Audimax 4440/4450 audio capacitors

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kcbooboo
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Audimax 4440/4450 audio capacitors

Post by kcbooboo » Fri Dec 18, 2015 12:45 pm

I've replaced the PS electrolytics on the 4440/4450 control/PS boards many times. Those aren't the issue.

There are dozens of black axial-lead capacitors on the audio boards. The positive end is chamfered/tapered. They seem to be entirely sealed and are either plastic or epoxy, which leads me to believe they weren't ordinary aluminum electrolytic caps. The manual has a Mallory part number. They look like tantalums but I'm not sure those were available when these units were made. They range from 4.7 to 10uF at 35V and were used as coupling and filter capacitors throughout the unit.

Any suggestions for what I should replace them with? It'll be harder to find axial electrolytics but that seems to be what would fit in the available space. Tear-drop tantalums would look dumb, as would radial lead electrolytics. They're all polarized.

Bob M.

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Re: Audimax 4440/4450 audio capacitors

Post by ChuckG » Fri Dec 18, 2015 4:07 pm

Like this only black?
Image

Those are probably tantalum caps. They've been available in dry form since the mid 50's and in "wet" form about 2 decades longer.
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TPT
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Re: Audimax 4440/4450 audio capacitors

Post by TPT » Fri Dec 18, 2015 4:14 pm

If you have nothing else to do, replace them with axial electrolytics. Otherwise, unless they are shorted, they probably can be left alone.

kcbooboo
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Re: Audimax 4440/4450 audio capacitors

Post by kcbooboo » Sat Dec 19, 2015 7:51 am

Yup, those pointed ones look correct. I just wasn't aware that solid axial tantalums were available that long ago. For some reason, Kemet brand comes to mind.

I know that when used as power supply bypass caps, they usually fail by shorting out and taking the series resistor (if present) or a foil trace (if not) with them. As these are mostly audio coupling caps and are working fine, I will just leave them, but I was wondering about a suitable replacement if/whn that is required. The few input stage caps that I measured were all at or above marked capacitance in-circuit, so they really can't be that bad. Low freq response was great too, and that's usually the first thing that goes with series coupling caps.

Thanks to all.

Bob M.

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Dale H. Cook
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Re: Audimax 4440/4450 audio capacitors

Post by Dale H. Cook » Sat Dec 19, 2015 9:20 am

For a late-model type TAC datasheet see:

http://datasheet.octopart.com/TAC474K03 ... 110167.pdf

I would not replace them if they are not bad. In a design of this vintage I would not replace them for audio coupling with aluminum electrolytics, especially inexpensive aluminum electrolytics. Tantalums are often seen in broadcast designs of this period for audio coupling, likely because of their small size and their ESR characteristics. Switching to aluminum electolytics may adversely affect frequency response. I have worked on old broadcast equipment where failed tantalums had been replaced with aluminum electrolytics with a deleterious effect. Replacement with new parts of the proper type restored proper functioning.

As for the failure of tantalum capacitors when used to bypass power supply rails, those that I have seen fail in that application have (IIRC) always been resin-dipped pearls where replacements with a higher voltage rating have not failed.
Dale H. Cook, Contract Engineer, Roanoke/Lynchburg, VA
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Deep Thought
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Re: Audimax 4440/4450 audio capacitors

Post by Deep Thought » Sat Dec 19, 2015 10:55 am

Those coupling caps rarely fail unless there was some other cataclysmic event in the circuit, so unless they are obviously bad I'd leave them be. They don't dry out over time like electrolytics, which is usually why those need to be replaced.
Mark Mueller • Mueller Broadcast Design • La Grange, IL • http://www.muellerbroadcastdesign.com

kcbooboo
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Re: Audimax 4440/4450 audio capacitors

Post by kcbooboo » Sat Dec 19, 2015 3:11 pm

Thanks. I wasn't planning on replacing them just because I could. The freq response and distortion meet specs (which aren't that good to start with, compared to today's equipment). If it ain't broke, don't fix it.

I don't know how much excess voltage a 22uF 20V tantalum can withstand, but I suspect it's a narrow peak that does them in. I've seen SMT parts go up in flames, smoking up the pcb pretty good and taking out a foil trace that feeds power to the board. I've seen some teardrop tantalums short out and discolor, but I've also seen some that appear fine, which can't be said for the accompanying series (10-22 ohm) resistor.

Someone gave me a few boxes of surplus teardrop tantalums. If I can figure out the voltage rating on one, I might just throw it onto my DC voltage calibrator and run the voltage up until it fails, to see where that might be, just for fun. I could also connect a pulse generator to some sort of DC supply and see how high I can go. They certainly don't seem to be as forgiving as aluminum electrolytics, which usually blow their tops or expand out through the rubber seal at the bottom, but rarely short and destroy other things around them.

I've also had some intermittently go bad, usually by shorting out very briefly, causing the power supply to shut down, but once the voltage has gone away, they continue to work fine as if nothing happened. I had one of those drive me nuts on a Fluke RF sig gen. The shorting out would cause the unit to reset itself and power right back up, losing all the settings. Eventually it failed permanently and solidly shorted out one supply, so it was then just a matter of unplugging boards until I found the board with the zero ohm short. Still, I had to pull a dozen boards on one of two castings, and even then there were a dozen blue tantalums on the board. I took a shot in the dark and unsoldered just one; luckily it was the shorted one. Good guess.

Bob M.

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Re: Audimax 4440/4450 audio capacitors

Post by ChuckG » Sun Dec 20, 2015 1:24 am

kcbooboo wrote: I don't know how much excess voltage a 22uF 20V tantalum can withstand, but I suspect it's a narrow peak that does them in..
IMHO if it's running at more than 50% of it's stated voltage rating, you're asking for it. Any spikes at all over the rated voltage, or momentary polarity reversal, or overheating during soldering can fracture the pellet, which reduces it's voltage rating. Repeat until flame appears. Typical failure is thermal runaway followed by a dead short. You're right-aluminum electrolytics are much more forgiving.
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