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GEC BT336 Selenium rectifier replacement

 
colourstar
(@colourstar)
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Hello all,

I've nearly finished restoring a GEC BT336, which features in the 1961-2 red Radio & TV servicing book. My example is suffering from a lack of width and a check on the HT reveals only 175v at best. The rectifier is of the finned selenium variety. At one point last week it emitted a loud 'pap' and some smoke (and a pong) but appears to be soldiering on regardless.

I'd like to replace this with a rectifier diode and resistor instead. I have a load of 1N4007 diodes - would one of these be suitable to use in this application? Also, as no figure for the HT is given (at least as far as I can see), what's the best approach as regards determining a value for the series resistor?

Many thanks,
Steve

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Topic starter Posted : 11/04/2015 3:05 pm
Refugee
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A 1N4007 will do fine with a resistor selected on test for correct width.
Without a new rectifier the set will continue to smell like a wet dog from time to time and the width will also get worse.

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Posted : 11/04/2015 3:12 pm
crustytv
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I'd like to replace this with a rectifier diode and resistor instead. I have a load of 1N4007 diodes - would one of these be suitable to use in this application? Also, as no figure for the HT is given (at least as far as I can see), what's the best approach as regards determining a value for the series resistor?

Many thanks,
Steve

I used to use 1N4007 which are 1000V 1A however I now prefer and always use 1N5408 1000V 3A, much beefier for that position. As for a surge resistor, normally 5W between 20R -100R by experimentation. I think I've mainly fitted 20R but it may have been 47R can't remember.

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Posted : 11/04/2015 3:36 pm
colourstar
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Excellent, thank you. I'll try a 4007 for now, but will upgrade to a 4008 as a more permanent repair.

Results to follow....

Steve

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Topic starter Posted : 11/04/2015 3:40 pm
crustytv
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will upgrade to a 4008 as a more permanent repair.

Results to follow....

Steve

Just for clarity I stated 1N5408 not 1N4008. The 4008 is 1200V but again only 1A

Edit:
Data for all the diodes mentioned here and others, are located in the component section of the Technical Library.

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Posted : 11/04/2015 3:42 pm
colourstar
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Duly noted! :)

Steve

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Topic starter Posted : 11/04/2015 3:48 pm
Terry
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... Also, as no figure for the HT is given (at least as far as I can see), what's the best approach as regards determining a value for the series resistor?

The HT voltage often isn't stated directly but can be found elsewhere if you look carefully.

For example, in the GEC BT304 (the only similar model, I think, for which data is available in the library) no HT voltage is stated but the main HT line can clearly be seen to feed the anode of the efficiency diode and this is clearly shown in the data as being 205V (for 240VAC input).

The screen grid of the field output valve is also fed directly from the main HT rail.

If you can't find a similar reference point in your set, look at at anode voltages in the sound and field output stages, for example, or any other transformer fed points (as there are more likely to be consistent from set to set). If you can get these voltages correct on average - you will rarely find 100% agreement! - you must be in the right area.

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Posted : 11/04/2015 6:53 pm
colourstar
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Thanks Terry, that's sound advice. I'll keep a note of that for future reference.

With a 20 ohm series resistor as a starting point, HT is at around 220v which gives slightly excessive width as shown. I've not run it for long like that and will find a resistor value that gives closer to 205v.

Steve

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Topic starter Posted : 11/04/2015 8:18 pm
colourstar
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Whilst I'm at it, here are some more pictures. The GEC came from an antiques centre in Norwich. With it was the original user manual and a copy of a previous owner's television licence from the '60s. By coincidence I already had the original GEC promotional leaflet for the range. The chassis is basically that of the original BT302, updated with a 19" 'squarer' tube and a pushbutton tuner.

The set is in super-clean condition and I love the unusual design.

Steve

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Topic starter Posted : 11/04/2015 10:49 pm
Nuvistor
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That's a very nice clean set, looks only a couple of years old. The circuit boards are laid out to be easily accessible to.
I always liked the look of the bow fronted sets from this era, Ekco T368 and T370 very similar in style to the bow fronted GEC BT 342 in the leaflet.
Frank

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Posted : 11/04/2015 10:58 pm
The_Teleman
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Wow ! I've been looking for one of those for years , I had one as a bedroom set when I was a kid , great set !

Chris

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Posted : 12/04/2015 2:58 pm
The_Teleman
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Hi Steve , If that's the leaflet I think it is it has the bow front BT342 & the transportable BT326 & your BT336 . I have the 342 but it's missing some parts I also have the 326 & that's a nice set too .

Chris

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Posted : 12/04/2015 7:16 pm
colourstar
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Hi Chris

I've never seen that portable before - only in the leaflet. Very nice indeed. Does it also have the same chassis as the 302 etc?

Attached are pics of the remainder of the GEC leaflet plus my BT314 - the very strangely shaped non-bow-front bow-front!

Steve

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Topic starter Posted : 12/04/2015 7:42 pm
Terry
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The chassis is basically that of the original BT302, updated with a 19" 'squarer' tube and a pushbutton tuner.

Steve, that's the service sheet I found in the library - I said BT304 in my previous post but it is the same as the 302, 303, 305, 312 and 316 so 205V HT it is, then!

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Posted : 12/04/2015 8:14 pm
colourstar
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Thanks Terry. The '300' series of GEC sets was far larger than I had guessed!

Also in the library is a very useful servicing guide from the redoubtable LLJ. Such a shame he's not around to enjoy the fact that we still refer to his articles all these years later. He'd be tickled pink I'm sure.

Steve

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Topic starter Posted : 12/04/2015 8:39 pm