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... 

A440B Preamp - parts replacement

After being in My Southern Exposure for a few weeks in June, I am back to work on the audio projects.

Over the last few weeks, the complete inventory took place for all audio units to see what capacitors needed to be ordered, what parts were in-house, and which parts would be left “as-is”. Since the most notorious component in vintage electronic equipment is the Electrolytic, paper, and non-ceramic capacitors, most of those were targeted to be replaced and ordered if not in-house.

The 440 has two can type Multi-section electrolytic capacitors. I could only find a suitable replacement for one of them – a 40/40/20/10 UF @ 300VDC can. The closest I could find was 40/40/20/20 UF @ 450VDC. This slightly taller replacement model could also replace cans in the tuner and power amp units so I went ahead and bought five of them for $38 each. Back in the day, I could probably have gotten one of these for $15 each, so this is just the [extra] cost of time past.

Picture of original (left) and new Electrolytic Capacitors

Anyway, three other electrolytic capacitors in the 440 were on the list, one I ordered (40 UF @ 250VDC), and the other two were "in stock" (50 UF @ 3VDC being replaced by 47 UF @ 16-and-20VDC respectively).

After receiving the ordered caps, I sat down with soldering iron in hand to swap them out. This is a skill you probably do not lose, even after some 20 years of not being in the trenches (what we used to call “smelling some rosin!”). It took about two hours to complete the replacements. 

 New Electrolytic Capacitor installed (right)

As can be seen above, the new can was almost too high to fit and I had to remove some screws that held the base in place to get it situated. The new one had twist tabs to hold it in place but they seemed to not have tab edges to lock it in place so it wobbles a little right now. I will probably glue the base to the Bakelite riveted mounting. I also had to bend the tabs over on one of the accessory power outlets so it would not hit the top of the can. It does clear it but I will probably put a piece of electrical tape on top of the can. 

The left hand electrolytic cap has a spec that I have been unable to locate a close match (1000/1000 Uf @ 15VDC) so at the moment, I am going with this one. I did order some small radial 1000 UF at 16VDC caps, which I may figure out how to use instead of the can. Amazing the tech advance!

Cleaning

After completing the retrofit, I could not help looking at the grunge so I decided to clean out the tube compartment and wipe down the tubes. Of course, the printing on the tubes mostly came off as I rubbed them lightly with a Que-tip dipped in water. Assuming the tubes are still good, I may hold onto them for now, and consider replacing later.

Still to work on before power up

Besides more cleaning, the two wire A/C power cable will be replaced with a new 3 wire plug (with ground). Hopefully, I will be ready to power up in a week or so.