9.1 Before Turning on the Power.There are certain dos and don'ts that should be observed when working on any piece of antique equipment. They are as follows.
1. Don't plug it in and turn it on.
2. Do replace paper and electrolytic capacitors and selenium rectifiers.
3. Do check all resistors for value and replace if more than 20% high or low.
4. Do check the power transformer by applying power with all tubes removed from sockets.
Replace Capacitors and Selenium Rectifiers.The photographs below show examples of the capacitors that should be replaced.
Figure 9.1 Paper capacitors in a radio chassis.These are wax paper capacitors. They are not just wax paper on the outside, they are wax paper on the inside as well. Refer to the construction of a capacitor in chapter 3. Instead of plastic film these capacitors use wax paper which is not unlike the stuff your mother used to wrap your school lunch sandwiches in. Paper is a hydroscopic material which means that it likes water. The wax is supposed to prevent the absorption of water but it won't work forever. Over time moisture in the air seeps into the capacitor and gets absorbed by the paper and its insulation property is compromised.
Capacitors are supposed to be a totally open circuit for DC but a paper capacitor conducts some DC current. This is referred to as leakage current and a capacitor that conducts is said to be leaky.
When you find these capacitors in radios or other old equipment replace them with modern plastic film capacitors. Don't bother testing, don't turn it on to see if it will work, don't hesitate, don't ask, don't tell, just replace them.
The photo below shows some capacitors that look nice but they are wolves in sheep's clothing. Or paper capacitors in plastic clothing.
Figure 9.2a Black Beauty and Paper Capacitors.
Figure 9.2b Dark Red Mallory Paper Capacitor.The one on the left in Figure 9.2a is a 47,000 pico farad, center is a 6800 pf and the one on the right is unknown. The 6800 pf is really blue, gray, red, but the blue and gray didn't translate well to the internet.
The oddball on the right is disguised as a mica capacitor but it is nothing more than alternate stacks of foil and wax paper. True mica capacitors rarely have to be replaced but the paper ones are usually thicker than mica. True mica capacitors are not very thick.
The type of capacitor shown in Figure 9.2b are especially troublesome. In the mid 1960s, 50 years ago, I stocked up on these thinking they were good capacitors. They were at the time but time has been their enemy. They look really good. The ends of the plastic tube are filled with epoxy and they even test good on a digital RLC meter. The only clue that something is wrong is that the value reads out of tolerance high. I just finished going through them and they are 100% too leaky to be used in any circuit. Every last one out of more than 200 capacitors was bad and had to be thrown away.
Figure 9.3 Selenium Rectifiers.Something else that should be replaced before turning on the power are selenium rectifiers. Selenium dioxide is not just a bad smell it is a rather deadly poison in high concentrations. Replace them with silicon diodes and a series resistor. More about this when 3 way portables are covered. Use the rectifier's mounting screw to mount a terminal strip and solder the wires, the resistor and silicon diode to the terminals.
Never, never, never, solder the diode across the terminals of the rectifier. It is going to short out some day and the results will certainly be catastrophic.
The section below was written by Mike (Mac) McCarty. Mac is a regular contributor to the Fun with Tubes email forum and Is highly knowledgeable in the areas of electricity, electronics, and mathematics. He has so many cogent thoughts on questions that are asked on the forum that I have prevailed on him to become a contributor to this book. I was going to write this section myself but when I remembered that Mac had written something along the same lines I read it over. I can't improve on what he said so why should I try to reinvent the wheel? I now place you in Mac's capable hands.
Steps To First Power On.
by Mike (Mac) McCarty.
*** C A U T I O N ***The high voltages associated with the older tube type equipment CAN KILL YOU!
Before I get into trying to power up a piece of equipment (especially an old tube type) that hasn't been powered up in quite some time, let me say that I presume that the person performing these steps is aware of the dangers presented by high voltages, and is capable of using high voltage power supplies safely. If you have any doubts about your ability to take measurements of high voltages including AC line voltages, then do not perform these tests. Get someone else who is comfortable and experienced with high voltages to do them for you.
Disclaimer.This little missive surely contains a mistake, or omission, or requires some other polishing. It is assumed that the reader has some electronic theory background. So use this guideline at your own risk. No one associated with the development of this document assumes any responsibility if you electrocute yourself, wife, cat, or dog.
Introduction.You've acquired a "new" piece of vintage equipment and want to restore it to service. What steps should you follow?
There are several ways to power up a piece of equipment that hasn't been used in a long time.
- The quickest and most likely destructive way, is just to plug it in and turn it on. This can result in anything from a working unit, to blown fuses, or smoked parts. NOT a good method.
- Another method is to use a light bulb limiter. This does not allow high current, even if there is a short in the piece of equipment being powered up. The light bulb is in series with the piece of equipment under test. So if the equipment has a short, the only current that flows is that required to light the light bulb. For a 60 W 120 V bulb, that is 1/2 A.
A good rule of thumb is that a light bulb limiter allows a maximum of about 40 % of the lamp's rated power to be dissipated in the load.
For information on building a light bulb current limiter see:
Light Bulb Current Limiter.
Or Current Limiting with a Dim Bulb Tester.
Though this method has much to recommend it over just plugging in the equipment, by itself it is not enough to ensure that the equipment does not get damaged.
- Another popular method is to use a variac to slowly bring the voltage up to "reform" the filter capacitors. Electrolytic capacitors have a tendency to loose their ability to store a charge if they have been lying around, not used, for a long time. I DO NOT RECOMMEND "bringing up voltage on a variac to reform capacitors". That is not a proper way to reform electrolytic capacitors.
Especially when the rectifiers are tubes, attempting to use that method can seriously shorten the lifetime of the rectifiers. Pulling current from the cathode of a rectifier running with reduced heater voltage can damage or destroy it.
Properly reforming electrolytic capacitors requires a current limited power supply, which a variac run through a transformer and rectifier is not. For more information on reforming electrolytic capacitors refer to Restoring Dead Capacitors.
This technique is superior to just plugging in the equipment, but it is also not enough by itself to prevent damage to the equipment.
- The recommended method takes a lot longer than the previous methods, but definitely reduces or limits the chances of doing any damage to the equipment. This requires disconnecting many parts of the equipment. Because of this, remember - the high voltages used in the old tube type equipment CAN KILL YOU. So proceed at your own risk.
Preparing to power vintage tube equipment for the first time.Before beginning work on vintage equipment which has not been powered in some time, first remove all tubes, and mark them as to where they go. Sometimes "identical" tubes don't perform identically. If you remove an IF amplifier tube from an FM set, you may need to tweak alignment if you replace it with an "identical" but not the same tube.
The heart of any piece of equipment is the power supply, and until the power supply is working properly and supplying proper voltages at adequate currents, one cannot hope to try to troubleshoot any of the other stages. Power supplies are like Mama. If Mama ain't happy, ain't nobody happy! So, I recommend you start with the power supply stage(s).
Preparing the Power Supply.Power supplies often supply several different voltages at different currents. Usually, there are one or more heater or filament supplies which may be AC, and one or more high voltage DC supplies, usually called something like B+1, B+2, etc. Since a variety of voltages is supplied, and also to provide isolation from the AC line, such power supplies usually contain a transformer with separate windings for each heater or filament supply, and one or more high voltage windings, usually center tapped, with a rectifier followed by filtering stages, and sometimes voltage dividers, to supply the various B+ voltages.
Many low end vintage radios have a so-called AC/DC power supply, that is a power supply without a transformer. If this is the case, then one may skip the steps necessary to test the transformer, and check the integrity of the insulation. Look down below to the section Testing the Input Side of the Power Supply. However, spend the time to read and understand the intervening sections, even if you are dealing with an AC/DC power supply, since the principles apply, and some of the techniques are used, in AC/DC supplies as well.
Otherwise, the first step in preparing the power supply is to test the transformer and ensure the power supply is safe to plug in.
Testing the Transformer.This describes how to check a "characterized" transformer, one whose characteristics are known. It's behavior is characterized, it's a known item, or one which is installed into a piece of equipment which is characterized, and from which we can derive approximate characteristics for the transformer.
If the transformer is "bare", we have no information about it other than perhaps some meaningless (to us) OEM part number, and it is "bare" in the sense of just the transformer, no equipment it is installed in, so we can't guess, except very vaguely like by its weight, what its characteristics might be, then some other checks must be performed.
However, since the transformer is in a piece of equipment which we want to power up, we omit some steps necessary for uncharacterized transformers in this procedure.
- Disconnect all windings from their sources or loads, or ensure that they were effectively disconnected when the tubes were removed, then test each winding for continuity using a multimeter. If any winding is open, then obviously there is a problem.
The resistance of the primary of a transformer intended for 120 VAC is usually of the order of tens to perhaps low hundreds of ohms. The higher the power, the lower the resistance, and high power transformers may have primary resistances of just a few ohms. Transformers for 240 VAC have primary resistances about double that of transformers for 120 VAC.
Heater and filament windings are usually much less than one ohm, and just a continuity check is all that gets done here.
The high voltage windings are usually center tapped, and may have resistances on the order of hundreds to a few thousand ohms.
Check for the center tap to be roughly centered for resistance. It will not be exactly balanced for resistance, because the winding is wound in layers, and is balanced for turns. The length of wire required for the outer turns is greater than that for the inner turns, so the tap is not precisely balanced for resistance.
- Check the integrity of the insulation between the various windings, and the core. To do this, apply a high DC voltage, of the order of 400 VDC, using in series a neon test lamp with integral resistor. Do not exceed the capability of the neon lamp tester with the DC voltage used for insulation tests. These lamps are available at many auto supply houses, or in the automotive section of Wal-Mart. This type of tester or equivalent is what you need:
Figure 9.4 Photo of Electrical Test Lamp.
Similar devices are available from Glow Circuit Tester. or Circuit Tester.
Check the test lamp, to make sure it lights up, by attaching one terminal of the power supply to a winding, the other terminal to the test lamp, and probing another end of the winding with the free probe of the test lamp. The test lamp should light indicating that there is a good connection. We don't want to think the insulation is good, when in reality it was not good, but the lamp didn't light because there was a bad connection to the winding. This check to ensure that the lamp lights up must be repeated for each winding connection.
Now, again using the free probe of the test lamp, probe all other winding ends not part of this winding, and the core. Expect to see a single flash, as the inter-winding capacitance (or to core) charges, and then little or no glow after that. If the lamp glows continuously, then there is an inter-winding short, or short to core. Such a transformer is not safe, and must not be used until the short is corrected, or the transformer must be replaced.
Note that some transformers use a single lead connecting to an internal shield, which may also be connected to the core. This lead is often uninsulated. I mention this so you won't think that there is a short from a winding to the frame, when you are actually just probing the shield, which may be intentionally connected to the core.
Repeat this test on every winding on the primary and the secondary side of the transformer.
Before applying 120VAC power, if there are line filter capacitors present on the primary side winding, then they must be replaced before doing any full power tests. This means that, if the capacitor is only across the line power, it must be X2 rated. If one or more connects from either side of the line to ground, or to the chassis, it must be Y2 rated. Below are some examples of X and Y rated capacitors.
Figure 9.5 Photo of X and Y Rated Capacitors.
The capacitors are, top left to right 0.047 uf Y2 250 VAC, 0.047 uf Y2 250 VAC, 0.1 uf Y2 275 VAC and the smaller one below is 0.047 uf X2 305 VAC.
This needs to be done between reattaching the primary winding to the switch, fuse, etc. present in the primary side and any further testing on the primary side.
Also, rarely but occasionally, there are paper capacitors in the power supply on the secondary side. These must be replaced with modern films before powering up the entire supply.
After verifying insulation integrity, try verifying voltages, and balance for tapped windings.
Instead of using 120V AC, try applying 12 VAC to the primary. This makes it easy to compute voltages at 120 VAC, and yet not deal with high voltage unnecessarily. Apply the voltage through a 12 VDC automobile tail lamp. If the lamp lights, there is a problem. If not, then check the windings to see that they produce approximately 1/10 normal voltage, and that center tapped windings are balanced for voltage. If all looks well, then you are ready to apply full voltage.
The full voltage is applied to the primary through a lamp current limiter. I use a 120 V lamp, and check for shorted windings this way. A variac is optional, but the lamp is not. I usually start with a 7 W lamp for small transformers, and a 40 W lamp for larger ones. Apply 120 VAC through the lamp, and expect to see little or no glow. With a 40 W lamp, you should expect to see no glow. If I use a variac (not usually, but sometimes) I ramp it up fairly quickly to 120 VAC.
If the lamp lights, then there is a shorted winding somewhere, or the transformer is not truly isolated from the circuit, or the voltage is applied to the wrong winding.
If all looks well, then another voltage check is done, and voltages should be somewhat high. For example, a 6.3 VAC winding may read 7.5 VAC with no load. To avoid unnecessarily checking high voltages, check tapped windings only from the tap to each end, and not end to end.
At this point, we have a working transformer. Reattach it, and then start to check integrity of insulation to chassis.
Testing the Input Side of the Power Supply.Using the same neon lamp technique as above, check the insulation from each side of the power line to the chassis. If all looks good, then we are getting close.
Check the fuse, and the power switch with a ohmmeter. Measure from prong to prong on the power line cord, and ensure that the switch can make it open, and also go down to the previously measured resistance of the primary, plus an ohm or two for the fuse. If this is an AC/DC power supply, then replace any line filter capacitors, or one across the rectifier(s) connected directly to the line with X2 rated capacitors as described in section 3 just above.
Testing the Output Side of the Power Supply.The next step is to investigate the secondary side of the power supply, that is the rectifier(s) and filter. Check all resistors and filter chokes for in tolerance resistance. Disconnect any filter capacitors, and disconnect all B+ outputs, and any resistive dividers, and check integrity of insulation to B- and chassis (if different), using the same neon lamp technique described above. Don't exceed the rating of the components of the rectifier, however.
- Attempt to reform the filter capacitors.
Any electrolytic capacitor which has been out of service for two years or more needs significant reforming before having full voltage applied to it. Some restorers don't like to reform capacitors if they've been out of service for several years, and simply replace them. In that case, the replacements must be reformed. One doesn't know how long an electrolytic capacitor may have been sitting in a vendor's shelf. If it's two years or more, then it needs reforming. That information comes from several manufacturers, like Sprague, Hitachi, and Nichicon. All agree with that assessment, and I believe them.
Reforming of electrolytic capacitors is worthy of a missive on its own, and is not covered in detail here. See Restoring Dead Electrolytic Capacitors, which describes some acceptable ways to reform electrolytic capacitors.
You may expect the reforming process to take a period of at least five minutes plus one minute per month of storage. So, if the equipment has not been powered for 20 years, expect to spend about four hours or more reforming.
After the filter capacitors are reformed (and possibly replaced) they can be reattached to the circuit. Then measure the actual resistance between B+ and B-. The positive probe from the multimeter goes to B+, the other to B-, observing polarity. That's not necessarily across the filter caps, since some sets use B- filtering or other unusual circuitry. The idea is to protect the rectifier. I've measured from the rectifier cathode to the return connection on the transformer (likely center tap of the HT winding). That's probably the best way. You want to see how much current the supply is going to demand from the rectifier. It'll take a while for the capacitors to charge up, so the reading will start out low and increase. Some meters reverse the polarity on ohms to that on volts, so that the black lead is positive. Check your meter.
The ultimate purpose, since we've already checked the capacitors themselves by reforming them, and also checked the insulation using the neon bulb, is to ensure that we haven't inadvertently introduced a short or near short into the supply when reattaching the caps.
The ohmmeter likely will read a low resistance, which gradually increases as the filter capacitors charge. Wait for the reading to stabilize. It should be such that not more than 2 mA of current would flow at full B+.
- If that's met, then I attach any resistive dividers, and it's time for First Power On of the power supply only.
If the rectifier is a tube, I install it after checking it for shorts. See below for how to do that. No other tubes get installed.
Ensure that the Device Under Test is not between you and the door.
With the possible assistance of a variac but definitely with a light bulb current limiter of appropriate rating, I apply power to the power supply. A bulb limiter permits a maximum dissipation of approximately 40 % of its rating. So, a 100 W bulb would permit a maximum of 40 W or so. That's too high for a First Power On. I normally start with a 20 W to 40 W bulb. If I use a variac, I ramp it up over a space of perhaps five or ten seconds to full voltage. Normally, I just use the switch on my bulb limiter.
The light bulb current limiter may light to perhaps yellow brilliance for a couple of seconds, then dim down to no more than a dull red glow. If that passes, then I check output voltages. They should all be "ballpark", but somewhat high. If that passes, then the power supply is probably in pretty good shape. If the output voltage has not risen to the point where it's not good for the filter caps, then I'll leave it that way, on a bulb limiter, for a few hours, to weed out any sudden changes which may occur.
So far, all that works is the power supply. It's time to move on to Part II, powering the equipment with the power supply connected to a load.
Good, we now have a working power supply. However, it still isn't time to plug the equipment in and turn it on. It may be that it was simply superseded, but the chances are that it was retired for a reason. Even if the equipment was in reasonable working condition when it was retired, there are components which deteriorate although they are not in use.
Many who have not dealt with vintage tube equipment much are inclined to believe that the tubes need extensive testing, or are likely to be bad. This is usually not the case. Tubes are essentially low power and low temperature light bulbs. Even the very early ones, like the 201, though they are not low temperature, are still essentially light bulbs. Light bulbs do not go bad sitting on a shelf.
There are three kinds of vintage components which are likely to be bad in vintage equipment. These are electrolytic capacitors, paper capacitors, and resistors. So, let's handle these likely problem spots first.
The case for electrolytic capacitors has already been covered in the description of preparing the power supply. They must be reformed or replaced. If they are to be replaced, then the replacements must be reformed before putting them into the equipment.
Paper capacitors are commonly subject to two kinds of deterioration. There are others, but these are the most common. Power line filter capacitors are subject to other stresses, but we covered replacement of them above.
One is electrolysis which takes place in the oils and waxes of the capacitor when it has a DC voltage impressed upon it. The other is corrosion which is a result of residual water which remains in the paper. Paper is extremely hygroscopic, and even when the paper has been dried by heating in a vacuum, as was done when papers were the common capacitor, some still remains. This corrosion takes place even when there is no voltage impressed upon the capacitor.
For this reason, all paper capacitors must be replaced. Some try to replace only those needing it, or to "save" them. I did that when I first started restoring vintage equipment to service, but found that, when the capacitors were properly tested at their rated voltage, 97 % of them were leaky. Even if they are not leaky enough to require replacement at the time the equipment is received, they will become so with use. They are like time bombs waiting for the moment when they explode. When a paper capacitor becomes leaky, the consequences may be disastrous for the equipment. They were commonly used for coupling capacitors from plate to grid. If one of them becomes leaky, it may cause a power amplifier to pull sufficient current through an output transformer that it damages the transformer. Transformers are expensive, when obtainable.
If you want to retain original look, then re stuff with Modern film capacitors. They are smaller than the originals, and can be put inside the old outer wrap. This procedure is not covered here, but it is not difficult, though it is time consuming. The same is true of electrolytics. Modern electrolytic capacitors may be fitted inside the cases of vintage electrolytics.
The third commonly bad component type is the carbon composition resistor. All resistors, carbon or not, should be measured to verify that they are within tolerance. It is very common for carbon composition resistors to change values, almost always increasing in resistance, sometimes by orders of magnitude.
I realize that some equipment may have hundreds of resistors. However, it is much easier to troubleshoot equipment when the easy to find problems have already been repaired. A screen resistor being two orders of magnitude higher than it should be can make a tetrode act like a triode, introducing weird "birdies" into a receiver, because it now requires neutralization, and there are no neutralizing components present. It's much easier just to measure a resistor, find it out of tolerance, and replace it, than it is to track down a birdie. A cathode resistor which is five times as large as it should be can reduce gain to unacceptably low levels, causing the equipment to work, but not work well. Partially functional equipment is hard to troubleshoot.
If you absolutely just can't wait, then perhaps you should be doing something besides restoring vintage equipment. There are many aspects to restoring vintage equipment to service which require patience. Knobs, for example, often require special care and time to remove without damage. Often switches or other controls have frozen shafts which require time and deliberation to free up.
Flexible wire resistors, looking like a piece of insulated wire, often with a terminal on each end, are subject to being open. I recommend checking all resistors for being in tolerance. Note that vintage parts often do not have the same appearance as their modern counterparts. If you are going to work on pre WWII equipment, then I suggest you familiarize yourself with the appearance of vintage components, like dog bone resistors, wire wound resistors which look like half of a mica capacitor having dots on them, and so on.
Something especially to watch for are paper capacitors having the appearance of a vintage mica capacitor. These usually have the trade name "Mica Mold" stamped upon them. While paper capacitors are frequently bad, and need replacement in any case, mica capacitors are very stable, and rarely need replacement. Ceramic capacitors, which often look similar to dog bone resistors, are also very stable, and normally do not need replacement.
The way to discern these is that vintage mica capacitors have reddish brown cases, but the paper capacitors mimicking them have black cases. Also, the dot in the upper left corner is a different color. So, familiarize yourself with the appearance of vintage components.
There is one other thing which requires checking before powering the equipment up as a whole. That is, the tubes need to be checked for shorts. This does not mean you have to have a tube tester. The reason tube testers exist has always been to sell tubes, that is to encourage customers to purchase replacements for otherwise perfectly functional tubes. If you were a repairman working for pay on someone else's set and wanted to maximize your profits, this would make sense. You are working on your own equipment and not for pay, and any tube replacements are going to come out of your own pocket.
What I suggest is to use test equipment similar to that used for checking the insulation resistance of transformers. Use approximately 100 VDC to 200 VDC and a neon test lamp with integral current limit resistor. Check each pair of tube pins which are not supposed to be connected internally. The lamp should not light.
Now, any reasonable tube tester can do this, perhaps a little faster, perhaps not. If you want a tube tester, then certainly feel free to use one to do at least the "shorts" test. If you want to do more extensive tests, then I can recommend one to use the Sencore "Mighty Mite" line of emissions testers for shorts, for rectifiers and other power tubes' emission, and for the grid leakage test for non rectifier tubes, at which it is superb. For testing gain, the Hickok circuit, used in certain Stark testers from Canada, is also excellent, and the Stark line, while internally a Hickok, will not cost nearly as much. However, bear in mind that the best test of the functionality of a tube is the equipment into which it is going to be placed. Unless a tester complains about shorted elements, or the tube has an open heater/filament, do not discard it just because a tester says to do so. Also, do not expect that a tube will perform adequately simply because a tube tester indicates "GOOD".
At this point, reconnect all the power supply connections, and re insert the tubes into their respective sockets. We are now ready for a true First Power On. Ensure that the equipment is not between you and the door. You should have a Master Power Off switch which is accessible from the door, and certainly not requiring you to reach over the equipment to reach it. Many like to use a variac, and if you wish to do so, then it comes first in the power chain. Next, and not optional, is a lamp limiter. Select the lamp to have a power rating approximately twice the expected power consumption of the equipment, or perhaps a little more. If the equipment to be tested has an AC/DC supply, then an isolation transformer comes between the variac and the bulb limiter.
Set the variac, if used, to minimum voltage. Turn the lamp limiter off, and set it to limit. Plug in the test equipment. Plug in the equipment under test to the test equipment, and turn it on. Turn the lamp limiter on, but leave it set to limit. If a variac is used, then ramp the voltage up to full voltage over a period not to exceed about thirty seconds.
Expect to see the limit lamp to light to perhaps half brilliance for up to several seconds, perhaps ten or so, and then dim down to yellow, and then to dull red glow. If the lamp lights brightly, or if the glow does not dim fairly rapidly, then there is a problem, which must be investigated. However, if the First Power On succeeds, it's time to start to do initial checkout, and troubleshoot any remaining problems in the equipment.
The equipment will probably function somewhat even with a limit lamp, and I normally will use a receiver and check each function, band, tone control, etc. If it appears that the equipment is mostly working, then I'll use it for a period of a few hours with the limit lamp to weed out early failures. If all looks good after that, then I'll set the limit lamp to full power, and after a few moments, begin real checkout.
Checkout and troubleshooting are outside the purview of this document, but a reasonable first step is to measure all socket voltages and ensure that they are all within 10 % or so of nominal. One thing to watch for is that often vintage equipment literature specifies a 1000 ohms/volt or 5000 ohms/volt sensitivity meter when making measurements. Use of a modern 10 Meg ohm meter would show all voltages as being too high. Parallel your meter with an appropriate value and rating resistor to avoid this problem.
Congratulations on successfully powering your equipment without damaging it!
Thank you Mac.
Return to Tips From Mac's Workbench page.
Return to Home page.