[sdiy] 1970's organ repair and multi-capacitor PS caps

Mike Beauchamp list at mikebeauchamp.com
Sun Jun 7 18:55:38 CEST 2020



On 6/7/20 12:25 PM, John Speth via Synth-diy wrote:
> Hi experts,
> 
> I'd like to attempt hum elimination on a 1972 Wurlitzer 4027 organ. I'm 
> pretty sure the bridge rectifier filter electrolytic caps are aged to 
> the point at which they don't filter so well anymore. The power supply 
> uses 4 inch tall multi-capacitor cans mounted on the PS chassis with 
> pins in the chassis and the cans external to the chassis. All caps are 
> employed for a total of 10 caps in three metal can packages. Are these 
> antique parts even available in new, recently manufactured form anymore? 
> If so, where?
> 
> There are three cans comprised of:
> 
>  1. 500uF/25V x 2, 500uF/35V, and 1000uF/25V (4 caps)
>  2. 1000uF/25V x 2 and 5000uF/25V (3 caps)
>  3. 3500uF/25V and 1000 uF/25V x 2 (3 caps)
> 
> Ideally, using exact replacement of new parts would be best (and maybe 
> costly, I fear). Non-ideally, I could wire new single cap electrolytics 
> but that would end up looking like a frankenstein fix (probably work but 
> bulky). Furthest from ideal is buy a new current technology PS and use 
> diodes or something to drop the highest voltage to obtain the multitude 
> of lower voltages (22.5V --> 20.0V, 19.9V, 19.5V, 17V, and 10V). That 
> might solve the problem but any engineering miscalculation could fry 
> other parts of the organ.
> 
> Another question: There is a 0.01uF/1400V cap across the PS transformer 
> primary coil. What is the function of that cap?
> 
> Thanks, John Speth

Hi John,

I haven't seen any multi-section low-voltage caps like that around for 
sale on any site I've used. Maybe others have spotted some..

I've seen a lot of restorations of radios/amps where the shell of the 
original multi-section caps are used to house modern individual 
capacitors though, and that looks very clean. I have never done this 
personally out of concerns of the contents of the can capacitors when 
cutting them open - not that I know what's actually in them.

The cap across the AC primary is likely for RFI/EMI and should be 
replaced by an appropriately rated "Safety Capacitor" ( 
https://www.allaboutcircuits.com/technical-articles/safety-capacitor-class-x-and-class-y-capacitors/ 
)

Mike






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