zenith trans-oceanic shortwave tube radio - my latest find.

[jshorva65]

Adding a bit to the sub-topic of capacitors in these sets, it's important to re-emphasize (especially to any novice restorers who might be lurking here) that ONLY electrolytic and paper (enclosed in wax, ceramic, or plastic) capacitors should be "shotgun-replaced" in these. Characteristics other than merely capacitance, working voltage, and tolerance were significant in the selection of the original mica and disc-type ceramic capacitors (this applies especially to mica types in tuned stages) and were often extremely critical ("Q" and "Tempertature Coefficient" characteristics in particular) in achieving the level of performance from which these sets earned their fame. Haphazard replacement with units of incorrect "Q" or mismatched "TC" spec can wreak havoc with stability. DO NOT mistake C37 (4000pF) and C39 (2750pF) mica capacitors for Micamolds (as I almost did). The Parts List specifies them as mica. Also, remember to turn the tuning condenser to its fully-meshed position before inverting the chassis for any reason to avoid bending the plates (learned that "the hard way" 30+ years ago, fortunately on a "nothing special" AA5 radio of the "25-cent garage sale find" variety).

Also note that some of the rotor plates are segmented. DO NOT disturb these. They were adjusted at the factory for optimum tracking, and their re-adjustment is both seldom-needed and an "experts only" endeavor. Even after 30+ years of practicing occasionally on "garage sale" sets, I myself still don't feel quite comfortable enough with my limited skills in this to dare test my mastery of the technique on any set as valuable as a Trans-O. It can best be described as a three-dimensional puzzle with billions of permutations and only one truly correct solution. I at least know how to avoid the most-obvious consequences of misadjustment (namely, accidentally creating regions of the tuning gang's travel in which it shorts itself out or where extremely nonlinear tuning occurs). Done properly by an exceptionally-skillful and steady-handed technician, though, the results can be quite impressive. Two of the "oldtimer" Radio & TV Repair guys who taught me back in the 1970s and 1980s and a Professor with Signal Corps experience from WWII and a PhD in Engineering were the only three people I've ever met who could calibrate a communications receiver or marker generator to track with laboratory-grade linearity.

 

[wa2ise]

Drop by the videokarma antique radio forum. More people there will be familiar with radios like this
http://www.videokarma.org/forumdisplay.php?f=16

 

[jshorva65]

Here are the pics after rust-removal, recapping, dial cord re-stringing, replacement of 5 out of its 6 tubes, a new line cord, dusting, etc. Its original line cord, although free of cracks or other problems that would have rendered it unsafe to continue using, was significantly more stiff than I consider acceptable and had some ugly chafing at the end farthest from the plug where the slot in the exit bushing would have gripped it when the cord was fully extended. Since there's no fix for it that I know of other than a new cord, I tore down the cord reel and re-assembled it after installing 76 inches of new AC zip cord and the side-entry replacement plug I mentioned earlier. Since the original selenium rectifier was concealed under the chassis and not visible from above the chassis, I removed it and installed a terminal strip in its place where I mounted a 1N4007 diode and the 250mA micro fuse which now protects the set's mains power supply. Since the normal mains current at 120V is about 125mA, the textbook procedure for overload protection of installing a fast-acting fuse rated double the normal mains draw works out to exactly 250mA. A 150-ohm 5-watt resistor substituted for the original R2 value of 130-ohm 3-watt yields 92 volts at the plate of the 3V4 tube with respect to B-, only 2V less than the 94V specified on the schematic. The polarized plug puts the chassis at mains neutral potential (0VAC) with the set operating. During operation, the exposed Wave Rod antenna is likewise at 0V, but reaches 10VAC when the radio is switched off while remaining plugged in. This is far more than the 0.75VAC worst-case value we prefer to achieve (National Electrical Code and DHEW Product Safety specifications as of 1975), but actually (due to our using a polarized line plug with Neutral on the B- side of the circuit) less than UL guidelines as of the radio's 1958 date of manufacture had regarded as permissible. The usual "Hot Chassis Fix" we apply to mains-only sets such as AA5 table radios, metal portable TV sets, etc. of connecting the line-connected chassis or B- bus directly to the Neutral side of a polarized line cord and re-wiring the Power switch so that the Hot is the fused / switched side of the line is not possible with three-way portable radio circuits without rendering proper functionality of the set's Mains / Battery change-over circuitry impossible (constant draining of the "A" battery through the 330K chassis-isolating resistor with the radio switched Off and left in Battery mode would result, instead of both the "A" and "B" circuits' opening with the Power switch in the Off position as occurs with the circuit wired per the radio's original schematic). Since a three-way portable radio which drains its batteries constantly when powered off can't be considered to be a properly-functioning radio, the next-best solution of making the line cord Polarized with the B- bus connected via the B-/A+ contact of the Power switch to Neutral is the only workable option short of adding another set of contacts to either the Mains / Battery switch or the Power switch.

With the 1R5 temporarily subbed for its missing 1L6, the unit plays better than I expected and the dial calibration is about as far off as I expected substitution of a 1R5 for a 1L6 to cause. Once I've got a good used 1L6 on hand (I know where to get one for under $20 and plan on ordering it first thing on Monday), I'll check performance thoroughly, check and correct its alignment as necessary, and pronounce it Basic Restored rather than the "Promising Work-In-Progress" status I've assigned it for now.

I used 630V Epoxy-Dipped Mylar caps to replace the Bumblebee and White-Time-Bomb caps because I've experienced better performance and reliability with them than I have with Orange Drop caps. Both Big Dave and I have gotten bad Orange Drop caps in batches we ordered from a certain vendor who we won't name but who we all would know by their higher-than-average prices. In my experience, the green epoxy mylars are great little caps and economical, so I use them in circuits where the original cap was rated 200V to 400V. I've never experenced a failure of a green mylar cap, but don't necessarily trust them in 600V applications. For 600V work, I prefer the yellow tubular axials (Mallory 150-series and the similar series made by Illinois Capacitor) from JustRadios and the 1000V types of similar construction for 1000V applications. Prices are reasonable, performance is excellent, great reliability, and I like the availability of tubular axials in both pre-WWII and post-WWII uF values. They've become "standard equipment" for our TV Restorations.

The set picks up stations on all bands, and actually gets better daytime SW reception with its simple built-in whip antenna than any other communications receiver I've owned has achieved with an indoor whip antenna of equivalent size. Performance with an outdoor longwire antenna (which I haven't built yet at this house, but plan to build eventually) and a genuine 1L6 tube must be simply phenomenal. I just might learn to fully appreciate the Trans-O and declare it worthy of its mystique once my 1L6 is in place. For someone who was never a big fan of three-way tube portables, I guess I've learned that the Trans-O is, at least, the best of them that I've ever seen.