Swap old tantalums for new electrolytics?

[stevehertz]

I have noticed on US vintage hifi forums that enthusiasts often swap old tantalums for new electrolytics. Is that a good thing? What do you think? What's the rationale? Why not fit new tantalums?

BTW, I don't normally swap electrolytics by the truck load, I just replace ones that cause an obvious fault - which isn't many in vintage receivers. However, I am going to do a complete electrolytic cap replacement on my Pioneer SX-1250 because of it's prestige as a set and I'm going to use it as my main receiver and want it to be 'as good as it can be'. I'm also curious as what difference swapping all the electrolytic caps make - probably not a lot despite what people say..

Anyway.. that's all background stuff, back to the prime question, why do people swap tantalums for new electrolytics? I'm guessing that earlier electrolytics were bigger and it was sometimes physically easier/preferable to fit a tant instead? or is it their low leakage current?

[PeterRoberts]

I once had a Racal RF signal generator that was stuffed with red tantalum bead capacitors. Every so often, one would go short circuit and necessitated dismantling the thing and then trying to track down the culprit. No sooner was the unit repaired and reassembled then another would go S/C. I didn't fancy replacing all of them as it would have been a VERY big job, so I got rid of it. I do have other bits of equipment that have similar capacitors that have never failed, so I don't know if it was just a case of poor manufacturing, but some are prone to going short circuit but others are happily working in bits of my kit after four decades!

I assume that some people are suspicious of these devices and replace them just in case.

Peter

[G6Tanuki]

Tantalums were a way to get lots-of-capacitance and low-ESR into a small package: great for decoupling supply-lines in digital circuits where there was lots of spikiness being introduced, and also good for taking the 'edge' off the ripple from early SMPS.

The downside is, like all electrolytics, reliability!

These days unless the circuit is *extremely* niche-market, I would replace old 1970s/1980s Tantalums with modern low-ESR high-temperature electrolytics of a reliable manufacturer.

Example: there were a number of small 130Volt-rated tantalums [metal-cased type] in my PRC320, associated with the +120V DC rail that feeds the varicaps in the frequency-synthesizer VCO. When one of these got 'tired' and a bit leaky it was causing a noticeable and most-offputting wanderiness on SSB . Scoping the rail showed the fluctuation was only 10-15 millivolts but that was enough to wobble the VFO by a few hundred Hertz.

I was able to source "Rubycon"-brand 160V/105C-rated electrolytics in packages that were no larger than the Tantalums. Sensible prices too, compared with the original tantalums (still available, but at a price around £75 +VAT - each!).

Half an hour with the soldering-iron and I was back on 14MHz without sounding like I was gargling porridge.

[Biggles]

Nasty little things that go with a big bang on occasion. I would guess that modern electrolytic design may have superceded the original advantages of tant beads. They were meant to be the bees knees in the day because of their high capacity in small size along with low leakage current but they always have been expensive and the low working voltage restricted things a bit.
Alan.

[stevehertz]

I'm getting the picture..

[paulsherwin]

I think it makes more sense to swap out old tants than it does old electrolytics - certainly, if you're changing all the electrolytics then you should do the tants as well. I'm surprised there are many tants in this receiver given that there will be plenty of space inside.

[Station X]

I've had tants go short-circuit and either explode or take out the PCB tracks feeding them. I hate the things.

[MrBungle]

They do indeed explode. It doesn't take much to do it either as demonstrated here:

https://youtu.be/VknpLq7DkSM?t=83

If you want to replace a tant with an electrolytic you're probably good. The general rule is 3x the capacitance and use a low ESR one. That applies to decoupling/filtering only. Other applications you can get away with a close value. TBH even the cheap naff Chinese ones from Tayda measure usually within 5% of tolerance.

Some of the newer brand name SMD tantalum capacitors such as AVX's TAJ line are impossible to blow up though. I've tried. They just get hot, desolder themselves and fall off the board.

However with electrolytic capacitors, those little 1uF 63v ones without any vents go with one hell of a bang as well, far louder and more destructive than any tantalums. They're little pipe bombs! Even the new Vishay and Rubycon ones like to wake you up. I occasionally blow one or two up when I'm bored. Last time, one blew a hole in SWMBO's tupperware box

[Radio Wrangler]

I now have a picture stuck in my mind of someone playing something like 'Whack-A-Mole' with a boardful of bead tants.

David

[evingar]

Originally Posted by MrBungle

Some of the newer brand name SMD tantalum capacitors such as AVX's TAJ line are impossible to blow up though. I've tried. They just get hot, desolder themselves and fall off the board.

I would agree that the modern SM tants are a totally different kettle of fish to the through hole "bead" types.

For modern SM commercial designs though I invariably specify multilayer ceramic caps for power rail decoupling on cost basis.

In the last few years availability of affordable higher values and voltages has increased markedly.

[MrBungle]

Yes I was quite surprised recently seeing the prices of MLCC low uF range caps. It's quite impressive really. Also much higher voltage ratings. No need for the tants then. Good riddance!

This is inversely proportional the price of tantalum! I tend to get my tantalum capacitors for £2-7 for a part used reel from a certain auction site. Lots of good deals there. To buy the same reel from RS would be £500+!

Really need to do more SMD stuff. The TH stuff is getting expensive now.

[McMurdo]

In a former life we were always changing tants that were used in a particular item as timing elements on the input of logic chips. They used to go leaky. I rarely buy them these days because standard aluminium electrolytics usually seem to do the job as well. Thee's always Oscons if you are stuck for space but these can also be susceptible to only slight overvoltages.

[Craig Sawyers]

The problem with using solid tantalum bead in local power supply decoupling is related to the original manufacturer's specifications. I too used them in that application back in the 1980's. In general 6V capacitors were used for 5V rails and 16V ones for 15V rails.

The problem is that those specs took no account of surge currents.

The best guidelines are from applications in the space industry, which calls for solid tantalums to be run at 60% of their ratings for voltage and surge current, and ripple current to be 50% of nominal maximum. Of course in commercial applications there is a cost associated with that conservative approach - with the consequence that a lifetime of taking current spikes and surges, along with insufficient voltage derating leads to burning tant syndrome.

I just replace them with aluminium electrolytic of around three times the capacitance.

Craig

Craig

[kalee20]

I use tants with caution, but haven't had a problem in the last 25 years (I generally use a tant on the output of negative 3-terminal regulators such as the 7915, and also on the supply rails of op-amps, with a 33 ohm resistor upstream).

I've actually done things the other way round - replace an old electrolytic with a new tant! A few of my radios with FM ratio detectors, I have replaced the stabilising capacitor (frequently 4.7uF 16V) with a tant. I generally use a 25V rated one. Again, no problems - and the position is benign with no massive ripple, but the low-ESR and long-term stability is attractive.

[Nuvistor]

Originally Posted by kalee20

A few of my radios with FM ratio detectors, I have replaced the stabilising capacitor (frequently 4.7uF 16V) with a tant. I generally use a 25V rated one.

I presume those are transistor radios. valve ratio detectors using say UABC80 can have higher voltages on them, typically on my radios about 40 volts depending on signal strength.
Internal aerial in the cabinet may give only 10 volts but a dipole in the attic gives around 40 volts.

Frank

[G0HZU_JMR]

Yes, I've been designing stuff with tants for over 25 years and have found them to be very reliable as long as the usual design rules are followed.
At my place of work tants get used a lot and they are great over temperature and very good for wideband decoupling.

However, go outside the rules (as some of my colleagues have) and tants can fail very quickly, often the first time the board is powered. But I've often produced designs with dozens of tants per board and we have made thousands of the boards and tant reliability has been excellent.

Electrolytic caps aren't always good replacements if you need low ESR and wideband decoupling, especially if you operate them at cold temperatures. Even more so if your equipment ever has to operate below -30degC as many electrolytic caps will cease to be effective capacitors at these low temperatures.

However, some caution would be needed if replacing old tants with new ones as there may be cases where a modern tant isn't the optimal choice.

[Bazz4CQJ]

Originally Posted by G0HZU_JMR

Yes, I've been designing stuff with tants for over 25 years and have found them to be very reliable as long as the usual design rules are followed.

That may be totally correct, but that suggests that there are designers out in the commercial world who don't know what the rules are. So if a piece of factory-made equipment repeated breaks down because the tants have gone, there comes a time when rather than take any more risks (that other components could also be damaged), going for modern electrolytics is the safer way to go - probably quite adequate for anything that lives in the undemanding world of the typical ham radio shack.

B

[kalee20]

Originally Posted by Nuvistor

I presume those are transistor radios. valve ratio detectors using say UABC80 can have higher voltages on them, typically on my radios about 40 volts depending on signal strength.
Internal aerial in the cabinet may give only 10 volts but a dipole in the attic gives around 40 volts

They actually ARE valve radios, one being an Ever Ready Sky Monarch, the other a GEC BC5842 (both however use germanium diodes rather than valves in the ratio detector). Good point though about high voltages with strong signals - aluminium electrolytics behave like a soggy Zener diode when overvoltaged whereas solid tants just permanently die.

[joebog1]

I note that kalee has a resistor in line!! I seem to remember reading that tants should always have a resistor in series. This would tend to ruin the ESR ratings, but saves the capacitor. As far as audio goes they dont sound very good !! Yes it sounds like hifi rubbish, but they tend to have a tinny, thin sound. I built a sand phono stage from Elektor magazine many years ago, and used exclusively tants. Some bead types and some solid types. All those that listened to it complained of an edginess or grating sound.
I built a second unit with low leakage and low ESR electrolytics and the problem was gone.

Joe

[G0HZU_JMR]

That may be totally correct, but that suggests that there are designers out in the commercial world who don't know what the rules are. So if a piece of factory-made equipment repeated breaks down because the tants have gone, there comes a time when rather than take any more risks (that other components could also be damaged), going for modern electrolytics is the safer way to go - probably quite adequate for anything that lives in the undemanding world of the typical ham radio shack.

Yes, often it will be fine to simply swap for an electrolytic. But there's never going to be a universal solution for this issue because of the wide variation in capacitor specs. Some tants will have quite high ESR and some very low and the same applies to electrolytics. So some caution and circuit analysis is probably a good idea when choosing a suitable replacement. Some of the old leaded tants have quite high ESR (even compared to electrolytics) and sometimes it isn't wise to swap for a part with much lower ESR as it may affect stability in a regulation loop for example. But in many cases things will be far less critical and anything with a reasonable ESR and reasonable stability over temperature will be fine