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Old 11th Oct 2014, 6:59 pm   #80
Phil G4SPZ
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Join Date: Apr 2005
Location: Bewdley, Worcestershire, UK.
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Default Re: Roberts RMB Problems

Quote:
Originally Posted by sobell1980 View Post
The two anodes of the 2 diodes are connected to the LT windings of the transformer. The two cathodes of the diodes are soldered together and joined into the circuit where they eventually meet L17.
That's your problem. Where is the negative connection? According to the Trader service sheet, the heater winding on the transformer doesn't have a centre tap. The circuit should be a full-wave rectifier, hence a bridge rectifier is necessary and you must have four diodes, not two. Wire the four diodes exactly as shown on the service sheet.

Be aware that modern silicon diodes may have a lower forward resistance than the original selenium rectifier, and the LT voltage to the valves may be higher than it should be. If you measure the LT current flowing from the battery as you have previously been advised, we can suggest a suitable value of series resistor to bring the voltage down to the correct value.
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