Metal rectifiers & HT smoothing components. - UK Vintage Radio Repair and Restoration Discussion Forum

Neil Purling · 30th May 2016, 6:30 pm

Inside a radio is a pair of Brimar RM3 in series rectfying the mains AC to make the HT supply.
Are they to be trusted at all, or do I regard them like I would the waxies I found within that receiver?. I wondered if one can tell how they'd cope under load.

How does one represent a Selenium rectifier on a circuit diagram?
I have the smoothing can to replace (it has puked). It was a 100+100uf @450V.
That sounds excessive. The can was not dated, so I have no idea if it was original or not.
Although the equipment was low end it has a choke between the capacitor sections rather than a resistor.

TonyDuell · 30th May 2016, 6:52 pm

I would be surprised if that smoothing capacitor was original. 100uF seems very large to me. You don't give the model (or approximate date) of the radio but most valve radios used 8uF or 16uF.

The normal symbol for a metal rectifier (or at least the one I have seen) is the same as that for a normal semiconductor diode. I guess you could write 'Se' alongside it to indicate it is selenium (I have seen 'Ge' and 'Si' alongside diode and transistor symbols to indicate germanium or silicon)

paulsherwin · 30th May 2016, 6:59 pm

The cap might be original with a metal rectifier, though it does seem rather large.

Metal rectifiers generally fail gradually, dropping more and more voltage and getting hotter. I usually change them as part of a refurb because it is good preventative maintenance and costs very little. One or two 1N4007s plus a 200 ohm resistor is normally about right. Some people don't change them in the absence of a fault though.

'LIVEWIRE?' · 30th May 2016, 9:36 pm

I normally regard Selenium rectifiers as 'replace on sight' because of the by now well documented possibility of failure. Somewhere stored away is a 'Standard' (Waltham') mains/battery record player which has a blue finned metal rectifier in it's mains PSU. If, or when,I get around torefurbishing it, that rectifier will go!

Neil Purling · 31st May 2016, 11:03 am

After removing them they were actually RM2's. One read 348K, the other 1.38M & that is the only direction I could get any reading. I know how putrid they smell when they die catastrophically from experience. Not an experience I wish to re-visit!
As the rectifiers were not in a good stage, electrically or cosmetically I binned them.
I have one Radiospares rectifier of similar appearance I could leave there for appearances sake.
The original RM2's had obviously been living somewhere damp.

30th May 2016, 6:30 pm	#1
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,087	Metal rectifiers & HT smoothing components. Inside a radio is a pair of Brimar RM3 in series rectfying the mains AC to make the HT supply. Are they to be trusted at all, or do I regard them like I would the waxies I found within that receiver?. I wondered if one can tell how they'd cope under load. How does one represent a Selenium rectifier on a circuit diagram? I have the smoothing can to replace (it has puked). It was a 100+100uf @450V. That sounds excessive. The can was not dated, so I have no idea if it was original or not. Although the equipment was low end it has a choke between the capacitor sections rather than a resistor.

30th May 2016, 6:52 pm	#2
TonyDuell Dekatron Join Date: Jun 2015 Location: Biggin Hill, London, UK. Posts: 5,220	Re: Metal rectifiers & HT smoothing components. I would be surprised if that smoothing capacitor was original. 100uF seems very large to me. You don't give the model (or approximate date) of the radio but most valve radios used 8uF or 16uF. The normal symbol for a metal rectifier (or at least the one I have seen) is the same as that for a normal semiconductor diode. I guess you could write 'Se' alongside it to indicate it is selenium (I have seen 'Ge' and 'Si' alongside diode and transistor symbols to indicate germanium or silicon)

30th May 2016, 6:59 pm	#3
paulsherwin Moderator Join Date: Jun 2003 Location: Oxford, UK Posts: 27,968	Re: Metal rectifiers & HT smoothing components. The cap might be original with a metal rectifier, though it does seem rather large. Metal rectifiers generally fail gradually, dropping more and more voltage and getting hotter. I usually change them as part of a refurb because it is good preventative maintenance and costs very little. One or two 1N4007s plus a 200 ohm resistor is normally about right. Some people don't change them in the absence of a fault though.

30th May 2016, 9:36 pm	#4
'LIVEWIRE?' Rest in Peace Join Date: Mar 2006 Location: N.W. Oxfordshire(Chipping Norton) Posts: 7,306	Re: Metal rectifiers & HT smoothing components. I normally regard Selenium rectifiers as 'replace on sight' because of the by now well documented possibility of failure. Somewhere stored away is a 'Standard' (Waltham') mains/battery record player which has a blue finned metal rectifier in it's mains PSU. If, or when,I get around torefurbishing it, that rectifier will go!

31st May 2016, 11:03 am	#5
Neil Purling Nonode Join Date: Jul 2005 Location: Hull, East Yorkshire, UK. Posts: 2,087	Re: Metal rectifiers & HT smoothing components. After removing them they were actually RM2's. One read 348K, the other 1.38M & that is the only direction I could get any reading. I know how putrid they smell when they die catastrophically from experience. Not an experience I wish to re-visit! As the rectifiers were not in a good stage, electrically or cosmetically I binned them. I have one Radiospares rectifier of similar appearance I could leave there for appearances sake. The original RM2's had obviously been living somewhere damp.