Polytone model 104 filter cap cans

[Randall]

Two 7300 uF 50v cans, 1.35" wide. Everything I can find is 2 or 2.5" diameter, which will not work. The amp is producing intermittent crackling noises when pushed hard, which I have yet to experience. Two ceramic disk caps have broken off on one of the pre channels and were knocking around inside the chassis. Since this is a 43 year old amp I suggested a re-cap as a place to start. I suppose I could double up pairs of 3700 uF and fit them inside the chassis, but since I don't really think the filter caps are causing the noise, I am inclined to do the preamp and PA board caps and go from there.

I have yet to find the pre amp schematic, so I'm flying blind at this point.

[52 Bill]

The amp is producing intermittent crackling noises when pushed hard, which I have yet to experience. Two ceramic disk caps have broken off on one of the pre channels and were knocking around inside the chassis.

Does the problem happen through both channels? Is it possible that the problem is mechanical? I've seen the power amp vibrate all of the screws loose until the heatsinks fell off.

Is the power amp pc board mounted by the filter cap clamps? If it is, you might have to remount the board with standoffs and then use smaller snap-in caps as replacements.

[Randall]

Don't know about any of the particulars yet as it hasn't done it in my care, and the owner isn't very astute in troubleshooting. So far, find no mechanical faults with chopstick probing or cold spray, past the normal. The filter caps are screw terminal type, so no physical contact with pc board. I don't find any snap in type caps at his rating at all. 7300uF at 50v is a big assed cap, and seems to me in the past.

[52 Bill]

I don't find any snap in type caps at his rating at all. 7300uF at 50v is a big assed cap, and seems to me in the past.

The value isn't all that important, 6800 or 8200 at 63 volts would work just fine. And they're probably in the 1-1 1/2" diameter sizes as well.

[g1]

35mm diameter is about 1.4 inches.
http://ca.mouser.com/Passive-Compone...z0wqsnZ1yx4avu

[Enzo]

Agree, even 10,000uf. Yes, search 63v. 7300uf is not a standard value, looking for it severely limits your options.

But I am concerned about one thing. You have intermittant crackles that you cannot replicate. So you have decided to replace the main filters. I would call that guessing. In my experience, fading filters tend to make humming or occasionally a little peak modulation on low notes, rather than crackles.

You can't replicate the complaint, any chance the crackles are from the owner's stage setup?

Have you performed the whack test?

[Randall]

I agree with Enzo, replacing the filter caps is not the likely cause of the problem. It was more of a "while we have it apart why don't we do the filters?". I have done the whack test, nothing.

[Enzo]

The reason I always recommend against while we were in there work, is that every part we replace, every connection we make is another opportunity to introduce new issues, which will only get in the way of solving the original problem. My analogy for this is if you are in the hospital with a bursting appendix, would you tell the doctor "While you are in there, please also do the liposuction on my belly"?


What is the owners rig like? Can you invite him to bring his setup to your shop and invite him to demonstrate the problem? Since you can't make it happen, you don;t really know that the problem is in the amp.

[Randall]

It's a keyboard rig, so I'm not sure I can get him to set up in my small shop. But I can wait while he uses a different amp at a few gigs and see where that leads.