How To Replace A Selenium Rectifier

This Zenith 7h822-Z, also known as the "Triumph", was on the Retro Radio Shop bench recently. Among a few other issues, this unit still had its original selenium rectifier in place which had failed. Follow along as we replace the item with a diode and explain the process. 

Replacing a selenium rectifier is a common and necessary task when restoring vintage tube electronics. While these components were standard in the mid-20th century, they are prone to failure and pose specific safety risks as they age. 

1. Firstly, understanding the Selenium Rectifier

A selenium rectifier consists of a stack of aluminum plates coated with selenium. It acts as a PN junction to convert alternating current (AC) to the direct current (DC) required for the radio's vacuum tubes. Over decades, the internal resistance of these plates increases, leading to a drop in output voltage and excessive heat generation. In the event of a catastrophic failure, they emit toxic hydrogen selenide gas, characterized by a pungent, "rotten egg" smell like my father made after a pint and a few pickled eggs. 

Here's the culprit and a schematic highliting the rectifier in circuit. On this radio, the 22 ohm resistor was blackened and the electrolytic capacitors were also destroyed, failing ESR testing dismally.

The reason this occurred is that when the rectifier failed, it subjected the polarized electrolytic capacitor to alternating current (AC) voltage. This causes rapid overheating, catastrophic failure, and therefore a possible small explosion.Because electrolytic capacitors are designed for direct current (DC) and rely on a thin oxide layer maintained by correct polarity, reversing the voltage (which happens every cycle in AC) destroys this dielectric layer and leads to a short circuit. When you see the YouTube videos of folks blowing up electrolytics, that’s the failure process.

OK, but why the damaged resistor you may ask? Selenium rectifiers inherently reduce inrush current due to their high forward resistance, which acts as a built-in current limiter during startup. The failed rectifier decreased the resistance, surpassing the resistor current rating limits and subsequently destroying it.

2. Safety First:  Isolation Transformers when considering how to replace a selenium rectifier

Before opening any vintage radio, especially "transformerless" AC/DC sets (often called All American Five or AA5 radios), you must use an Isolation Transformer. We also recommend using a Dim BulbTester as a current limiter and visual tool.

In these designs, one side of the AC power line is often connected directly to the metal chassis and referred to as a Hot Chassis radio. If the plug is inserted "backward," the entire chassis becomes electrically "hot" relative to the earth ground. Touching the chassis while grounded can result in a lethal electric shock. An isolation transformer physically separates the radio from the mains earth ground, preventing this shock hazard during servicing.

** As well, an inline fuse should be installed for added protection. A common failure mode for silicon diodes is to short and not fail in an open state. Therefore, a fuse prior to the power switch should be installed. **

3. Selecting the Replacement Diode

The modern standard replacement for a selenium rectifier is a 1N4007 diode. 

• Voltage Rating: The 1N4007 exceeds the peak inverse voltage (PIV) requirements of most consumer radios, with a rating of 1000 V. 

• Current Rating: It handles up to $1\text{A}$ of continuous current, which is more than sufficient for the typical $50\text{mA}$ to $150\text{mA}$ draw of a tube radio.

4. Determining the Series Resistor Value

With respect to how to replace a selenium rectifer, silicon diodes are much more efficient than selenium rectifiers. A selenium rectifier typically has a forward voltage drop of $7\text{V}$ to $12\text{V}$, whereas a silicon diode drops only about $0.7\text{V}$. Without a compensation resistor, the DC "B+" voltage in the radio will be too high, potentially damaging the vacuum tubes or causing the electrolytic capacitors to exceed their voltage ratings.

Don't let the math frighten you; if required, you can easily use an Ohms Law calculator online. To find the correct resistance (R), follow these steps:

• Step A: Measure the Voltage. Measure the DC voltage at the output of the new diode ($V_{\text{new}}$) and compare it to the voltage specified in the radio's service manual ($V_{\text{spec}}$).

• Step B: Measure the Current. Measure the total DC current (I) drawn by the radio in amperes.

• Step C: Calculate Resistance. Use Ohms Law

$$R=\frac{V_{\text{new}}-V_{\text{spec}}}{I}$$

5. Determining the Resistor Wattage

The resistor must be able to dissipate the heat generated by the voltage drop without failing. To calculate the power ($P$) in Watts

$$P=I^2\cdot R$$P = I(2)⋅R

Safety Margin: When considering How To Replace A Selenium Rectifier, ALWAYS use a resistor with a wattage rating at least twice the calculated value to ensure longevity and prevent the component from becoming a fire hazard.

Example Calculation:
The new DC voltage is $145\text{V}$, the required voltage is $125\text{V}$, and the radio draws $60\text{mA}$ ($0.06\text{A}$):

R = 145V - 125V  / 0.06A   =      20V / 0.06A = 333.33 ohm         

Given this, a standard $330\mathrm{\Omega }$ resistor is used. For the wattage:

P = .0036 X 330 = 1.18W

Applying the $2\times$ safety factor, a 3W or 5W resistor should be selected. 

6. Installation Summary

1.  Disconnect Power: Ensure the radio is unplugged and capacitors are discharged.

1. Mounting: You can leave the old selenium rectifier in place for aesthetic reasons but disconnect all wires from its terminals.

1. Orientation: Solder the 1N4007 diode in place on a terminal strip. The cathode (the end with the stripe) must point toward the first filter capacitor (the positive DC side).

1. Resistor Placement: Solder the calculated power resistor in series with the diode on the terminal strip. It is usually placed after the diode's cathode.

1. Testing: Power the radio through a variac or Dim Bulb Tester while monitoring the B+ voltage to ensure it matches the manufacturer's specifications.

 Thanks for reading our article on How To Replace a Selenium Rectifier. If you would like to obtain formal online Antique Radio Repair training, check out the Antique Radio Repair Academy (ARRA) courses.