Scott 222c power supply

jkaley

If it ain't broke, fix it until it is.
First Scott amp I'm working on. What is the correct value for the AC capacitor to ground? The original circuit has a .01/10KV (10,000pf?) and the modified, 3-prong has a 10n/1KV? The unit I have has a IEC 3-prong. I'm attaching an original and modified schematic for reference. Thanks!org.png mod.png .
 
0.01 microfarad, 10,000 volts DC was the original. I half suspect that voltage is wrong, a 10KV cap would be pretty big. 1KV or 1.4KV were fairly common in that spot though.

10 nanofarad is the same as .01 microfarad.

I'd use a modern .01uf X1/Y1 rated cap, probably at 250 VAC.
 
Thanks for the clarification! Is the CL200 in the modded circuit a Thermistor?
 
Original capacitor was a disc. They are not extremely big because they are disc. 10KV looks correct. I can check later.

As for modifying the power plug, I would leave it the way it was designed by Scott Engineers. It worked in 1961 and should work OK today. There is a remote chance of ground loop hum with a three wire mod.
 
Thank you. Yeah, hum is part of the current problem. It came to me with the IEC 3-prong, and I have no problem putting it back to the original circuit if that's the overall consensus.

Replacing all caps now. The hum follows the volume control a bit. I can hear both 60 and 120hz. The 120hz goes away when the amp is run at 110 volts.

Another question: Should the transformer center tap be grounded to the chassis at all?
 
...should have said: The 120hz goes away when run under a current limiter. 60hz still present.
 
Thank you. Yeah, hum is part of the current problem. It came to me with the IEC 3-prong, and I have no problem putting it back to the original circuit if that's the overall consensus.

Replacing all caps now. The hum follows the volume control a bit. I can hear both 60 and 120hz. The 120hz goes away when the amp is run at 110 volts.

Another question: Should the transformer center tap be grounded to the chassis at all?
If you are talking about the high voltage secondary center tap, yes it must be grounded. That's the main circuit ground (common).
 
Currenty, the secondary center tap connects directly to the ground of the first filter cap. That cap rests on a fiber board...So it should not be connected to earth ground in any way?
 
Not that one. It is probably part of the bias circuit and should not be grounded to the chassis....but, there should be a center tap somewhere as it will not work without it.
 
The only center tap is connected to the ground on the first filter cap. I have continuity between the chassis and the ground on this first filter cap. Seem like there shouldn't be.
 
It probably ties to chassis somewhere else then. Grounding can be a bit of a dark art. Tying the transformer CT to the filter cap negative is good practice because it keeps the ripple currents from flowing across the chassis. Often the cap happens to be where the CT ties to chassis, but it doesn't have to be. Sometimes moving that connection point to another place is done to make things quieter.
 
Thanks for the clarification. After the caps are replaced and reverting back to the 2-prong, I'll see how things turn out.

This is my first Scott project, and things are a bit unfamiliar to me as I'm more of a guitar amp guy. Fun to learn, though.
 
Can someone please look at the attached photo and tell me what you think of the voltage readings I'm getting? (Scott 222c)

I have DC voltage on the transformer secondaries to the silicon rectifier. Normal?
222c_voltages.jpg
 
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