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Old 3rd Sep 2019, 6:57 pm   #1
sven_swe
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Join Date: May 2018
Location: Cologne, Germany
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Default RPM370/1 Valve Tester - Selenium rectifier replacement

Hi experts!

I just got hold of a German Neuberger RPM370/1 valve tester that has a selenium rectifier as original. This is for the negative grid voltage in ranges of 5, 10, 50 and 100V with a rheostat for fine adjust.
The section for 100V(110V) has been replaced before by someone with a 1N4007 diode since that section seems to be dead. I can notice some "flickering" when in the 5 and 10V range so it seems that this one is about to give up and I would like to change it.

My question is how to get about to replace this selenium rectifier correctly. I have read that 100-200Ohm high wattage resistor is often used in series with a diode to limit current surge and to compensate for the voltage drop. I do not know what the voltage drop "was" before since a section has already been replaced with one diode but without the resistor.

Would a 1N5408 diode with a series resistor of about 100Ohm 5W be sufficient as a replacement for each section? Which side of the diode is preferred to have the resistor?

I have attached an image of the rectifier and schematics.
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Old 4th Sep 2019, 8:48 am   #2
Silicon
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Default Re: RPM370/1 Valve Tester - Selenium rectifier replacement

Some moving coil meters will show needle vibration if they are connected to a half wave rectified voltage. It is effectively 50Hz pulsating DC supply.

A resistor in series with the new rectifier may help.

It looks like smoothing capacitors are used across the rheostats.
Have you replaced capacitors C4 and C5?
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Old 4th Sep 2019, 10:35 am   #3
Herald1360
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Default Re: RPM370/1 Valve Tester - Selenium rectifier replacement

If it provides bias voltages <100V, it'll be low current load so 1N4004 will be fine for all sections. Series resistors can go either end of the diode, it makes no difference in a series circuit. What their values might be you'll need to determine by experiment. If the voltage ranges are variable and checked with a meter anyway, you probably won't need to bother.
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Old 5th Sep 2019, 2:59 pm   #4
sven_swe
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Default Re: RPM370/1 Valve Tester - Selenium rectifier replacement

It is a lot better now after replacing the C4 and C5 and changing for diodes. No more vibrations.
100 Ohms is way too much though, voltage drop too high. I Read that it is highly recommended to have a resistor in series to limit surge current.
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