[Uram-rejections] Transistorising older receivers?

Thu Mar 3 15:30:41 GMT 2016

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> From webstump at chiark.greenend.org.uk Thu Mar 03 14:31:08 2016
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> Date: Thu, 3 Mar 2016 14:30:56 +0000
> From: Ian Jackson <ianREMOVETHISjackson at g3ohx.demon.co.uk>
> Newsgroups: uk.radio.amateur.moderated
> Subject: Re: Transistorising older receivers?
> References: <nb7c0s$u53$1 at dont-email.me> <YX$BcZCa$z1WFwZ+ at g3ohx.demon.co.uk>
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> In message <nb94iu$6qi$1 at dont-email.me>, Brian Reay <no.sp at m.com> writes
> >On 03/03/16 08:53, Ian Jackson wrote:
> >
> >> The BC453/4/5 range of Command range of receivers didn't have a
> >> 'conventional' power supply. Instead they were designed to be powered by
> >> a small rear-end rotary converter, mounted on rubber feet.
> >> http://www.tuberadio.com/robinson/museum/command/
> >> It is highly unlikely that many amateurs used them like this, but
> >> instead provided their own power. These days, a 12V wall-wart supply
> >> would probably suffice. If not, as the valves had 12V heaters, an
> >> original homemade power supply could probably still be used by
> >> disconnecting the high voltage stuff, and re-wiring it to use what had
> >> been powering the heaters.
> >
> >While everything you say is true, the devil is in the detail.
> >
> >While, in your approach, you have assumed you will essentially need a 
> >new PSU, alternative approaches of using the existing rectifiers 
> >overlook the detail that these are generally valves/tubes themselves.
> 
> Well, quite. It all depends on how much of the existing PSU is usable, 
> and if you actually want to use it (even if it is just to preserve the 
> appearance). As I said, those Command receivers were intended to use a 
> 'sit-on-the-rear-shelf-of-the-chassis' rotary power supply (dynamotor), 
> but this was generally not suitable for amateur use. Most on the surplus 
> market were 'sans dynamotor' (maybe they were flogged off separately?). 
> If so, any power unit used by the amateur would not be in the original 
> style, so why try and retain it when the receiver was transistorised?
> >
> >Also, what about the various Electrolytic capacitors scattered around 
> >the radio? They require 'form', especially as they age. Will the much 
> >lower supply voltage enable this or will they end up exhibiting wilding 
> >different characteristics?
> 
> I had actually thought of the effects of the lower voltage on the 
> electrolytics. I don't think that the Command receiver has many - but 
> any it has will certainly be old, and will need reforming. [AIUI, modern 
> electrolytics don't really need this reforming process.] But even if the 
> electrolytics ARE old, and are potentially as leaky as old boots, I 
> would expect that if instead of being used to having (say) 200V across 
> them they now only have 10V, they would probably still reform 
> satisfactorily - if only to a new working voltage of 10V. [Yes, there IS 
> a school of thought that says that you should run electrolytics at maybe 
> 80% of their specced working voltage (ie the working voltage should be 
> around 120% of the running voltage), but I'm not really convinced that 
> this is necessary.]
> >
> >You've already touched on the impedance issues, so I won't revisit those.
> >
> >I can see why people may want to preserve the 'look' of these wonderful 
> >old sets but I suspect the work involved is perhaps more involved and I 
> >wonder if trying to 'replace the valves' is the best way, when it 
> >brings so many inherent issues.
> >
> >Would a better approach not be to ask: "What do I need to keep to 
> >preserve the look?" or possibly "What do I need to keep to preserve the 
> >look and feel?"
> >
> >That way, you may well end up with, say, one of your Command Receivers 
> >with modern, solid state, receiver, built inside, using all new 
> >components, with the old control knobs and dials etc. reused.
> 
> It's all a question of how much of the original actually needs to be 
> changed. After 70 years, it could indeed be quite a lot. On the other 
> hand, there often isn't really an awful lot inside this old equipment, 
> so there's not too much to go wrong.
> >
> >Sacrilege? Some may think so. But some may say the same if your replace 
> >the valves with transistors. Unless the equipment is 'one of a kind', 
> >at least rare, or of some historical interest, it would probably have 
> >been scrapped years ago but for your being an avid geek (and that is a 
> >compliment).
> 
> While I have no doubt that almost any modern receiver would out-perform 
> these 70 year-old relics, there must be great satisfaction in being able 
> to revitalise some of them - and get at least the same performance out 
> of them that they had originally.
> >
> >
> >
> >
> >
> 
> -- 
> Ian
> 
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