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problem with treble pot


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#1 stevenrob

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Posted 03 December 2016 - 03:07 PM

Hello guys,

 

I'm repairing my nicotined RM70. The treble pot did not work( no change when I moved it). I wait for another pot  but I tried to repair it.

I opened it and clean. Now it works better but it is not the perfection : treble starts to increase only near the max, the is no progressivity. Do you have an idea ?

 

Best regards



#2 Superduper

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Posted 03 December 2016 - 10:19 PM

Search the archives.  I have repaired and rebuilt many slider pot boards and shared how I did it in the forum, you just need to find it.

 

The problem could be the slider fingers are loose.  Or it could be the resistor board has broken connection.  You need meter to check, but best time to check is when the pot is opened.  Now that you put it back together, it's a little risky to disassemble again because the metal tabs easy to snap off after too many bends.



#3 stevenrob

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Posted 04 December 2016 - 12:47 AM

OK, How can I test the pot with a meter ?  There is only one sense to mount the pot on the board ?  I find 2 capacitors mylar 0.015 uf  (C804 and C704) with R = infinity :  is it normal ?

 

Thanks a lot



#4 baddboybill

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Posted 05 December 2016 - 09:43 AM

Honestly you're better off sending it in for a rebuilt slider control board from superduper (Norm)

#5 stevenrob

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Posted 05 December 2016 - 02:14 PM

Hello,

Perhaps you're right but before I want to try to repair it myself.  :-)



#6 Superduper

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Posted 05 December 2016 - 10:11 PM

I will give you advice based on memory so no guarantee.  The pots can physically go in both ways but only 1 way is correct.  If you reverse it, it will work funny.

 

1.  Look at the housing carefully and you will see a #1 stamped on the tin shell.  That end should be installed on the board which also has #1 printed.

 

2.  On the bottom, there are 2 pairs of 3 contacts. They are numbered 1, 2, 3 (left side) and 1,2,3 (right side).

 

3.  1-3 should read approximately 88k - 100k ohms.  If you get infinity, then either the resistor board trace has a break, or a bad solder joint, or a broken pin rivet.

 

4.  1-2, and 2-3 should both have continuity.  With slider set in middle position, one side should read low (about 15-20k), the other side should read high (about 75-85k).  

 

5.  Now with probes steadily connected to either 1-2 or 2-3, slowly move slider from one end to another.  Analog meters are easiest to read motion.  Digital meters will be confusing to read because the autoranging feature will probably re-set several times during the length of the travel.  Because these are log pots and not linear, you should get steady increase or decrease without breaks although the rate of increase or decrease will not be linear.

 

As for the capacitors, they should read initially read some resistance and very quickly increase to infinity as it charges up.  This may happen quicker than your meter can display with such small caps.  Final reading of infinity is normal.  Very seldom do these go bad.  I do however replace the .15uf electrolytic caps with audio-grade ones while I am in there.  0.15uf is very hard to find but you can use .1uf.



#7 stevenrob

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Posted 06 December 2016 - 08:53 AM

You're the best, I tell you the results of my work



#8 stevenrob

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Posted 11 December 2016 - 03:03 PM

Hello,

 

Finally, the 2 second hand m70 pot I bought was dead too : with my tester I find R=INF in  any way (but the metal fingers were good...)

 

 I have tried to use the pot of a JVC RC-328 : I compare the measure with those Superduper told me and it is the same that RC-M70 !  :thumbsup:

 

Thanks to Superduper