Essence 5 - preamp problem

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Author Message markusbass New Username: markusbass Post Number: 2 Registered: 9-2010 Posted on Tuesday, September 14, 2010 - 5:52 am:    I'm working on Essence 5 bass that has some problems with the preamp. I found out that turning the Volume pot causes audible "scratches" in the amp. I checked that the reason for this is the fact that there is 40 mV voltage on the pot. I assume that the pot is connected between the output of the op-amp (TLE2022) and "virtual" ground. But shouldn't there be a capacitor that isolates the pot from any DC offset of the op-amp? Should I susspect that the capacitor is "leaky", or is it a problem with the op-amp? The preamp is the one with Volume, Balance and Filter pots (without switches). BTW, where can I order such volume pot? Its shaft seems to be not in the best condition (but as I said the main problem is caused by a DC on the pot)? Mark mario_farufyno Senior Member Username: mario_farufyno Post Number: 519 Registered: 9-2008 Posted on Tuesday, September 14, 2010 - 10:30 am:    Isn't a matter of just cleaning the Pot? There is a tread about that on "FAQ & Must Read"... markusbass New Username: markusbass Post Number: 4 Registered: 9-2010 Posted on Tuesday, September 14, 2010 - 11:07 am:    I don't think so. How can you remove voltage (electricity) by cleaning (mechanical) the pot? Every pot that has a DC voltage on it causes scratches in the amp when you turn the pot. The should be exactly 0 V on the pot and this is usually achieved with a (good quality) capacitor in series with the pot. Mark terryc Senior Member Username: terryc Post Number: 1349 Registered: 11-2004 Posted on Tuesday, September 14, 2010 - 2:02 pm:    mario... I think markusbass knows a little bit more about electronic theory than the rest of us here, he needs advice directly from Alembic. mario_farufyno Senior Member Username: mario_farufyno Post Number: 520 Registered: 9-2008 Posted on Tuesday, September 14, 2010 - 2:28 pm:    Yeah, I should have noticed that... (Message edited by Mario Farufyno on September 15, 2010) mica Moderator Username: mica Post Number: 6937 Registered: 6-2000 Posted on Tuesday, September 14, 2010 - 6:08 pm:    There is a capacitor on the circuit card in series with the pot as you expect. As you've observed, it needs replacing. It's mounted under the pot, so you'll have to desolder the pot to access the cap. Over the years, we've used both 1.0 and 1.5uF. You can use either with confidence. The volume pot is available from us. What happened to the shaft? Was there an incident with the collet knob? markusbass New Username: markusbass Post Number: 6 Registered: 9-2010 Posted on Wednesday, September 15, 2010 - 1:19 am:    Mario, I'm not an expert but I fix bass amps (Ampeg, Eden, Trace, Aguilar,...). Sometimes I get bass guitars to be fixed (including several nice 5, or 6-strings Alembics). Usually they were treated bad by their owners :-) and they needed some adjustments. Mark markusbass New Username: markusbass Post Number: 7 Registered: 9-2010 Posted on Wednesday, September 15, 2010 - 1:32 am:    Mica, I've seen this capacitor. Can you give me the type? I'm asking about it because for the moment I gave the bass back to the owner - he's making money with it :-). In the meantime I could order the capacitor (and the jack). The shaft of the pot moves up and down for let's say 0.8 mm, which is not the case with other pots. Should it be replaced? Is it available on the market, or should I order it from you? I'm from Poland and I would prefer to buy parts in Europe :-). Mark