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Old 5th Jul 2018, 3:43 pm   #21
tamemin
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Default Re: Mystery 1930s 3 valve battery radio chassis

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Originally Posted by ms660 View Post
Yes, reaction is back into the RF amps anode circuit, both grid and anode circuits of the RF amp are tuned, if the tuning gang isn't grounded properly to the chassis the tuning/sensitivity will be very poor. Looks manufacture made to me...Any cabinet/other bits etc?
About the cabinet... there was a plaque on the back saying that something like the radio was made under license from Mullard.
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Old 5th Jul 2018, 3:52 pm   #22
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Default Re: Mystery 1930s 3 valve battery radio chassis

Scroll down to variable capacitors for a differential capacitor symbol, that's how to draw one, sometimes they are drawn with the plates slightly curved, either/or will do:

https://www.electrical-symbols.com/e...or-symbols.htm

Lawrence.
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Old 5th Jul 2018, 11:07 pm   #23
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Default Re: Mystery 1930s 3 valve battery radio chassis

Hi there,

I've tested the tuning capacitor, and the rotor is well grounded, so no quick win there, for my lack of MW tuning and selectivity

I've updated the circuit diagram as well as I could by hand, the latest is attached. I believe this to be a true reflection of what that chassis is.

I'll probably do more physical fiddling with it at the weekend, but if any one has any other comments on this diagram though, please feel free to post.

Thanks

Tam
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Old 6th Jul 2018, 10:08 am   #24
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Default Re: Mystery 1930s 3 valve battery radio chassis

The screen decoupling capacitor on the first valve is actually across the H.T. supply, and due to this, forms part of the second tuned circuit. It is (from an RF point of view) in series with the tuning coil, providing the RF return path to negative, thus putting the coil effectively across the tuning capacitor (which is correct) at RF. I suggest that you make sure that this capacitor is a really good modern one of low self-inductance. Ensuring this will make the tracking of the tuned circuits more accurate, and easier to adjust if you decide to do an alignment on the set. Tony
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Old 6th Jul 2018, 10:22 am   #25
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Default Re: Mystery 1930s 3 valve battery radio chassis

The tuning rotor is normally connected to the chassis by the fixing screws, but I did once have a KW 2000 ham radio HF rig which wouldn't tune due to a high resistance connection between the tuning capacitor and the chassis, even though the screws were tight, removing, cleaning and refitting cured the fault.

Peter
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Old 6th Jul 2018, 10:54 am   #26
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Default Re: Mystery 1930s 3 valve battery radio chassis

Does the detector oscillate at max regen?

Lawrence.
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Old 7th Jul 2018, 10:23 am   #27
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Default Re: Mystery 1930s 3 valve battery radio chassis

Yes it does oscillate at max regen, I'll fiddle a bit with it today.

Tam

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Old 7th Jul 2018, 10:28 am   #28
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Default Re: Mystery 1930s 3 valve battery radio chassis

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The screen decoupling capacitor on the first valve is actually across the H.T. supply, and due to this, forms part of the second tuned circuit. It is (from an RF point of view) in series with the tuning coil, providing the RF return path to negative, thus putting the coil effectively across the tuning capacitor (which is correct) at RF. I suggest that you make sure that this capacitor is a really good modern one of low self-inductance. Ensuring this will make the tracking of the tuned circuits more accurate, and easier to adjust if you decide to do an alignment on the set. Tony
Hi Tony,

That cap is actually a modern one, which someone (who knows what they're doing) must have replaced recently. I'll wire up the chassis again in a moment, and see what readings I can measure while it's running.

I tried to use my scope on it a few days ago, and it's very much affected by the scope probe on it
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Old 7th Jul 2018, 3:14 pm   #29
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Default Re: Mystery 1930s 3 valve battery radio chassis

Try the effect of a small 100k resistor in series with the probe tip. It may help reduce the small extra capacitive load on the point you're probing. If you know the scope probe stray C you could allow for the first order rolloff effect you'll be adding.
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Old 9th Jul 2018, 8:54 am   #30
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Default Re: Mystery 1930s 3 valve battery radio chassis

** Some Success**

Well, I tried to rule out possible noise from my LT power supply last night, and replaced it with 2 D cells and a power diode (or two), which gave me about 1.8v under load.

Anyway, the radio seems to pick up quite a few channels on MW (five live is quite clear) Which is great. The regen and tuning seem to work quite well, although there does seem to be a constant 12-16 khz on the audio out on some channels, I guess maybe that's the nature of the beast?

So I guess, the challenge now, is to make a nice clean 2V supply, and 90v supply. I've ordered some dc-dc buck converter modules from ebay, but I suspect these are going to be really noisy and cause the same problem as before. I'm hoping to power the whole thing from a single 12v plugin power supply.

Tam.
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Old 9th Jul 2018, 2:49 pm   #31
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Default Re: Mystery 1930s 3 valve battery radio chassis

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although there does seem to be a constant 12-16 khz on the audio out on some channels, I guess maybe that's the nature of the beast?
More than likely this is interference from a nearby switch mode supply. Most of us can't hear 12-16kHz so not a problem!
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Old 9th Jul 2018, 3:02 pm   #32
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Default Re: Mystery 1930s 3 valve battery radio chassis

That reminds me..I'm still on the lookout for a lazy old paper cone speaker to put into this thing. (It was missing the speaker and transformer)

I'm currently using a small modern this that has a very wide frequency response. (ouch!)
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Old 12th Jul 2018, 10:33 am   #33
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Default Re: Mystery 1930s 3 valve battery radio chassis

What diameter is your speaker? Is there an output transformer fitted on the chassis?
I have a few old (but good) speakers, and I can look amongst them if you say what is required. If I find something suitable, it is yours at cost of postage. Tony.
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Old 12th Jul 2018, 11:02 am   #34
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Default Re: Mystery 1930s 3 valve battery radio chassis

Hi Tony,

There is a square space that'll take an 8-inch speaker easily. I've ordered a cheap 8" from eBay, but as far as transformers go, that was missing too, so I've been using a 100 volt line transformer with a 2k resistor in series (not perfect I know, but a proper one from RS is nearly 20!)

I would definitely be interested in a more vintage appropriate speaker if you have one.

Tam
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Old 27th Jul 2018, 12:02 pm   #35
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Default Re: Mystery 1930s 3 valve battery radio chassis

Cabinet Update
-----------------

I've almost finished the cabinet restoration (which was quite badly woodworm eaten)

As soon as the weather cools down a bit, I'll re-fit the speaker cloth, and the new speaker with transformer (thanks Tony!)

I've attached before and after pictures, hope you like.

Tam
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Old 27th Jul 2018, 12:27 pm   #36
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Default Re: Mystery 1930s 3 valve battery radio chassis

Very nice too I add.
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Old 27th Jul 2018, 5:55 pm   #37
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Default Re: Mystery 1930s 3 valve battery radio chassis

That cabinet is looking good.
Well done, Tam.
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Old 27th Jul 2018, 6:47 pm   #38
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Default Re: Mystery 1930s 3 valve battery radio chassis

I have this model and mine is complete but no maker! As much as I can find is VT/S/3.

Wether it is Mullard or not I am not sure.
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Old 27th Jul 2018, 7:02 pm   #39
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Default Re: Mystery 1930s 3 valve battery radio chassis

Great job on the cabinet Tam

Just a thought did you get this set from RWB bring and buy ?

Cheers Mike T
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Old 27th Jul 2018, 7:03 pm   #40
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Default Re: Mystery 1930s 3 valve battery radio chassis

Is that a label inside the cabinet?
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