radiomobil: Radiomobile 52T convert positive to negative ground?

Hello all,

new person here.  I recently purchased the above radio.  On the bottom of it there is a red plug, which according to the adjacent information sticker converts the radio to positive ground.  Indeed, I did use a 12 VDC source (an old computer power supply), and connected the +12 VDC to the ground, and the ground wire from the power supply to the power wire of the radio, and it worked!  Actually, it was surprisingly clear AM reception for the sparse AM radio we have here in Prescott, AZ.  According to the sticker if I were to have a black plug, the radio could be converted to negative ground.  I wish to use this radio in a restored automobile that will have a negative ground.  Here are my questions, please:

 

 1.  I am presuming there is no way I could find a vintage black plug to convert to negative ground.  Would there be a way to add jumper wires inside the radio to convert to negative ground?  I am handy with a soldering station, in the past I have diagnosed and repaired pinball machine circuit boards, but I am brand new to tube radios.  Just observing the schematic I would think it would be possible, but I do not wish to blow capacitors!  :-)  I am thinking the SK3 notation on the schematic is referring to the red/black plugs I was referring to.

 

2. Could someone please observe the schematic and tell me where the loudspeaker output is? Again, from my newbie observation I would guess the notation SK2 is it, but I wish confirmation (or rejection) of this assertion.  :-)

3. If I do have to open up the box for jumpers, would it be helpful to upload photos of the electronics?

Thank you in advance for any and all information you can give me.

Regards, Dan

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Hi Dan.

Taking some detailed pictures of your set and uploading them to the model page will probably bring out more reliable answers to your questions.  It will also provide you with UACS points.  If you can unplug your existing power selector, that is the red plug on the bottom, from the SK3 socket so we can get a clear view of the socket, that will help as well and we may be able locate hardware necessary to create a "black plug".  I have looked at the schematic of the Radiomobil 402T which contains the pinouts for the red and black plugs.

The radio diagram shows a fuse, F1.  You would want to locate that and make sure that it has not been bypassed.  I don't see a value for the fuse on the schematic, but I would guess that somewhere between 5-10 amps would be the value.  When installing into your vehicle, an extra inline fuse for extra piece of mind won't hurt.

You are right wth the assesment of SK2 being your speaker output.  According to the schematic, this should be a 3 pin socket.

I realize that I haven't directly answered your questions, but let's try to start here and I'm sure we can make the appropriate modifications to your set for a negative ground.

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Yes Dan some nice 933 pixel wide photos!

 

From R&TS 1959/1960 for Model 402T

Not all models actually do have polarity reversal.

The 52T is same as 42T with 5 component changes, which does allegedly use the same polarity plug as the 402T. It's just a plug with wires.

The four valve heaters are in parallel series and not affected by the polarity plug. They will liight up with it unplugged. If you get the "plug" wiring wrong you short the supply out! The top connection is to the chassis. either side of that is Radio valve -ve rail and audio amp -ve rail. The two bottom connections go to audio amp and RF section +ve rail.  The two pins across the middle are the feed from battery via fuse, filter and with lamp and heaters across it.

Which makes the left wiring make sense. Also it's irrelevant that the connection is "seen from rear" as it's symmetrical!

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Front panel, Top, Bottom, rear, clear shots of any labels.

Insides is good too. But absolutely nothing needs changed there. Don't adjust, it's working!

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