another Zenith Transoceanic H500 Receiver
Also two Transoceanic clones
While not heavy metal boatanchors, the Zenith Transoceanics are favorites for short wave listening. The bands are push-button selected and cover primarily the international "world-band" shortwave. The bands are arranged so that no bandspread is needed. Click here to view an H500 that I repaired a few years ago.
The sets are quite sensitive for shortwave listening with just the built-in telescoping whip antenna if the radio is working well. Tubes in this set are 3V4 audio output, 1S5 detector/AVC/ audio preamp (the 1S5 indicates it was early in the model run, the 1U5 was used in later releases of this model), two 1U4 for RF and IF amplifiers, and the expensive 1L6 converter.Schematic, alignment and other information for the H500 can be found in Sams Photofact 152-12 and for download at Nostalgiaair . A close schematic with some variations and lots of good information on the circuits and operation can also be found in the manual for the military version of this Zenith Transoceanic, the R-520/ URR radio receiver. I recommend using the alignment info in the military manual. A link to download the large PDF of that manual is available on the home page. The military version R-520 uses the large filament-type ballast tube 50A1 that does better at keeping the tube filament voltages constant with varying power line voltages and was included on some later H500 models as well as the 600 models. That 50A1 ballast is difficult to find new. This earlier H-500 just uses a sand-style power resistor.
I bought this set at a hamfest (amateur radio swapmeet) knowing it was not functioning. I prefer dead sets since part of the pleasure is "sweat equity", getting one of these old sets to work well again. For that reason, a functioning set is less FUN for me and usually requires just as much work to bring it to up to specs. Cosmetically, this set was average. The front plastic trim had no visible cracks and the gold-color metal inserts in the knobs were still in place. It had no accessories inside and no paperwork or instructional manual. Before buying, I double checked to confirm the presence of the expensive 1L6 tube.
Nearly all of the Transoceanics which I have repaired have some problem with the AC power supply. This model uses a selenium rectifier, dropping resistors, and electrolytics to provide the equivalent of "A" and "B" battery power from the AC line. Be aware that these are transformerless AC-DC sets with one side of the power line connected to B- floating ground. The floating ground is connected to the chassis through a resistor and capacitor. Use of an isolation transformer is strongly advised when working on these sets.
On pulling the chassis, found that the .047 MF cap across the power line connections had split in two at some time in the set's life. I will replace it with a cap rated for line voltage. I suggest replacing this cap in any Transoceanic.
External power for testing
After pulling the chassis on these sets, I typically feed power from external power supplies for both the low voltage "A" source and the approximately 90 volt "B" source. This lets me check and repair the radio proper before tackling the AC-DC power supply. I also get a sense of the quality of the circuits. I monitor both the voltage and the current draw on the external supplies. The maximum current for the "A" supply should not exceed about 50 milliamps or so. I normal set the "A" supply for current limiting at 50 Ma. For the "B" supply, I use a variable regulated Heathkit PS-4 with a 25 milliamp lamp in series with the hot lead in case of short circuits. The lamp gives me a visual indication of current draw while reforming caps and actual use in this set. It also can act as a "fuse" if necessary. B+ draw in normal use should remain well below 25 Ma.
A 3 Pole Double Throw (3PDT) switch is activated by plugging the AC plug into the chassis. This switches the radio for battery power. These switches sometimes need a bit of contact cleaner such as deoxit. Checked this switch and found good continuity. The 3 wires from the battery plug are color coded. The black lead is the chassis ground connection which is also the "A-" and the "B-" when connected for battery power. The Red wire is the "A+" and the Blue wire is the "B+" .
Not surprisingly, the set started playing with the external power applied via the battery connections. I did some tests to verify the quality of the 1L6 and its circuit on the highest frequency range, the 16 Meter band. The highest frequency seems to demand the most from this tube. I found that the "A" supply could be set to as low as 6.51 volts with the 1L6 filament itself as low as 1.08 volts before the oscillator dropped out at the highest frequency range with either an off-air signal tuned in or a signal generator source. I found the tuning accuracy to be off considerably on this band but was obviously pleased with the test result as it indicated a very good 1L6 tube. On the 4 to 8 MHz band which I used for early testing, the set was happy with only 5.75 volts on the filament chain. I checked the voltage across each of the tube filaments to determine if the chain was well balanced. The filaments carry not only the heating current, but because they are directly-heated cathodes, also carry the return current for the rest of the tube. An unbalanced filament chain is a good indicator of improper current draw and an off-value resistor or some other problem in the circuit for one of the tubes.
Checking the AC-DC power supply
When these sets are connected to an AC-DC source, the "A" filament supply is simply fed by the high voltage through a couple of power resistors. For the next test, I pulled the plug from the chassis switch, and fed DC from my Heathkit PS-4 variable regulated high voltage supply to the output side of the selenium rectifier and to the B- connection for the power cord. I closely monitored the filament voltage to check the dropping resistors. With the external DC supply at 102 volts, the filament voltage read about 7.5 and the set was working fine. Next, fed the input side of the selenium rectifier to see how much voltage was required to bring the set to the same normal operating voltage. It required 142 volts of DC to bring the filament voltage back to 7.5. The rectifier voltage drop was 40 volts. The conclusion was that this set had a totally worn out selenium rectifier, a typical problem.
Disconnected the wires from the selenium rectifier but left it in place. Mounted a small terminal strip on the end of the rectifier's mounting bolt. Wired in a silicon diode and experimented with the variac to determine the necessary dropping resistor. A silicon diode is much more efficient than the selenium rectifier. An additional dropping resistor is needed to prevent over-voltage to the delicate filaments. Final result was about 75 ohms to keep the filament voltage maxed at about 8.0 with 122 volts AC input. This meant that for normal line voltage, the filaments would be fed about 7.7 to 7.9 volts. The circuit design is 8.4 volts maximum on the series filaments, but I try to keep a conservative safety margin on these delicate tubes.
Had previously cleaned the bandswitch with a bit of deoxit and worked the push buttons. This set was performing decently even before alignment but the alignment tweaking improved sensitivity and dial accuracy. Found that fully accurate alignment on the 16 Meter spread band may not be possible on this particular set without circuit changes since the adjustment range on the coil is not quite sufficient. Let me know if you have had a similar experience with the top band and how you resolved it. All other bands aligned properly. Although I have not found the detail in any of the manuals, the oscillator settings for the spread bands are all at 455 KHz under the dial marking while that for the three wide bands (broadcast, 4 to 8, 2 to 4 MHz) are 455 KHz higher (the latter is noted in the mil manual). A frequency counter makes it relatively easy to adjust the oscillator for the various bands and to check tracking.
Before putting the receiver back into its case, I checked the conections to the "Wave-Magnet" loop antenna which is mounted on the lid. The hinges on the H-500 are part of the electrical connections to the loop as are the loop's thumbscrews. I found one side intermittent. Unscrewed the wood case top section to see where the problem was. Finally found the problem in the lid itself. Carefully peeled back a bit of the plastic fabric trim and carefully soldered the bad connection where the hidden braid connects to the hinge. Loop connections now show good continuity
Like all tube Transoceanics, this set is proving to be a decent performer at pulling in shortwave broadcasts with good sound given its circuitry. It is also excellent at BC broadcasts.
Do not pull any of the tubes while the set is powered by AC. Plugging the tube back in will discharge the electrolytic caps, which have been charged to full voltage, through the delicate filaments, risking a burn out. Jim, WN1A, wrote of problem he encountered with a suppressor grid to screen grid short in the output tube that would send B+ to the filament string. Suggest checking that tube to avoid such a possibility. Other restorers recommend shunting the filament string with an 8 volt or so zener diode to swamp the circuit to prevent filament over-voltage. Others suggest a 7.5 volt zener to further extend the life of the tubes. Wire the zener appropriately in the circuit, especially If you run such converted sets on battery or you might drain the battery to the zener voltage or burn out the zener. Wiring in a couple of silicon diodes in series with the A battery would reduce the voltage a bit when the batteries are new.
Send in the Clones
There were a number of tube Transoceanic "clones" made. My definition of a clone is a 3 way power shortwave portable with similar tube line-up including the 1L6 oscillator/converter. Common ones are the Hallicrafters World-Wide and the RCA Stratoworld.
Sears (below) made the handsome Silvertone model 7224 chassis 528.33202 which has 4 bands including a VLF band and bears a strong resemblance to the four-band version of the Hallicrafters, especially the controls, the band coverage and the world map.
Sonora Radio and Television Corp. made model # GTU, No. 604, which has only two bands, Broadcast and shortwave. Shortwave covers 6 to 18 MHz. The dial markings are enhanced, perhaps to imply more coverage on the shortwave. Also note the (in)accurary of the grayed Eastern time zone on the world map. Inside is a very nicely designed chassis and circuit complete with tube and section shields.
"I Like Ike"
Transoceanic Ad - Listening to the 1952 election results on an H-500 with lid removed.
A Rider Chanalyst was the previous item on the bench.