[sdiy] demagnetising spring reverbs

[James Dunn]

Thanks for the replies.
I have changed the resistor back to the original value and the sound is 
the same (muddy and distorted) so I assume the damage has been done. 
Looks like I may need a new tank then.

Quoth Barry Klein, on 12/05/10 17:00:
> 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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