question on this use of variacs

[nosaj]

I have a PT that burned up. One with a 14vac 3a and one 7.5vac 0.5.

Is there anything wrong with taking 2 variacs setting one for 14vac and the other for 7.5vac To troubleshoot out the rest of the keyboard?

I don't see an issue with it for troubleshooting, but wanted to make sure I wasn't overlooking something.

Thanks,
nosaj

[The Dude]

I see no reason why not. I often use my variac as a low voltage AC supply when people don't bring in AC adaptors. It's no different. The only hitch would be if the PS is looking for a center tapped transformer.

[g1]

I thought to use both in the same circuit you would need at least 1 isolation transformer.

[nosaj]

Ok Thats what I figured. It sure is great to have ham friends who are hoarders... I just went an picked up 2 5amp variacs for free.

If in the case I ever came across needing a center tapped transformer I guess I will cross that bridge when I get there. But suggestions would be very welsome.
nosaj

[nosaj]

I thought to use both in the same circuit you would need at least 1 isolation transformer.

Well my B&K power supply is a isolation / variac does that count?

nosaj

[g1]

Well my B&K power supply is a isolation / variac does that count?

nosaj

Yes, I think you should be good.

[Jon Snell]

Most variacs are not isolated, they are Varialbe Auto Transformers. Be careful or you will destroy the keyboard.

[J M Fahey]

I have a PT that burned up. One with a 14vac 3a and one 7.5vac 0.5.

Is there anything wrong with taking 2 variacs setting one for 14vac and the other for 7.5vac To troubleshoot out the rest of the keyboard?

I don't see an issue with it for troubleshooting, but wanted to make sure I wasn't overlooking something.

Yes, everything wrong.
The typical Variac is an Autotransformer so NOT isolated from mains.
Ever heard the word "widowmaker"?

I see no reason why not. I often use my variac as a low voltage AC supply when people don't bring in AC adaptors. It's no different.

????????????????????????????????????????

I thought to use both in the same circuit you would need at least 1 isolation transformer.

BOTH need to be isolated from mains.
A live ONE is one too much.

Well my B&K power supply is a isolation / variac does that count?

It counts as ONE. And the other?

Most variacs are not isolated, they are Varialbe Auto Transformers. Be careful or you will destroy the keyboard.

^^^^^^^^ THAT.
But besides the keyboard, I worry much more about our Friend´s life.

[nosaj]

Can you elaborate on this? Please? And how it could destroy keyboard? I had a b&k 1655.
but if you could explain why 2 regular non isolated variacs could be a problem. Call it a teachable moment
thanks
Nosaj

[nosaj]

Ok Juan answered it .
Nosaj

[Jon Snell]

Can you elaborate on this? Please? And how it could destroy keyboard? I had a b&k 1655.
but if you could explain why 2 regular non isolated variacs could be a problem. Call it a teachable moment
thanks
Nosaj

I refer to Williams keyboard power transformer replacements?
Because it is not isolated from the mains and therefore is a risk of shock.
If you have two variacs, the outputs are connected together,hopefully with the neutral side of the mains power, so if you require 14volts at 3A and 7.5v . The 14v AC supply goes through a bridge rectifier to producing about 20v DC and there is no common connection within the power supply, then there will be a AC power across half of the bridge rectifier destroying it.
If the voltages did not need to be isolated, there wouldn't be two separate windings but one 14v with a 7.5v tapping, I am assuming isolated AC supplies are required.
By all means fill your boots but man made faults are the worst!

[nosaj]

After thinking about it all day and what I have on hand. I used the Isolated variac on the 7.5 vac rail and checked the voltage at the bridge rectifier. Removed the variac and put a DC power supply at the bridge rectifier set to the voltage I found. Then I put the variac on the 14vac rail with the dc power supply setup on the 7.5 rail and powered it up. Everything looks ok but no sound. Couple that with not being able to source a power transformer for it and I think its done for.

Hopefully Jon you didn't take my questions the wrong way. But I do thank you for taking the time to explain what could go wrong.

An Juan thanks for reminding me using the variac essentially makes a widowmaker.

Thanks,
All.

nosaj

[Helmholtz]

Of course you could use some (small) PT having a rated secondary voltage higher than required after the variac and use the variac to adjust the output voltage to the desired level.

[nosaj]

Unfortunately I didn't have any small salvaged ones around.

nosaj