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"scratchy" beam intensity adjustment


 

The beam intensity adjustment on my Tek 475 appears to have a bunch of spots along the first 30% of its travel where it jumps from near zero intensity to full intensity, but the "jumps" only occur if I rotate the knob quickly. If I rotate the knob slowly the intensity smoothly increases or decreases without any points of high intensity. I've noticed similar behavior with the vertical position adjustments where rapid adjustment seems to result in discontinuous behavior but slow adjustment does not reveal any discontinuities (and I've seen this behavior with the trigger level adjustment, but I figure that's because there is some sensitive comparator involved and jerking the reference voltage around make the comparator momentarily output a trigger signal until it has adjusted to the new reference voltage).

I'm a little puzzled by what is going on with the beam adjustment. I have been trying to clean and lubricate the pot, with limited success, but I'm wondering if, in fact, the pot is just fine (aside from being quite stiff) and I should just expect rapid adjustments to produce this discontinuous behavior. Can anyone explain why the pot behaves this way, and whether it is normal?


Renée
 

Hi Jeff-
I surmise that upon quick rotation the wiper is skipping across the resistive element causing intermittent opens hence the full beam brightness.
on my 475 I replaced the pot element since it was assembled with screws....I was just lucky to have an assembly with correct resistance only wrong hardware.
Renée

On 11/10/20 10:04 AM, Jeff Dutky wrote:
The beam intensity adjustment on my Tek 475 appears to have a bunch of spots along the first 30% of its travel where it jumps from near zero intensity to full intensity, but the "jumps" only occur if I rotate the knob quickly. If I rotate the knob slowly the intensity smoothly increases or decreases without any points of high intensity. I've noticed similar behavior with the vertical position adjustments where rapid adjustment seems to result in discontinuous behavior but slow adjustment does not reveal any discontinuities (and I've seen this behavior with the trigger level adjustment, but I figure that's because there is some sensitive comparator involved and jerking the reference voltage around make the comparator momentarily output a trigger signal until it has adjusted to the new reference voltage).

I'm a little puzzled by what is going on with the beam adjustment. I have been trying to clean and lubricate the pot, with limited success, but I'm wondering if, in fact, the pot is just fine (aside from being quite stiff) and I should just expect rapid adjustments to produce this discontinuous behavior. Can anyone explain why the pot behaves this way, and whether it is normal?




 

Renée, so you're saying that this is the pot equivalent of switch bounce? Now that you say it it seems almost obvious, and I was already trying to imagine a way that the pot could bounce like a switch, since the behavior looks a lot like switch bounce.

Are you saying that you unscrewed the back cover on your beam intensity pot and replaced the entire guts with the guts of some other pot?


Renée
 

Jeff-
yes and yes.
the pot in mine is a AB modular one could be made to be stacked etc. front and rear comes off....IIRC (it was 15 years ago ) I did remove the board to replace the section. I cannot remember if it had rivets...but it has screws now!
Renée

On 11/10/20 11:41 AM, Jeff Dutky wrote:
Renée, so you're saying that this is the pot equivalent of switch bounce? Now that you say it it seems almost obvious, and I was already trying to imagine a way that the pot could bounce like a switch, since the behavior looks a lot like switch bounce.

Are you saying that you unscrewed the back cover on your beam intensity pot and replaced the entire guts with the guts of some other pot?




 

All the pots on my scope are AB stackable pots with screws (though some of the screws are blocked by other components on the board, which means that I would have to remove the main board in order to unsolder the beam intensity pot). I am planning to unscrew the back plate on the beam intensity pot so I can get it open just enough to inject some lubricant. I just didn't realize that replacing the inner components was even an option.

Were the pot guts that you used from another AB pot, or is that just a standard size?


Renée
 

pot parts were from another AB pot multiple stack obtained from surplus place sadly no longer in business and that was several years ago.
parts may be still available? not sure....
Renée

On 11/10/20 1:45 PM, Jeff Dutky wrote:
All the pots on my scope are AB stackable pots with screws (though some of the screws are blocked by other components on the board, which means that I would have to remove the main board in order to unsolder the beam intensity pot). I am planning to unscrew the back plate on the beam intensity pot so I can get it open just enough to inject some lubricant. I just didn't realize that replacing the inner components was even an option.

Were the pot guts that you used from another AB pot, or is that just a standard size?




 

I found apparently "new-old stock" for the beam intensity pot on eBay, so they're still available after a fashion. I also found some other sellers online that advertised the pots, but most of them did not actually have them in stock. I guess they're getting pretty rare on the ground.