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Vintage Synth Repair

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Isolation transformer / variac during restoration

Isolation transformer / variac during restoration

2013-09-13 by Tom Butcher

Hi list,

I'm getting my feet wet with restoration and repair. I mistakenly bought a broken Oberheim DMX from Ebay and wound up accumulating some more parts that should give me enough to get it going.

I've read in various places that it's a good idea to use a variac or isolation transformer with voltage control to gradually bring up the device after cleaning and drying etc. Is this a good practice? I found an isolation transformer with voltage regular for about $200 I'm considering buying. From what I can tell, this would give me the ability to gradually increase the AC voltage to the power supply as well as circuit isolation from the AC mains.

So if I have that thing there's no need for the variac, right? Seems like the advantage to the variac is just more current capacity.

t

Re: [vintagesynthrepair] Isolation transformer / variac during restoration

2013-09-13 by 65 Lotus

I'd like to hear someone's explanation why you would want to do this. It you're concerned about thermally shocking an old circuit, simply unplug the power supply circuitry from the load. If some component on the power supply board can't handle the inrush of an unloaded system, you would want to know anyway and want that part fail without being plugged into the system.
A better idea would be to put a power rheostat across the load and slowly load up the power supply circuitry after it's powered up.
As for the incoming AC mains, I can't see the benefit. If you're concerned about the age of the parts, just make sure they're a nice room temperature then put the poop to it. Anything else is snake oil IMO. I sure wouldn't variac anything. It's designed for 110 or 220, (depending). Run it there.
All an isolation transformer does is get rid of minor line noise or spikes. Unless you live in a really crappy power area, it's not really going to do anything. It certainly can't hurt of course, but I'd say the money would be better invested in a solder vacuum system (Hakko) or cheap o-scope.
Scott
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----- Original Message -----
Sent: Friday, September 13, 2013 12:21 PM
Subject: [vintagesynthrepair] Isolation transformer / variac during restoration

Hi list,

I'm getting my feet wet with restoration and repair. I mistakenly bought a broken Oberheim DMX from Ebay and wound up accumulating some more parts that should give me enough to get it going.

I've read in various places that it's a good idea to use a variac or isolation transformer with voltage control to gradually bring up the device after cleaning and drying etc. Is this a good practice? I found an isolation transformer with voltage regular for about $200 I'm considering buying. From what I can tell, this would give me the ability to gradually increase the AC voltage to the power supply as well as circuit isolation from the AC mains.

So if I have that thing there's no need for the variac, right? Seems like the advantage to the variac is just more current capacity.

t

Re: [vintagesynthrepair] Isolation transformer / variac during restoration

2013-09-14 by kslinkard@...

I've heard a couple of sources recommend using a variac to gradually increase the voltage from zero to full over 24 hours to "break in" new or long unused electrolytic caps in the power supply, but then I've heard even more sources say this is completely unneccessary. I've actually tried it both ways and can't tell any difference.

Re: [vintagesynthrepair] Isolation transformer / variac during restoration

2013-09-15 by Tom Butcher

Thanks for the info and feedback! I guess I'll just throw the switch when I'm ready and see what blows up.

On Sep 14, 2013, at 11:43 AM, kslinkard@... wrote:


I've heard a couple of sources recommend using a variac to gradually increase the voltage from zero to full over 24 hours to "break in" new or long unused electrolytic caps in the power supply, but then I've heard even more sources say this is completely unneccessary. I've actually tried it both ways and can't tell any difference.


RE: Re: [vintagesynthrepair] Isolation transformer / variac during restoration

2013-09-15 by <sneakyflute@...>

I think the point of using a variac is so that you can gradually reform old, dried out capacitors. If they get the full voltage when dry, you kill them. if you bring the voltage up slowly, they can reform, or something to that effect. Plenty of discussion on this on the EICO yahoo group forum by old time radio servicing guys. The folks on that list know their stuff.



--- In vintagesynthrepair@yahoogroups.com, <vintagesynthrepair@yahoogroups.com> wrote:

I'd like to hear someone's explanation why you would want to do this. It you're concerned about thermally shocking an old circuit, simply unplug the power supply circuitry from the load. If some component on the power supply board can't handle the inrush of an unloaded system, you would want to know anyway and want that part fail without being plugged into the system.
A better idea would be to put a power rheostat across the load and slowly load up the power supply circuitry after it's powered up.
As for the incoming AC mains, I can't see the benefit. If you're concerned about the age of the parts, just make sure they're a nice room temperature then put the poop to it. Anything else is snake oil IMO. I sure wouldn't variac anything. It's designed for 110 or 220, (depending). Run it there.
All an isolation transformer does is get rid of minor line noise or spikes. Unless you live in a really crappy power area, it's not really going to do anything. It certainly can't hurt of course, but I'd say the money would be better invested in a solder vacuum system (Hakko) or cheap o-scope.
Scott
Show quoted textHide quoted text
----- Original Message -----
Sent: Friday, September 13, 2013 12:21 PM
Subject: [vintagesynthrepair] Isolation transformer / variac during restoration

Hi list,

I'm getting my feet wet with restoration and repair. I mistakenly bought a broken Oberheim DMX from Ebay and wound up accumulating some more parts that should give me enough to get it going.

I've read in various places that it's a good idea to use a variac or isolation transformer with voltage control to gradually bring up the device after cleaning and drying etc. Is this a good practice? I found an isolation transformer with voltage regular for about $200 I'm considering buying. From what I can tell, this would give me the ability to gradually increase the AC voltage to the power supply as well as circuit isolation from the AC mains.

So if I have that thing there's no need for the variac, right? Seems like the advantage to the variac is just more current capacity.

t

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