Power Supply voltages has gone up after recap


2008-09-23 9:52 pm
Hi All

I just recapped and replaced the rectifier diodes on 2x of my vintage power supplies for the tube radio, and all the output voltage readings are way higher than the circuit diagram. I couldn't tell you what they were before because they were non working.

But looking at the original circuit diagrams voltage quotes and my DMM readings now, there is big difference.

Circuit Diagram
Pin 3 = -80V
pin 9 = 280V
pin 11 = 700V
pin 4 - 6 (heater) = 12.6VAC

Power Supply 1
Pin 3 = -107V
pin 9 = 380V
pin 11 = 870V
pin 4 - 6 (heater) = 13.4VAC

Power Supply 2
Pin 3 = -106V
pin 9 = 484V
pin 11 = 900V
pin 4 - 6 (heater) = 12.88VAC

Is this normal? Or is there some problems with the recapping or the replaced rectifying diodes?
Will it cause damage to use the power supplies as they are now with those readings?

What could be done about it? - add resistors to reduce the voltages?
Or any other ways?

Thanks in advance.
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2008-09-23 9:52 pm
Are the power supplies connected to the radio and supplying current?

If you can upload the schematic, that would help.

Here is the link to the manual and circuit diagram.
http://www.arizona-am.net/PHOENIX/W7CPA/National NCX-A_manual.pdf

The voltages were read without the load.
And after that I connected the power supply to the rig, and was working. But then now, it is gone silent.

All the tubes light up, but no audio. I can hear very weak hum when volume at max.
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2008-09-23 9:52 pm
I wonder what the make and model of the equipment is.
Don't forget in the dark ages, pre 1980, all multimeter readings were based on the AVO model 7 or 8 and they have an impedance of 20kOhms per volt, not 10MOhms as most DVMs have.
It could make a difference.
Is the rectifier solid state, selenium or valve I wonder.

It is a vintage ham radio. NCX-3 and NCX-A power supply.
The radio is tube, but the psu is 4x diode rectifier with heavy mains transformer.
The power supply voltages without load will be higher.

You can test the power supplies first, independent of the radio, to make sure that they are working by connecting an appropriate test load resistor to each power supply output. Use Ohm's Law to calculate a resistor value to nominally load each output and measure voltages. Make sure the resistor can handle the power (2 * I * I * R).

If the power supply checks out, then you can work on the radio section.
Modern capacitors have lower ESR and the old ones had probably risen even higher than they were when new.

At these voltages rectifier Vf would not explain this great change in voltage.

Play around with DUNCAN AMPS power supply tool and change the ESR values of the capacitors and see what happens. You will be surprised.