scott 399 rebuild.. rectifier question

[ducati2]

can anyone recommend a silicon diode bridge rectifier and resistor set up to replace my original selenium rectifier in my scott 399 receiver?

and source would be great for the silicon.. maybe lots of places carry those, but not sure.

 

[kvflyer]

I don't have a schematic for a 399 to look at for this receiver. I assume that you are referring to the Siemens Selenium Bridge that is used in many Scott amplifiers for the bias and the preamp filament supply.

I know that Radio Shack is almost gone so I hesitate to tell you that they had a silicon bridge that went for about ~$2-3. Something like 200 Volt, 8-Amp. It would bolt in place of the existing bridge without making any more holes in the chassis. As for the resistor, often Scott used an 18Ω 1-watt resistor first after the bridge. It can usually be changed to a 22-33Ω 5-watt resistor and the voltages will usually be spot on. You may need to adjust the value up or down to get them correct.

Hope that helps.

 

[ducati2]

I am all good, except for figuring out exactly where the resistor would go. 'After the bridge' does not help this rookie.. would it be between the bridge and what item you think?

 

[kvflyer]

Sorry! There may be a resistor, either 10Ω (Brown-Black-Brown-Silver) or 18Ω (Brown-Gray-Brown-Silver) connected from the negative (-) terminal of the Siemens Selenium bridge to the first capacitor section/lug/terminal in the bias/filament power supply.

That capacitor will be a four section metal capacitor which has either 75µFD or 100µFD sections. Mind you, this electrolytic capacitor is odd! It has a common positive where most electrolytic capacitors have common negative, referring to the outside of the aluminum can.

This resistor, the one connected from the bias bridge to the capacitor may be changed to a resistor value of 22Ω, 27Ω or 33Ω 5-Watt wire wound resistor, which ever one drops the voltage to the correct amount.

Hope that helps

 

[drew_t]

The bridge rectifier has four terminals - two marked with a "~" sign, one with a "+" sign, and one with a "-" sign. The wires from the power transformer connect to the "~" terminals. The "+" terminal goes to ground. Follow the wire from the "-" terminal; the thing it connects to is probably going to be that 18 ohm resistor that kvflyer refers to, probably on a tag strip.

The Radio Shack part number for the 400V, 8A silicon bridge rectifier you can use to replace the selenium one is 276-1181.

 

[ducati2]

Thanks for the lesson! kflyer was trying to remember silicon spec and thought maybe 200v 8a, would a 400v 8a be better? the SM has this selenium item at .15 amp... Does this matter?

 

[drew_t]

Better, no; just more over-spec'd. The Radio Shack part is an easy replacement because it's dimensionally similar to the selenium one, and either is, or used to be, readily available (depending on whether they've closed all the Radio Shacks in your locale).