Replacement caps for DC 505A


Dave Peterson
 

Since we seem to be on the subject of filter caps:

Last night I got a DC 505A apart so I could confirm that the +5v filter caps C25 and C26 are bad. Oh yeah, they're long gone. According to my Peak Atlas ESR70 one is open and one is "leaky" with an ESR of 6.8. Nominal capacitance of these is 5000uF and Tek manual calls for 10v. The cans say they are 15v.

Mouser has just a couple of Vishay axial caps, but they are 1" (25mm) in diameter. The caps in the unit are 0.75" (20mm). As this is a plug in, there is no room to spare. So the Vishay's on Mouser are a no-go. Searching axial caps on Mouser only yields these couple of Vishays.

Aside from getting some work done for my paying job, a goal for today is to broaden my search across the web for suitable replacements. But I thought I'd ask y'all here as I suspect someone might have some experience with this. Where might I find suitable axial replacement caps for these? Or will I have to adapt some radial electrolytics into place?

Thanks,
Dave


Mark Vincent
 


Dave Peterson
 

Using UHEs was what came to mind while sleeping on it. They definitely have the small diameter.
I'd prefer an axial solution. Superficially for the aesthetics, more significantly for the long distance between leads/trace locations: about 70mm. Using a radial will require wiring a jumper.
Not egregious, but my first choice would be an axial, if available.

On Wednesday, October 20, 2021, 07:45:10 AM PDT, Mark Vincent <orangeglowaudio@gmail.com> wrote:

Dave,

Would these fit? https://www.mouser.com/ProductDetail/Nichicon/UHE1C562MHD6?qs=%2Fha2pyFaduhn2raJlu2%2F0FQLbgkwz6LZB1%252BS6xn%252B0yk%3D

Mark


Mark Vincent
 


Dave Peterson
 

That looks pretty good!

I've been studying Vishay's website, and the 500D series was looking promising, but couldn't find the right size available for sale. It's been kind of maddening how this particular size seems to skirt the boundaries of availability. I keep getting close, but then the series/size combos that would work turn out to be the ones that aren't available.


I guess I was excluding things below 5000uF. Do you think it's ok being 300uF below nominal? The book calls for +75%, -10%, so as low as 4500uF. Obviously it would depend on what actually got delivered. A 4700uF 20% low would be only 3760uF. I suspect they come a bit closer to nominal than the extreme ends of spec.

Looking at the datasheet for the Vishay 120 ATC looks like the lead length is enough to span the trace distances. It's something to contemplate.

The frustrations with the axial availability is leading me to reconsider the Nichicon UHE. I think it's a better capacitor, and the need to run a jumper is really a small aesthetic thing only. Wouldn't I prefer a better cap over aesthetics? I guess I'd need to consider physical mounting as well - a radial would require something to hold it in place. The axial would be lead mounted. But long leads (the 120 ATC means lead lengths of about 20mm + 1 radius ~= 30mm) seems a bit flimsy as well.


Thanks for the research and inputs Mark.


Mark Vincent
 

Dave,

Take one lead and bend it so it goes in one hole. The other lead bend it so it is over or under, under if the bare wire does not touch other traces. Then use a solid wire as the extension to the other hole. the solid wire keeps the body stiffer and reduces movement. Wire spaghetti can be put on the wire for insulation. An adhesive between the can and board can be used to further secure it. I have done this axial to radial conversion before.

Mark


 

Dave,

How close are the mechanical tolerances? The highest values for each voltage range (e.g. 6800 uF @ 16 V) is only 21mm in diameter. Would that still fit in the space?

Otherwise, my (relatively ignorant) understanding is that 4700 uF is today's equivalent to 5000 uF from the 1970s. Also, you are not designing this to be manufactured in bulk. If you order several more caps than you need you can select for highest capacitance, and not have to worry that the parts might be 20% below spec. You might get lucky and get a couple of parts that are 10% above spec.

-- Jeff Dutky


Dave Peterson
 

Blame it on me: I wasn't doing a proper job of searching Mouser. Not even sure what I did to get only two results. Using Mark Vincent's search as a start point I found the variety of Vishay axial capacitors. Looks like the 125 ALS line 6800uF cap is a good fit.

Still good to get thoughts and ideas from others. I'm still a newbie at capacitor selection, so don't completely trust myself.

On Wednesday, October 20, 2021, 05:29:02 PM PDT, Jeff Dutky <jeff.dutky@gmail.com> wrote:

Dave,

How close are the mechanical tolerances? The highest values for each voltage range (e.g. 6800 uF @ 16 V) is only 21mm in diameter. Would that still fit in the space?

Otherwise, my (relatively ignorant) understanding is that 4700 uF is today's equivalent to 5000 uF from the 1970s. Also, you are not designing this to be manufactured in bulk. If you order several more caps than you need you can select for highest capacitance, and not have to worry that the parts might be 20% below spec. You might get lucky and get a couple of parts that are 10% above spec.

-- Jeff Dutky


Dave Peterson
 

Repair complete!

I ordered two: https://www.mouser.com/ProductDetail/594-MAL212516682E3

And finished reassembly of the plug-in after work yesterday. Ran through the check-out procedure in the manual, and things (mostly) worked. So, repair complete, but some calibration to be done. Just some small adjustment of trigger levels. Also the ribbon cables connecting the boards at the rear need replacing. I'm surprised that I'm not finding 0.1" pitch ribbon cable readily available. If anyone knows where I can get a foot of it I'd love to replace them. The adhesive holding the plastic is failing. Simple contact cement is doing a fair enough job to allow temporary use.

I also mistakenly thought the plug-in went to 250MHz (spec is 225MHz). I tried to test it with an SG503 to 250MHz, but it only went to 240MHz. Oh darn!

So it's a nice victory. Feels like I might be getting the hang of this (again). I agree, this was a fairly simple affair. But it has been very satisfying to work through the process methodically, with the right tools, find and repair a fault, and have a serviceable unit ready to go again.

Thanks for the inputs and help finding the right parts!
Dave