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 Emerson model 25A 1932
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 Return to top of page · Post #: 1 · Written at 9:16:58 PM on 8 April 2019.
Tallar Carl's avatar
 Location: Latham, ACT
 Member since 21 February 2015
 Member #: 1705
 Postcount: 1574

I am fixing this little gem for a good friend and what a education. Valves are 36, 37, 38 and 39. Ac/dc set but not a hot chassis.
I recapped it but not working . All resistors ok except for the 305 ohm dropper resistor. I have ordered a 300 ohm 30 watt dropper resistor from rd components. This is another beaut little set and hoping to have it up and going soon.

Emerson 25A Valve Radio
Emerson 25A Valve Radio
Emerson 25A Valve Radio
Emerson 25A Valve Radio
Emerson 25A Valve Radio
Emerson 25A Valve Radio


 
 Return to top of page · Post #: 2 · Written at 9:49:35 PM on 8 April 2019.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4337

Interested to see the circuit. 36 is interesting as its a Tetrode. I would suspect that this is an Autodyne (Screen grid). Some of them can be cantankerous. same rules as for a superheterodyne if it is?

I normally use a Signal Generator & Oscilloscope as a signal tracer on them when they loose signal. Watch the plate detector, they are great for causing distortion especially with changed voltages & dodgy components. Occasionally they need adjustment. If 37 is the Plate detector is should be set to draw 0.2mA. The voltage a cross the cathode resistor will give you that figure (High impedance meter).

IF may be as low as 175kHz


 
 Return to top of page · Post #: 3 · Written at 10:00:58 PM on 8 April 2019.
Tallar Carl's avatar
 Location: Latham, ACT
 Member since 21 February 2015
 Member #: 1705
 Postcount: 1574

Marc I have sent the circuit as well will email it to you.


 
 Return to top of page · Post #: 4 · Written at 10:11:25 PM on 8 April 2019.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6010

Schematic is available on Radiomuseum:

https://www.radiomuseum.org/r/emerson_25a_25_a_chg4.html


 
 Return to top of page · Post #: 5 · Written at 8:50:02 AM on 9 April 2019.
Wa2ise's avatar
 Location: Oradell, US
 Member since 2 April 2010
 Member #: 643
 Postcount: 766

That is a hot chassis design. Though they used a capacitor to the chassis from the line to make it a little less dangerous.

I'm assuming you are using a 240 to 120V autotransformer to feed this set.

It looks like a TRF set. If the 205 ohm dropping resistor is open (bad) then the tube heaters will not be powered, and the set will not operate.

The resistor will "burn" a lot of power. A better way, if you can find a place to place it, would be a transformer to go from 240VAC to 24V secondary to feed the tube heaters, and a 120V secondary to provide the rectifier tube to make the B+. This would make the set "cold chassis".

Or use a 240V to 24V transformer to run the heaters, but the set is then "hot Chassis".


 
 Return to top of page · Post #: 6 · Written at 7:28:45 PM on 9 April 2019.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4337

NBN was down for a large part of the day just got to the circuit. They are supposed to be upgrading? To what I don't know? The tower should have been put in the nearby citrus orchard.


 
 Return to top of page · Post #: 7 · Written at 9:00:08 PM on 9 April 2019.
GTC's avatar
 GTC
 Location: Sydney, NSW
 Member since 28 January 2011
 Member #: 823
 Postcount: 6010

They are supposed to be upgrading? To what I don't know?

Smoke signals (CCITT approved, of course).


 
 Return to top of page · Post #: 8 · Written at 9:07:49 PM on 9 April 2019.
Marcc's avatar
 Location: Wangaratta, VIC
 Member since 21 February 2009
 Member #: 438
 Postcount: 4337

I have sent the RCA data for the four valves. The last time I dealt with a triode wired as a rectifier as the #37 is; was a #24 in a Lyric 70 suppling the somewhere around 50-80V to the grid of a #50.

#37 plate looks like it only handles around 7.5mA but the rest can draw up to 20mA.. However, the cathode resistors tell the story. Like 7mA for #38. If you persist with mains C1 & C3 should be line caps.

Thinking aloud: Heaters as they are 25.2V . 300mA draw: 24V would suffice. NB the American mains voltage has increased from 110 to around 120V so the resistor may be too small Dropping from 110V to 6.3 suggests 0.34A; I squared R = 35 Watts to dissipate. With the new voltage we drop near enough to114V. To get 300mA we need 380 ohms. So a chassis mount resistor of 390 Ohms and 40W is probably getting closer.

I am thinking a transformer may be more practical or a PSU for battery sets? I will check with one I know makes them.

If the voltage is changed R3 on the plate detector may need adjustment to get it back to 0.2mA?

Marc


 
 Return to top of page · Post #: 9 · Written at 7:16:22 PM on 16 April 2019.
Brad's avatar
 Administrator
 Location: Greenwich, NSW
 Member since 15 November 2005
 Member #: 1
 Postcount: 6347

Photos uploaded.


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

 
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