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Old 22nd August 2006, 10:02 AM   #1
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Default Old radio quick question re PS

Hello there!

I bought myself an old radio from the 60s, typical AA5 circuit using miniature 7B tubes.

I've changed the smaller caps, as mandatory, but didn't change the large electrolytics as i don't hear hum on the speaker.
As this radio was meant to 110V, it uses a 2uF cap connecting rectifier plate to ground to lower the B+.
Click the image to open in full size.
I've only listened to it twice as i have the chassis out for cabinet restoration, but today i had it playing for around 1/2 an hourwhile taking care of the cabinet.
The radio starts to fade slowly, and not only the tubes, but the chassis gets hot! I think i have traced it to the 2uF cap, as 1- it was hot as hell, 2- it has a metal tab connected to the chassis and that area is the hottest. B+ measured around 60V.
I couldn't measure any resistance across the cap using the DVM, but it should be culprit no?
One other thing: the cap says 2uF/220WVAC and the filter 30uFx2/150WV. What is the voltage rating i should ask when buying the caps?

Thank you so much
"There's something organic about a happy cloud of electrons" - Neutron Bob
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Old 22nd August 2006, 11:04 AM   #2
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are you sure that the circuit drawing is correct?

Filaments are overheated (valves are going to dead in a short time, if not already gone) and that poor 2uF cap shorting line voltage to ground dissipates about 40 watt.... obviously the chassis goes hot.

Take away the 2uF (you could use a 2nF instead, just to suppress HF interferencies), connect the filament series after the main switch and use a 240/110 step-down transformer, which will act also as a safety device. That kind of radios, connected directly to the main voltage, can be quite dangerous.

regards, Piero
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Old 22nd August 2006, 11:06 AM   #3
SY is offline SY  United States
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That kind of radios, connected directly to the main voltage, can be quite dangerous.
Can be? IS. This sort of circuit is DEADLY. The reason they aren't made like this anymore is that this arrangement is deadly dangerous, especially when run with the cabinet open. ABSOLUTELY use an isolation transformer- touching the chassis at the wrong moment will KILL.

Piero, thanks for adding that warning!
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Old 22nd August 2006, 11:31 AM   #4
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Yes, the circuit is pretty much wired like that, it's just an image i picked up from the web illustrating the "basic" connection of it.
I am aware it is lethal, i replaced the isolating cap from circuit to chassis for that
Will do as suggested. For the filter caps, 220V rating is enough?

"There's something organic about a happy cloud of electrons" - Neutron Bob
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Old 22nd August 2006, 07:05 PM   #5
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[For the filter caps, 220V rating is enough? [/B]
220 VDC WV? I'd say yes, the peak voltage should be about 170V at the rectifier cathode with a nominal input of 120VAC .

regards, Piero
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Old 23rd August 2006, 04:32 PM   #6
kevinkr is offline kevinkr  United States
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This looks to me like someone's REALLY BAD idea for powering a 120V AA5 directly off of 230V. The 2uF cap provides a series reactance (reactive divider) that is supposed to effectively drop the filament voltage to 120V. The B+ is derived from the difference between chassis ground and that cap - theoretically if the currents all balance the chassis will be at half the supply voltage. DANGER! It is unlikely to balance, and the circuit will be extremely sensitive to very small variations in the value of the cap. The losses in the 2uF cap theoretically are in the imaginary plane, but the losses from ESR and DA are not. A very large cap of just the right value would be required to make this work properly. I would find the correct schematic reconfigure it for 120V operation and for HEAVEN'S SAKE use an ISOLATION step down transformer. Do NOT use this with the usual auto-transformer, it could kill you.
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Old 23rd August 2006, 04:55 PM   #7
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Hey, I hope this isn't thread jacking, so i'm sorry if it is

I have an AA5 radio that I jolted myself on once (i should be dead), If I use it with an Isolation transformer will It keep me from getting Jolted?

also, how do I attatch a Third prong to an AA5 radio? because it says on the back of my radio in big letters "DO NOT ATTACH A GROUND TO THIS RADIO"
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Old 23rd August 2006, 05:06 PM   #8
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Every advice taken into consideration and capacitor removed. One of the paper caps i replaced connected the 35W4 pin 4(filament) leg to ground. Can i do without this one too? It doesn't seem to be doing much, unless referencing the filament to ground?
One of the hardest things will be finding an isolation transformer, as all i can find is autoformers Will probably end asking to custom wound one for me.
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Old 23rd August 2006, 05:50 PM   #9
Tweeker is offline Tweeker  United States
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Its more source R, but you can use a pair of any identical stepdown/stepup transformers back to back.

Edit: this just gives isolation, not stepdown. It may be possible to get 2:1 stepdown if the transformers have dual secondaries. Or an autotransformer could be used in series with this. Really low performance/iron though.
Be sure your foil hat has a good low impedance ground.
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Old 23rd August 2006, 05:53 PM   #10
anatech is offline anatech  Canada
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Hi Simpleton,
You need a step down transformer! This radio must be supplied with 115 VAC. You have overheated all the filaments and stressed the supply caps. I'm going to guess your 2 uF cap is history too.

You may have guessed at all this as previous posts have mentioned these concerns.

So, to begin with, you need a step down transformer with insulated windings between primary and secondary. Look in catalogs for transformer companies. I think I saw some at Hammond as well.

Hi alexmoose,
A 1:1 isolation transformer for you will prevent shocks due to a live chassis.

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