Selenium rectifier replacement (Roberts RMB). - UK Vintage Radio Repair and Restoration Discussion Forum

howard · 23rd Dec 2006, 9:04 am

Hello,

Ron Bryan is currently checking over the very nice Roberts RMB which I won at the last Wootton Bassett auction. It works but it doesn't play very loud. It has a couple of faults. Firstly the HT voltage is only around 40 volts due to failure of the selenium rectifier. I understand that that can be overcome by attaching in parallel a diode and changing the decoupling resistor but are these rectifiers still available as new old stock anywhere ? Secondly, it has a faulty 40uF+40uF double ended capacitor, again are original capacitors still available anywhere ?
Thanks.

Howard

Darren-UK · 23rd Dec 2006, 1:15 pm

Selenium rectifiers can be obtained from the likes of http://www.gdrectifiers.co.uk/ but it appears only the spindle mounted type would be suitable for your application. An internet search will reveal several USA suppliers too. I wouldn't take the decidedly dodgy route of salvaging one from a scrap radio, but it's not impossible a good one could turn up this way.

Double ended capacitors. If appearance is important, I'd just buy a couple of modern axial types and shove them in the gutted original can. That is, unless someone comes up with a suitable and servicable original from a scrapper.

Not 100% relevant to this thread, but you may find this of interest .... http://hhscott.com/cc/rectifiers.htm

howard · 24th Dec 2006, 11:23 am

Thanks Darren,

That's very useful information. It would be nice to retain as many of the original components as possible in this RMB which has a mint case, but sometimes I guess one has to compromise and use modern parts to keep these old sets going.

I will contact those suppliers to see if they keep a good match for the RMB's rectifier, and will try and reuse the original can from the double ended capacitor so it at least looks original

Howard

Radio_Dave · 24th Dec 2006, 11:36 am

Hi Howard

I always change selenium rectifiers on sight because I don't trust them. However I never throw them away, so it might be worth posting a picture of it... Actually it might be worth posting a picture of the cap too

Regards
David

howard · 24th Dec 2006, 11:42 am

Thanks Dave,

Either myself or Ron will do that ASAP. By the way, where do you get your new selenium rectifiers from ?

Howard

Radio_Dave · 24th Dec 2006, 12:19 pm

Hi Howard,

I replace selenium rectifiers with a IN5404 or IN5408 and a resistor of around 220R

.

Regards
David

ronbryan · 27th Dec 2006, 11:46 am

David

Here are two pictures of the Roberts RMB selenium HT rectifier. The first shows the RM0 rectifier in Howard's set, with the temporary 1N4004 and extra series resistor I've tacked on to it. Forum member Igranic has suggested the RM0 may well be a service replacement for the original rectifier.

The second picture shows the HT rectifier in Igranic's RMB. It's considerably longer than the RM0 in Howard's set and is probably the original type.

Ron

igranic · 27th Dec 2006, 6:24 pm

On further research, the recommended HT rectifier for the Roberts RMB is a SenTerCel H18-12-1RW, which I believe is that illustrated in the second picture of ronbryan's post, but as yet I can find no further information in my collection of data books.

Perhaps another forum member could provide either a replacement, or a copy of the data sheet?

According to the 'Brimar Valve and Teletube Manual No. 8' of June 1959, the RM0 is rated at 30mA with a maximum RMS input of 125 volts.

Radio_Dave · 27th Dec 2006, 8:36 pm

I've had a good look through my spares and there's nothing like it... sorry

.

It might be worth keeping an eye on ebay. I've just done a search for 'selenium rectifier' and it came up with two results, however they're not quite like yours, but you never know wht might turn up in the future

David

Sideband · 27th Dec 2006, 11:35 pm

Quote:

Originally Posted by howard

It would be nice to retain as many of the original components as possible in this RMB which has a mint case, but sometimes I guess one has to compromise and use modern parts to keep these old sets going.

That is what I always try to do Howard. However with selenium rectifiers and double electrolytics, it's a question of reliability versus originality. If a NOS electrolytic capacitor (either double or single) is fitted, chances are it will need reforming and MAY be unreliable. I would tend to fit a modern replacement neatly either in the original can or under the chassis; in the latter case retaining the original component on top.

With a selenium rectifier it would be replaced by a modern silicon job for the same reasons above. I would not want to rely on a 50-odd year old NOS selenium rectifier...if you have never smelled one of these when it fails.....well an open sewer comes to mind

! I would far rather fit a 1N4007 and a 220 ohm resistor in series or something like that and keep the original mounted in place but disconnected. Chances of one of these failing is very remote.

Rich.

Mike Phelan · 28th Dec 2006, 9:14 am

Howard
If you were going to swap a selenium rectifier for a silicon diode + resistor, the original reccy ought to be disconnected, as they tend to go leaky/short circuit, giving that wonderful eau-de-merde aroma.

To preserve appearance, you could make a "selenium rectifier" of the cylindrical type as in Ron's picture using a bit of threaded rod and a stack of washers, then spraying it grey.
Not a good idea to connect it, though!

howard · 28th Dec 2006, 10:19 am

Quote:

Originally Posted by Mike Phelan

Howard
they tend to go leaky/short circuit, giving that wonderful eau-de-merde aroma.

and the Selenium fumes are also very toxic, as described here .....

http://antiqueradio.org/tvshow.htm

Having read the notes above it seems that selenium rectifiers are doomed to eventual failure and aren't really worth replacing with another one

I think the best thing is to leave it in place to retain its original appearance but to disconnect it and discretely replace it with a diode and additional resistor.
Thanks all

Howard

23rd Dec 2006, 9:04 am	#1
howard Nonode Join Date: May 2005 Location: Godalming, Surrey, UK. Posts: 2,593	Selenium rectifier replacement (Roberts RMB). Hello, Ron Bryan is currently checking over the very nice Roberts RMB which I won at the last Wootton Bassett auction. It works but it doesn't play very loud. It has a couple of faults. Firstly the HT voltage is only around 40 volts due to failure of the selenium rectifier. I understand that that can be overcome by attaching in parallel a diode and changing the decoupling resistor but are these rectifiers still available as new old stock anywhere ? Secondly, it has a faulty 40uF+40uF double ended capacitor, again are original capacitors still available anywhere ? Thanks. Howard

23rd Dec 2006, 1:15 pm	#2
Darren-UK Retired Dormant Member Join Date: Apr 2003 Location: Blackpool, Lancashire, UK. Posts: 4,061	Re: Roberts RMB problems Selenium rectifiers can be obtained from the likes of http://www.gdrectifiers.co.uk/ but it appears only the spindle mounted type would be suitable for your application. An internet search will reveal several USA suppliers too. I wouldn't take the decidedly dodgy route of salvaging one from a scrap radio, but it's not impossible a good one could turn up this way. Double ended capacitors. If appearance is important, I'd just buy a couple of modern axial types and shove them in the gutted original can. That is, unless someone comes up with a suitable and servicable original from a scrapper. Not 100% relevant to this thread, but you may find this of interest .... http://hhscott.com/cc/rectifiers.htm Last edited by Darren-UK; 28th Dec 2007 at 5:28 pm. Reason: Removed ref to defunct supplier.

24th Dec 2006, 11:23 am	#3
howard Nonode Join Date: May 2005 Location: Godalming, Surrey, UK. Posts: 2,593	Re: Roberts RMB problems Thanks Darren, That's very useful information. It would be nice to retain as many of the original components as possible in this RMB which has a mint case, but sometimes I guess one has to compromise and use modern parts to keep these old sets going. I will contact those suppliers to see if they keep a good match for the RMB's rectifier, and will try and reuse the original can from the double ended capacitor so it at least looks original Howard

24th Dec 2006, 11:36 am	#4
Radio_Dave Nonode Join Date: Jun 2004 Location: Norwich, Norfolk, UK. Posts: 2,543	Re: Roberts RMB problems Hi Howard I always change selenium rectifiers on sight because I don't trust them. However I never throw them away, so it might be worth posting a picture of it... Actually it might be worth posting a picture of the cap too Regards David

24th Dec 2006, 11:42 am	#5
howard Nonode Join Date: May 2005 Location: Godalming, Surrey, UK. Posts: 2,593	Re: Roberts RMB problems Thanks Dave, Either myself or Ron will do that ASAP. By the way, where do you get your new selenium rectifiers from ? Howard

24th Dec 2006, 12:19 pm	#6
Radio_Dave Nonode Join Date: Jun 2004 Location: Norwich, Norfolk, UK. Posts: 2,543	Re: Roberts RMB problems Hi Howard, I replace selenium rectifiers with a IN5404 or IN5408 and a resistor of around 220R . Regards David

27th Dec 2006, 11:46 am	#7
ronbryan Octode Join Date: Jan 2003 Location: Guildford, Surrey, UK. Posts: 1,960	Re: Roberts RMB problems David Here are two pictures of the Roberts RMB selenium HT rectifier. The first shows the RM0 rectifier in Howard's set, with the temporary 1N4004 and extra series resistor I've tacked on to it. Forum member Igranic has suggested the RM0 may well be a service replacement for the original rectifier. The second picture shows the HT rectifier in Igranic's RMB. It's considerably longer than the RM0 in Howard's set and is probably the original type. Ron Attached Thumbnails

27th Dec 2006, 6:24 pm	#8
igranic Rest in Peace Join Date: Jan 2005 Location: North Bedfordshire, UK. Posts: 395	Re: Roberts RMB problems On further research, the recommended HT rectifier for the Roberts RMB is a SenTerCel H18-12-1RW, which I believe is that illustrated in the second picture of ronbryan's post, but as yet I can find no further information in my collection of data books. Perhaps another forum member could provide either a replacement, or a copy of the data sheet? According to the 'Brimar Valve and Teletube Manual No. 8' of June 1959, the RM0 is rated at 30mA with a maximum RMS input of 125 volts.

27th Dec 2006, 8:36 pm	#9
Radio_Dave Nonode Join Date: Jun 2004 Location: Norwich, Norfolk, UK. Posts: 2,543	Re: Roberts RMB problems I've had a good look through my spares and there's nothing like it... sorry. It might be worth keeping an eye on ebay. I've just done a search for 'selenium rectifier' and it came up with two results, however they're not quite like yours, but you never know wht might turn up in the future David

28th Dec 2006, 9:14 am	#11
Mike Phelan Dekatron Join Date: Nov 2003 Location: Near Leeds, West Yorkshire, UK Posts: 4,609	Re: Roberts RMB problems Howard If you were going to swap a selenium rectifier for a silicon diode + resistor, the original reccy ought to be disconnected, as they tend to go leaky/short circuit, giving that wonderful eau-de-merde aroma. To preserve appearance, you could make a "selenium rectifier" of the cylindrical type as in Ron's picture using a bit of threaded rod and a stack of washers, then spraying it grey. Not a good idea to connect it, though! __________________ Mike.