[sdiy] "Zerolytics"

Steve Lenham steve at bendentech.co.uk
Tue May 5 13:09:47 CEST 2020

On 05/05/2020 11:31, Gordonjcp wrote:
> You'll never need to "recap" a power supply.  Ever.  Unless you've done something crazy and actually damaged it, of course.

Sorry, but you post this over and over and it just isn't true. Saying 
that you never need to recap a power supply is every bit as extreme - 
and mistaken - as saying that you should always do it.

Two real-world examples:

1. I worked on some jolly expensive Apogee converter units that wouldn't 
turn on. They used off-the-shelf switch-mode power supply modules and 
didn't have a proper hard power switch, only a standby mode. This meant 
that the SMPS was running all the time that power was connected; the 
output reservoir caps in an SMPS work fairly hard and these were dead. 
It's a situation that is increasingly common and will cause the 
premature demise of a lot of gear, so Quantec IMO have the right idea.

The best electrolytic caps have a rated lifetime of around 12000 hours. 
Yes, this is at their max rated temperature and lifetime doubles for 
every 10 degrees below that temp, but there are 8760 hours in a year so 
you cannot rely on them for too many years if in constant use.

I am told that the same problem afflicts the power supplies in Mac Pro 
workstations, with computers costing a five-figure sum being put out of 
action by a handful of worn-out electrolytic caps.

2. I work a lot on vintage Lexicon effects and they use late 70s/early 
80s tantalum caps and poor quality electrolytics. Same goes for e.g. 
Prophet 5s. The tants are prone to spontaneously failing short-circuit - 
not all of them, and not after any particular length of time, but 
statistically it is quite common. Presumably in your world you would 
wait for that to happen, then find the one bad cap and replace it - and 
repair all the other damage it might have done. But a lot of people - 
those that can't do repairs themselves, or are already having work done 
on their gear, or just need their kit to keep working because they rely 
on it - prefer to eliminate the possibility of such a failure happening 
any time soon by fitting better quality modern caps. It is a question of 
risk management.

I could go on. I accept that good-quality electrolytics seeing moderate 
100Hz ripple in a well-ventilated piece of equipment can go on for a 
very long time. But those conditions arise in a diminishing number of cases.


Steve L.
Benden Sound Technology

More information about the Synth-diy mailing list