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Post by Kerry Tue Apr 13, 2021 11:27 pm

Hello Everyone,
I am new to the forum and inexperienced in electronics.

I am restoring a 93BR-714B Airline tube radio.  I did research on the net, replaced the filter and paper capacitors, replaced the resistors, replaced three bad tubes, and plugged it in. Lights work, tubes light up and get hot, no signal anywhere.

I started testing the pin voltage according to the schematic.  The transformer is outputing 780 V on tube 5Y3G pin two and the schematic says it should be 320V.  Pin 8 has a funny "E" on the schematic and the meter reads 780 volts on that also. Pin two goes to most the other tubes so they all read high on AC voltage.

The transformer insulation looks like brittle old paper and looks like it has been hot (brown).  The transformer does not get hot to the touch.

Am I reading the meter correctly?  Why would the voltage be more than double?  How do I correct this? Do I need a new transformer?  
I don't dare run the radio too long as I'm afraid the high voltage will damage tubes and other components.

Any help would be greatly appreciated.

Thanks,
Kerry

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Post by mr_ed01 Fri Apr 16, 2021 2:03 pm

The funny 'E' is actually a 'C'.  Note C says 5.2 volts AC measured across pins 2 and 8 of the 5Y3.  What AC voltage do you measure across pins 4 and 6 of the 5Y3? Schematic indicates that AC voltage should be about 625 volts.

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Post by Kerry Fri Apr 16, 2021 5:23 pm

Thank you for that clarification. Very Happy

I get 640 V measured across pins 4 and 6 of the 5y3, 320 v on each pin separately. I get 780 v each on pins 2 and 8 of the 5y3, which I assume is the problem.

I took the transformer apart. It is pretty fried. Probably not worth messing with.

I found tube amp transformer
110V 250W 4A 300B power transformer amplifier 320V-0-320V 0-5V 0-6V
on ebay. The specs seem to make sense now with your insights. They have two 320 V leads with a common ground, two 6.4 v Leads and grounds, and two 5.0 V leads and grounds. Would that transformer work? If so, how would I wire the 320 v leads and ground? I assume I would need the two 320 V connected to pins 2 and 4 and the ground to pin 3 to get the 640 V across pins 2 and 4. One of the 5.0 V leads to pin 8. One of the 6.4 V leads and ground to tube 6SA7. But where would pin 2 get its 320 V from? Also, how would measuring across 2 and 8 get 5.2 v if pin 2 has 320 V as on the schematic?

Sorry for my ignorance, but I am learning a lot (half the point of the restore).

Thank you so much for your help,
Kerry

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Post by mr_ed01 Fri Apr 16, 2021 9:17 pm

When you were measuring the 780 volts, was your meter set to AC volts or DC volts?  You replaced filter capacitor C19 with a 450 volt electrolytic?  

That 300B transformer is way overkill for that radio.  The transformer you need:  340-0-340 @ 70 ma,  6.3V @ 3 Amp, 5 V @ 2 Amp.
A Hammond 272X Transformer 272X might be what you need.

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Post by mr_ed01 Fri Apr 16, 2021 11:11 pm

When you were measuring the 780 volts, was your meter set to AC volts or DC volts?  You replaced filter capacitor C19 with a 450 volt electrolytic?  

That 300B transformer is way overkill for that radio.  The transformer you need:  340-0-340 @ 70 ma,  6.3V @ 3 Amp, 5 V @ 2 Amp.
A Hammond 272X Transformer might be what you need.

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Post by Kerry Sat Apr 17, 2021 11:20 am

The meter was set to AC. I replaced C19 with a 10 uf 450 volt electrolytic.
I am confused on the transformer wireng on pin 2.
Thank you so much for taking the time to help me,
Kerry


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Post by mr_ed01 Sat Apr 17, 2021 1:50 pm

When measuring the voltage at the 5Y3 pins 2 or 8 to ground, the meter should be set to DC.  The only AC voltages to be measured are the transformer high voltage winding and the 2 filament windings, for everything else, DC.

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Post by Kerry Sun Apr 18, 2021 12:32 pm

The 2 and 8 pins are definitely reading AC volts. The replacement transformer you recommended, 272x, also lists the 5 V outputs as AC.
I hooked up the old transformer with the case off to a house outlet. All the outputs are reading correctly. The insulation on the transformer is fried, brittle and breaking. It must be shorting out against the case. I ordered the 272x. I'll post how things go once it arrives.
Thanks again for all your help. I think I finally understand the power supply.
Kerry

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