What is The Purpose of Restoration?

So, you have caught yourself a fine example of a Zenith Trans-Oceanic from Ebay. Well done! 'Great condition', the auction copy said. 'Works well', it went on...if you were lucky.

But, what arrives is a large, dusty, somewhat dirty black box with a clouded dial and tarnished metalwork. Sure, it picks up one or two local AM broadcast band stations, but they put out so much RF power anyway that even your maiden Aunt's dental fillings will pick them up.

But don't despair! Some elbow grease, patience and a few hours work will soon have your ZTO playing and looking like new.

'Like new' is a phrase to remember here for we must make some decisions as to what we want from our 'new' ZTO. Some folk say there is a 'sliding scale' to describe the level of restoration - I say there are really only three positions....
  • If you want a 'museum quality' radio - a 'shelf queen' - that looks like it came out of the box that very morning then be aware that such a radio may not actually work. True authenticity means the use of original components and these may well have degraded beyond their original specification. On the other hand, some museums insist on showing their exhibits 'warts and all'!
  • If you want a shelf queen that can be used in the way it was intended to be used then nothing short of a full electrical and cosmetic restoration will do. This means that modern components may be used but, possibly, disguised to look like the originals.
  • There is a 'third way' of restoration. This involves replacing all examples of certain types of components - regardless of condition - and checking the tolerance of others, replacing where necessary. No attempt is made to disguise the fact that modern replacement components have been used as the point is to create a 'practical classic' - a radio that sounds great, works as good as when new and looks like it has been cared for. 
Whatever your aim and whatever purpose you have in mind for your 'new' radio, you will probably be told (assuming the set does not work) that...

'It Probably Just Needs A Tube...'

In all likelihood, the very last thing that your newly acquired ZTO needs are new tubes. Frequently, the three most common reasons for the set not working are:

  1. When using battery power, the contacts of the power changeover switch (in to which you must insert the mains plug) are tarnished and not conducting OR 
  2. The tuning switch contacts for the Broadcast Band (through which all antenna signals pass, regardless) are tarnished and not conducting well OR 
  3. The tube pins are tarnished and not conducting well, so preventing some filaments from working.

More likely, the reason for an inoperative or malfunctioning  radio is the failure of the electrolytic and/or paper/foil capacitors -- the former is usually indicated by the presence of a loud hum, the latter by poor performance, erratic tone control operation and distorted, weak output. The electrolytics are contained in the (usually) black cardboard "cans" visible at the top of the chassis. 

Tip: Note that failed electrolytic capacitors and discussions of zener diodes, dropping resistors, fuses, silicon diodes, selenium rectifiers, current regulators, inrush current limiters, varistors, thermistors, adjusting the filament voltage, dangers of the 'hot chassis' design and so on, etc. etc., ad nauseam ad infinitum is only an issue when using the radio on the AC line - powering your radio with a battery is possibly a good alternative to power supply repairs and electrolytic capacitor replacement.

Failing electrolytic capacitors are common and to be expected with all vintage electronics (sometimes even new equipment). They were initially of decent quality but not intended to be in service 50 or 60 years later; they tend to degrade with long periods of disuse rather than regular operation. If the radio was put away with some other fault and has been without power for many years they will almost certainly be bad. Several sections are contained in one unit, and shorting between sections is a common failure mode. Some experienced technician claim they are able to "reform" the aged electrolytics if power has not yet been applied but success is not consistent.

Also known as "filter" capacitors, their job in the ZTO is to smooth out, or filter, the rippled DC voltage rectified from the original AC line current. The device which makes this conversion to DC is called the rectifier - either a tube, in the earliest models, or a selenium based solid state device. Either provides direct current from half of the original alternating current waveform. The filter capacitors store and release this energy, much like a watertank, to provide filtered DC as required to power the radio's various circuits. The 8G005, for example, has five separate electrolytic sections varying from a rated 150 volts DC to 10 volts DC. 

Excessive hum in the radio when running on house current is usually the result of one or more of these filter capacitors no longer being able to perform their job - they are "leaking" electrically. This is hard on tubes and other components as well as the ears. Do not operate the radio on the AC mains line if excessive hum is evident. Typically, tube radios do not have audible hum when idling unless your ear is just adjacent to the speaker. Often one or more of these sections of the can capacitor are shorted, or perhaps 'open'. Again, the radio may otherwise work quite fine on battery power; consider this alternative if you don't necessarily want to replace those electrolytic capacitors - modern alkaline "D" and "AA" cells can easily be made into a long life battery to power the radio.

he wax or paper dielectric coupling, decoupling and bypass capacitors will also adversely affect battery operation though, tending to act more like resistors as they are now decades old. During your inspection of the chassis you may have noticed wax leaking or evidence of overheating. Powering the radio by a battery or D-C bench supply is an excellent diagnostic method of determining the overall health of the radio. Remember Trans Oceanic radios are primarily battery sets, the power supply is full of large wire-wound power resistors to lower a portion of the line voltage down to the nine volts required for the battery-type tube filament string. Incidentally, if you have access to a tube tester and one or more of the tubes test "weak" do not throw them away! - they will likely work fine when installed and that's the definitive test of a tube. Even very weak tubes often work well depending on the application. Tube testers were practically designed to sell tubes! However, while a tube may well test "bad" and work fine in the radio, the opposite is also true. Note that shorted rectifiers are commonly found, particularly 117Z3, a result of operation of the radio with leaky or shorted filter capacitors. A similar effect is observed with selenium rectified radios, if the filters are bad the rectifier is sure to be bad as well.

If you want to run the radio via the AC line replacing the electrolytic filter capacitors is the first step to restoring the radio to reliable high performance operation. Even so, replacement usually makes other faults more apparent, esp. the small value paper capacitors used for coupling and stage isolation. Given that the radio may not have worked at all prior to electrolytic replacement this is to be expected. Fresh electrolytic and paper capacitors generally restores full performance, tone and reliability. Once you hear a fully restored Trans-Oceanic you won't want to settle for anything less. The TO's are a bit time consuming compared to other, less complicated radios due to the cramped chassis, but be assured it is well worth the time and effort to do so. Multi-section replacement "can" type electrolyic capacitors of recent manufacture are available but alter the outward appearance of the set and are expensive relative to discrete individual components. Most repairmen elect to install modern replacement capacitors under the chassis as they are much smaller than in years past and there is plenty of room. Alternatively, some restorers "restuff" the can with modern capacitors.

It's good practice to leave the original filter cans in place but disconnected electrically. Soldering the new capacitors in parallel across the originals, though tempting, is always bad practice so don't do it for anything other than test purposes.

Radio Shack no longer carries high voltage capacitors, electrolytic or otherwise. The "MFD" (MicroFarad) term has also been renamed "µF", but new designation is equivalent. Periodically, engineers apparently rename all arbitrary designations to sow confusion. The actual capacitance values and voltage rating of replacement capacitors are different as well, e.g. the 40MFD@150's closest replacement most often found today is 47µF@160 and the 20MFD@150's new equivalent is 22µF@160, and so on. These values are well within original design tolerance and work fine. You can use 250 volt rated units in lieu of the 150 volt rated units, they are only a bit larger, far smaller than the originals, and offer better reliability. You can also use a 40µF unit to replace a 20µF unit, for example. Radios are easy to please - do not, however, under any circumstances, use a lower voltage rating than was used originally. With large corporate "bean counters" watching every penny spent, you can be assured that if a 600 volt part was specified and installed in a given location, its replacement must at least meet that value, they used that value for a reason. Sometimes manufacturers "got a deal" buying in bulk and used over-rated parts in a given location but until you are experienced enough to make that call, be sure to closely match capacitance and voltage values. More often, it would seem they ran the ragged edge of suitability. We can do better - use the highest quality you can find of recent manufacture. The "bargain basement" no name types may have quality control issues and fail prematurely, electrolytic capacitors are a weak point in any electronic device and highly stressed so trying to save a few pennies here is false economy. For an excellent tutorial visit Phil's Old Radios web page for tips on Replacing Capacitors in Old Radios.

All tubed transoceanics have negatively grounded electrolytics; (Remember that electrolytics have a + (positive) and - (negative) polarity!) you won't go wrong terminating the replacement capacitors - (negative) leads right to the twist lugs of the original can capacitor -- they are convenient and electrically equivalent. Be aware that this is not equivalent to chassis ground! Terminating the negative leads to the chassis will result in hum and distortion as the entire "can" is supposed to "float" above chassis ground, not be connected directly to it. Typically, the first couple sections would now be terminated on the positive side directly to the "Candohm" strip resistor sections riveted to the side of the chassis on the earlier models, and if you follow the circuit carefully you'll realize that the first couple sections can be relocated far from the old can capacitor on later models. Electrically it makes no difference, with one caveat - avoid placing your replacement electrolytics near sensitive RF sections. Work neatly, you'll likely require a solder sucker and at least one 'pick' to remove old leads, take your time, keep lead lengths to a minimum; and always use "spaghetti" insulation over capacitor leads prior to soldering. Stripped insulation from plastic coated wire works fine for this purpose. A selection of 2 or 3 lug terminal strips come in handy for securing the new capacitors.

Vacuum tubes have in many cases an undeserved reputation for unreliability and generally very long lived, at least in radios - assuming filter and paper capacitors are replaced. While of course it's possible one or more tubes are bad, this is somewhat rare outside of rectifier types. Radios were often run hard until they completely failed. If you need replacements or spares, they are commonly available with one or two exceptions. In general vacuum tubes for all vintage equipment are easily found and offered by numerous large scale vendors and private sellers, tube availability is pretty much a non issue surprisingly.

Trans-Oceanic tube filaments are connected in a "series-string" arrangement - if one tube filament goes, none of them will light, similar to certain Christmas tree light strings. You won't see the tubes light up in a Trans-Oceanic, though! When initially inspecting your "new" radio do not assume that the installed tubes are the correct type or in the right socket. If you are unclear you should refer to the schematic or tube layout diagram. Some models will have the tube type embossed on the socket. Incidentally, the only tube that should visibly glow is the A-C rectifier tube, on the right in the following picture; 117Z6GT. Later models utilize 117Z3, a somewhat cranky miniaturized tube type, or contain no tube rectifier at all, so if you want to gaze at glowing tubes, you'll have to find a different radio!

Why no glowing tubes? Remember, filament voltage in a Trans-Oceanic is only nominally 1.4 volts, not enough to see in daylight, if at all, and loctals usually have a generous silver "getter flash" covering most of the glass anyway. Removing and reinstalling loctal tubes while the radio is in operation will probably blow the tube filaments as the 200µF@10WVDC filter string electrolytic charges itself to a high level and then discharges across the filament string. Maybe then the tubes will glow, if only briefly! Remove tubes only with the power OFF by rocking back and forth at the base, not by the glass. Sometimes loctal tube pins are near welded to the sockets - a shot of DeOxit between the tube base and socket will help to free the tube, to be pried upwards gently with a small screwdriver or similar tool between the base and socket.

Loctal tube pins in particular are prone to thick layers of corrosion. Cleaning each pin throroughly with a fine emery paper will improve operation and overall performance. Prior to this, the tube will likely test badly in a tube tester, never deep-six a loctal that tests weak, dead, or "sounds bad" till the pins are cleaned up, try again - you might be pleasantly surprised

last update: 31st May 2008