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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 27th Oct 2014, 12:38 pm   #1
belerecords
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Default Capacitor? Replacement?

This component is one of two in an early one valve set. It looks to me like a capacitor but if so it is leaky as it has a resistance of about 500K. The resistance slowly increases - charging? There is no marking on it other than the name Readi Rad.

Should I replace them both? If so what with guessing I won't be able to source identical components?
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Old 27th Oct 2014, 1:07 pm   #2
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Arrow Re: Capacitor? Replacement?

Interesting: it looks like a high-value resistor that one finds in a vintage one-valve set - typically a 1 Meg. ohm grid leak - yet by your test, it appears to behave like a capacitor, albeit a leaky one. And the fact that it has no indication of its electrical size (resistance or capacity) doesn't help either.
However, if you can send in a sketch of where it (and the other one you have) appears in the circuit diagram, that information will go a very long way to identifying what it is and an intelligent guess of its electrical value can then be made.

Al.
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Old 27th Oct 2014, 1:12 pm   #3
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Default Re: Capacitor? Replacement?

What does it look like from the side. I have seen capacitors like this, but from what I remember they're about 2" square when viewed from the side.
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Old 27th Oct 2014, 1:15 pm   #4
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Default Re: Capacitor? Replacement?

I vaguely recall that in the 1920s some 'kit' radios made use of a combined grid-leak-and-grid-coupling-capacitor: typically 100pF and 1Megohm in parallel.

Given the quality of components back then, and the intervening years, it could now be way off-resistance and/or capacity.
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Old 27th Oct 2014, 2:31 pm   #5
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Default Re: Capacitor? Replacement?

What set is it? or home made? PM sent.

John.
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Old 27th Oct 2014, 2:45 pm   #6
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Default Re: Capacitor? Replacement?

It is a Readi Rad 1 valve set with a Mullard PM2DX valve.

One of these components goes from the anode of the valve to earth. The other is from the hot side of the tuning capacitor to the grid - that would suggest its a grid leak of some kind.

There is a separate resistor of 5Meg from grid to the + side of the filament. Thats a resistor that looks rather like a large black fuse.

The idea of it being a combined grid leak capacitor/resistor combination seems possible.

Thanks for all the help!
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Old 27th Oct 2014, 2:54 pm   #7
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Default Re: Capacitor? Replacement?

It is extremely well made if it is home made. It has a metal plate inside the lid with the name Readi Rad on it. All solid wood case black ebonite front panel.
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Old 27th Oct 2014, 3:05 pm   #8
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Default Re: Capacitor? Replacement?

Is it easy to remove the insides? If so I would pop new bit(s) in there, no one will know!
 
Old 27th Oct 2014, 3:10 pm   #9
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Default Re: Capacitor? Replacement?

Yes, they do look like condensers, being the archetypal mica/tin foil type. These were small value, between 100pf and 300pf. In your case they are likely to be 100pf, one being the grid-coupling condenser mentioned by G6Tanuki, the other being a fixed series aerial condenser. It is an easy matter to dig out the pitch, remove the old condenser and insert a modern mica one (as illustrated in the picture) and then refill with pitch.
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Old 27th Oct 2014, 4:53 pm   #10
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Default Re: Capacitor? Replacement?

I have just re-read your post #6 in which I think you say one of the capacitors goes from the anode of the valve to earth. Is it possible this was married with a resistor in the HT line to prevent distortion and oscillation? It would be most helpful to see the circuitry or even a hand drawn circuit diagram.
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Old 27th Oct 2014, 5:18 pm   #11
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Default Re: Capacitor? Replacement?

I have not heard of Readi Rad before, and believe it may be American. Maybe worth asking on the USA Forums if anyone has a circuit for your set.

John.
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Old 27th Oct 2014, 6:49 pm   #12
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Default Re: Capacitor? Replacement?

As requested here's a hand drawn circuit diagram. Apologies for my poor draughtsmanship. Some of the coil windings are broken at the point where they pass through the former.

The two components marked ? are identical in appearances and both give a resistance of about 500K which slowly increases. At first I thought the 5M resistor was the grid leak but I can now see it isn't which further supports G6Tanuki's suggestion.


Thanks for all the suggestions to date I will try the American forums as well and will post details of any success I have.

The valve seems to fine and my next step is to rewind the coil. It does bother me a little that it will not be entirely original but i am determined to make it work!
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Old 27th Oct 2014, 7:12 pm   #13
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Default Re: Capacitor? Replacement?

When refilling the case don't use a hard e.g. epoxy filler these can crack the component. Best not to fill the case at all, the gap will be hidden once upon the baseboard.

(edit) The ?? is more than likely a 100pF capacitor.

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Old 27th Oct 2014, 8:40 pm   #14
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Default Re: Capacitor? Replacement?

OK. Your 5 megohm resistor is a grid leak. It's quite high, a 2 megohm would be more usual. The capacitor going to the grid of the valve is the grid-coupling condenser. 0.0002uF (200pF) would be a good idea. The other capacitor from the anode to HT/LT neg should be 0.0001uF (100pF). It's a standard circuit. There is a very similar one in FJ Camm's book 'Practical Wireless Circuits'. I have included a snapshot below. If you would like decent scans of the relevant pages PM me your email address.
Nick
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Old 27th Oct 2014, 10:48 pm   #15
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Default Re: Capacitor? Replacement?

Hi, as the others have said it is a cap, for my money I would say both caps would be 100pF,
The coils could be re-wound, but if you look out for Telsen baseboard mounting dual range coils they would also work in your circuit and give both MW and LW. Often advertised and not expensive.
They would look right with the other parts.

Ed
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Old 27th Oct 2014, 11:35 pm   #16
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Default Re: Capacitor? Replacement?

Hi belerecords

If you want to find the actual circuit for your receiver you might do well to start hunting under the name Ready Radio. Founded in 1926 and based in Blackheath, London they were rather short-lived, closing in 1934. During the eight years they traded they were suppliers of Kendall and Kendall-Price Radio kits. They also published small booklets of radio designs by Mr. G.P. Kendall BSc. & Mr H.D. Price.

ReadiRad was one of their brands. In fact the company's telegram address was "Readirad" Blackvil London.

If I were you I would try and find one of these booklets, or search for some of the designs of Mssrs Kendall and Price.

I found this information on the RadioMuseum website (http://www.radiomuseum.org/dsp_herst...mpany_id=15079)

Nick
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Old 28th Oct 2014, 8:29 pm   #17
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Default Re: Capacitor? Replacement?

Many thanks to all for your comments and suggestions - I'm going ahead with the restoration.
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Old 29th Oct 2014, 1:17 pm   #18
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Quote:
Originally Posted by Skywave View Post
If you can send in a sketch of where it (and the other one you have) appears in the circuit diagram, that information will go a very long way to identifying what it is and an intelligent guess of its electrical value can then be made.
Quote:
Originally Posted by belerecords View Post
As requested, here's a hand-drawn circuit diagram.
Thank you for that - which is quite adequate. And - as is usual in cases like this - a picture says a thousand words!

Al.
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