Pots - basis for failure?

[Guest BarryH]

In cleaning my Aetna-Pollak pots I used a 20X magnifier on the springy contact arm and noticed that the two small contact pads which were formed during manfacturing have microscopic stress cracks that go all the way through the pad. The material was taken to failure by the impact of the forming tool. I wonder if the silver plating of the arm was done before or after the forming. If the plating was done beforehand, the raw unplated substrate would certainly be exposed by these cracks and the copper content of the substrate would begin its fatal green corrosion. If the impact was before plating, the rough interior of the fissure might end up being unplated or poorly plated. If impact damage was typical, this may be the basis for the widespread failure of this component. Have any of you magnified and seen the same fractures? If someone has a "dead" pot you could examine, I'd be curious if you find the same cracks.

Thanks-

BH

[bitbandit]

Hi Barry,

That's a very interesting observation. I'll have 4 out for refurb in the next week or two so I will examine them and take pics if possible for you.

Dave

[Guest BarryH]

Hi Barry,

That's a very interesting observation. I'll have 4 out for refurb in the next week or two so I will examine them and take pics if possible for you.

Dave

Great! Mine are coming out of a Deoxit soak today. I'll see if my camera can get a deep enough close-up. Might need better optics. The fissures are very tiny.

Barry

[genek]

In cleaning my Aetna-Pollak pots I used a 20X magnifier on the springy contact arm and noticed that the two small contact pads which were formed during manfacturing have microscopic stress cracks that go all the way through the pad. The material was taken to failure by the impact of the forming tool. I wonder if the silver plating of the arm was done before or after the forming. If the plating was done beforehand, the raw unplated substrate would certainly be exposed by these cracks and the copper content of the substrate would begin its fatal green corrosion. If the impact was before plating, the rough interior of the fissure might end up being unplated or poorly plated. If impact damage was typical, this may be the basis for the widespread failure of this component. Have any of you magnified and seen the same fractures? If someone has a "dead" pot you could examine, I'd be curious if you find the same cracks.

Thanks-

BH

I've disassembled a number of these, and also the other version in which an unplated formed brass wiper pivots on a hinge and is loaded with a small coil spring. I have not noticed any significant difference in the amount of corrosion that either of these versions accumulates. From what I can see, the primary source of corrosion is actually the brass end terminals and center hub embedded in the ceramic backpiece.

[Mexicomike]

Just clean them, coat the contact surfaces generously with di-electric grease and put them back together. They'll probably outlast the current owner!

[bitbandit]

In cleaning my Aetna-Pollak pots I used a 20X magnifier on the springy contact arm and noticed that the two small contact pads which were formed during manfacturing have microscopic stress cracks that go all the way through the pad. The material was taken to failure by the impact of the forming tool. I wonder if the silver plating of the arm was done before or after the forming. If the plating was done beforehand, the raw unplated substrate would certainly be exposed by these cracks and the copper content of the substrate would begin its fatal green corrosion. If the impact was before plating, the rough interior of the fissure might end up being unplated or poorly plated. If impact damage was typical, this may be the basis for the widespread failure of this component. Have any of you magnified and seen the same fractures? If someone has a "dead" pot you could examine, I'd be curious if you find the same cracks.

Thanks-

BH

Hi Barry,

Pics attached for you as promised, one before and one after cleaning. I don't know if they get close enough for you but it's the best I can do.

I'll try and get hold of a magnifier for when I do the other speaker and let you know if I can see anything.

Dave

[Guest BarryH]

Hi Barry,

Pics attached for you as promised, one before and one after cleaning. I don't know if they get close enough for you but it's the best I can do.

I'll try and get hold of a magnifier for when I do the other speaker and let you know if I can see anything.

Dave

In the "after" image, I think I can see a fissure at the upper right of the contact...approx. 2 o'clock.

On my contact arm each pad had two fissures and they show light through if held before a bright source. Poor control of quality from A-P.

What's the material of your arms? Is the gold color an image artifact or are they actually a brassy hue? Mine are silver plate over what is probably beryllium copper.

Good photos, thanks.

B

[bitbandit]

In the "after" image, I think I can see a fissure at the upper right of the contact...approx. 2 o'clock.

On my contact arm each pad had two fissures and they show light through if held before a bright source. Poor control of quality from A-P.

What's the material of your arms? Is the gold color an image artifact or are they actually a brassy hue? Mine are silver plate over what is probably beryllium copper.

Good photos, thanks.

B

I struggled with the lighting, they are in fact silver plate, it's just that I couldn't use the flash as I was so close to the arm the lens of the camera was putting the thing in shadow. It was taken under halogen lights and I didn't correct the colour temperature, hence the yellow hue.

The second pot was more corroded and had a hole in the centre of the contact but it was a definite corrosion hole, and made worse by the flow of current across the junction I guess. The mid always seems to corrode worse than the tweeter which I assume is due to the greater current across the junction. Anyway, nicely greased and re-assembled now, appearing to work. If they give me any grief later I'll just meter the setting I like and replace them with 2 power resistors.

Dave