Tantalum vs. Electrolytic Caps

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kc8gpd
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Tantalum vs. Electrolytic Caps

Post by kc8gpd » Sat Jul 09, 2016 8:25 am

so i am going to go through the A-50 and replace all it's caps.

do Tantalums need replacing after so many years like Electrolytics do?

i have my list together for the A-50

Electrolytics

470uF/16V x4
100uF /25V x136
10uF/63V x2

Tantalum

1uF/25V x 12

has anyone done these before? am i missing any caps in the list?

i looked over manual and got parts count for the main and ext boards as well as the dmp50 module(s)

i also have a crimper and pins coming :-)

this will be my first time doing a complete cap replacement on a piece of equipment with this kind of parts count.

any tips for me?

since i probably wont do it all in one shot I'm thinking of black tape on all the caps before i begin so i can quickly see what has been changed and what hasn't?

thanks.
kc8gpd

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Re: Tantalum vs. Electrolytic Caps

Post by TPT » Sat Jul 09, 2016 8:33 am

Definitely replace the electrolytics. Get 105C caps to replace the bigger ones.
Solder braid is helpful--I like the kind sold by Radio Shack (Yes, there still is a Radio Shack--buy on-line)

There has long been an argument about the virtues of tantalum vs. electrolytic caps in audio circuits--but at 1 uf I suspect these tantalum caps would have no effect (unless shorted) even if they are in the audio path. Not having a schematic, I would guess they are not. I would leave them alone.

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Re: Tantalum vs. Electrolytic Caps

Post by kcbooboo » Sat Jul 09, 2016 11:42 am

Tantalums are getting expensive. If they are across the power supply rails, they'll short out when they fail, which doesn't take much, and that may or may not take out series resistors. A schematic would tell you if they're audio coupling caps or PS bypass caps.

Electrolytic caps are smaller than they used to be, and most are radial leads (both coming off one end). Axials are getting rare. With the number you have to replace, do one entire board at one sitting. [u][b]Do one cap at a time.[/b][/u] Carefully note the polarity, either by taking a photo, printing it out, and marking the positive end, or by using something permanent on the circuit board. If the manual has pictorial or parts placement diagrams, print those and mark them accordingly.

Many times the boards have a large ground foil on one or both sides. These will suck all the heat out of your iron when you attempt to unsolder the caps. I usually suck the solder out of the ungrounded hole first, then wiggle the lead until it floats freely in the hole. Then I heat the grounded hole and pull the cap out when the solder melts. I then go back and suck the solder out of the lead-free hole.

I just finished replacing 22 electrolytic caps in a Motorola R2001B communications service monitor power supply. In each case the new cap was 2/3rd the size of the old one, both in diameter and height. I bought all Panasonic brand. I used a 48w iron set for 700F and a plunger-style vacuum desoldering tool. Clean the board with 91% alcohol and a toothbrush when you're finished replacing the caps.

Bob M.

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Re: Tantalum vs. Electrolytic Caps

Post by COMMENG » Sun Jul 10, 2016 9:13 am

My advice is, if the Tantalum's are being used to couple audio from one stage to the next, replace them with non-polar or bi-polar electrolytic's, with at least the same voltage rating and designed with a 105 degree temp rating.

COMMENG

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Re: Tantalum vs. Electrolytic Caps

Post by Dale H. Cook » Mon Jul 11, 2016 4:49 am

COMMENG wrote:... if the Tantalum's are being used to couple audio from one stage to the next, replace them with non-polar or bi-polar electrolytic's ...
I have read that replacing tantalum audio coupling caps with electolytics can affect frequency response, and found that to be the case in one old Gates console. Would anyone care to comment?
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Re: Tantalum vs. Electrolytic Caps

Post by kc8gpd » Mon Jul 11, 2016 7:15 am

i ordered tantalums and electrolytics. i forgot to order for the PSU these are just for the the main boards in the console. was only $30.00 from ebay to reorder caps. i did select usa only parts and nothing from china.

i still need to order for the PSU. luckily the psu parts count is small but they are large caps.

it powers up fine so my suspicion is the caps were replaced at least once in it's history.
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Re: Tantalum vs. Electrolytic Caps

Post by TPT » Mon Jul 11, 2016 12:42 pm

Unless you are comparing dried up electrolytics to tantalum caps, I can't see how changing one for the other (same capacitance) would have effect on frequency response.

I know there have been "golden ears" arguments of one versus another. Years ago there was a fad--in amps using discrete transistors--of piggybacking several different capacitor values in parallel (e.g. 20 uf/2 uf & .2 uf). Now that might have some interesting effects-tried it once on an old Gates cart machine and it did sound different.

On the power supply--I would order the big electrolytics from a reputable source like Mouser or Digikey.

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Re: Tantalum vs. Electrolytic Caps

Post by kc8gpd » Mon Jul 11, 2016 12:46 pm

should i reorder everything from mouser or digikey? or do you think the smaller caps are okay from ebay? like i said i ordered from usa sellers, no china stuff. i want this done right the first time around. this will be my personal board for my studio.
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Re: Tantalum vs. Electrolytic Caps

Post by PID_Stop » Mon Jul 11, 2016 3:06 pm

A couple of things to consider that I haven't see posted:

1) Are the existing tantalum caps wet or solid? Wet caps are generally tubular and look more like electrolytics, while solid tantalum caps are generally epoxy dipped and are more spherical (though there are a few that do resemble diodes, but in odd colors). If you have wet tantalums, then you should definitely replace them (I have the tee shirt for replacing hundreds of them on an analog video routing switcher years ago). On the other hand, solid tantalums tend to be very stable over a long time; unless you are looking at part of an R-C timing network, or a bypass capacitor next to a voltage regulator, I would leave them alone.

2) Tantalums and electrolytics do behave differently in some ways, but for simple DC decoupling, you can probably use either type. On the other hand, if these are part of an EQ circuit, you probably should use the type that was originally designed.

3) I would stick to Digikey or Mouser; for one thing, you will probably end up with fairly new stock from a reputable manufacturer like Kemet.

4) Beware of no-name capacitors. Aside from the bad-cap fiasco, there have been cases where tantalum caps have been mismarked. Years ago, the NBC station across town bought a brand new Central Dynamics routing switcher... and after several weeks of random failures, they discovered that they had gotten a batch of caps with the polarization marked wrong. They sent a field engineer from Montreal with a big box of caps, and replaced every single one.

5) Be very careful that you're reading the polarity markings correctly: most of the tantalums I use in new designs have the strip on the positive lead, whereas most electrolytics stripe the negative lead.

Just some random things that might save you from some of the problems we've encountered over the years!

-- Jeff

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Re: Tantalum vs. Electrolytic Caps

Post by Dale H. Cook » Mon Jul 11, 2016 4:02 pm

TPT wrote:... I can't see how changing one for the other (same capacitance) would have effect on frequency response.
It has been decades, but IIRC axial wet tantalums were used in feedback loops in some Gates consoles, and someone had replaced some with aluminum electrolytics, which affected the frequency response. I will have to dig through my Gates catalogs, figure out which model of console that was, and pull up the manual. I do well remember that replacing the electrolytics with the proper tantalums cured the problem. The electrolytics were the proper value and voltage, but perhaps they were very old stock that were not reformed properly before being installed.
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Re: Tantalum vs. Electrolytic Caps

Post by COMMENG » Mon Jul 11, 2016 5:50 pm

Dale H. Cook wrote:
TPT wrote:... I can't see how changing one for the other (same capacitance) would have effect on frequency response.
It has been decades, but IIRC axial wet tantalums were used in feedback loops in some Gates consoles, and someone had replaced some with aluminum electrolytics, which affected the frequency response.
Never heard of that one. If the capacitor value and the capacitive reactance are correct for the frequencies of interest at the 3dB points, it shouldn't matter.

Polarized capacitors will have higher leakage currents than non-polarized, which may affect biases. If one is coupling FET discretes or analog FET IC's, the last thing you want is leakage current upsetting the operating point bias voltages. And non-polarized coupling caps wont act as a short circuit if reversed.

For power supply filtering, polarized electrolytic caps with low ESR are indeed warranted.

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Re: Tantalum vs. Electrolytic Caps

Post by Dale H. Cook » Tue Jul 12, 2016 5:52 am

COMMENG wrote:If the capacitor value and the capacitive reactance are correct for the frequencies of interest at the 3dB points, it shouldn't matter.
In some applications the leakage resistance or effective series resistance may matter. Although we generally treat capacitors as pure reactances (which they effectively are in most applications) they are actually impedances.

It may be some time before I can determine which Gates console that was and find the print - we recently had a death in the family and I am still wrapped up in the aftermath.
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Re: Tantalum vs. Electrolytic Caps

Post by kcbooboo » Tue Jul 12, 2016 1:45 pm

I would go out on a limb and say that Effective Series Resistance (ESR) is a factor worth considering primarily for power supply filter capacitors. For ordinary signal coupling or on-board power supply bypass caps, it doesn't matter nearly as much.

The capacitance value and ESR are totally independent. If either is way out of speck, it makes sense to replace the cap. These days it's cheaper and faster to replace a cap than to pull it, test it, and try to put it back into the board. There are ESR testers (Peak ESR70+) that can usually test caps in-circuit, which speeds up the troubleshooting process.

I know people who, after finding one shorted tantalum bypass cap, will replace every single one (over 40) so they never have that problem again. I wouldn't go that far, but I suppose with some critical piece of equipment it might be worth doing. I just have never been in that situation.

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Re: Tantalum vs. Electrolytic Caps

Post by ChuckG » Tue Jul 12, 2016 8:58 pm

kc8gpd wrote: do you think the smaller caps are okay from ebay?
There's really no way of knowing unless they're from a merchant you know and trust. I use DigiKey even though it costs a little more because I know their stock is fresh. If they check OK on a cap meter then I wouldn't worry about it.
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