Preamp - parts replacement and maybe power up!

When we last met, I had replaced a number of the capacitors in the preamp, done some cleaning, and threatened to replace the AC cord before power was applied. Well, more research had me determine that I needed to replace more parts in the power sections.

Rectifiers
To provide some background, a Rectifier converts Alternating Current (AC) to Direct Current (DC). What comes out of the wall in your house is 120 Volts AC. However, most electronics built in the past 75 years or so uses Direct Current (DC) to power the tubes, transistors, and chips in them.

Rectifiers have taken many forms over the years, from Tube Rectifiers (like the one in the Altec A-340 Amp we will get to later), to Solid State rectifiers of various flavors which include current day devices often referred to as "Diodes".

"Selenium Rectifier"
In the Preamp, a number of "Selenium" Rectifiers are used in the power supply sections. These devices were state of the art back in the 50's and they were used as a small alternative to the use of tube type Rectifiers.

  Selenium Rectifiers for the high voltage power supply

 
 

  "Dual" Selenium Rectifier for the tube filaments

 
While these were an improvement over the use of Tubes, they have several disadvantages. They are notorious for having short life spans and their operational specs would vary over time. Worse, they are known to fail, which could be anything from opening up (no voltage through) to shorting out, burning up, etc. Since Selenium is toxic, breathing the fumes from one that is failing is not recommended....

Bottom line: replacing these devices is cheap insurance.

Selenium Rectifier replacements and new wiring
Fortunately, silicon rectifiers that will perform the same function are very inexpensive. I picked up 20 pieces of the 1N4007 rectifier for $1.20 ($0.06 each). Since the normal voltage drop across the old and new Rectifiers differed, I also ordered some 100 Ohm "Wirewound" Resistors but will not install those until I have powered up the supply and checked some of the voltages (more on these Resistors later...).

Since the existing rectifiers were mounted to the chassis with screws and nuts and the new rectifiers are solder pigtails, some terminal blocks were needed. While I ordered the rectifiers online, I figured I could find the Terminal Blocks at the local Radio Shack store...well, that took some work with the "Shack" becoming a scarce commodity. Anyway, in the end, I was able to "travel"  to a Radio Shack that carried the Terminals.

After receiving these, I lit up the soldering iron and got to work on the rewire job. It took a few hours to remove the Selenium's, install the Terminal Blocks, add the new rectifiers and hook everything back up. 

 Old and new Rectifiers

 
After completing and checking the wiring, I disconnected the 3 wires that go from the power system to the main circuitry which is good practice before powering up such equipment. I wanted to check all voltages coming out of the power systems before applying it to the circuits. Further, I might break out the Oscilloscope to look at the power filtering.

Power Up?
Normally, the recommendation is to bring things like this up under a Variac (Variable voltage source) to ensure that if something serious is wrong, the voltage can be cranked down quickly. But I do not have one of those, and what fun is it finding out the easy way...

Since anything can happen with such ancient equipment under first power up (and me being so brave!!!), I ran an extension cord to the Preamp power cord and plugged it in on the other side of the room! 

You will have to wait for the next update to find out what happened...