Bad Alpha pots

markm · February 01, 2007, 11:19:30 AM

I was working on a build last night and took 2 brand new Alpha 16mm pots out of the parts drawer in my cabinet, soldered 'em up and got some really strange results.
So, in thinking it was my build I spent about 1 hour trying to de-bug the circuit to no avail.
I checked the pots for ha-ha's and here's what I found;
The 100K measured at some points of the rotation almost 3M!
The 5K maesured 8K.....
What the hell!

Anybody else had this thing happen?
I got 'em from Steve at Smallbear last year for "stock" and they were brand new.
I immediately ordered replacements from him last night but, I'm a bit discouraged with these as this is not good!!

John Lyons · February 01, 2007, 11:32:46 AM

I'd ask steve (smallbear) about it. All this info is helpfull so he can address a problem if it's a certain batch or whatever.
He may have had similar problems with others. But if he doesn't know about it there is nothing he can do.

One posible reason the pot was so high in resistance is that the carbon track is printed on the round wafer, if an area was printed really thin or an almost blank spot then maybe it showed up as a high resistance...??

It's a good thing to measure the pot anyway (not that I usually do...) to see what you get, and as a possible problem curb in marks case!

John

Ronsonic · February 01, 2007, 11:47:36 AM

Typical pot tolerance is 20% so Hmmm, 8K makes it about triple that. That's a very normal fault.

Did you try a quick flush and clean on the one going open? At least open it and take a look, or ship it to your supplier so he has the chance. The multiple contacts used on modern pots make it rare for a simple dropout in the carbon track to cause problems. I'd expect to find some sort of contamination in there. Tiny crumbs of plastic from packing and wrapping material are the usual offenders.

Anyway, just a few thoughts.

Ron

MartyMart · February 01, 2007, 12:36:44 PM

I used to have loads of bad pots from another supplier but since getting them from steve
I've only had one "duff" one, which was a reverse log 1M .... I think.
Now I test 'em before fitting .... more to see the real value than a condition test !

Those 16mm alpha's are not the most "robust" pot IMO, I've knackered a few

MM.

MKB · February 01, 2007, 07:24:48 PM

The last two 12mm Alpha pots I used went bad immediately after soldering to them; the lugs seem to have lost contact to the resistive elements. Very disappointing. However I haven't had any problems with the 16mm ones. The 12mm and 16mm pots were recently ordered from Mouser.

dxm1 · February 01, 2007, 10:39:42 PM

The only bad Alpha pots I recall in the last couple of years were also some 1M-Cs. Two bad out of ten. I took them apart, and they both looked fine mechanically. The taper was definately reverse, but I'd call it "random" rather than "log". They were 16mm units.

Processaurus · February 02, 2007, 04:31:03 AM

I was introduced to the ways of pot trouble by several of the cheap 16mm alphas with the black plastic bottoms, I haven't had trouble with the metal case 16mms yet. A bad pot can be a hard one to debug, if you aren't ready for it. I might have my first bad metal alpha pot on a pedal I made for a friend, acts strange, and seems to get normal by pushing on the knob?

?

rockgardenlove · February 02, 2007, 05:12:53 AM

12 millimeter pots? Never heard of these. Interesting.
I am fascinated. Cramming stuff in a box is always hard for me, I think I'll give these a shot next Mouser order. You your report is somewhat discouraging.

MartyMart · February 02, 2007, 05:16:29 AM

I've noticed that the metal "rivet" that attaches the "lugs" to the wafer can come loose
if too much pressure is applied, but can be squeezed back together.
I have a 1M log that goes "intermittent" if the "lugs" are slightly pushed against.
I've tried to be more gentle with pots, but still get problems.
My 24mm Citec's with grey plastic shafts seem to fail a lot, and they were 89 pence each !

MM

smallbearelec · February 02, 2007, 05:37:42 AM

I've not had any other complaints about the RV16 series, but I will keep my ears open.

SD

MKB · February 02, 2007, 08:22:03 AM

Quote from: rockgardenlove on February 02, 2007, 05:12:53 AM
12 millimeter pots? Never heard of these. Interesting.
I am fascinated. Cramming stuff in a box is always hard for me, I think I'll give these a shot next Mouser order. You your report is somewhat discouraging.

http://www.mouser.com/catalog/625/510.pdf

They look and feel nice and are inexpensive. I'm disappointed as well. Been using Alpha pots for many years, these two were the first bad ones I have ever seen. It might be that they are too sensitive to heat, the lugs are very small and it's easy to get too much solder on them. Might help to use as little solder as possible so the solder doesn't go up the lug to the rivet area.

I forgot to mention, these were the 12mm standard wire lug single pots, I also used several dual 12mm PCB mount pots from the same line and they worked very well. The dual pots were PCB mount, and the bad single pots had standard wire lugs. And I had to add wire to the PCB mount tabs of the dual pots to use them. Hmmm.... wonder if it was just a bad batch of wire lug pots...

Mark Hammer · February 02, 2007, 08:30:26 AM

When I bring them home, I immediately pop the backs off, wipe them down with some Stabilant, give the lug rivets a little squeeze with the needle nose pliers (easier to do with the backs off) and reassemble them.

Clearly, if you have a business to run, this should NOT be your obligation, but it's what *I* do as a response to inconsistent performance of pots in past. It may only really be necessary in very rare instances, but I'd rather not find that out after hours and hours of hair-pulling and troubleshooting. So, I just do it.

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Bad Alpha pots