Part 1: The power supply capacitors
It’s not a secret that one of the more economical ways of building a HiFi system is to purchase used or vintage gear. There’s a plethora of old gear available that can be found on the second hand market from companies such as Audio Research, Conrad Johnson, Luxman, PS Audio, Dynaco, Marantz, Sansui, Quad, and many more. Unfortunately, older gear will likely sound inferior compared to the day it was purchased, as certain electronic components under the cover age over time.
During my early college years when funds were tight, I purchased a Marantz Model 1060 off of Craigslist for a few hundred bucks. I remember feeling ecstatic about the deal and rushed home as soon as possible to hook it up to my system. When I first powered it up, I was in plain and utter shock. The soundstage was nonexistent, the treble was rolled off, and the bass sounded like someone was kicking a cardboard box. “What the hell had gone wrong?”, I thought. It was indeed one of the worst sounding amplifiers that I had ever heard!
After browsing some audio forums on the topic, I learned that the capacitors had probably aged, dramatically compromising the sound quality. With help from some of the forum members, I was able to get a schematic and order the correct replacement capacitors. Despite being concerned of my lack of soldering skills or understanding, I was surprised with how easy and exciting it was. This project was actually enjoyable and gratifying, like the feeling of refinishing an antique piece of furniture.
What I didn’t know while I was working on it was that the best reward was yet to come. After replacing all the necessary capacitors, all the negative aspects that I heard before were gone! The sound now had that warm and detailed sound Marantz was famous for. My hard work added a sense of sentimental attachment to it that I had never experienced with a piece of audio gear before. This project not only made me comfortable with DIY projects that would follow, it gave me the confidence to go and seek out new projects to the point of building and designing my own equipment. It’s safe to say that without that good old 1060, I’m not sure I would have ever gotten into DIY.
I found that one of the biggest obstacles for someone starting out is determining what components need to be replaced and how to choose those that replace them. Although the list of components for a full in depth restore may be long, capacitors are usually the first to cause audible or functional related issues. In this article, I’ll cover some basic theory and advice on the component most likely to age in a piece of equipment, the power supply capacitor.
What is a power supply capacitor and why do they age?
There are multiple stages within a power supply. Firstly, a transformer is used to change the mains voltage that comes out of the wall to a voltage or various voltages on the secondary. These are chosen based on the application that the power supply will be used for. The secondary of the transformer is then connected to what’s known as a rectifier. A rectifier is comprised of diodes that convert AC (50 or 60Hz) to DC (0Hz). Whenever the AC waveform crosses its zero intersection point, there is a period where the rectifier is completely turned off. This causes a parasitic AC component at twice the mains frequency to appear after the rectifier. This waveform is what is known as ripple.
Capacitors are used after rectification for energy storage and filtering purposes. When the diodes in a rectifier become forwardly biased, they charge the capacitor. During the zero crossing, the rectifier stops conducting and therefore ceases to supply current to the capacitors. At this point, the capacitors become the only source of current for the load of the power supply and start to slowly discharge. As they discharge, the output voltage starts to drop until the next conduction cycle. This process of adding capacitors post rectification decreases ripple and increases the DC voltage. Although some of the ripple has been reduced, remember that it’s still present due to these charge and discharge cycles.
This leads us to the second application for power supply capacitors, filtering. When placed from the DC rail to ground, capacitors filter by forming what is called a low pass filter.
A first order low pass filter commonly used in power supplies
Since capacitors are higher impedance as they approach DC and lower impedance as they approach high frequency, the high frequencies are shunted to ground passing the majority of frequencies that are lower than the cutoff frequency. The cutoff frequency, also known as the corner frequency, can be defined as fc = 1/(2πRC) and is the frequency in which there is a 3dB reduction of amplitude.
The power supply capacitors are usually electrolytic due to the desire for high capacitance in this application. The higher the capacitance, the lower the cutoff frequency becomes; the more the ripple is attenuated. Since electrolytics essentially use fluid as an electrode, this fluid eventually starts to dry up, increasing the series resistance and decreasing the capacitance of the component. This decreases the effectiveness of the low pass by shifting the cutoff frequency up as well as reducing its ability to attenuate higher frequency noise. The decrease in capacitance also prohibits it from storing adequate charge during the non-conducting periods. The consequence is higher ripple and lower DC voltage, both of which will worsen under higher output currents.
The result is hum, high frequency noise, a loss of dynamics, decreased output power, and well…poor sound. These capacitors will eventually fail completely so it’s the highest priority to change them in older equipment.
Electrolytics that have failed or are about to fail will often bulge at the top due to the release of hydrogen gas, causing electrolytic fluid that hasn’t dried up to leak out. This is only in extreme cases and many capacitors that are in need of changing will not show physical signs.
How do I know which power supply capacitors to replace and what do I replace them with?
The first step when planning to recap a component is to attempt to get the schematic or service manual. The best database that I have found on the internet is www.hifiengine.com/. It’s a free database and all you have to do is register to get a username and password. If hifi engine doesn’t have what you need, you could also call the manufacturer if they are still in business. If the component is old enough, they may just email you the schematic.
Once a schematic is obtained, it’s time to identify the power supply capacitors. I recommend looking for the transformer and that should lead you directly to the rectifier and power supply capacitors. The schematic below shows a tube heater power supply in an Audio Research D51 amplifier. Notice the transformer windings and bridge rectifier on the left hand side. To the right of that is a low pass filter similar to the one we discussed earlier. In this particular area of the circuit, we would be interested in replacing C24, C25, and C26. Look at the rest of the power supply and repeat the same process. It’s almost always helpful to follow the voltage rails to the signal path since many times some local decoupling capacitors might be placed in this area as well.
Next, we must decide which capacitors to put in place of the originals. When selecting new capacitors, it’s important to note their max voltage rating, capacitance, type, and size. It’s fine to pick a capacitor that exceeds the voltage rating of the original capacitor. Regarding capacitance, I would advise not to pick a capacitor that has a higher value than the original capacitor. Adding excessive amounts of capacitance will increase the inrush current upon start up, which could damage the rectifier or blow the fuse.
Generally, you want to keep the same type of capacitor that the original manufacturer used. The only exception to this rule- in my opinion- is when it comes to plastic capacitors. Many designers use plastic capacitors to bypass large electrolytics in order to achieve better noise suppression at high frequency. Unlike electrolytics, plastic capacitors only come in lower values and are non-polarized. Older types of plastic capacitors such as mylar are inferior compared to new polypropylene types that have lower parasitics and therefore have better high frequency filtering capabilities. For low values less than 1uF, I would recommend using metalized polypropylene offerings from companies such as Wima or Epcos.
Depending on the available space inside of the chassis, the size of a capacitor can also be a major factor. Locate the capacitor in your component and measure the circumference, height, and lead spacing. Due to the advancement in modern technology, you’ll notice that capacitors with the same voltage and microfarad ratings will be considerably smaller. If the capacitors are chassis or PCB mount, it might be necessary that you find a similar size. The best route to take here is to look for a cap that has a higher voltage rating. It’s likely you’ll find the size you need.
Using the Audio Research example discussed earlier, C24 and C25 are both labeled as 500uF 15V capacitors. Since 500uF is not a common capacitor value, I would prefer to replace this capacitor with a 470uF 16V electrolytic capacitor. Now repeat this process for each remaining power supply capacitor.
That about sums it up! Your vintage component is on its way to sounding as good as the day it was first plugged in. But there’s more work to be done. Start with getting your schematics and identifying the caps you need to purchase. In the next issue, we’ll take it a step further by talking about another type of capacitor that is also extremely important to replace; the coupling capacitor.
Warning!
I am sure you have heard the dangers of working on electronics. My advice to you is to read up on how to be safe when working on equipment. While working inside of electronics, always treat everything like it has the potential to kill you. Make sure the AC cord is unplugged and discharge all power supply capacitors before you start working. This reinforces correct habits and may very well save your life.
If not clearly marked, always notate the correct polarities on the PCB before removing any electrolytic capacitors. If electrolytics are reversed, they could explode and cause severe injury. Please be cautious!
Below are links to information to learn about proper safety practices.
Safety links:
- General Safety Information:
http://diyaudio.com/forums/showwiki.php?title=DIYSafety
- How to discharge a capacitor:
http://www.learningaboutelectronics.com/Articles/How-to-discharge-a-capacitor
- How to make a capacitor discharge tool:
https://www.ifixit.com/Guide/Constructing+a+Capacitor+Discharge+Tool/2177
Links to popular capacitor venders:
