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Steve Purcell
Hi All, ? I'm getting set up to work on tube equipment and I'm under the impression that with old and unknown tube instruments it's best to bring up the voltage slowly using a variac while monitoring the current for anything unexpected.
? Does that sound reasonable? As an integrated circuit guy I wouldn't have thought that very useful but perhaps it is for tube circuits. ? Another question I have is about what variac to purchase. There are some cheap Chinese ones on e-bay but the real thing appears to be 10 times the price. Since I'm just a hobbyist would the cheap Chinese one work well enough or should I?search for a used one of higher quality since I can't afford a $900 Variac.
? Thanks in advance Steve |
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On Tue, Jan 22, 2013 at 2:04 PM, Steve Purcell
<steve.purcell.k@...> wrote: I'm getting set up to work on tube equipment and I'm under the impression that with old and unknown tube instruments it's best to bring up the voltage slowly using a variac while monitoring the current for anything unexpected.The Chinese variacs I've checked out have been OK. You would probably want to derate them a bit, especially for extended use. On the other hand, used ones often go for not-unreasonable prices on eBay, for example: Also, you'll almost always find well-priced variacs at hamfest swaps. |
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Richard Solomon
Variac was a trademark of the General Radio Company. There are also Powerstats out
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there. There are several over on the "evil empire", just search on "Variac". Don't gamble your priceless antique on something of questionable pedigree. 73, Dick, W1KSZ On Tue, Jan 22, 2013 at 2:04 PM, Steve Purcell <steve.purcell.k@...> wrote:
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--- In TekScopes@..., Steve Purcell wrote:
Steve, This has been discussed MANY times in this forum. Search on the messages for "variac". Consensus is that use of a Viarac is a bad method for anything with a SWPS in it. For linear supplies, there are two camps - use the Viarac to protect the caps, or - just plug it in and look for smoke. I am in the second camp. In turning on over 75 old scopes, some for the first time in decades, I have never had a cap short. Maybe I am lucky. Important- if you see smoke, avoid the almost impossible instinct to instantly cut the power switch. If a resistor is burning, give it a few seconds to actually burn. Carbon comps smoke quite a bit before they discolor enough the find them once the power is off. Surplus viaracs can be had for an affordable (but not cheap) price, especially if they are just the raw Viarac and not mounted in a box with a nice voltmeter. Be sure to get one with enough power capability for what you want to test. Old vacuum tube scopes can use up to 1000 W. If searching, also use the term "powerstat" a competitor's trademark for a variable autotransformer. - Steve |
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On Tue, 22 Jan 2013 13:04:27 -0800, Steve Purcell
<steve.purcell.k@...> wrote: Hi All,I have never worked on tube equipment where I thought it was necessary. I usually use my variacs for power supply troubleshooting where I already know something is not working correctly. Another question I have is about what variac to purchase. There are someI pick them up used at garage sales, swap meets, and Craigslist and build an enclosure if necessary. None of mine have meters as I have found the dials to have fine accuracy. In one case I ground a replacement brush from the carbon rod of a carbon-zinc D battery. |
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Alex
Keep an eye on your local Craigslist. I found a cheap authentic Variac
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from a ham radio operator. But something to consider about used Variacs is that the brush can be worn out, and it's a tough part to find. --- In TekScopes@..., Steve Purcell wrote:
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Rob
¿ªÔÆÌåÓýI read the various replies and think you got some good advice. If I can be so bold I would take the conversation in a couple of directions it didn¡¯t go. I would have perhaps hopped if you are indeed not versed in tube based equipment and just starting, etc. Trust me I am not expert but there are a few things this forum has helped me understand and I would like to pass on. ? I actually find a variable transformer most useful for solid state power supplies and amplifiers where the solid state devices; especially power transistors can short. This way one can bring up the rails slowly which can help immensely in troubleshooting such equipment. ?Especially if the variable transformer has a rudimentary amp meter with it. However one must be careful not to apply this same technique to switching power supplies as bringing them up slowly is not good to do. ?In tube based equipment the same applies for shorted filter capacitors, etc. I suppose but tubes by their nature need to have a relatively higher current, etc. So for me personally a variable transformer isn¡¯t quite as useful. ????? ? Another method applied a lot for TV repairs and tube equipment is a light bulb in series with the load. It is a technique I recommend you research if you haven¡¯t already. You can find a lot of information on that. ? Finally it is important to keep in mind that most variable transformers are not isolation transformers. I went ahead and wired mine to one. ? Anyway just trying to be helpful. I am sure that others with more experience may expand on this somewhat. It¡¯s important to keep in mind my electronics experience is somewhat dated so please listen to me with a grain of salt as there are true experts in the forum ranks. ???Rob ? ? From: TekScopes@... [mailto:TekScopes@...] On Behalf Of Steve Purcell
Sent: Tuesday, January 22, 2013 3:04 PM To: TekScopes@... Subject: [TekScopes] Variac's ? ? Hi All, ? I'm getting set up to work on tube equipment and I'm under the impression that with old and unknown tube instruments it's best to bring up the voltage slowly using a variac while monitoring the current for anything unexpected. ? Does that sound reasonable? As an integrated circuit guy I wouldn't have thought that very useful but perhaps it is for tube circuits. ? Another question I have is about what variac to purchase. There are some cheap Chinese ones on e-bay but the real thing appears to be 10 times the price. Since I'm just a hobbyist would the cheap Chinese one work well enough or should I?search for a used one of higher quality since I can't afford a $900 Variac. ? Thanks in advance Steve |
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--- In TekScopes@..., David wrote:
Use of a Variac with old tube equipment is to protect the power transformer as mush as possible. Vintage equipment, especially antique radios and test equipment, many times have unique power transformers that can't be found as replacement parts (i.e., unobtanium). The power transformer is often the single most expensive component in the equipment. Don't risk it any more than absolutely necessary. (Experience talking here.) If there is a shorted of very leaky filter capacitor on the seccondary side, the transformer is in serious danger unless care has been taken to lessen the risk. A Variac can help reduce that risk. There are caviats to that... the rectifier on the secondary side might be a tube as well, and might not start passing current until the primary voltage is pretty high, posing significant risk. If the unit under test has been on the shelf (unused) for a number of years, I have wired in a separate filanment transformer to get the filments hot so that the power supply can start to work normally. If the equipment has been on the shelf for several years, the filter caps might need to be reformed. Starting off with a low voltage helps to reform them properly rather than hitting them with full voltage, and possibly causing an immediate short. With solid-state equipment, the same risks exist, but you can start everything at low voltage and monitor voltages and currents without damage to the power transformer. One last comment... a fuse sonetimes doesn't work in time to avoid damage to the power transformer, so don't be lutred into complacency by the presence of a fuse. If you value your equipment, take the precaution! It might take a bit more time, but it could save you $$$ or scrapping a nice piece of equipment. Cheers, Dave M |
¿ªÔÆÌåÓýI use both, a Variac and a "Dim Bulb" tester.Sort of a belt and suspenders approach, but it works! Of course you need to stock up on 100 watt bulbs while you can still find them! Just Google? "Dim Bulb Tester" to get lots of hits. I also use an isolation transformer if it a transformerless set under revival. Hot Chassis style. Frank DuVal On 1/22/2013 8:13 PM, Rob wrote:
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Steve Purcell
You guys are awesome.?
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I've been an electrical engineer for a long time but feel like I'm just getting started.? The sad news is that the URM 25D I bought off of flea bay showed up with a big dent so experimentation is on hold.
But that will give me time to study tube circuits a bit more before actually opening something up and hopefully another unit will come up for sale. If anyone on the list has a working unit they're willing to sell for a decent price let me know.
Steve On Tue, Jan 22, 2013 at 6:50 PM, Frank DuVal <corvairduval@...> wrote:
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Richard Solomon
As long as the dent did not result in internal damage, it may be OK. Those
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things are built like a tank. I have had several URM-25's over the years. Manuals are available for download, just Google the model number. Have you tried to open it up yet ? 73, Dick, W1KSZ On Tue, Jan 22, 2013 at 9:01 PM, Steve Purcell <steve.purcell.k@...> wrote:
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¿ªÔÆÌåÓýA genuine Variac typically sells for more today ($35-$50) than it did when it was first introduced in the mid-1930s by General Radio. ? ? In 1976 I was built my first computer, an S-100 system, from scratch. All of the S-100 cards required +8VDC. My motherboard had 12 S-100 card slots and it was not uncommon to have all 12 slots occupied in those days when 8KB of static RAM was typically what you would find on an S-100 memory card. ? +8VDC at 20 amps was not readily available from commonly available transformers, and space was at a premium in the portable case I built for my system. I could not find a transformer that met the power and space requirements for next to no cost. What to do? While I was looking through my parts boxes I came across a small form (120V 3 Amp PowerStat) and realized that the 120V winding was the perfect 120V transformer primary I needed. And because the core is a toroid I could wind as many turns as I needed around it to make my secondary output any voltage I wanted. ? I removed the rotor and wound mylar electrical tape around the primary to prevent anything from touching the exposed copper where the rotor brush rotates across it. Next I ran the heaviest gauge magnet wire I had through the core enough times until I got 8V at 20 amps out of my full wave rectifier / filter capacitor. I soldered taps on the primary above and below the 120V winding to allow for Hi and Lo line voltage situations just like my Tek 453 could do. ? My problem was solved for next to nothing thanks to those great Variacs. ? I still have plenty of uses for my metered Variac to this day. There is just nothing else like it. Dennis ? |
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sipespresso
The "just plug it in" method is reasonable only after the fuses have been checked to make sure they are the correct type and amperage. I wouldn't take this for granted. -Kurt
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For linear supplies, there are two camps - use the Viarac to protect |
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Mark Wendt
Isn't that the truth. Picked up a non-working SC502 that had 2 - 3 amp slo-blo fuses in the place of a 0.3 amp slo-blo and a 0.75 amp slo-blo. Also someone had replaced the R943 1.1 ohm 1 watt transistor on the -20V rail with a 1 Mohm resistor, which in turn caused R960 to burn up.
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Interesting stuff out there. Mark On 01/23/2013 12:52 AM, sipespresso wrote:
The "just plug it in" method is reasonable only after the fuses have been checked to make sure they are the correct type and amperage. I wouldn't take this for granted. -KurtFor linear supplies, there are two camps - use the Viarac to protect |
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Ray
Regarding the comments mentioned below: The problem here is the "smoke test" may be precipitated by a shorted filter capacitor, resulting in a burned out power transformer. It's a helpful precaution to measure the B+ circuits to insure there is at least a thousand ohms or more of resistance to ground, before applying power.
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When using a Variac, I monitor the power transformer secondary voltages with a multimeter to insure they track the applied voltage as I increase it. If they don't, it indicates there is a possible bad capacitor that didn't show up per the above test, on high voltage circuits. An bad capacitor located by testing before power is applied, is usually is a simple repair. A failed power transformer is often not so easy to replicate. Ray, W4BYG For linear supplies, there are two camps - use the Viarac to protect |
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Rob
Hey some of us just like green stripes..... I like to sprinkle a few green
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striped resistors in on any repair I do. Makes um pretty. In addition you get way more fuse for the bigger numbers. I know you cannot be complaining about getting some free fuse in the same size package. Well and actually you got a lot of free resistance just thrown in the deal as well by going from black to green. Shies you just cannot please some people.... *I know I'll pay for this * Rob -----Original Message-----
From: TekScopes@... [mailto:TekScopes@...] On Behalf Of Mark Wendt Sent: Wednesday, January 23, 2013 5:35 AM To: TekScopes@... Subject: Re: [TekScopes] Re: Variac's Isn't that the truth. Picked up a non-working SC502 that had 2 - 3 amp slo-blo fuses in the place of a 0.3 amp slo-blo and a 0.75 amp slo-blo. Also someone had replaced the R943 1.1 ohm 1 watt transistor on the -20V rail with a 1 Mohm resistor, which in turn caused R960 to burn up. Interesting stuff out there. Mark On 01/23/2013 12:52 AM, sipespresso wrote: The "just plug it in" method is reasonable only after the fuses have ------------------------------------ Yahoo! Groups Links |
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Peter Gottlieb
Green technology is big now. If I ohm would work, wouldn't 1 million of them work that much better?
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I wonder if the previous repairer misread the resistor, put it in, it didn't fix the problem, and he gave up? On 1/23/2013 6:52 PM, Rob wrote:
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Mark Wendt
It's like the SC504 From Hell I have. I finally gave up on it, after finding something different not working each time I fired it up. It's know a parts pig. I swear someone picked up a bunch of bad boards, found a frame and a CRT, lashed it all up, and sent it on it's merry way.
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Mark On 01/23/2013 06:52 PM, Rob wrote:
Hey some of us just like green stripes..... I like to sprinkle a few green |
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Mark Wendt
"Green" just grates on my nerves, I hear it so much.
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He/she probably gave up after smoking the other resistor. Doesn't explain the two 3 amp slo-blo fuses though. ;-) Mark On 01/23/2013 08:52 PM, Peter Gottlieb wrote:
Green technology is big now. If I ohm would work, wouldn't 1 million of them |
David Hess