Hi G²,
Okay, we disagree on the solder joint. Assume the worst and be safe. I can't tell exactly, but if there is anything odd about a connection - redo it. It's not like this is difficult. Newbie or not, the truth is far more helpful than a pat on the back no matter what.
We are not attempting to bring the amp up to full power. In fact, we do not want to do this on the first power-up with a new technician. Partial power is the point where errors can be salvaged, an opportunity not to be squandered. Time to learn about carefully bringing up a circuit that is not known to be in working order. Caution is intelligent in these circumstances, and I follow the same procedure myself.
If your variac does not include a current meter, you can measure it and later add a current meter to the variac. This can go in a box with a cord and outlet, or right on the variac if you can find room somewhere. You do not care about watts, this requires calculation. Read current directly, because it is this that tells you if something is going sideways in your repair. You can have high current - low voltage (= low wattage) and be in trouble.
Just keep your eyes open and your nose sniffing (I'm not kidding!). If you hear hissing, duck first, then cut the juice immediately. An exploding capacitor can be dangerous and will certainly rattle the nerves.
-Chris
Okay, we disagree on the solder joint. Assume the worst and be safe. I can't tell exactly, but if there is anything odd about a connection - redo it. It's not like this is difficult. Newbie or not, the truth is far more helpful than a pat on the back no matter what.
We are not attempting to bring the amp up to full power. In fact, we do not want to do this on the first power-up with a new technician. Partial power is the point where errors can be salvaged, an opportunity not to be squandered. Time to learn about carefully bringing up a circuit that is not known to be in working order. Caution is intelligent in these circumstances, and I follow the same procedure myself.
If your variac does not include a current meter, you can measure it and later add a current meter to the variac. This can go in a box with a cord and outlet, or right on the variac if you can find room somewhere. You do not care about watts, this requires calculation. Read current directly, because it is this that tells you if something is going sideways in your repair. You can have high current - low voltage (= low wattage) and be in trouble.
Just keep your eyes open and your nose sniffing (I'm not kidding!). If you hear hissing, duck first, then cut the juice immediately. An exploding capacitor can be dangerous and will certainly rattle the nerves.
-Chris
Okay, that makes sense (to be clear, I wasn't questioning your procedure, just trying to understand what I'm doing and why).
It appears those Kill-A-Watt meters can measure/display in amps. Are they accurate enough for this work? They're so cheap I have to wonder. I also wonder if they work with less than 115 voltage. In other words, can I just plug one in between my Variac and the equipment under test and use it like an ammeter?
My only other option is to use the ammeter of my DVM (B+K 2707A), but I don't think that's a good idea since it would be hard to connect (I don't have a clamp-style meter) mechanically and I need it for other measurements besides.
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Hi Fred,
There are a couple ways to approach this.
Use a 0.1 or 1 ohm resistor and measure the voltage across it. Be careful to build it in such a way that insulates everything but a couple snug measuring points. That way you can still use your meter for other things. You will need to use your meter to confirm various things.
Better, use a panel ammeter. The old analog style and mount that in a safe manner. Make sure no one can get a shock. You need to measure the actual current entering the unit under test. You correctly pointed out that this will be a low voltage situation most of the time. These "Kill-A-Watt meters" will probably not function correctly at low applied voltage. They have other uses though, you could use it when testing a piece of equipment after it's serviced. That way you can detect an increased power draw as an early warning. I may just pick one up for that use.
-Chris
There are a couple ways to approach this.
Use a 0.1 or 1 ohm resistor and measure the voltage across it. Be careful to build it in such a way that insulates everything but a couple snug measuring points. That way you can still use your meter for other things. You will need to use your meter to confirm various things.
Better, use a panel ammeter. The old analog style and mount that in a safe manner. Make sure no one can get a shock. You need to measure the actual current entering the unit under test. You correctly pointed out that this will be a low voltage situation most of the time. These "Kill-A-Watt meters" will probably not function correctly at low applied voltage. They have other uses though, you could use it when testing a piece of equipment after it's serviced. That way you can detect an increased power draw as an early warning. I may just pick one up for that use.
-Chris
I don't think I can do the Ohm's Law math fast enough to make your resistor solution work reliably!
How about this meter:
NEW 0-3A 3 A AMP AC ANALOG CURRENT PANEL METER AMMETER - eBay (item 380201150177 end time Feb-25-10 08:14:22 PST)
Hi Fred,
Looks fine. What is your variac rated for again?
Hi Glenn,
You are kidding me ... right?
The bulb method may work in a pinch, but there are too many unknowns when using this method. The only thing the bulb thing has going for it is price. It can only give you a rough idea of applied voltage and current flow. The only thing this method is good for is "flipping the switch and watching what happens as far as I'm concerned.
If you do repairs and charge for the work, get the proper gear to do the job. Learn how to use a variac and you'll never go back. A variac will also double as an adjustable power supply by using the pulled power supply from a large amplifier. This is something else I do to run units with "issues" in it's own power supply.
Hi djk,
What can I say? You know I completely disagree with you, but you are also an experienced guy I respect. I don't know how you work, but I do know that using a variac is a far more refined way to power up an unknown. I suspect that the way you do things, the variac with a lamp in series with the primary may give you everything you are looking for.
I have tried light bulbs a couple times. Yes, they can work. However, they do not give me as much information as a metered variac will. That is something that can't be denied. From there on in it's a personal decision.
-Chris
Looks fine. What is your variac rated for again?
Hi Glenn,
You are kidding me ... right?
The bulb method may work in a pinch, but there are too many unknowns when using this method. The only thing the bulb thing has going for it is price. It can only give you a rough idea of applied voltage and current flow. The only thing this method is good for is "flipping the switch and watching what happens as far as I'm concerned.
If you do repairs and charge for the work, get the proper gear to do the job. Learn how to use a variac and you'll never go back. A variac will also double as an adjustable power supply by using the pulled power supply from a large amplifier. This is something else I do to run units with "issues" in it's own power supply.
Hi djk,
What can I say? You know I completely disagree with you, but you are also an experienced guy I respect. I don't know how you work, but I do know that using a variac is a far more refined way to power up an unknown. I suspect that the way you do things, the variac with a lamp in series with the primary may give you everything you are looking for.
I have tried light bulbs a couple times. Yes, they can work. However, they do not give me as much information as a metered variac will. That is something that can't be denied. From there on in it's a personal decision.
-Chris
I have a Variac (22A US-made Staco) and I know well how to use it. I simply find it's a time-waster for about 98% of my repairs.
Your suggestion for using the Variac with a PS from another big amp is a cool idea though...I have a big Bando toroid & caps sitting here somewhere that would do a fine job.
The more you use it, the better you can interpret what you're looking at. It's not the simple dumb tool that you may suppose. I've become quite adept at reading the results regarding bulb brightness.
Your suggestion for using the Variac with a PS from another big amp is a cool idea though...I have a big Bando toroid & caps sitting here somewhere that would do a fine job.
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That meter would work but if you watch the sales you could get a Kill-A-Watt for about the same price and it will tell you a lot more for the same bucks. Just a thought.
G²
You can't plug it in the output side of the variac but it could be plugged is as the source for the variac and read the current that way. For an inexpensive gadget it's remarkably good. I consider my Fluke 8060As as my reference and the Kill-A-Watts are very close - certainly sufficient for checking the Adcom.
G²
No math to be done Fred. With a 1 ohm resistor it will be 1 amp/volt so if you see a 2.5 volt reading, it's 2.5 amps. With the 0.1 ohm resistor , you just multiply times 10 so 2.5 volts would be 25 amps and blow the 15 amp breaker. When you get more experienced it becomes pretty easy. You've got a good start.
G²
Is there another 'Glenn' here? I didn't suggest light bulbs and have never used them for that purpose. I'm a variac guy but am so darned cocky that I don't use them much anymore. Keep in mind my audio work is a hobby now though I used to be 'factory authorized' for Marantz, Phase Linear, H-K, Sony blah blah blah. I work on broadcast gear now and most of that uses switch mode power supplies where you can't use a variac even if you wanted to. "Baptism by Fire" in this business.
G² aka Glenn Gundlach
Hi G²,
Yes, thank you for explaining that one. A good job if I may say so.
Plugging the Kill-A-Watt into the primary side will not read the proper current. Remember the variac is a step down autotransformer. The cheap and cheerful analog meter will show you trends and a quick visual can tell you all you need to know. Digital instruments are not that intuitive, you have to read the numbers and figure them out. The meter pointer position only requires a quick glance to give you all the information you need. Remember that with a variac, great precision is not the goal. It's the trend in current that you are really interested in, and the rough level of current.
-Chris
Yes, thank you for explaining that one. A good job if I may say so.
Plugging the Kill-A-Watt into the primary side will not read the proper current. Remember the variac is a step down autotransformer. The cheap and cheerful analog meter will show you trends and a quick visual can tell you all you need to know. Digital instruments are not that intuitive, you have to read the numbers and figure them out. The meter pointer position only requires a quick glance to give you all the information you need. Remember that with a variac, great precision is not the goal. It's the trend in current that you are really interested in, and the rough level of current.
-Chris
Hi G²,
Hi Glenn (EW)
So, how does this take any extra time from a repair? You have to much about replacing lamps, I just turn the knob and go. Not unless you aren't testing, and hat is an entirely new issue.
-Chris
Yes there is. Glenn, meet Glenn (EchoWars).
Hi Glenn (EW)
I'm sure you do know how to use it. You may not be using yours in the same way I do. One thing to consider with me is that I generally get the repairs that someone else got stuck on. Panicking technicians and "experts". I don't trust any to plug in, so I simply plug a unit into my bench mounted variac and power up. Really, there is no extra work involved here, and nothing to break or burn out. I often catch problems before even 10 Vac has been applied by watching the current meter. The other point is that I can easily test in brownout conditions and also high line voltages.
So, how does this take any extra time from a repair? You have to much about replacing lamps, I just turn the knob and go. Not unless you aren't testing, and hat is an entirely new issue.
-Chris
With the DBT, I can spot a (major) issue in about 2 seconds. I don't think I can beat that time with the Variac. I also like it that the DBT limits the current, so even in a worst-case scenario the chance of damage is vanishingly small. The Variac doesn't do a thing to limit current.
One of the main reasons I recommend the DBT to novices is simply that most casual DIY'ers are going to have a hard time justifying the cost of a Variac, and the DBT can save them from an expensive mistake for a very small investment.
Tell ya what. I'll spend a few months using the Variac instead of the dim-bulb tester (I suppose that will also help to justify its cost). At the end of that time I'll be able to make a more informed judgement. Fair enough?
One of the main reasons I recommend the DBT to novices is simply that most casual DIY'ers are going to have a hard time justifying the cost of a Variac, and the DBT can save them from an expensive mistake for a very small investment.
Tell ya what. I'll spend a few months using the Variac instead of the dim-bulb tester (I suppose that will also help to justify its cost). At the end of that time I'll be able to make a more informed judgement. Fair enough?
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A Crown DC300A (and many others) will pretend to be latched to the rail when brought up on a variac, even though it is OK.
It fires up fine on a bulb.
A variac will offer no protection on intermittent problems (bias going open), a bulb will flash and limit the current.
I can't tell you how many times after a re-build that with run-in on the bench for several hours, the lamp starts flashing. With a variac you are replacing the output stage (again), and you still haven't found the real problem.
PS
I used to be Crown authorized service, and they REQUIRED me to have a variac (even though their amps didn't like them).
It fires up fine on a bulb.
A variac will offer no protection on intermittent problems (bias going open), a bulb will flash and limit the current.
I can't tell you how many times after a re-build that with run-in on the bench for several hours, the lamp starts flashing. With a variac you are replacing the output stage (again), and you still haven't found the real problem.
PS
I used to be Crown authorized service, and they REQUIRED me to have a variac (even though their amps didn't like them).
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Well, I don't know enough to weigh on the various merits of Variac vs. DBT. Even though I am "casual DiY'er", I ponied up for the Variac and meter b/c it struck me as a basic, necessary tool. Beyond this repair, I hope to learn enough to someday be able to fix aging electronics to keep me and my friends' gear running. So it seemed to me that I needed to equip the shop with the basic tools of the trade.
That said, I think I will construct a DBT and experiment with it to see how it works.
That said, I think I will construct a DBT and experiment with it to see how it works.
5 amps. It's a Starco type 3PN501. Should I add a fuse to protect the meter?
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So when the amp first failed, the remaining channel (R), was playing just fine. Given my lack of experience, I think it will be very useful to have a good channel to compare readings with. So, I *think* the first order of business is to verify that the good channel is still good. To do that, it seems safest to disconnect the bad channel. What's the best way to do that? Is it enough to remove the fuses from the output boards or should I disconnect some leads?
Is this the right approach?
You know, I realized that after I went to bed. I probably woke the wife with the forehead slap. *sigh*
It's taken a lot of discipline on my part to avoid referring to myself as the "dim bulb tester" in this thread...
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