[sdiy] [AH] Arp Solina with excessive chorus noise

[Chris Juried]

In the 30 years I've been recapping SSL, Neve, API, Helios and Trident consoles along with MCI, Studer, Ampex, Otari and 3M tape machines, I can assure that everything Steve states is right on target. 

Best,

Chris 
http://www.JuriedEngineering.com  
 
  On Mon, Feb 1, 2016 at 4:55 AM, Steve Lenham<steve at bendentech.co.uk> wrote:   The re-capping thing comes up every six months or so, becomes polarised 
(like the caps, ha ha) and peters out with this kind of nonsense.

As usual, the truth is neither of the two extremes.

The right question is: what is going on in electrolytic caps that means 
we might want to replace them?

Near-as-dammit all the aluminium electrolytic caps that are found in 
synths, etc, contain a wet paste electrolyte that forms one of the two 
plates of the capacitor. This is held inside by a polymer/rubber seal. 
Over the years, the electrolyte paste gradually dries up, a process 
which is accelerated if/when the seal starts to degrade. Look at the 
bung on the bottom of a 40-year-old cap and chances are it has hardened 
significantly, or even cracked - do you think it is still acting as a 
good seal?

This is an inherent limitation on the lifetime of electrolytic caps 
which simply does not apply to other types of component. It is 
accelerated by raised temperatures, for obvious reasons, but happens anyway.

As the electrolyte level diminishes, the capacitance value of the part 
reduces and the impedance/ESR rises. However, they rarely fail open- or 
short-circuit. That is why those who assert that "bad caps do not cause 
synths to fail" have a point. They don't die, they just fade away. But 
as they do so, they test the design of the equipment to the limit. Not 
many designs can survive a whole bunch of their components reducing in 
value by 90% without some kind of performance degradation.

That is why people do a blanket re-cap of equipment that they wish to be 
long-term reliable. There is no way to predict which caps will/won't be 
affected, by how much and in what timescale. You can't measure it 
without taking each cap out and testing its value and ESR - at which 
point you may as well solder in a new one anyway.

There is not a magic age, before which everything will be fine and after 
which everything should be re-capped. It is a statistical spread and the 
choice of when (or if) to re-cap is a matter of personal preference, 
often influenced by how important a piece of equipment is to you, its 
value and how long you intend to keep it.

A few other random points:

- Electrolytics in SMPSUs do indeed degrade quicker. It is the same 
effect, but the large, high-frequency ripple currents to which they are 
exposed causes internal heating which hastens the process. It's also 
fair to say that many SMPSUs do not have the same design margins as 
equipment of old.

- Sometimes seal degradation allows electrolyte to leak out. This can 
corrode PCB tracks and is one instance where cap failure can damage a 
unit. 70s/80s Korgs seem to be particularly prone to this; I have seen 
it in PS3x00 synths and SDDx000 delays. It also kills display modules in 
Lexicon PCM80/81/90/91s!

- Sure, bodgers are attracted to re-capping because it is an easy thing 
to try (and to understand). But equating re-capping with bodgery is BS. 
Bodgers bodge all kinds of things, not just caps. And I can assure you 
that the units I re-cap (and I do) are extremely reliable.

FWIW, I personally would be considering a total re-cap for equipment I 
value if it is more than 30 years old. Between 20 and 30 years old, I 
might just change the main PSU reservoir caps. Secondary-side reservoir 
caps in SMPSUs I would give no more than 10 years!

Hope this sheds some light!

Cheers,

Steve L.
Benden Sound Technology



On 31/01/2016 23:20, rsdio at audiobanshee.com wrote:
> The whole damn synth is 40 years old. By that logic, you'd be better off replacing the whole thing!
>
>
> On Jan 31, 2016, at 2:04 PM, Tom Wiltshire <tom at electricdruid.net> wrote:
>> I don't see what's "premature" about putting fresh caps in the power supply of something from the 1970's. Those caps are probably 40 years old!
>>
>> Tom
>>
>> On 31 Jan 2016, at 19:30, rsdio at audiobanshee.com wrote:
>>> Just say "no" to premature re-capping.
>>>
>>> On Jan 31, 2016, at 11:21 AM, <tprivitera at nc.rr.com> wrote:
>>>> I recently picked up a 70's Arp Solina String Ensemble. It has the button on the right cheek block to turn the chorus on/off and it is not the later version with LEDs.
>>>>
>>>> It's in pretty good condition, every function works and it sounds great, but there is, in my opinion, excessive noise and 'swooshing' when the chorus/modulation circuit is engaged.  It is dead silent with the chorus turned off.  Not a big deal when you are playing notes or chords, but enough to be annoying when it's idling.
>>>>
>>>> I downloaded the service manual/schematics and it looks like there are adjustment pots on the modulation boards.  Anyone had one with a similar issue adjust these or can suggest some ideas to reduce the noise?  I will be re-capping the power supply later this week for preventative maintenance, as I do with all my synths, combo organs, etc.
>
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
>

_______________________________________________
Synth-diy mailing list
Synth-diy at dropmix.xs4all.nl
http://dropmix.xs4all.nl/mailman/listinfo/synth-diy
  
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20160201/ced9f8bb/attachment.htm>