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 Post subject: Using a variac on a solid state receiver or amplifier
PostPosted: Jan Wed 11, 2017 4:37 pm 
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I saw in one of the audio groups on Facebook a discussion of a new in box 40 year old receiver where someone mentioned the receiver should be slowly powered up with a variac.

I myself don't think it is a good idea.

What do y'all think?


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 Post subject: Re: Using a variac on a solid state receiver or amplifier
PostPosted: Jan Wed 11, 2017 4:48 pm 
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I'm not sure I see what the harm would be, but I'm also not sure why it would be necessary. Solid state stuff is perhaps a bit more robust in that most of the circuits operate at higher currents and so the effect of a shorted cap is not quite so catastrophic.

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 Post subject: Re: Using a variac on a solid state receiver or amplifier
PostPosted: Jan Wed 11, 2017 5:18 pm 
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Even low voltage electrolytics can age, so slow starting may be useful.

Many solid state devices have regulated power supplies. The regulators will not work at low voltages, so the product will not perform. On a tube set, you might get lower volume at low voltages, but a solid state set may not work at all. Probably won't harm it, unless there is some kind of sequencing during power up.... ie, drive signal must be available before the power amp is booted up. Some ICs may latch if control voltages are low or missing.

Rich


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 Post subject: Re: Using a variac on a solid state receiver or amplifier
PostPosted: Jan Wed 11, 2017 5:26 pm 
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My thought was with dc coupled amps where certain voltages have to be correct or within a certain tolerance to keep the amp from going nuclear and taking out a few transistors in the process.


Exactly, Rich or in the case of amps and receivers with speaker protection circuitry that engages a relay after a short delay it may not function at lower B+ voltages.


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 Post subject: Re: Using a variac on a solid state receiver or amplifier
PostPosted: Jan Wed 11, 2017 6:12 pm 
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I asked that question too. I have an I-Com solid state communications receiver that had not seen power in 15 years. I was told by an "authority" that low voltage turn-on could cause a runaway condition in some of the logic. Advise was, fuse it correctly and turn full power on...

YMMV

Chas

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 Post subject: Re: Using a variac on a solid state receiver or amplifier
PostPosted: Jan Wed 11, 2017 6:15 pm 
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Another thing I wonder is this.

In amps that have bias and idle current adjustments will powering them up slowly lead to a condition that will affect those enough to cause a transistor to fail?


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 Post subject: Re: Using a variac on a solid state receiver or amplifier
PostPosted: Jan Wed 11, 2017 7:06 pm 
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I do this constantly with conventional analog preamps, power amps & receivers. Never a problem, but always check the manual as a few designs aren't tolerant of it. That would include anything with switching supplies and unconventional power management, Carver amps perhaps, and probably units with logic or uPs.

Almost all power amps will happily run on very low supply rails. Even if current sources and bias controls aren't functioning as they should, the circuits work well enough for passing signals and troubleshooting. Modern power amps are mostly similar to opamps in design. Opamps may work at a maximum of +/- 18 or more volts, yet also do fine at +/- 2 volts, albeit with limited capability. In general, a circuit that can't survive brownout conditions or slow power-up isn't going to be reliable in the field, so smart companies/designers avoid them.

Unless caps are hopelessly bad, they won't have a problem until the highest voltages are reached. Thus you can usually go smoothly up to 75-100 VAC, and then slowly go up to 120 VAC over 10 minutes to an hour, depending on the condition of things. In my experience, if a cap has high leakage, it skyrockets on the last 20% or so of the supply voltage. If it takes too long to reform, it's junk and should be replaced, as the "reforming" likely won't last.


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 Post subject: Re: Using a variac on a solid state receiver or amplifier
PostPosted: Jan Wed 11, 2017 9:12 pm 
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Quote:
I asked that question too. I have an I-Com solid state communications receiver that had not seen power in 15 years. I was told by an "authority" that low voltage turn-on could cause a runaway condition in some of the logic. Advise was, fuse it correctly and turn full power on...


Most "modern" ham gear (last 20 yrs.) has a UVLO feature. Under Voltage Lock Out. My Icom-756 drops out completely below about 10 Volts.

Rich


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 Post subject: Re: Using a variac on a solid state receiver or amplifier
PostPosted: Jan Thu 12, 2017 4:50 am 
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Tube Radio wrote:
Another thing I wonder is this.

In amps that have bias and idle current adjustments will powering them up slowly lead to a condition that will affect those enough to cause a transistor to fail?


Good question and the short answer is: YES, it can (and does) happen... sometimes. From my experience in repairing hundreds of SS amplifiers I can say about 95% will pass a slow variacing without harm but for the remaining 5% you're playing Russian Roulette. Depending on the specific circuit design some SS amplifiers don't react well to severe under-voltage condition and due to improper biasing could be set in a locked state condition and self destruct. I've seen this happen more than once, thus my best advice is: THIS RISK MUST ALLWAYS BE TAKEN INTO CONSIDERATION BEFORE SLOW VARIACING ANY SOLID-STATE CIRCUIT.


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 Post subject: Re: Using a variac on a solid state receiver or amplifier
PostPosted: Jan Thu 12, 2017 5:22 am 
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Bringing solid state amps--especially DC coupled ones--up on a variac used to be S.O.P. in many shops when dealing with shorted or defective transistors in output stages. It gives you a chance to see if things are drawing too much current or getting hot before damage occurs. It was also sometimes done to catch shut-down problems that occur too fast to troubleshoot at full voltage. Maybe it is possible for something in a bias circuit to latch up when an amp is brought up slowly, but I've done it myself thousands of times and never had any trouble unless something in the circuit was previously damaged--in which case the variac didn't make much difference anyway.

However, as Conrad pointed out, you can't do this if there is a microprocessor or logic involved, and most switching power supplies start erratically (or don't start) if brought up slowly.

The big problem with older solid state amplifiers and receivers that have been out of operation for long periods of time is, the large can electrolytics used as filters in many of them had very high ripple current ratings and it is not easy to find adequate replacements today. When you can find new ones they tend to be very expensive. Bringing them up slowly and trying to save them is definitely worth trying. But I find it preferable to disconnect the caps from the equipment and bring them up on a bench power supply so I can monitor the leakage current as they reform, measure dielectric absorption, and test ESR or tan-delta. If the caps look okay after a few hours on the power supply, you can usually reconnect them and power up safely without a variac--unless the amp has something else wrong with it, that is.

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 Post subject: Re: Using a variac on a solid state receiver or amplifier
PostPosted: Jan Thu 12, 2017 12:54 pm 
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Tubologic wrote:
THIS RISK MUST ALLWAYS BE TAKEN INTO CONSIDERATION BEFORE SLOW VARIACING ANY SOLID-STATE CIRCUIT.


So basically one should either be very careful if they slow variac any solid state circuit with hard or impossible to source transistors or IC chips with no suitable replacement or just not do it at all.

That was my main point on that Facebook page. Why risk hard or impossible to find parts with no known substitutes unless the person knows the circuit well enough to know it won't be damaged by slow variacing it.


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 Post subject: Re: Using a variac on a solid state receiver or amplifier
PostPosted: Jan Thu 12, 2017 1:44 pm 
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As opposed to risking unobtanium transistors, ICs, and in some cases electrolytics by jamming the plug in the wall?

In troubleshooting, like most other things in life, there are no one-size-fits-all easy answers. Only complicated ones with exceptions, subtleties, and sometimes contradictions. That's what experience is all about. Experienced technicians know what they are looking at--or find out as much as they can about it before doing anything irreversible--and have a variety of strategies, techniques and tools available. They tailor their approaches for each situation they find themselves in.

In other words if you think you are dealing with a shorted output stage or deformed electrolytics, slowly bringing the unit up on a variac is probably a wise idea--unless the unit contains a microprocessor or a switch mode power supply. But if the outputs and power supply are okay and you're trying to troubleshoot a preamp problem, the variac is probably superfluous and you're better off just plugging the thing in.

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 Post subject: Re: Using a variac on a solid state receiver or amplifier
PostPosted: Jan Thu 12, 2017 3:46 pm 
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My position is assuming all the electrolytics have already been replaced first and basic checks have been done at which point if nothing stands out as being bad or questionable I see no reason not to just plug it in and turn it on.


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 Post subject: Re: Using a variac on a solid state receiver or amplifier
PostPosted: Jan Thu 12, 2017 3:58 pm 
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I always bring up vintage equipment with a variac AFTER I've
done a complete inspection for obvious things like burned wires,
shorted caps, diodes and transistors.

Solid state stuff is far easier to damage than tube equipment.
Steve


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 Post subject: Re: Using a variac on a solid state receiver or amplifier
PostPosted: Jan Thu 12, 2017 4:25 pm 
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Tube Radio wrote:
My position is assuming all the electrolytics have already been replaced first and basic checks have been done at which point if nothing stands out as being bad or questionable I see no reason not to just plug it in and turn it on.

Agree---and the same with tube equipment.

I routinely replace all parts known to be high risk--before applying power. Even then, I bring up the tube stuff slowly** while monitoring input current and B+ voltage. Success rate so far ~ 98%.

I think with SIMPLE S/S, that an initial startup at reduced voltage might make sense. eg. start at ~ 90-100, check all the power supply voltages, and then go to full input voltage. "SIMPLE" means no delay relays, microprocessors, etc. I think equipment like this is designed to protect itself.

**not super-slow......up to ~80% as soon as the B+ starts to come up---usually within a minute or so. Run at ~90% for initial checkout, looking for overheating, funny noises, etc.

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 Post subject: Re: Using a variac on a solid state receiver or amplifier
PostPosted: Jan Thu 12, 2017 4:58 pm 
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Quote:
My position is assuming all the electrolytics have already been replaced first and basic checks have been done at which point if nothing stands out as being bad or questionable I see no reason not to just plug it in and turn it on.


Really? What do you replace the electrolytics with? If you are talking about solid state audio gear above about 40 watts per channel, the main filter capacitors are usually highly specialized components. They have ripple current ratings of a few amperes or more, custom voltage and capacity ratings, and in many cases the physical diameters and heights of the cans were custom too. Most of these parts are simply unobtainable today, and it is not even that easy to find practical substitutes because that kind of heavy duty analog power supply is seldom used or made any more. It is often necessary to put two, three, sometimes four smaller electrolytics in parallel to get the necessary ripple current capacity.

With more recent gear having switch mode power supplies, you have to worry about the fT and ESR's of the caps (some SMPS circuits won't work if the ESR is either too high or too low), but there's a lot more to choose from in replacement parts.

Solid state power amps are not the same AA-5 radios where you can stick pretty much any electrolytics you want in there and expect it to work--not for long, anyway.

Speaking as somebody with 40 years' experience as a technician, there is no way I would replace the electrolytics first and plug in second. The first thing I look at are the fuses! If they are blown, missing, or the wrong values, you know that there is a high probability that something is shorted or damaged, and there is no way I'd consider applying power without thoroughly testing the output stages for shorts. Since you cannot know if there's a bias problem or other voltage/current fault without powering up, I would use a variac where possible, at least the first time I applied power. I cannot even begin to count how many times that saved me from blowing output transistors or doing other damage.

As for the electrolytics themselves, if the unit has been in use recently, and there are no signs of electrolyte leakage or bulging, I just don't worry about them too much. If there are any problems with them, it will become apparent in subsequent testing. But if the unit has been in storage a long time, I always do an excess electrolyte test* to see what shape they are in.

*Excess electrolyte test: Disconnect one end of the cap and apply 1/2 rated WV with a bench supply until the reforming/leakage current stabilizes. May take several hours in some cases. Once current stabilizes, bring voltage up rapidly to full WV and note how long it takes for reforming/leakage current to stabilize again. A good capacitor should stabilize very quickly, a few minutes at most. A worn-out capacitor will require much more time to stabilize at the new voltage. It takes some practice since the times vary with the ratings of the cap in question.

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 Post subject: Re: Using a variac on a solid state receiver or amplifier
PostPosted: Jan Thu 12, 2017 5:52 pm 
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I always bring up an unknown vintage amp on a variac. (Switch mode supplies, no!) Also I do not connect any speakers to the amp, SS amps can operate with no load since they don't have output transformers to spike if not loaded. At the same time I monitor the voltage between the output transistors; it should stay zero for amps with +/- rails, or half of VCC on single ended power supplies. Some amps may not have proper voltages at low voltage, but also note the low voltage condition will not exceed the voltage specs of the outputs under those voltages either. If I get to full voltage and everything looks good, then try it with the speaker.


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 Post subject: Re: Using a variac on a solid state receiver or amplifier
PostPosted: Jan Thu 12, 2017 6:14 pm 
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I definitely check the fuses first.

When I recapped my Fisher 202 Futura Series receiver I had no trouble finding the proper replacement caps.

Same with the Carver TFM-15 I recapped. Someone had listed the proper replacement caps and I used those.

For high current power supplies and other higher current areas such as speaker coupling caps I always select the caps with the highest ripple current rating and lowest ESR. So far I've had no trouble finding caps that are a proper replacement, but I don't mess with that many solid state receivers and amps though.

The reason I will recap a vintage receiver is because of the age of the caps. It's much the same thought as someone replacing the electrolytic caps in an antique radio even though the original caps show no sign of failing.


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 Post subject: Re: Using a variac on a solid state receiver or amplifier
PostPosted: Jan Sun 15, 2017 5:19 am 
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Chris108 wrote:
Quote:
My position is assuming all the electrolytics have already been replaced first and basic checks have been done at which point if nothing stands out as being bad or questionable I see no reason not to just plug it in and turn it on.
...If you are talking about solid state audio gear above about 40 watts per channel, the main filter capacitors are usually highly specialized components. They have ripple current ratings of a few amperes or more, custom voltage and capacity ratings, and in many cases the physical diameters and heights of the cans were custom too. Most of these parts are simply unobtainable today, and it is not even that easy to find practical substitutes because that kind of heavy duty analog power supply is seldom used or made any more.

Agreed, although I don't have much trouble finding a suitable substitute perusing the online mfrs. catalogs, sometimes having to raise the voltage rating to get to the right diameter can size; height is less of a problem and there's the benefit of larger cap current with a higher voltage rating. The problem I encounter is the availability of such parts in small 2x or 4x quantities. Often when I find a replacement the minimum order of tens or hundreds prevent my buying it and what is available is often the only one to be found at the highest price. Admittedly, I don't do a lot of this so maybe that is the problem or I'm just looking in the wrong places for parts.


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