Here's the deal: it was cracking before but it got worse recently. It's worse at start-up, presumably because the (large, 470u) input decoupling cap is charging slowly through the 100k pot and it's arching (I read that this kills them). Trouble is it's part of a commercial amp and the shaft is fluted. It looks like it's impossible to get another motorized fluted shaft pot. How would you go about this? I'd try taking it apart but just in case something goes wrong I want to have a backup. Some have suggested using a pen to short the damaged portion of the resistor. What about grease? I had success in the past using generic (silicone?) grease in cheap pots but I'd not experiment with this one.
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It would help to see the circuit and how its used, and also the DC voltage present on this 470uf cap.
It sounds more of a design fault at this stage.
It sounds more of a design fault at this stage.
Well 470uf looks like overkill tbh. The input impedance of that stage is high, even a 2.2uf (in total) coupling cap wouldn't compromise the bass response all that much, less than -1db down to below 10Hz.
I would try that for starters. Take those two caps out and replace with a good film cap of around 2.2uf
I would try that for starters. Take those two caps out and replace with a good film cap of around 2.2uf
Yes, I was surprised too. It takes maybe more than 30s for the cracks to reduce in intensity after power-up.
DC on the input is worth checking too.
What about teh pot itself? How would you go about the shaft problem in case it needs replacement?
That's the RK27, carbon track I believe, you can still use the deoxit. You might find ones with the right shaft on Ebay, but as you say most are round. I've never tried dismantling one
Where? They are available from RS etc RK27112MC01P | Alps RK27 Series Potentiometer with a 6 mm Dia. Shaft, 50k?, ±20%, 0.05W, Panel Mount (Through Hole) | Alps
Oh yeah, it ain't cheap! There are small tubes you can get DeoxIT® Fader Squeeze Tube, 100% solution, 2 ml I've found this in the UK for about £5 inc. p&p
If you replace the pot then you will just have to use some mechanical ingenuity... the sort of thing you need in front of you to see what is do-able and what is not.
I think you will find replacing the cap for something more suitable solves the problem.
I think you will find replacing the cap for something more suitable solves the problem.
I had a similar issue on an old 90's Marantz and couldn't afford deoxit so purchased a can of WD 40 's new ( at the time ) specialist electronic cleaner , fixed the problem with no negative effects ,from memory. i paid around GBP 8 via Amazon (400 ml ).
lt's listed as WD 40 SPECIALIST FAST DRYING CONTACT CLEANER .
Worth a go maybe , i've used it on loads stuff now doesn't appear to rot or damage any plastics etc and it's non conductive.
lt's listed as WD 40 SPECIALIST FAST DRYING CONTACT CLEANER .
Worth a go maybe , i've used it on loads stuff now doesn't appear to rot or damage any plastics etc and it's non conductive.
just use standard wd then as recommended by hi - fi 'experts ' on you tube 🙂
* Disclaimer * DON'T !!!!
Alps pots
I have taken the pots apart for my alpine 7909 units. For the most part they just need a good cleaning. I would avoid any type of lube/grease/silicone on the part. The pot is a carbon resistor, and the oils may penetrate the carbon film, and if that happens the part is done.
What I have found, is that the wipers get tarnished and also some oil from the pot shaft seeps onto the resistor.
A Q-tip with isopropyl alcohol does the cleaning well, and go to a crafts store and find an eraser pencil and use that to clean the wipers on the pot.
I am not going to say it works every time, but I have had good luck doing the repair.
REGARDIND ALPS, I wanted the new assembly, Alpine obsoleted the part. I contacted Alps, and sent them pictures of the part. Alps did find the correct part, in fact sent me the original drawings and specifications of the part, then told me. Oh well that part is obsolete and we don't make it anymore sorry.
They will not even consider another short run of the part.
I have taken the pots apart for my alpine 7909 units. For the most part they just need a good cleaning. I would avoid any type of lube/grease/silicone on the part. The pot is a carbon resistor, and the oils may penetrate the carbon film, and if that happens the part is done.
What I have found, is that the wipers get tarnished and also some oil from the pot shaft seeps onto the resistor.
A Q-tip with isopropyl alcohol does the cleaning well, and go to a crafts store and find an eraser pencil and use that to clean the wipers on the pot.
I am not going to say it works every time, but I have had good luck doing the repair.
REGARDIND ALPS, I wanted the new assembly, Alpine obsoleted the part. I contacted Alps, and sent them pictures of the part. Alps did find the correct part, in fact sent me the original drawings and specifications of the part, then told me. Oh well that part is obsolete and we don't make it anymore sorry.
They will not even consider another short run of the part.
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