[sdiy] demagnetising spring reverbs

[Barry Klein]

You didn't say how it sounded with the resistor back to original value...

I wonder if the author is correct suggesting the driver transformer 
magnetics
get permanantly damaged.
I think the truth is you will get distortion by saturating the driver 
transformer -. much like if you
over saturate the inductor of a DC/DC converter.  By doing so you would see 
the current (with a current probe)
waveform change as amplitude is increased.  You then also can just be 
overdriving the transducer, resulting in a
distorted signal passing through the reverb spring.  Lastly you could have 
an issue with the frequency
profile of the driver circuit...
You could instrument tests for this but your ear is probably a better 
diagnostic tool.
A pickup has a magnetic core but a reverb driver transducer will not.

Barry



----- Original Message ----- 
From: "James Dunn" <james at 4thharmonic.com>
To: "synth-diy at dropmix. xs4all. nl" <synth-diy at dropmix.xs4all.nl>
Sent: Wednesday, May 12, 2010 03:04
Subject: [sdiy] demagnetising spring reverbs


> Hi list,
>
> Some time ago I came across a suggestion to increase the signal to noise 
> ratio in the ARP 2600 reverb unit by reducing the value of R273 by half. I 
> tried this mod and it was indeed louder but after a while the reverb 
> seemed to deteriorate. It now sounds very muddy and distorted. I looked at 
> the signal being sent to the reverb tank on a scope and it was also 
> distorted, although it only had a minor glitch when I disconnected the 
> tank. So I concluded that the tank must be faulty (probably by overdriving 
> it). This <http://sound.westhost.com/project34.htm> website would seem to 
> confirm this - it says:
>
>> ...but you must be very careful, because overdrive causes the small pole 
>> piece to become magnetically saturated, leading to gross distortion that 
>> increases with decreasing frequency
>
> So my question is how can I demagnetise it or otherwise repair the tank? I 
> know there was some discussion a while ago about repairing tanks, but not 
> specifically demagnetising...
> On the other hand, should I just buy a new tank? Where can I get one?
>
> many thanks
>
> James
>
> PS - the transducer input impedance is 175 ohms and I temporarily 
> connected another tank which worked fine.
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