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RESTORING A PIECE OF VINTAGE GEAR

Doug Bannard(TelePARC)

Restoring a piece of vintage gear is a great way to spend a few evenings when the weather isn't conducive to participating in outdoor winter activities. You might have an old transmitter or receiver that you'd like to bring back to life, if for no other reason than nostalgia. Or perhaps you've always wanted an exotic piece of test equipment and have picked up an old surplus "beater" and would like to get it up and running.

Since I do a lot of this sort of work, I decided to pass along the steps that I go through in the next three President's Messages. Some folks in the club are no doubt already familiar with restoration, and some may have different ideas on the subject. The methods I'm describing are one's that have stood me in good stead for a number of years and even let me use the gear on a daily basis without fear of burning the house down!

The assumption, at least in this first part, is that you are starting off with a piece of equipment which either:

1) Is unknown condition (you have never seen it powered up).

2) Was good the last time you used it...but that was 20+ years ago.

A) Examine the line cord, as well as the wiring between line cord, "on-off" switch, fuse, and power transformer (if used). Replace any wiring with burnt or deteriorated insulation. If the piece of equipment has no fuse, it is certainly a good idea to add one, particularly if you plan to leave the piece of gear plugged in all the time. Although adding a fuse will certainly mean that the piece of gear is no longer "original". If you use a "pigtail fuse", you can conceal it pretty well and only you will know!

Pay particular attention to any capacitors which are used across the AC line, or from one or both sides of the AC line to chassis. Capacitors used in these locations in modern gear must conform to strict UL and CSA requirements to minimize both fire and shock hazards. Those used across the AC line are called "X" caps and those from line to chassis called "Y" caps. It would be prudent for you to replace these in your equipment with the latest and greatest (and safest). They're not expensive and are available locally.

B) Now that you have any bad AC line wiring taken care of, do a resistance check between both sides of the AC line and chassis. The resistance should be infinite unless you have either a fault or an AC/DC set.

C) Time to check the power transformer for shorted turns. Nothing puts a damper on a restoration project faster than a bum power transformer. To check it (without any fancy equipment) do the following:

1. Pull out the rectifier tube (or disconnect any solid-state rectifiers. Also pull all other tubes as well.

2. Power up the piece of equipment under test using a 100 Watt light bulb in series with the AC line and the equipment. If you have a metered Variac, this works well too, in which case you don't need the light bulb, just bring the voltage up slowly and watch for excessive current (it will be obvious).

3. The lamp should light dimly. If it lights with full (or close to) brilliance, you have either a shorted turn in your transformer, or the wiring from one or more of its secondaries is shorted somewhere in your piece of gear. So if your transformer fails this test, look for secondary wiring shorts before wringing your hands.

D) So now we hope you have a good power transformer. Now, with all power off, check the resistance from the cathode of the rectifier to chassis. If you measure in excess of 5000 ohms for tube gear you're probably OK and can proceed as below:

1. Plug in the rectifier tube, (all other tubes still out).

2. Change your series light bulb to a 40 W one and turn on the power. The purpose here is to SLOWLY "re-form" the electrolytic caps in the power supply (which may not have been used for years. It is useful to have a look at the voltage across these electrolytics as you're doing this, it should be slowly rising.

3. CAREFULLY feel the side of each electrolytic cap in the power supply after about 15 minutes of this. If none of them are warm to the touch, replace the 40 W bulb with a 60 W and let sit, powered up, for another 15 minutes.

4. Now measure the DC voltage across each of the power supply caps and make sure that its within the cap's rating. We are testing the power supply with no load on it and its output voltage may be considerably higher than normal (particularly if it uses a chokeinput filter).

5. If nothing is hot, and all caps are within their voltage ratings. go the last step to a 100W series bulb. After a check to ensure that the capacitor voltages are safe, let the equipment stay powered up this way for several hours to complete the capacitor forming process.

At this point, we could simply power up the piece of gear, evaluate its performance, then troubleshoot it and replace components as necessary. This is the route that I would suggest if you are restoring either: 1) An elderly piece of solid-state equipment, or 2) A piece of test gear which used extremely good quality components such as a vintage Tektronix 'scope. (Vintage tube type HP equipment may not fall into this category, read on).

In early solid state gear there did not tend to be as much deterioration in passive components such as resistors and capacitors as occurred in vacuum tube equipment, due to the lower generated heat and lower operating voltages. Similarly, the passive components used by Tektronix in their 'scopes were high quality components which withstood the tests of time, heat and high voltage very well, they rarely require replacement.

If your equipment doesn't fall into the above categories, then first start off by having a look at all of the capacitors. Disk ceramic caps and dipped micas fail very infrequently (although not unheard of), these seldom require replacement. Molded micas are a bit less reliable, but failures are still pretty rare. Not so with paper dielectric (paper) capacitors, the most unreliable component you will run into in old electronics.

Paper capacitors come in many different shapes and sizes. Many are packaged in the familiar waxed cardboard tubes with wax end seals. Others are molded in plastic. Some molded paper caps even resemble molded micas (the ones in the BC348 receiver for example). Still others are packaged in ceramic tubes with epoxy end seals. They all have one thing in common however, (except for those in metal packages with glass end seals such as the Sprague "Vitamin Q" capacitors), moisture eventually gets into all of them. Once this occurs the capacitor degrades badly due to the reaction betweeen the acid in the paper dielectric and the aluminum foil plates. Some aluminum actually migrates through the paper, causing the cap to become electrically "leaky". This deterioration occurs whether the cap has been in use or not. Unused paper caps which have been in stock for many years will usually measure leaky. The other insidious problem with leaky paper caps is that the leakage current usually increases as the equipment warms up (the capacitor's leakage resistance decreases).

The only way to deal with paper capacitors (if you want your piece of restored gear to be reliable) is to replace them all. I can't emphasize this enough. In the words of many a service tech: "There are only two types of paper capacitors, those that have failed and those that will soon fail". Remember that if a paper coupling cap is leaky, it will upset the bias on the grid of the tube to which it is connected, shortening that tube's life. Replacement capacitors are quite cheap. You should be looking at the plastic film types. The SBE (formerly Sprague) "Orange Drop" series are very good and widely available in voltage ratings from 100 V to 2000 V, but any of the plastic film caps (polyester, polypropylene etc.,etc.) will do the job nicely, and will last forever.

Electrolytic caps do degrade with time as well, but unless I see evidence of high ripple voltage, (hear excessive hum), or high leakage current, (heat in the capacitor), I don't replace them as a matter of course as I do with paper caps. With electrolytics, look at the ends of the capacitor and in particular around the terminals. If you see a white deposit, this is a dead giveaway that electrolyte has been leaking from the capacitor,and it definitely should be replaced. A hobbyist type capacitor checker (such as a Heathkit IT-28) is a great aid in checking all capacitors. These show up at ham flea markets from time to time. I picked mine up for the grand sum of $10.00 at the ham flea market in Kingston in 1993, works just dandy!

Resistors in old equipment aren't as big a source of problems as are capacitors, although carbon composition resistors can change value quite dramatically with use and time (and either up and down in value too). I tend to only check the value of resistors that look like they have been dissipating a fair bit of power, (look discoloured), but you may choose to check them all, to be rigorous. Remember that resistors from the 1930's tended to use an identification scheme different than the coloured bands used today. The first digit was given by the body colour. The second digit was given by the colour of the end of the resistor and the multiplier by the coloured dot on the resistor body. When replacing any out-oftolerance resistors, you will find that by far, the most commonly available resistors today are the metal-film types. Although excellent resistors, they do not suffer overloads, (excessive dissipation), gladly. So, for example, if you need to replace a badly discoloured 2 Watt carbon composition resistor, and you can only find a metal-film type, it would be prudent to err on the safe side and use a 3 Watt unit. It will probably last longer.

Changing resistors and capacitors requires some care if you wish to keep the piece of equipment looking "original". For resistor replacements, it is impossible to find modern resistors which have the same appearance as original 1930's units. About the only way that I know of to find resistors of this type is to either cannibalize an old "junker" or to frequent the vintage radio flea markets.

Capacitors present less of a problem. It is quite possible when replacing a waxed paper type of capacitor, to melt the wax from the ends using either a hair dryer or an inexpensive heat gun such as Black and Decker's "Heat and Strip", force the old innards out of the capacitor, then replace them with a new plastic film unit (it will easily fit inside). The leads are brought out and the ends may be re-sealed with the wax that you have melted out. Electrolytic caps in paper cases may be refurbished in similar fashion. The "wet electrolytics" used in old receivers may also be re-built. These are often held to the chassis of the receiver with a clamp. If you cut them with a pipe cutter in the area normally concealed under the clamp, you can withdraw the contents, replace it with a new dry electrolytic capacitor (they're much smaller these days), and re-mount the capacitor. No one but you will ever know! Multi-section electrolytics in cans are almost impossible to re-build at home, but if you MUST retain that original appearance, there's a company in the US, Frontier Electronics, who will re-build them for you. You also might get lucky and find a replacement on the shelf from Antique Electronics Supply in Tempe, Arizona.

When cleaning cabinets, (either the metal cabinet on a vintage transmitter or piece of test gear, or the wooden cabinet on a vintage broadcast receiver), don't use any cleaners that are too aggressive, without first trying them on an inconspicuous area. Some receiver dial glass marking for example are particularly vulnerable to solvents and even detergents. Try just dusting these off. One cleaner that is pretty safe is "waterless hand cleaner" (the greasy stuff, NOT the abrasive stuff), which is even pretty effective at cleaning finished wooden cabinets without damage. I'm not going to get into re-finishing here as it is a topic in itself, but one word of advice, don't re-finish unless you absolutely have to. It can be a lot more effort than the electrical restoration if you want to retain a vintage appearance. As far as cleaning chassis goes, just dust whenever possible. Blowing them out with compressed air works nicely too. Just wear your safety glasses so that you don't blow debris into you eyes. Last month we covered the replacement of components that, in my experience, are the most likely to have failed in the piece of equipment that you are restoring. Up until now, I haven't mentioned the active devices (either transistors or vacuum tubes) and their influence on performance.

Let's have a look at transistors first. If the performance of your piece of gear is not up to par, and you suspect one of the transistors, how do you determine its condition?. Many early transistors (particularly those in plastic packages) died slow deaths from sodium poisoning, with severe alteration in their characteristics. The only bullet-proof method of determining whether or not a transistor's characteristics have changed is to check it on a transistor curve tracer. Few of us are fortunate enough to have access to one of these however, and luckily there is a test that we can easily carry out using any old style 20000 ohm/volt VOM to determine at least whether the transistor has failed catastrophically.

1) Switch your meter to the R X 1000 ohms range (do not use R X 1, or you risk damaging the base-emitter junction of some transistors due to the relatively high current used on this range by some VOMs.

2) Identify the base, collector and emitter leads of the transistor under test.

3) Connect the meter leads between the base and emitter of the device, first in one direction, then in the other. In one of the two directions you will measure a relatively low resistance, and in the other direction a VERY high resistance, or open circuit. This is the indication of a good base-emitter junction.

An open circuit in both directions indicates a bad transistor. A low resistance reading in both cases also indicates a bad device.

4) Repeat the same test as above, but this time between the base and collector of the transistor. Once again, you should get the same result, low resistance in one direction, and high resistance or open-circuit in the other. Any other result and you have a bad transistor.

5) Finally, repeat the same test between collector and emitter. This time you should see an open circuit in both directions (except for germanium transistors which can show a moderately high resistance reading in one direction and open circuit in the other). If your readings differ from this, your transistor is shot!

Although I have specified a 20,000 ohm/volt VOM for this, a DVM may also be used on its "diode" setting (if it has one) to perform the same tests.

If you do find a failed transistor in a piece of early solid state gear, the chances are excellent that it is a germanium one. This is bad news for the restorer unless he/she has been doing a lot of scrounging at flea markets or has a very well stocked junked box, as germanium bipolar transistors have not been made in production quantities for at least 15 years. It is possible in many circuits to replace germanium devices with more modern silicon ones, if minor biasing changes are made. Some circuits however (such as local oscillators) may be rather intolerant of transistor substitutions, so keep your eyes peeled at the flea markets for early solid-state junk that you can strip for the transistors. There's lots of early solid state gear up at Bill Ford's as well.

I've saved vacuum tubes for the last. Although you will be hard-pressed to obtain germanium transistors (and also many early integrated circuits), you can STILL obtain almost any vacuum tube ever made. This should not surprise any of us, for after all, tubes have been with us for so much longer than transistors, and there were so many millions of them made. Many are still sitting in warehouses today.

There are two basic types of tube checkers which may be used to evaluate the condition of your vacuum tubes. The most common type seen in past days (as in drug stores etc.) was the emission tester. An emission tester straps all grids to the tube plate and evaluates the performance of this connection as a rectifier (diode). It also checks for shorts between the various elements of the tube. Although this type of test is a good indicator of the condition of a tube's cathode, it will not tell you if the transconductance, (Ib/Eg), has fallen off due to other factors. Hence the second type of tube checker known as the transconductance or "mutual conductance" checker. A transconductance checker is supposed to measure:

(small signal change in plate current, Ib)/ (small signal change in grid voltage, Eg) = gm. (With Eb = const.)

If you believe that most of them really make this measurement with any degree of accuracy, I have a very nice bridge in Brooklyn that I'd just love to sell you! Often the "small signals" that they use for grid voltage exceed the negative bias that they apply, driving the tube into grid current and giving erroneous readings, (particularly for tubes tested at low bias such as 12AX7’s and 12AT7’s). Most also use either unfiltered or poorly filtered DC for the plate supply (and Eb is supposed to be constant), so readings are to be taken with a large grain of salt.

The very best test of all for a tube is how it works in your piece of gear. If you want to determine how effectively a given tube is performing in your equipment, then get hold of a new one and substitute it in the equipment in question. If there is any significant improvement in performance (receiver sensitivity, transmitter power output etc.) over the original tube then the original should be replaced with the new one. Otherwise, the old tube should be considered to be good and left in place, no matter what the tube tester says. Do this with each tube in turn. Most tubes are still inexpensive enough (< $5.00 US each) that you can afford to keep a complete set around for this type of test (and for spares). There are some tubes that are quite expensive however. These are the ones used in audio equipment (things like 12AX7’s etc.) and some tubes used by amateurs (sweep tubes like 6JS6C’s etc.). The prices have been artificially raised by gougers, since there is a high demand. Hams still tend to charge reasonable prices for them, however, so check out the flea markets and also ham radio related Web pages. For MOST tube purchases, I would recommend that you try Antique Electronic Supply in Tempe, Arizona. They have a good stock, fast delivery and their prices are for the most part pretty reasonable. Also, check with Jerry Dillon, VE3AVI, the custodian of PARC's collection of tubes. If Jerry can't help you out, please check with me and I'll see what I can do. Remember, you don't have to go wild accumulating spare tubes for everything. The use of tubes in television receivers, where tube counts (and total power dissipations) were very high, gave the vacuum tube a bad reputation for reliability. In a well-designed circuit however, life times of 50000 hours are not uncommon for small-signal tubes (that's over 5 years of CONTINUOUS use). Lifetimes of output tubes are somewhat shorter. The ways that you can maximize the lifetimes of tubes in your restored piece of equipment are:

1) Make sure that heater voltages are within +/- 10% (ideally +/- 5%) of their rated values.

2) Do not apply B+ voltages until heaters are thoroughly warmed up. This is to prevent "cathode stripping". In a receiver, this can be easily done by replacing any indirectly heated cathode rectifier tubes (such as 5Y3) with controlled warm-up directly heated cathode types of similar ratings (such as 5V4GA, that's 5 Victor 4 not to be confused with 5 Uniform 4. A slow warm-up replacement for 5 Uniform 4 is the 5AR4.

A heater warm-up of 15 seconds is quite sufficient for receiving tubes to prevent cathode stripping, but some transmitting tubes may require longer, and mercury vapour rectifiers longer still. You may want to add a delay relay to hold off the application of B+ for this longer warm-up period. I will be glad to provide anyone interested with an inexpensive-to-build delay circuit, since the Amperite relays used in days gone by are prohibitively expensive, although still available.

3) Give your tube gear lots of ventilation. It really helps tubes live longer when their bulb temperatures are lowered. Make certain that all ventilation holes are clear and unobstructed.

 

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