the full current instantly? This will rush into the smoothing capacitors, but the valves will need to warm up before they start pulling HT current. I think that silicon diodes have a lower forward volt drop than metal rectifiers, so the HT voltage wiil be higher unless a dropper resistor is used. __________________ Graham. Forum Moderator Reach for your meter before you reach for your soldering iron.

25th Aug 2007, 8:44 pm	#6
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: How to replace a metal rectifier on a pre-war radio? Hi Gents, the easly rectifiers were usually copper oxide. These slowly went higher resistance and did not die with the release of toxic gasses like selenium rectifiers. They did have an higher forward resistance than modern diodes, hence the series resistor. The parallel capacitor (ideally a C/R combination) is there to reduce the effects of incoming mains spikes, there will be no switching hash from a "slow" 1N4004 diode on 50Hz supplies. An X cap is not the ideal component here. These are usually rated at 250V AC. This cap will "see" the full Peak mains voltage (when the supply swings negative), plus the peak rectified DC on the smoothing caps. A 1000V or 1500v component would be best here, with a value of between 1000pF and 10nF max. Too high a value may increase hum levels. Having said the above, an X cap will probably work perfectly well and has the advantage of dying quietly when they do expire. Ed

25th Aug 2007, 9:32 pm	#8
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,936	Re: How to replace a metal rectifier on a pre-war radio? Strictly speaking you don't need a surge limiting resistor if the radio has been fitted with modern caps. The idea is to mimic the electrical characteristics of the metal rectifier being replaced, which as others have said would have a significant forward resistance even when new. A resistor makes life easier for the other components. The value of the resistor certainly isn't critical. Personally I think it's not worth losing sleep over. I fit something like a 100-220 ohm resistor, whatever I have in the spares box, and check the HT voltage to make sure it sin't too far out (it usually isn't). Paul

26th Aug 2007, 1:01 am	#9
PJL Dekatron Join Date: Sep 2005 Location: Seaford, East Sussex, UK. Posts: 5,997	Re: How to replace a metal rectifier on a pre-war radio? I usually choose a resistor that sets the HT to the trader sheet value. I think the resistor is a good idea - there is otherwise the potential of a massive surge current when the mains switch is turned on and I suspect this is not good for the rectifier, reservoir capacitor or the mains switch and is likely to generate plenty of EMI.

26th Aug 2007, 9:50 am	#10
Ed_Dinning Dekatron Join Date: Jan 2004 Location: Newcastle upon Tyne, Tyne & Wear, UK. Posts: 8,194	Re: How to replace a metal rectifier on a pre-war radio? Hi Gents, agreed with Mesery Swimmer that the Y cap is probably the best, and generally available in the correct sort of values. Ed

26th Aug 2007, 11:19 am	#11
DoctorWho Rest in Peace Join Date: Apr 2005 Location: Devon Posts: 850	Re: How to replace a metal rectifier on a pre-war radio? Thanks guys for the advice, I'll experiment with something in the region of 100-220 ohms to get the advised HT voltage, where available. All of my sets have all had their condensers replaced (with the exception of mica ones) so hopefully they should all be alright. Peter.