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 Testing filament circuit.
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 Return to top of page · Post #: 1 · Written at 1:13:04 PM on 21 February 2015.
Airzone's Gravatar
 Location: Maclean, NSW
 Member since 30 May 2008
 Member #: 291
 Postcount: 341

I have a Franklin radio I brought back from Argentina and I want to ask a question.
Is it OK to disconnect the filament, lamp and rectifier circuit from the rest of the radio?
Parts of the dropping resistor were missing and I want to test currents and voltages. The red cross is where I want to disconnect.
See picture attached here:

Franklin Radio

Peter


 
 Return to top of page · Post #: 2 · Written at 3:28:26 PM on 21 February 2015.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4506

That tells me you do not understand that radio. This is a "Hot" chassis radio which is not intrinsically safe. The globe is a specific one, of a specific number and removing it will adversely affect the heater string & any voltages you get will then, likely be wrong.

C1 needs to be a mains approved "X" type, any wax paper, or oil type caps replaced and most likely the electrolytics and wiring inspected, before powering it up.

If you have a Variac that will lower the mains volts: But!

But! ... Being a transformerless set the chassis has to be considered "Alive" and dangerous. Normally these are powered up via an isolation transformer. "Variacs (Slide regulators) and other brands are not an isolation transformers, they are "auto transformers"

As there is a 50/50 chance of the chassis being "active": There is a good probability of it killing you if you touch it.

Marc


 
 Return to top of page · Post #: 3 · Written at 3:45:43 PM on 21 February 2015.
Brad's avatar
 Administrator
 Location: Greenwich, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 6573

The power switch appears to control the neutral connection rather than the active. If the radio is to be put into service I would recommend a double pole switch so that a cut off of the active is guaranteed, no matter which way the power point is wired.


‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
A valve a day keeps the transistor away...

 
 Return to top of page · Post #: 4 · Written at 8:12:05 PM on 21 February 2015.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4506

Very common for some reason, for the Americans to do that. I do from time to time tell them to fit double pole switches to them, when refurbishing.

They seemed to be of the opinion the 110V did not kill, so I guess the change to around 117V mains was to eliminate those of that opinion.

Marc


 
 Return to top of page · Post #: 5 · Written at 9:14:12 PM on 21 February 2015.
Airzone's Gravatar
 Location: Maclean, NSW
 Member since 30 May 2008
 Member #: 291
 Postcount: 341

Hi Guys,
Marcc, I am fully aware of the HOT chassis. I have an isolation transformer and variac built into my test bench. I wanted to see who would recognise and comment first.

I was merely asking if I break the output from the rectifier UY41, is it OK to just run the filament, lamp and rectifier valve circuit on their own. I just want to measure voltages and current draw.

I take point on C1, I have changed also the usual paper caps etc.

With the double pole switches, are they a double pole volume pot/switch ?

Peter


 
 Return to top of page · Post #: 6 · Written at 9:55:29 PM on 21 February 2015.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4506

They did / do make volume & tone pots with double pole switches on them, they tended to be the dominant ones in some brands.

The only reason I would break the B+ is to insert a milli-amp meter & that's not, to me, really necessary.

You can get the AC current from R14, and a good guide of the DC current from the sum of the cathode resistor R18, & R13's current

The important B+ voltages are the ones on load. & the one on startup to make sure there is no voltage surge in excess of the filter cap ratings. All the rest: On load.

Marc


 
 Return to top of page · Post #: 7 · Written at 11:43:22 PM on 21 February 2015.
Airzone's Gravatar
 Location: Maclean, NSW
 Member since 30 May 2008
 Member #: 291
 Postcount: 341

OK, also found the old diode across the rectifier valve trick


 
 Return to top of page · Post #: 8 · Written at 6:48:57 PM on 22 February 2015.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4506

Brilliant idea that if its not compensated for. Apart from increasing voltage, as consequence of it being more like a filament rectifier in its behaviour, you are confronted with PIV.

B+ is instantaneous and that means that the filter caps have to be able to withstand the additional voltage. Should this not be the case, they will punch through.

I have already had that happen recently, when a dodgy new one, rated at 500V let go just past 450V. I have since tested it & B+ . 400V, with the heaters off and everything so far says cap fail.

Marc


 
 Return to top of page · Post #: 9 · Written at 9:11:51 PM on 22 February 2015.
Airzone's Gravatar
 Location: Maclean, NSW
 Member since 30 May 2008
 Member #: 291
 Postcount: 341

I am going to replace the faulty UY41 and remove the diode, so then I need to ask, does anyone have a good or new UY41 they would like to sell.
C1 on the aerial measures OK, it is a ceramic, would you leave this or replace it.


 
 Return to top of page · Post #: 10 · Written at 10:02:55 PM on 22 February 2015.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4506

Personally, I would be rid of it. Whilst Ceramics & Mica types here are fairly reliable, they are not infallible & therein lies the problem.

"X" Types are an approved type for mains and are designed to fail safe (open), hence my pedantic enthusiasm, for being rid of caps exposed to the mains, that were not approved, or designed to be there.

Marc


 
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