Hello, I have a JVC RX-6000 which works well except for the volume control, which behaves pretty oddly. If I turn the volume down slowly, everything works fine. If I turn the volume down quickly, the volume often ends up higher than it was. I assume the pot has some problems, though I cannot imagine what would make it behave like this. So I have the unit open, and there are no identifying marks on the pot. Its square, top mounted with a metal bushing 3/8" long. The pot stem stick out another 3/8" past the bushing. I think its 10k, but I can't be sure until I pull it out. I have looked all over for this pot, and I can't find anything. Does anyone know how I can identify the pot, and where I can find a replacement?
Are you sure it's a pot, and not a rotary encoder ? Does it have end stops at anticlock and clock ends ? If it goes round and round without stops, then it is an encoder, not a pot.
The symptoms you describe are very typical of the 'stirring treacle' grease that they put in the shaft, having migrated down onto the encoder disc and contacts. If it is one of these, they are cleanable, but you need to carefully dismantle it, to do so.
Thanks. I will look into this. As you describe, the pot has no stops. I cannot imagine what would cause a pot to behave this way, so it makes more sense that it is the encoder. Would the encoder be inside the pot housing?
On Sat, 13 Dec 2008 20:37:33 -0800 (PST), Andrew put finger to keyboard and composed:
I have an amp with a rotary encoder that doesn't respond correctly if I turn it too quickly. I suspect this reflects the processing speed of the uP, and is not a deficiency in the encoder. If it's a mechanical rather than optical encoder, maybe there are time delays to allow for debouncing. If so, could the uP be confused if a new contact is made before the previous one has been debounced?
- Franc Zabkar
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Agreed. It is very very common. I would guess that on average, I do one every couple of weeks. Lots of different makes use them, but they are almost always the same type with the green square body, 3 electrical legs, and two mechanical stability pins, one at either side in the middle. ALPS make, I think.
The one that's commonly found in many hifis Dave, comprises a printed circuit disc with broken rings of (gold plated?) print on it. This is attached to the shaft. The plastic body of the encoder has a set of contacts on it which I think are also gold plated. The whole thing puts out Gray code or some such, that can be used to derive a rate and direction of rotation. The system control micro then looks after turning this into an I2C data stream to work the digital pot IC for volume control, and the display decoder so that it can put up a pseudo dB display. The useful thing about using such an encoder, is that it can be used to control other functions too, such as tone and radio tuning and so on. It's also cheap compared to optical devices.
As far as problems with them go, I don't think it is so much that they are unreliable by design, more that their problems have been caused by the manufacturers being a bit enthusiastic about giving them a high-end 'feel' in use. To give them this 'stirring treacle' feel, they fill the shaft with some kilopoise type grease, and this seems to migrate down onto the disc and contacts, where it wreaks havoc with the operation. Once it has been carefully cleaned off chemically, the contacts and disc are usually bright shiny clean, as you would expect gold-plated contacts to be. I usually finish off before reassembly and refitting, with a small drop of cleaner/lubricant on the disc. They always work perfectly after this treatment, and I can't remember ever having had one fail to be recovered.
I've had one or two that didn't clean quite so well. One was on a JVC shelf system which caused a BUNCH of extra labor to re-do, which involved replacement rather than cleaning the second time around. I wasn't going to risk another re-do.
BTW I don't think it's grease migration. I think it's silver content in the switch tarnishing like a VCR mode switch. A good cleaning with De-Oxit and a fiberglass nick-sander brush usually gets it but the parts are actually pretty cheap, and there's only a couple types in common use. I get them from Onkyo.