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Old 6th September 2011, 08:45 PM   #11
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Quote:
Originally Posted by Xoc1 View Post
Download a service manual from
ElektroTanya | Service manuals and repair tips for electronics experts
C501 and 502 after the volume pot are probably worth replacing with new parts.
Hope I found the right schematic!
Regards
Martin
don't use electrolytics again. I would prefer WIMA MKS like this:
http://files.tradoria.de/41a03f69e52...132_247910.jpg
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Old 7th September 2011, 12:55 AM   #12
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Remove pot and C A R E F U L L Y unbend tabs holding it together. Buy only Deoxid/Cramolin Red contact cleaner. Gently swab both resistive elements and contact fingers with quetip wetted with Deoxid/Cramolin. Reassemble and C A R E F U L L Y rebend tabs in place. Reinstall. Noise gone.

(Cramolin -now Deoxid- is the only substance I ever found that will successfully clean up an analog oscilloscope position pot. I was reffered to it in the 80's by John Fluke Company while looking for a replacement for the old Rycon they used to use in their voltage calibrator controls. Amazing stuff! Used to be manufactured by Caig industries, but Caig and Deoxid merged. What used to be called Deoxid went away and Cramolin got rebranded to Deoxid.)
Doc
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Old 7th September 2011, 12:32 PM   #13
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After a third attempt at cleaning with spray and a look at the tracks they don't look dirty, I'm starting to suspect this is something else as the right channel is perfect yet the left is so very wrong, the solder joints looked ok and taking the pot completely apart looked like it'd be quite a bother.

This problem also seemed to happen suddenly, there was no gradual creep in just suddenly it started doing it. As I mentioned earlier the noise makes the bass cones throw themselves out in a frankly scary way that I never see when listening to music within sane volume ranges, it makes me think that somehow higher voltage is getting in there somehow, definitely something way out of the range of line level audio.
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Old 7th September 2011, 02:50 PM   #14
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Put back the pot or put an equivalent resistor in place ( with slider and top of pot connected together ) and see if there is a dc voltage on it when the amp is powered up and if so does it drop with time.
Compare both channels. If there is nothing, then probably the pot is just bad !

Try using external wires to wire the pot terminals to the pcb so that left and right channel sections are reversed. If the problem goes to the other channel the pot is surely gone. If not ,look elsewhere ......like above ....
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Old 10th September 2011, 01:28 AM   #15
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Spray cleaners applied from the outside aren't always very effective. That's why I recomended taking the pot out and apart to clean it. Dust and even carbon from the pot element itself get stuck between the wipers and the elements creating variable resistance noise. Pots are usually held together three different ways; small bent metal tabs, sledged rivets and (the best) tiny screws. (A fourth method is epoxy encapsulation, but if that is so you are SOL). Most used in audio applications are bent tab types. The tabs can be unbent if done very slowly and gently to avoid even more metal fatigue (some of which exists from the initial crimp).
I've successfully taken apart, cleaned-lubed and reassembled literally hundreds at this point. The system I recomended works... nothing else ever did. Like I said it has worked with noisy oscilloscope position pots, which give instant visual feedback as to the success of cleaning the pots.
Doc
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Old 10th September 2011, 02:53 AM   #16
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I've unbent and cleaned quite a few myself, but I always thought, why am I doing this, can't they just stop trying to save pennies and make equipment that LASTS. Unfortunately, consumers are so greedy it blinds them of subtle differences they don't understand. Literally, $10 could make a huge difference in long term viability of products, but all the consumer sees is "it costs more and I see no reason why".
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Old 10th September 2011, 03:45 AM   #17
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Per my spec, all Racal Dana IEC 845 fully progamable .01Hz-15Mhz function generators now have pricey Spectrol calibration pots as standard issue. Why? To save on multiple service calls to Germany.

As to why I would repair a cheap style bent tab pot on commercial test equipment; because it was much quicker than putting a unit HFP (hold for parts) and got the equipment back to the person that needed it to ship product.

Doc
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Old 10th September 2011, 08:33 AM   #18
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it uses small bent metal tabs, I sprayed directly onto them and have inspected the tracks and can't really see anything. I've taken it apart a few times now and am trying everything that I can that's not potentially destructive first, if it comes to it I'll take the thing apart as best I can.
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Old 10th September 2011, 11:28 AM   #19
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oh, and if anyone is curious this is the pot

original produced by alps

35001070 GTP-500II Volume Control Motorized 50KBx2 W/Motor

A modern clone I found, though I can't find any way to buy a single unit, just quantities of 1000.

TOP-UP INDUSTRY CORP. -- MMD16M1HBRT-B3
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Old 10th September 2011, 11:37 AM   #20
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Quote:
Originally Posted by thaumaturge View Post
Spray cleaners applied from the outside aren't always very effective. That's why I recomended taking the pot out and apart to clean it. Dust and even carbon from the pot element itself get stuck between the wipers and the elements creating variable resistance noise. Pots are usually held together three different ways; small bent metal tabs, sledged rivets and (the best) tiny screws. (A fourth method is epoxy encapsulation, but if that is so you are SOL). Most used in audio applications are bent tab types. The tabs can be unbent if done very slowly and gently to avoid even more metal fatigue (some of which exists from the initial crimp).
I've successfully taken apart, cleaned-lubed and reassembled literally hundreds at this point. The system I recomended works... nothing else ever did. Like I said it has worked with noisy oscilloscope position pots, which give instant visual feedback as to the success of cleaning the pots.
Doc
I am looking arround to manufacturers from variable resistors (pots), that I can completly disa'ssy into its all individual parts (as usually only possible by rotary switches like ELMA).
Do you know such brands with economy price class?

here an overview of various brands:
Potentiometer Overview - Alps, Penny+Giles, TKD and Vishay and more, what's your fav?
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