Lenco MC-4000 mini-stereo, weard Volume control

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- BRASTO
  
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posted
17 years ago

Sat, Feb 10, 2007 10:32 AM

This LENCO MC-4000 stereo receiver when powered on will read Volume position 16, when rotating the volume control CCW the output volume increases rather than decreasing. It will stop at MAX 40 and cannot be reduced. Pulling the powercord for at least one hour will reset the initial VOL to 16 and the problem can be duplicated at will.

I have inspected the VOL control, which appears to be a sort of digital shaft encoder rather than the old days potentiometer. Presumably something is wrong with the logics in the set, can anybody help to find the cause of the problem or knows its schematics ?

TY Brasto

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- Arfa Daily
  
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posted
17 years ago

Sat, Feb 10, 2007 10:50 AM

This is a *very* common problem on hifi's using a rotary encoder for control of volume. It is caused by the grease that is put in the shaft bearing to give it that "stirring treacle" feel, migrating down into the control body, and contaminating the encoder disc and its pickup fingers.

It is usually a straightforward job to remove the control (two metal tabs and three pins), bend open the four metal tangs that hold the control closed, and clean it out. If you use electronics grade isopropyl alcohol on a cotton tip to deal with the contact fingers, it will usually cause the grease to 'ball', which makes it easy to remove from them with tweezers or a pin. Take care with the contacts, as they are quite delicate.The encoder disc can be cleaned with a cotton tip and alcohol too.

When it's all clean, //slightly// re-bend the fingers to restore their original contact pressure, put a small amount of switch cleaner / lubricant on the disc, and reassemble. When refitted to the hifi, you should then have a working control again. I do probably a couple a month.

Arfa

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posted
17 years ago

Sat, Feb 10, 2007 2:25 PM

This was exactly my experience with my high end Pioneer receiver, which was driving me mad because its sound volume was jumping from normal level to maximum , almost blowing up the speakers, by just moving the volume knop slightly. Simply cleaning all vaseline from the contacts in the rotary encoder with some alcohol did indeed the job ... I guess all "digital volume controls" with grease on the shaft will suffer from this sooner or later... unless they are constructed in a different way, preventing the leaking of lubricant to the interior.