Oscope ?

[Leo_Gnardo]

Power resistors again? We were here just a couple days ago. "I"m swimming in a circle" to borrow a line.

There are some good answers, just look back on the thread.

FWIW I started out over 30 years ago (as Stan suggested) with a couple of 10 ohm 100W "hot dogs." Got'em for a buck apiece from a surplus shop. Series, parallel, or alone, close enough for rock n roll.

At a later date I made up the panels I described with a total of 40 75 ohm 50W resistors, same surplus shop, 50 cents each. Switches to pick 15 ohms @ 250W, 7.5 at 500 or 3.75 ohms at 1000W. Again close enough for rock n roll. Yours need not be this elaborate. Just an illustration of what can be done with a timely cheap find, some money, and a couple hours with a drill and screwdriver and soldering iron. That WAS 30 years back, prices generally go up over time.

You have to count in your time scrounging and what that's worth, more than nothing. Indecision costs.

Review the ideas in the thread. Pick a plan & go for it.

- - - - - - - -

Now count me too lazy to look back, but here's an important point about variacs. Someone mentioned they are transformers. Maybe some are, but I can't say I've ever seen one of those. All the ones I've ever seen are autoformers. What's the difference? In a transformer, the secondary winding is electrically separated from the primary. Put an ohm meter on = no connection. Energy is transferred from primary to secondary via magnetic field. In an autoformer this is not the case. You are still connected to the AC line directly and the device confers no isolation the way a transformer does. (You can think of an autoformer as just the primary of a transformer, with no secondary. There may be taps to pick various voltages OR a special case where there's a movable tap that can select any voltage: this is a variac.) This MAY make a difference some day if you think you are isolated and find out you are not. Maybe this is "too much information" for now, but I thought I better put it out there. If I need to be corrected or advised about variacs, I'm sure we'll be seeing it right here real soon. Stay tuned! And have a good Sunday everybody!

[olddawg]

Power resistors again? We were here just a couple days ago. "I"m swimming in a circle" to borrow a line.

There are some good answers, just look back on the thread.

FWIW I started out over 30 years ago (as Stan suggested) with a couple of 10 ohm 100W "hot dogs." Got'em for a buck apiece from a surplus shop. Series, parallel, or alone, close enough for rock n roll.

At a later date I made up the panels I described with a total of 40 75 ohm 50W resistors, same surplus shop, 50 cents each. Switches to pick 15 ohms @ 250W, 7.5 at 500 or 3.75 ohms at 1000W. Again close enough for rock n roll. Yours need not be this elaborate. Just an illustration of what can be done with a timely cheap find, some money, and a couple hours with a drill and screwdriver and soldering iron. That WAS 30 years back, prices generally go up over time.

You have to count in your time scrounging and what that's worth, more than nothing. Indecision costs.

Review the ideas in the thread. Pick a plan & go for it.

- - - - - - - -

Now count me too lazy to look back, but here's an important point about variacs. Someone mentioned they are transformers. Maybe some are, but I can't say I've ever seen one of those. All the ones I've ever seen are autoformers. What's the difference? In a transformer, the secondary winding is electrically separated from the primary. Put an ohm meter on = no connection. Energy is transferred from primary to secondary via magnetic field. In an autoformer this is not the case. You are still connected to the AC line directly and the device confers no isolation the way a transformer does. (You can think of an autoformer as just the primary of a transformer, with no secondary. There may be taps to pick various voltages OR a special case where there's a movable tap that can select any voltage: this is a variac.) This MAY make a difference some day if you think you are isolated and find out you are not. Maybe this is "too much information" for now, but I thought I better put it out there. If I need to be corrected or advised about variacs, I'm sure we'll be seeing it right here real soon. Stay tuned! And have a good Sunday everybody!

Most benches I worked on used these or something like it:
http://bit.ly/14rtV9V Full secondary isolation plus an AC leakage test function. Most manufactures required it.

[km6xz]

Leo. although there are lower power isolated variacs, most variacs have a common from "primary" to "secondary" connections. The tap is called a secondary even though there is only one winding. So in a shop dealing with higher power loads such as amplifiers it is common to treat voltage selection and isolation as separate functions. If you need isolation, connect a device or variac to an isolation transformer. A popular bench variac that did have built-in isolation was the 3 amp HeathKit model from the 60s. The bulk of an isolation transform made integrated isolation + variable transformer less common in stand alone units for high power loads.
Variacs do not need as much iron or copper for any specific power level, so 1000va variacs are pretty practical and less expensive than 1000va isolation transformers.
Variac's used to be expensive when they were trademarked but low cost versions are available from Chinese manufacturers that have metered variable transformers up to 20 amps for less them $200. I have a 8 and a 10 amp 240 input metered variacs that I bought locally here for less than $100 each so I expect them to be cheaper in the US.
Used high quality bare variable transformers that can be put into a box and meter are found in surplus outlets for $20 or a bit more. If surplus make sure it is for 60 Hz, some are 400 Hz which is used in aircraft.
One note, since they use less magnetic flux density for operation than a standard transformer, they have low inductive reactance. That requires realistic fuse values for primary since a shorted load can pull very high current

[km6xz]

Threads like this contain a lot of practical information about setting up a home or commercial workbench to do music electronics so out to have its own forum or made into a FAQ. Tips and techniques contributed by members can save a lot of time, money and potential frustration for someone just getting into it.

[Leo_Gnardo]

Leo. although there are lower power isolated variacs, most variacs have a common from "primary" to "secondary" connections. The tap is called a secondary even though there is only one winding. So in a shop dealing with higher power loads such as amplifiers it is common to treat voltage selection and isolation as separate functions. If you need isolation, connect a device or variac to an isolation transformer. A popular bench variac that did have built-in isolation was the 3 amp HeathKit model from the 60s. The bulk of an isolation transform made integrated isolation + variable transformer less common in stand alone units for high power loads.

And the rest, just want to make sure our OP and others understand a "standard" Variac has no isolation. Wasn't aware of that Heathkit unit, and haven't seen a Chinese variac yet but they make just about everything else so why not a variac too.

And olddawg, I'm aware of the leakage test & if not mistaken that it was/is a California requirement for electrical repairs. Link went nowhere, can you give a title & I'd like to search that item on ebay. FWIW there was a local repair shop in the 70's-80's that was very good at applying the leakage test & wrote results on every repair ticket. Each one was marked Zero on a scale of 0 to 99 milli or micro amps I don't recall. Charged $75 minimum to have a look-at all the way back then. Didn't get much fixed. Guess where people brought their gear after being frustrated over there. Ah but the gear's safe, see - no leakage. The shock people got was in getting the bill. Long since out of biz. I think they did a lot of "acquiring" abandoned gear from people who didn't want to pay their look-at bill. Held a BIG auction when they closed, cashed in on all that stranded gear. Heckuva way to treat people... quacks.

Thanks Stan & olddawg!

[Steve Conner]

Yes, every variac I've ever seen has been an autotransformer. I have a box on my bench with a variac permanently hooked up to an isolating transformer and some volt and amp meters. Great for working on SMPS and similar things that aren't isolated from the line.

The isolating transformer secondary can be switched to deliver 120V at twice the current, which comes in handy now and again.

I won't even mention the hack I use to get 60Hz when testing frequency-sensitive gear that needs to ship to the US. It involves a large audio power amp and crossed fingers.

[bkahuna]

I won't even mention the hack I use to get 60Hz when testing frequency-sensitive gear that needs to ship to the US. It involves a large audio power amp and crossed fingers.

Yikes!

[bkahuna]

Ok...I may have jumped the gun ,but I made an offer on a scope ,and it was accepted. (really didn't expect it) It is a Tektronix 2213. Says it works ,but may need calibration. Comes with one probe ,and power cord.
Oh ,and thanks Enzo. Now I want a new Les Paul

I have this same exact model. Been using it about 5 years without any issues. It's a fairly simple scope, lacking a lot of the bells and whistles, but for my application, it's perfect. I would think it would be a fine addition to your arsenal!

[Leo_Gnardo]

I won't even mention the hack I use to get 60Hz when testing frequency-sensitive gear that needs to ship to the US. It involves a large audio power amp and crossed fingers.

OH I've done that one Steve. On tour in Italy '83. Oscillator and a bridged SAE 2500 borrowed from sound company Hornquakes. The sound techs were about to have a cow but it all worked out OK. And up in my attic there's a Bogen MO-200 which was a fave amp to run VSO tape or turntable back in the day, from one of the higher output taps meant for distributed PA. Letsee..... 25V, 70.7V, ah 125V yes that's the one!

[km6xz]

That was the standard way of varispeed operation of synchronous capstan motor decks in the days before phase locked servos. Had a C37 Studer and a bunch of old Ampex's 350's which were VSO's with a HP 200D tube audio generator and MacIntosh power amp to drive the capstan motors using the 70.7 volt line. The C37 came from Abby Road. Starting with the 3m M64, the whole industry went to DC servo capstan motors. I had a bunch of M79s which were replaced by Studer A80 and A800's and mastering decks were all ATR100 series Ampex. All these later machines had DC servo motors so could be adjusted over a wide range of tape speeds.

[gtrplayr1976]

Variac

So what would the minimum output I would be looking for in a variac as far as amps?

[olddawg]

And the rest, just want to make sure our OP and others understand a "standard" Variac has no isolation. Wasn't aware of that Heathkit unit, and haven't seen a Chinese variac yet but they make just about everything else so why not a variac too.

And olddawg, I'm aware of the leakage test & if not mistaken that it was/is a California requirement for electrical repairs. Link went nowhere, can you give a title & I'd like to search that item on ebay. FWIW there was a local repair shop in the 70's-80's that was very good at applying the leakage test & wrote results on every repair ticket. Each one was marked Zero on a scale of 0 to 99 milli or micro amps I don't recall. Charged $75 minimum to have a look-at all the way back then. Didn't get much fixed. Guess where people brought their gear after being frustrated over there. Ah but the gear's safe, see - no leakage. The shock people got was in getting the bill. Long since out of biz. I think they did a lot of "acquiring" abandoned gear from people who didn't want to pay their look-at bill. Held a BIG auction when they closed, cashed in on all that stranded gear. Heckuva way to treat people... quacks.

Thanks Stan & olddawg!

The shop I worked the most in charged $2 for the AC leakage test as another $2 for cleaners and lubricants. It was a way of padding the bill IMHO, but most shops did it. Like billing major labor on warranty repairs, lol. Here is the link again:
http://item.mobileweb.ebay.com/viewi...d=161079538438 The other one worked fine for me. I just used it. It's a Sencore and I think it is overpriced.

[gtrplayr1976]

The shop I worked the most in charged $2 for the AC leakage test as another $2 for cleaners and lubricants. It was a way of padding the bill IMHO, but most shops did it. Like billing major labor on warranty repairs, lol. Here is the link again:
http://item.mobileweb.ebay.com/viewi...d=161079538438 The other one worked fine for me. I just used it. It's a Sencore and I think it is overpriced.

Way too much for my pockets. I have also been told I can use a dimmer switch by an old Fender repairman ,but by the time I put that all together I can buy an inexpensive one. Just need to know what the output amperage should be rated ?

[Leo_Gnardo]

Way too much for my pockets. I have also been told I can use a dimmer switch by an old Fender repairman ,but by the time I put that all together I can buy an inexpensive one. Just need to know what the output amperage should be rated ?

Before there were silicon SCR/Triac dimmers like the ones you can get for a couple bucks at the hardware store, Variacs WERE the dimmers. If that repairman meant SCR/Triac he's full of beans. If he meant the old fashoned type, he's right on the money.

Smart studios install Variacs as dimmers because they don't throw buzz noises into the audio. Also, you've seen those late night shows with a city background? At the old NBC Letterman set, the lamps in that city display were all run off Variacs. They must have had 40 of them in there at least. Smart choice - they don't put noise in the audio.

A 5 Amp variac has been serving me for many years without a hitch. I fuse the input AND output for safety. Rarely pop a fuse. I mounted a "raw" variac into the box for an old piece of test equipment that was a discard. A Vedolyzer, if anybody remembers those. If you happen to find a larger one that's affordable then get it. A real good find would be a variac that has a switch, fuse, and outlet all built-in, with a metal cover over the whole thing.

Now another "fine point" about Variacs. Some let you vary voltage from zero to 100% line voltage. Other models let you dial beyond line voltage by @ 15-20%. Up to 135-140V. You may want to avoid the second kind if only to prevent accidental damage to whatever you're testing. Glad I remembered to mention that. Might save you some headaches.

Snoop local electric/electronic surplus places if you have any. Good idea to call first BUT sometimes the help doesn't know what they have anyway. Around here P&D Surplus in Kingston NY was a good source. They've changed name and moved down the street but if I needed a variac I'd go there. You probably have something like P&D somewhere not too far away. Let's hope.

Happy variac hunting!

[gtrplayr1976]

Before there were silicon SCR/Triac dimmers like the ones you can get for a couple bucks at the hardware store, Variacs WERE the dimmers. If that repairman meant SCR/Triac he's full of beans. If he meant the old fashoned type, he's right on the money.

Smart studios install Variacs as dimmers because they don't throw buzz noises into the audio. Also, you've seen those late night shows with a city background? At the old NBC Letterman set, the lamps in that city display were all run off Variacs. They must have had 40 of them in there at least. Smart choice - they don't put noise in the audio.

A 5 Amp variac has been serving me for many years without a hitch. I fuse the input AND output for safety. Rarely pop a fuse. I mounted a "raw" variac into the box for an old piece of test equipment that was a discard. A Vedolyzer, if anybody remembers those. If you happen to find a larger one that's affordable then get it. A real good find would be a variac that has a switch, fuse, and outlet all built-in, with a metal cover over the whole thing.

Now another "fine point" about Variacs. Some let you vary voltage from zero to 100% line voltage. Other models let you dial beyond line voltage by @ 15-20%. Up to 135-140V. You may want to avoid the second kind if only to prevent accidental damage to whatever you're testing. Glad I remembered to mention that. Might save you some headaches.



Snoop local electric/electronic surplus places if you have any. Good idea to call first BUT sometimes the help doesn't know what they have anyway. Around here P&D Surplus in Kingston NY was a good source. They've changed name and moved down the street but if I needed a variac I'd go there. You probably have something like P&D somewhere not too far away. Let's hope.

Happy variac hunting!

Cool bit of info there. Very interesting. Would this work ? I can get a Chinese one with a meter for about the same cost ,but not sure how reliable it would be.