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Vintage Test Gear and Workshop Equipment For discussions about vintage test gear and workshop equipment such as coil winders. |
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3rd Sep 2019, 6:57 pm | #1 |
Pentode
Join Date: May 2018
Location: Cologne, Germany
Posts: 104
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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. |
4th Sep 2019, 8:48 am | #2 |
Nonode
Join Date: Jun 2007
Location: Coulsdon, London, UK.
Posts: 2,163
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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? |
4th Sep 2019, 10:35 am | #3 |
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,535
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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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5th Sep 2019, 2:59 pm | #4 |
Pentode
Join Date: May 2018
Location: Cologne, Germany
Posts: 104
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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. |