[NJARC] Selenium Rectifier Question (Heath SG-8)

[antqradio at juno.com]

Joe
If you are measuring 400 VAC RMS on the transformer side of the rectifier
to ground, then it is quite close to calculated voltage.

According to the SG-8 manual downloaded from BAMA, pin 1 of the 12AU7 is
at approximately 80 DCV.  Assuming that the 3.3k 1 watt resistor between
the filter capacitors is actually dissipating 0.5 watt, then about 12 mA
is flowing through it.  The voltage dropped across the 3.3k is then
approximately 40 volts.  This plus the 80 volts on pin 1 equals about 120
VDC.  Divide 120 VDC by 0.318 gives you approximately 377 VAC RMS.  Close
enough.

Resistors pass current, not voltage.
If there is no voltage dropped across the 3.3k resistor, then there is no
current flowing through it.
If both sides of the resistor read approximately 120 VDC to chassis
ground, then the tubes are not drawing current.  Are the filaments lit?
If both sides of the 3.3k resistor read 0 volts to chassis ground, then
the selenium rectifier is open.  Try a 1N4007 as a substitute but
remember to remove the selenium rectifier from the circuit.  Selenium
rectifiers are a disaster just waiting to happen.
Regards,
Jim


On Sun, 26 Feb 2006 21:51:05 EST TrainBee at aol.com writes:
> Visit our web site - See http://www.njarc.org
> _______________________________________________
> Hello all,
> 
>     I am having a problem with my Heath SG-8 Signal Generator.  The 
> fillament 
> voltage is OK.  The other side of the transformer goes to the 
> Selenium 
> Rectifier.  There are two problems here.  
>     1) I have no reference for the voltage that is going into the 
> rectifier 
> and what should be coming out of it. I am getting a negative reading 
> on the 
> input to the rectifier and a 400v +voltage on the output side.  
> Strange since the 
> electrolytic is rated for 150volts.  Any ideas what voltage I should 
> be 
> seeing? 
> 
>     2) The output from the rectifier goes to a 1W 3.3Kohm resistor 
> to serve 
> the oscillator section.  With the unit off the resistor checks OK.  
> With it on, 
> the resistor does not pass any voltage.  Any ideas of what is going 
> on here?
> 
>     The Heath SG-8, uses a 6C4 and a 12AU7.  Both check OK.  The 
> unit is 
> wired correctly, double checked.  All of the other components check 
> OK.  This 
> leads me to the question above.
> 
> Thanks for any guidance.
> Joe Devonshire