'Noble' TRF radio (Portadyne Princess 'lookalike'). Any info please?

[Yorkie]

Thanks for the link Joe - 12 bids but it only achieved £25.00, so the punters didn't think it was 'rare' or sought after. I notice that the dropper looked intact, but it had a home made back. Yes Rob, it looks like a Champion of that era too. I'll have to have a poke around to see if Champion made TRF sets in the mid 1950s. The 25 uF 25V cathode bypass cap on the Princess was dated 1954 - I guess the Noble will be a similar date. Modern ones look so puny by comparison that I took the unusual step (for me) of stuffing it.

(Woke up this morning to bright sun streaming through the hotel window at 6.30 am and not a cloud in the sky. How bizarre, but most welcome! I hope it's the same in Fenay Bridge, in which case Colin will have his happy face on today! )

[Yorkie]

A forum member who is restoring one of these dinky little sets has pm'd me for some info. As a consequence, I've revisited this thread and realised that I didn't post a pic of the back that I made on a router jig. The only pic referring to the (missing) back is the fifth one along in my first post to this thread, which shows a piece of cardboard cut to shape to use a template.  I wouldn't want anyone who happens across this thread to think that the cardboard was my idea of a suitable back! Hence, for the avoidance of doubt I've attached a couple of pics of the back that I made, along with a rather manky label I cobbled up, based on the one on the Portadyne Princess, which is basically the same set by a different name.

Crikey, the three years since I started this thread have passed quickly - that's scary - I still haven't decided what I want to be when I grow up!!

[Aerodyne]

Very nice work, David. I think it makes all the difference to a restoration when items such as the back cover are properly made, rather than rushed afterthoughts. I too have routed back covers for sets, a fact that makes me appreciate all the more the amount of effort needed to create a facsimile.
Tony

[Radio Fixer]

Talk to Brett at Blore Edwards, really helpful guy. He will make you an anti-log pot with switch or without as needed. It will be in metal, with a shaft of 1/4" or 6mm if needed and look much like a small original. Far better than those plastic efforts. And you are supporting a small Brit company.

A little more expensive than usual but its not always about the money with a hobby is it.

Gary

[Yorkie]

Thanks for reading the update Tony & Gary.

I did indeed talk to Blore Edwards,  (it may have been you who suggested them to me Gary), but at the time couldn't help with an anti-log track, so I spoke to Omeg who are based in East Grinstead. They could have made them, but  if the model selected consists of parts already in existence (as they were at the time), then the minimum order quantity is 10 pieces and the minimum order value is £25 + carriage + VAT if applicable. The very helpful lady to whom I spoke said she'd wave the MOQ but couldn't budge on the MOV. Hence, the price with carriage as I recall came to close of £40.00, which was way beyond what I was prepared to pay for a not very special radio - well two actually, counting the Portadyne Princess. As with anything, economies of scale come into play with larger orders. I dare say that if Id ordered twenty I'd have got them for less than a fiver each, but I'd have been lumbered with them. (I did ask if any of their usual clients bought anti-log switched pots, but none did).

OMEG  make most of the pots supplied by the likes of Maplin, Farnell, RS etc. OMEG pots are Similarity to Radiohm potentiometers. From 1958 to 1983 East Grinstead Electronic Components Ltd, under the direction of the company's founder, imported component parts from the Radiohm factories in France (Melek) and Tunisia (Tunirama) and assembled potentiometers in Sussex for the UK market. The current managing director gradually steered the company away from reliance on the Radiohm components by encouraging in-house development of a technology and ethos that is now evident in the current product range. The trade name of Omeg was established and at the end of 1993 the company trading name was changed from East Grinstead Electronics Components Limited to Omeg Limited.   Nowadays their pots are 95% made in the UK. They manufactures a range of 16mm and 20mm potentiometers that are equivalent to original Radiohm products. Worth a look at their website:

http://www.omeg.co.uk/profile.htm

I must say that I was actually quite impressed with the performance of this simple little TRF, given that it only uses a handful of components. So few in fact that I can imagine that they designed a radio that worked, then started removing parts till it stopped working, then put the last part back! the caps and resistors together only amount to about twenty. I would have been a convenient set to tune in, with no reaction control in the conventional sense - just the combined volume/reaction control. As with many AC/DC sets of that era, it has a single pole switch in the neutral line - not the live. I think the reason they did that was due to arcing of the contacts when the sets were used on DC mains, but I could be mistaken - I often am . The downside to this is that when the set is switched off, part of it is live.

I think it's the same basic set sold in kit form in the late 50s/early 60s - presumably bought up as surplus stock by the likes of Premier.

[Terry]

It doesn't seem to be commonly known but there is a solution to the log/antilog/special law potentiometer problem and it surprisingly simple!

If you put a resistor between the slider and one end of the pot, you will alter the law of the pot. For a log law, the resistor should be between the slider and the earthy end. For an anti-log law simply move the resistor so that it is between the slider and the 'hot' side of the pot.

When calculating the resultant characteristic it is important to consider the external circuitry.

For example, I once calculated that putting a 15k resistor from slider to earth on a 47k pot would modify the law to match a varicap diode's characteristic.* In this case, however, the top end of the pot was connected to the 30V varicap PSU.

If there is external resistance at either end of the pot - usually at the top end - this will need to be taken into account and, if the total resistance is important, a higher value of pot may need to be used as the basis of the exercise.

I would suggest temporarily tacking a resistor across an existing linear pot of the same or higher value as the original, then using a 'suck it and see' approach to determine how close to the desired law the result is.

If you think this approach is somewhat crude, you might like to ponder on the fact that many log law pots are nothing of the sort! Essentially, the track is laid down in two overlapping linear  sections of different values so that the selected portion of the track increases slowly in value until about the halfway stage, then increases rapidly to maximum!

Your ears will tell you when you've got it right!

* The varicap diode law is much sharper than the normal log characteristic so a starting value of ~50% of the linear pot's value might be a useful starting point.

[pwdrive]

The tapering resistor trick works for sure, I did that for a lin pot used for a regen control in a regen receiver, from memory it was a 50k lin pot with something around 6.8k from slider to top.

Lawrence.

[Yorkie]

It doesn't seem to be commonly known but there is a solution to the log/antilog/special law potentiometer problem and it surprisingly simple!

If you put a resistor between the slider and one end of the pot, you will alter the law of the pot. For a log law, the resistor should be between the slider and the earthy end. For an anti-log law simply move the resistor so that it is between the slider and the 'hot' side of the pot.

Thanks for reading the thread Terry and for your helpful comments.

The first of these little sets I worked on was a 'Portadyne Princess', on which I wrote a thread on the forum, I think back in 2012 when the topic of adding a resistor to create an anti-log pot was discussed. The website 'Secret Life of Pots' was discussed, and Lawrence pointed out another useful site which explained how to do it. I gave it a spin but couldn't create the effect of an anti-log law plot - maybe because I wasn't measuring it correctly?

Another topic is that of 'tapped' pots which occasionally crops up - I  had a problem with a Stella 105U which used a tapped pot and was duff. I'd wondered if that too could be solved by external resistors, but the consensus was no - it couldn't.

An interesting topic worthy of discussion in its own right, so I'll start a new thread on it so it doesn't get buried in this thread as it's not just common to the Noble, Princess or Stella radios, and as time passes, and radios become even older and pots go to the 'workshop in the sky', it will perhaps crop up more often.

[Radio Fixer]

Well BE say they do anti-log pots now David.

They made me pots with 4mm metal shafts, again special and they were about a fiver each.

[attachment=12688]

Gary

[Yorkie]

That's good to know Gary - very reasonably priced too.

I know from our previous discussions that they're very helpful.