Electrolytic capacitors - a perspective

[laurie]

Over in this thread I talked about changing the filter capacitors in a 30 year old Peavey amp.
http://www.bossareaforum.com/Forum/view ... f=14&t=789

There are two schools of thought about this:

• Anything over about 20 years old, just replace the capacitors - if they haven't failed yet, it is imminent.

• Measure the capacitor and if it is within specification, leave it until it does actually fail (years in the future?).

In that other thread I said "I decided to change the main filter caps. 30 years is too old... whether I keep it or not, the caps are going to fail.".

But here is the conundrum. The readings for the capacitors I removed are below. They are still perfectly within specification.

So what is the right approach? In this amp the cost of the replacement caps was about USD$10.

So what I'm trading off is:

• $10 now when I have the amp open anyway.

• The risk/certainty of failure "in a couple of years" with the time to dismantle the amp plus the clean-up of the almost inevitable associated mess (leaked electrolyte).

That Peavey amp, made in the USA 30 years ago, likely had "good" components. So there are probably years (and years) of life left in the capacitors.

I had the amp open anyway. The cost was only $10. And I threw away components that tested as perfect.

If it was a tube amp and the capacitor replacement cost was $50-to-$100, I might have decided to leave them...
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[Pepe]

So what I'm trading off is:

• $10 now when I have the amp open anyway.

• The risk/certainty of failure "in a couple of years" with the time to dismantle the amp plus the clean-up of the almost inevitable associated mess (leaked electrolyte).

This is exactly why I do the same thing. I have the synth (or whatever) open anyway. The parts don't cost a fortune and they will ensure that it will work for the next 25 years or more.

I purchased good radial "Long Life" electrolytic capacitors from Panasonic for the '80s Roland Alpha Juno-2 synthesizer that I purchased lately. The three capacitors didn't even cost 5 EUR. They might prevent some headache in the future.

[Luke Drifter]

My 30 year old Blues Deluxe is in for a major 100,000 km service at the moment. My guy said the capacitors still are all good and not leaking but that it can only be a matter of time. He said (if I heard him right) that the ones Fender used were cheap horrible ones that probably cost 50 cents if that each when thrg were put in, and he wants to use $15 units as replacements. He said ot will make a big noticeable difference. I've not spent a cent on it in 15 years (apart from the Cannabis Rex i put in there, so i'm happy for him to do that

[laurie]

My 30 year old Blues Deluxe is in for a major 100,000 km service at the moment. My guy said the capacitors still are all good and not leaking but that it can only be a matter of time. He said (if I heard him right) that the ones Fender used were cheap horrible ones that probably cost 50 cents if that each when thrg were put in, and he wants to use $15 units as replacements. He said ot will make a big noticeable difference. I've not spent a cent on it in 15 years (apart from the Cannabis Rex i put in there, so i'm happy for him to do that

This seems like a great idea. he is correct - it is only a matter of time until the old ones kark it.

However ... I'd be hesitant about the statement "a big noticable difference". There will likely be something, but depending on the state of the old ones it might not be a "big" difference.

That being said, the amp will be rock-solid for years to come after this work!

[laurie]

I've just been through this process again... repairing an Ibanez SDGR bass for someone locally. It came to me with "no sound".

The SDGR has active electronics - well, kinda. The tone controls are passive and the opamp is used to buffer the pickups and drive the output.

The issue was the 10uF output capacitor. It was leaking and almost open-circuit. Fixed that and the treble control didn't work. Another 47uF electrolytic capacitor open circuit.

The bass is 30(?) years old, so at that point I just swapped all the electrolytics. Cost pennies because of the low values (uF and voltage), but, clearly, it was time.

[Zerksies]

I go with the more by feel on replacing caps. If they are not puffy or leaking anything or giving sometype of issues I am probably leaving thm alone. I've got a 66 deluxe reverb I should of changed caps on three times in the 20 years i've had it but i have only done it once.

[snufkin]

I think it also entirley depends the function the cap is doing in the circuit, power supply caps and also caps in control and power functions elsewhere are usually under more strain than caps in the audio path which may have been carefully selected.

It also depends the time, resources and level of knowledge you have to really dig in to the circuit/faults etc.

Those lilac colour electrolytics in allot of old JP gear are known to go bad as are "capacitor plague" era caps from certain brands.
Tantalum caps in power supply functions are also known to fail after years of service in such a way as to knock out other parts during failure.

However there are lots of caps I would think carefully before changing, caps in an audio filter may change it's characteristics even if you select for exactly the same material and values.

Hope that makes sense.

[bigtone23]

Yeah, I look at 20 year old caps as something to keep an eye on. Once they hit 30 years, maybe not a bad idea to start replacing, and look at some of the resistors, too.
I have found that it's not a massive difference when replacing working filter caps with direct replacements, unless they were upped in value, and that is often just a touch more than noticeable. To be fair, I'm not a golden eared cork sniffer, but do notice performance and tone things, especially in an A-B comparison.
I have two Peavey Century 200 heads, one from 77, the other 78. These are my favorite bass head. So simple, so unassuming, but sound so good for a cool, semi-old school sound. Plus, the distortion circuit is the best thing in the world!
The 78 had every out of spec component replaced with the same value component 8 years ago. The only mod is one suggested by some fellow Century nerds on TalkBass where the preamp board connects to the power amp board. I had it capped a little bigger with 3 parallel caps that were a smaller value, bigger voltage, but summed to a bigger value. Also added an extra film cap into the parallel cap mix. The multiple caps increased inductance, but the film cap allows more highs to pass through.
The 77 is bone stock/original.
Comparing them, they are 80-90% the same. The 78 is a little tighter and has slightly more bandwidth than the 77. I can do small EQ adjustments to make them sound pretty much the same, but that coupling cap mod is certainly adding a little sauce.
However, the 78 keeps it's wits about it after 2-4 hours play, the 77 gets a little saggy when heated up. In the grand scheme of things, I also trust the 78 more, so it's my live head and the 77 stays in the practice space.