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

rsdio at audiobanshee.com rsdio at audiobanshee.com
Sun Jun 7 22:49:59 CEST 2020

I realize that it might seem like a big investment if you don't already have one, but if you're thinking that the power supply is letting through 60 Hz on the outputs, then your best bet is to take a look with a 'scope and see for sure. The hum could actually be coming from somewhere else. A vintage Tektronix might cost as little as $100, and then you can confirm you suspicions as well as confirm any attempts you make to improve things.

For the most part, power supplies are not picky about exact capacitance. You don't really need precise replacements. There is a minimum capacitance where the supply will not work so well below that value, but as long as you have "enough" capacitance there won't be a problem. I suppose you could go ridiculously too high and then the power might take a really long time to start, so I wouldn't recommend going much beyond two times the original capacitance.

It's hard to tell without pictures, but I'm wondering if those metal cans help shield hum from other parts of the circuit. In other words, the cans might be the only part of the original that you would need to maintain when repairing this guy. Vintage organs tend to have a lot of space between subsystems, so perhaps shielding isn't that critical.

By the way, it wouldn't be terribly hard to custom-design a supply that delivers 22.5 V, 20 V, 19.9 V, 19.5 V, 17 V, and 10 V. It might be a bit too tedious, though. Hint: Rather than put a power diode on the output to drop the voltage, use a regulator and place the diode on its reference pin to shift the output. This requires far less current to flow through the diode, and then the regulator just puts out the right voltage without the same amount of waste. Alternatively, you could buy adjustable power supplies, and use their internal circuitry to dial in the desired voltage. These options will cost a lot more than buying a cheap 12 V computer supply, though.

I'm assuming that this is not a precision-regulated supply, given the age. The original schematics would be really helpful in determining how fancy the replacement needs to be.


On Jun 7, 2020, at 9:25 AM, John Speth via Synth-diy <synth-diy at synth-diy.org> 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:
> 	• 500uF/25V x 2, 500uF/35V, and 1000uF/25V (4 caps)
> 	• 1000uF/25V x 2 and 5000uF/25V (3 caps)
> 	• 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

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