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 Post subject: Sparton 30 Power Supply Question
PostPosted: Dec Tue 03, 2019 2:59 am 
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Joined: Sep Sat 24, 2011 2:47 am
Posts: 58
Location: Orange, CA
I'm having trouble with the B+ voltage. I get 0v across the B+ voltage divider (2.9K & 3K). - see attached schematic - And the voltage where the center taps for the B+ and 2.5V for the 45s join the filter caps is -400v with respect to ground. Seems to me that center tap voltage should be somewhere close to ground, but I don't understand how it is generated. Appreciate any help.


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 Post subject: Re: Sparton 30 Power Supply Question
PostPosted: Dec Tue 03, 2019 4:43 am 
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Joined: Nov Mon 02, 2009 7:01 am
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Location: Lincoln City, OR 97367
Greetings to jwd and the Forum:

The schematic is very hard to follow due to the phono-radio switching and the overall drawing style. However, my first question would be: Do you have the speaker plugged in? The negative return for the power supply appears to be through the speaker field coil. If the coil is open or the speaker is not connected, the voltages will be as you measured.

If the speaker is plugged in and OK, see if you can print out the schematic and highlight the path from the 800 volt winding center tap to ground. Many of the tube cathode circuits are already connected to the negative buss as it leaves the speaker field coil, but I do not find a ground connection. It is possible that there isn't one, in which case the radio has a floating B- and you will have to connect your meter negative lead to that point to measure voltages.

Good Luck,

_________________
Jim T.
KB6GM


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 Post subject: Re: Sparton 30 Power Supply Question
PostPosted: Dec Tue 03, 2019 6:21 am 
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Joined: Sep Sat 24, 2011 2:47 am
Posts: 58
Location: Orange, CA
Thank you Jthorusen for replying. Attached is the schematic of a Sparton 45. This has the same power supply configuration as the 30, but easier to follow. The B+ path to ground is in the voltage divider following the filter choke and the 3rd filter cap which is also referenced to ground. In my measurements the speaker is plugged in and the field coil has continuity. Would appreciate and further thoughts.


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 Post subject: Re: Sparton 30 Power Supply Question
PostPosted: Dec Tue 03, 2019 9:20 am 
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Joined: Nov Mon 02, 2009 7:01 am
Posts: 4611
Location: Lincoln City, OR 97367
Greetings to jwd and the Forum:

The Sparton 45 schematic is indeed easier to follow. However, we are not interested in the B+ path to ground. We are interested in the B- path to ground. In the 45 schematic, it will be seen that the center-tap of the high voltage winding (which is the most negative point in the system) is routed to the speaker field coil and thence to ground. If the speaker field coil were open, the B+ line would be at zero volts and the B- line (negative terminals of the filter caps) would be at -400 volts or so, exactly as you are measuring.

This is why I asked if the speaker were connected and if its field coil had continuity. In the 45, the speaker is apparently permanently wired in; the 30 schematic shows a connector in the circuit.

You can restore the set to normal operation to prove this by grounding the negative terminals of the filter capacitors to the chassis with a clip lead. You will not have much if any audio, because their will be no speaker field, but all of the B+ voltages should revert to normal, or close to it.

If you go to the ground terminal on the far left of the schematic and follow the wire into and through the tangle of the main part of the drawing, you will discover that it is the ground return for the field coil and the B- of the power supply. It was so hidden that I couldn't follow it from the field coil end. Anyway, some part of this path is open. You will need to find out where.

By the way, if you follow the B- lead from the negative terminals of the filter capacitors up to the 100K resistor in the 45 grid circuit, you will see that it crosses six horizontal wire runs on the schematic. Observe very carefully; it hops over FIVE of these but it is CONNECTED to the second one down from the top of the bundle. If you follow that wire to the right and up, you will find that it connects to the speaker field. From there, the path is across the whole drawing to the ground terminal just under the "Aerial" terminal. VERY bad drawing.

Good Luck,

_________________
Jim T.
KB6GM


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 Post subject: Re: Sparton 30 Power Supply Question
PostPosted: Dec Tue 03, 2019 7:18 pm 
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Joined: Sep Sat 24, 2011 2:47 am
Posts: 58
Location: Orange, CA
Thanks Jim for taking the time to help me. Great explanation! I'll check it out and let you know what I find. -John


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 Post subject: Re: Sparton 30 Power Supply Question
PostPosted: Dec Tue 03, 2019 8:55 pm 
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Joined: Sep Sat 24, 2011 2:47 am
Posts: 58
Location: Orange, CA
Hi Jim - Found the open you described. I didn't see the dot on my schematic o the line below the 100K resistor from the input audio transformer and the B- side of the filter caps that connected them to the field coil. Thanks again - you've been a big help!


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