How To Fix A Digital Volume Control

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There’s a common issue that affects many amplifiers or integrated stereo systems with a digital volume control, where control of the volume on the front of the unit itself is via a physical Knob. If the volume on your stereo randomly turns up by itself, or doesn’t work at all, this article is for you.

Yamaha Crx M170 Front View

Traditionally the volume knob was an analogue device – a potentiometer, which is a type of variable resistor. To put it simply as a potentiometer is rotated its electrical resistance changes, thus changing the gain, or volume, of the amplifier.

Many modern devices from the last 30 years or so have switched to digital control for their functions. Thus this change in resistance is handled not by an analogue part but by either an electronic implementation of a potentiometer, a microcontroller, a digital to analogue converter or even a series of relays switching banks of physical resistors. Fronting all of this is the rotary encoder – a tiny part that serves as a means to send user input to the digital brain behind these clever volume controls. In essence, the encoder is really just a fancy term for a digital knob. Despite being one of the least complex and cheapest parts in the device in which they are used, they’re probably the most prone to failure and the cause of many volume-related faults in stereo equipment. But they can be fixed, and herein I’ll show you how.

The rotary encoder is named so because it takes a user input, I.E the rotation of a knob, and converts it into a series of electrical pulses that can be understood by a microcontroller – the ‘computer’ that is controlling things behind the scenes, as it were. When an encoder is not being turned it outputs nothing – or at least, when it’s working properly. There are many types of encoder but the most common encoder used in a volume control application is a very simple construction.

Crx M170 Side View Of Rebuilt Pot

Attached to the rotating shaft is a metal contact wheel which has equidistant slots cut around its circumference. Inside base of the encoder are small flexible metal fingers. When the shaft rotates, the wheel brushes against the fingers in turn, thereby producing an electrical pulse. There are three pins – a, b, and ground, and by determining the direction of the pulse (A to B or B to A() we can determine the direction that the shaft is being rotated. This is a simplified explanation – the scientific version involves discussion of waveforms and phase offset, but that’s not the purpose of this article.

I recently fixed a Yamaha CRX-M170 CD receiver. One of its faults was erratic function of the volume knob. More often than not the volume would continue to rise to full at the slightest touch, no matter how much the knob was moved or which way it was turned. This is the most common symptom of this fault, though the opposite (the volume turning down to nothing) can occur, and in rare cases the knob does nothing at all. You may also find that if the knob behaves sporadically, the remote control (if you have one) will work just fine. This is a clear indication that the fault lies with the encoder.

The cause of the issue is two-fold. The encoder is normally lubricated with a grease to give it a smooth feel when it’s rotated. But when the heat produced by the stereo system causes the grease to soften, it gradually drips down the encoder shaft and coats the metal contacts, inhibiting their conductivity. The springy metal fingers in the encoder base can also become flattened with repeated use, and when this happens they don’t consistently make contact with the rotating contact disc. When either or both are true, the pulses from the encoder are inaccurate or inconsistent, and the microcontroller cannot determine the rotation of direction or when you’ve stopped turning the knob. Thus, most microcontrollers assume you’re still rotating the control clockwise to raise the volume, so they keep raising the volume until the stereo is as loud as it will go.

Fixing the problem requires that the stereo be disassembled to gain access to the front of the encoder. This normally involves removing the front panel and removing the circuit board that contains the encoder itself and usually the display and other buttons. You’ll need to apply mechanical intuition and common sense as it’s impossible to document specific instructions for every stereo system, but the principle is the same.

You undertake any work at your own risk. There are mains voltages present in these devices that can cause serious injury or death. Always ensure that the power is unplugged, and allow some time to pass for capacitors to discharge. Use common sense. I’m not responsible for any damage you cause to yourself or your equipment. This article is provided for those who feel confident enough to safely undertake the steps outlined. If that isn’t you, put down the screwdriver and find someone to do the work on your behalf.

In the case of this Yamaha and most machines with a drawer-loading CD player, the front fascia of the CD tray should be removed first. It’s easier to do this by powering up the system, opening the drawer, shutting off the power and then unclipping the decorative fascia before you start disassembling things. The decorative trim usually either lifts up or is unclipped and slid to the side to remove it from the tray itself.

In the case of the Yamaha six screws remove the top panel – one at each side, and 4 at the back. There are five screws holding the front panel in place – one on each side and three at the bottom. The panel then slides off and there are two cables to disconnect; one small bundle of wires connecting to the circuit board on the bottom right, and a flexible ribbon connecting to the back circuit board.

Once you have the front panel removed, it’s a good idea to remove the knobs themselves. They normally pull off (some are secured with a small grub screw in the side of the knob), and more often than not you’ll find some nuts behind them securing the control shafts to the fascia. Remove these before flipping the front panel face down and removing any screws holding the circuit board in place.

You might find other hardware holding the board in too. In the case of the Yamaha, there are a number of small plastic clips which have to be pushed out of the way while simultaneously lifting up on the board. Careful examination of the board itself and the surrounding panel is the only way to determine how to remove it without breaking anything. When you do have it removed, you’ll be treated with a site that looks something like this.

Crx M170 Pot On Board Rebuilt Tabs Apart

The encoder shown is the simple three-pin encoder described above. You’ll note that I’ve begun to disassemble this one. On most encoders there are four metal tabs attached to the bass that are bent over to secure the top of the encoder. These can be pried up and to the side using a jeweller’s screwdriver. Only push them far enough to clear the top of the encoder, and try not to flex them too much – they will break. If you do break one it’s not a dealbreaker. You can either replace the encoder for pennies, or hot glue it back together.

Crx M170 Volume Pot Opened

Once you’ve pried open the four tabs you can separate the two parts of the encoder. You then have access to the metal contact fingers and the contact disc.. Remove the shaft from the top housing and clean away the grease with a liberal dose of electrical contact cleaner. I recommend Servisol Super 10 switch cleaner as it has a lubricant that will give you a nice smooth encoder when it dries. You can use deoxit too if you can stomach the cost.

Clean the contacts in the base of the encoder too and bend them up slightly to ensure they’re making good contact. Don’t go too far with this – a slight tweak is all you need. Then, reassemble the encoder in reverse order by carefully aligning the top plate and setting it in place, and bending the tabs back down. Make sure you press the tabs tightly into place so that the clamping pressure ensures that the sprung contacts maintain pressure against the contact disc.

Crx M170 Close Up Of Pot Port On Board

You can then put the stereo back together, reversing the steps you took to get it apart. Ensure all cables are routed and connected properly, and that the screws go back in the right holes. Once you’re confident, power it up and if all went well you should find you have a working volume control once more.

Crx M170 Close Up Of Volume Pot

There is no definitive list of models that can suffer this problem, though it’s probably safe to say most that use a rotary encoder will at some point. You can tell if you have e rotary encoder by rotating the knob by more than one complete turn. If the knob has stops at either end of its travel, it’s a traditional potentiometer. If it rotates infinitely it’s an encoder.

Most mini hi-fi systems have used encoders for decades including models from Sony, Yamaha, Technics, Teac, Aiwa, Panasonic, Kenwood, Pioneer, JVC, Sanyo and many others. The same goes for pretty much every AV receiver ever made, as well as integrated home cinema systems and a lot of modern hi-fi amplifiers. I have personally fixed a number of systems in this way including the Cambridge Audio 840A V1 / V2, 851A, 851E, various all-in-one hi-fi systems from Sony, Aiwa, JVC and Kenwood, this Yamaha and a few others. Obviously this is not a definitive repair for every possible volume issue, but if you have a system with volume issues and it has an encoder it would be the first place you should look to find the fault.


About Ashley

I founded Audio Appraisal a few years ago and continue to regularly update it with fresh content. An avid vinyl collector and coffee addict, I can often be found at a workbench tinkering with a faulty electronic device, tweaking a turntable to extract the last bit of detail from those tiny grooves in the plastic stuff, or relaxing in front of the hi-fi with a good album. A musician, occasional producer and sound engineer, other hobbies include software programming, web development, long walks and occasional DIY. Follow @ashleycox2

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