I am recapping a vintage tape recorder and I have noticed MANY low value (≤ 1uf) electrolytics (some tantalum) being used, especially around Dolby encode-decode circuits. I am curious as to WHY they would opt for this, as film capacitors would easily fit and have many advantages. Why use electrolytics when they are known to be prone to failure? Is there some advantage that I am not aware of?
Cost could be one reason, small 'lytics (aluminium ones, not tants) are cheaper than equivalent value film caps.
I think I just answered my own question: THEY ARE CHEAP!!
sO...WHAT'S THE NEXT BEST ALTERNATIVE? COG? X7R?
sO...WHAT'S THE NEXT BEST ALTERNATIVE? COG? X7R?
Last edited:
C0G isn't cheap when above 100nF, prices go up pretty steeply there. X7R are microphonic and non-linear with respect to applied voltage.
Looks like I'll go with PET caps---more reliable and less distortion than electrolytics or tantalums; much smaller than polypropylenes (space is absolutely CRITICAL in my application); however they ain't cheap!
I’d be inclined to analyze the circuit before just dropping ”more expensive therefore better” capacitors into it. If it’s got a decent (at least several volts) DC bias on it there’s nothing wrong with a new electrolytic. Nichicon, Panasonic, Kemet, even CDE - can’t go wrong compared to what was available when the unit was made. If there is little or no DC bias to form the electrolyte I’d want to find a way to use a film cap. 50 volt green mylar “chicklet” types even. They are small enough in 0.47 uF-ish sizes. DA wont be any worse than vintage aluminum electros, and even if it has 15 ohms of ESR it’s probably working into at least a 15k impedance. If the impedance is a lot lower, you may need a better cap. Just think about what the cap is doing in the circuit and you can figure what compromises are transparent.
Wima polyesters from 0.1 to 4.7uf are reasonably small. Below that the Wima polypropylenes are a reasonable size
Or just, you know, use electrolytics... As stated, they are cheap and work just fine - no reason to fix what isn't broken after all.
Quite true. There may be a blocking cap with only a tenth or two of volts on it, where an electro develops a couple hundred ohms of ESR from not having enough bias on it. And you might not even notice (or even be able to measure the result). I’m not OCD enough to go replacing caps without cause, but the engineer in me would want to prevent this sort of soft failure if it doesn’t cost $20+ or routing wires off the PCB to do it.
Well, the ESR won't really change with the bias but the nontrivial ESR of a small value electrolytic does play a role in the circuit, which was designed around that value. That is one very good reason not to just shotgun replace electrolytics with film or ceramic (both of which have an extremely low ESR) as it can cause all manner of circuit problems - that resistance is functional part of the circuit. It's a good way to completely change the frequency response of the circuit and vastly increase the chances of oscillation. All of that is not to say that electrolytics can't be replaced with other dielectrics but you should first be very sure of how they, and their parasitics, affect the circuit before making changes. And even then, only change one capacitor at a time - then test thoroughly to see what - if any - changes in performance happened.
As an anecdote, I found E-caps as small as 47nF (in a Sony VCR U-matic), but they were tantalum. The lowest value of Al Ecap I ever found is 100nF, and they were quite common in many equipments from many manufacturers.
In general the sensible thing to do is to replace them with PET, which are much more durable and polarity insensitive, but in some cases the higher losses were part of the function (knowingly or not)
In general the sensible thing to do is to replace them with PET, which are much more durable and polarity insensitive, but in some cases the higher losses were part of the function (knowingly or not)
Sad and very desperate way of doing business.
All for the money, even just the relative crumbs and throw all pride out of the window as well 🙁 👎
One reason for cap ”failure” in the first place is ESR going up over time when the cap has no bias. Apply bias and reform them and they can recover. When the circuit operates them with no bias they just get steadily worse with time and heat. Can you hear it? It depends. In series with 47k ohms ESR can go up by a factor of 10 and no one would care. In series with only 220 ohms, your amp/preamp can noticeably lose gain or bass response, or even drop closed loop gain enough to go unstable. Fractional or single digit uF parts would tend to be used at higher impedances, if response down to 20-ish is expected. But not universally.
All this respect for worn out old garbage! Electrolytics are a time fuse for products that are designed to be thrown away in 1, 2, 5, 10, 15 years. Only the epoxy ones in fender amps and the ones in sardine cans in forties Hammond tone cabinets are worth two cents after that time.
I've replaced 1, .47 and .22 uf electrolytics in a FM radio, no alignment, it started working like it was 1978 all over again. Mostly used ceramics. COG is great stuff but no distributor ever stocks any. X7r does not exhibit all the bugaboos that people are warning about. Use 50 v rated ones on 2 v circuits. Microphonic, this tape recorder does not have a speaker in the case turned up to 11 as guitar amps do.
I've replaced 1, .47 and .22 uf electrolytics in a FM radio, no alignment, it started working like it was 1978 all over again. Mostly used ceramics. COG is great stuff but no distributor ever stocks any. X7r does not exhibit all the bugaboos that people are warning about. Use 50 v rated ones on 2 v circuits. Microphonic, this tape recorder does not have a speaker in the case turned up to 11 as guitar amps do.
Last edited:
Most electrolytics last longer than 15 years - its more like 30+ years, as the '78 radio testifies
News to Farnell, Digikey, Mouser, etc. Plenty available and to much higher capacitance values these days, upto 100's of nF or more. Gets expensive though, 470nF is reasonably available in THT and SMT.
Digikey, Mouser, LCSC, TME, Elpro, RS Online, Farnell by the thousands and thousands.
Most of these places also sell to private consumers and hobbyists
Possibly very cheap ones from one-hung-lo brands - quality ones from a mainstream manufacturer will usually outlast the equipment. Note that this is not the case for those used in high current and/or temperature applications such as bulk filtering - for electrolytics, temperature is the killer.