ZENITH ROYAL 500   SEVEN TRANSISTOR RADIO  (1955)

The Royal 500 turned out to be a very successful radio and remained in production for nearly ten years, continually being upgraded in terms of circuitry and external appearance, but maintaining the same basic size of 3-1/2" wide, 5-3/4" tall, and  1-1/2" deep. Hundreds of thousands were ultimately made. The unique Zenith design applied to the volume and tuning knobs is thought by many people to resemble the appearance of an owl's eyes. This terminology is commonly used as a nickname for the first generation Royal 500s.

Identifying characteristics of the earliest Royal 500s are volume and tuning knobs with a bar through the middle and which are tapered to be thinner towards the outer edge (later knobs are round and uniformly thick across their diameter), non-vernier tuning (no gear reduction), the Zenith logo painted in white rather than gold, and the battery cover retaining screw hole split at the extreme top edge rather than being a full hole placed about 1/2" lower.

Below is the Zenith logo as it appears in black and maroon on the back of the hand wired Royal 500 nylon plastic cases. The information was engraved into the plastic (very obvious when actually observed) and filled with gold paint. The name Zenith was outlined. The words "UNBREAKABLE NYLON" were heat stamped at the very bottom of the battery cover on early production sets. I have a later production hand wired set which doesn't include the "UNBREAKABLE NYLON" message at all.

On later printed circuit models, shown above, the "Tubeless All Transistor" text was revised from the original script to the block lettered "TUBELESS - 7 TRANSISTORS". The information was simply heat stamped (making only a slight depression into the plastic) in gold paint with no outlining. The words "UNBREAKABLE NYLON" were still heat stamped, but moved from the battery cover back onto the main body of the case. These changes were undoubtedly a cost saving measure on what had to have been a very expensive set to manufacture. Based on my observations I believe this change may have occurred during the transition from hand wired to printed circuit models somewhere in the 90,000 to 95,000 serial number range. As always, this is only a general guideline and "crossover" examples do exist. I have a hand wired set (s/n 87106) which has the second design "TUBELESS - 7 TRANSISTORS" back.

If one is curious enough to remove the back cover of a Royal 500 he may face a detective job since this particular model was produced with several chassis variations. The following information is from the Zenith Royal 500 factory service literature which was printed in 1956.

CHASSIS IDENTIFICATION

The "Royal 500" seven transistor portable has been produced with four
basic chassis. This expedient was necessary to enable us to produce
sufficient quantities by using transistors from many sources. All
chassis have the chassis number stamped on them as well as a color
identifying code on the battery compartment just above the battery
installation instruction label. They are as follows:

                   Chassis 7XT40   - (Black) code dot
                   Chassis 7XT40   - (Maroon) code dot
                   Chassis 7XT40Z  - (Red) code dot
                   Chassis 7XT40Z1 - (Green) code dot

The two 7XT40 chassis are very similar with the exception of the
different coding on transistors. The transistors in these two chassis
are manufactured by Sylvania.

The 7XT40Z uses transistors manufactured by Raytheon Mfg. Co.

The 7XT40Z1 uses transistors manufactured by Texas Instruments Inc.

In addition to this, each receiver has its individual transistor layout
label and the color of the printing on these labels as well as the
chassis number on these labels conforms respectively to the color dot
and chassis number.

(N=NPN, P=PNP)

7XT40 (CIRCUIT 1) SYLVANIA    7XT40 (CIRCUIT 2) SYLVANIA
--------------------------    --------------------------
121-22 MIXER  N 2N94          121-22 MIXER  N 2N194
121-21 OSC    N 2N94          121-21 OSC    N 2N193
121-6  I.F.   N 2N94          121-6  I.F.   N 2N216
121-7  DRIVER N 2N35          121-7  DRIVER N 2N35
121-8  OUTPUT N 2N35          121-8  OUTPUT N 2N35

7XT40Z RAYTHEON               7XT40Z1 TEXAS INSTRUMENTS
---------------               -------------------------
121-14 MIXER  P 2N112/CK760   121-16 MIXER  N 2N145 *
121-9  OSC    P 2N112/CK760   121-15 OSC    N 2N145 *
121-10 I.F.   P 2N111/CK759   121-17 I.F.   N 2N145 *
121-11 DRIVER P CK725 **      121-18 DRIVER P 2N185 *
121-12 OUTPUT P CK725 **      121-19 OUTPUT P 2N185 *

*  The only published RETMA equivalents for these Zenith types
   were found in a Sencore transistor testor cross reference chart.

** RETMA equivalents for the CK725 are 2N65 or 2N132.

Here are some items which may help identify the earliest Royal 500s from late 1955 to mid 1956. These characteristics only seem to be present on the earliest sets, but there may be exceptions:

A green dot on the ferrite antenna support (visible from the front of the chassis).

The orientation of the antenna wires as they pass over the front of the tuning capacitor (spread out, not all grouped together).

The lack of any colored wax/paint on the tops of the I.F. transformers.

Electrolytic capacitors marked RETMA 911. Later capacitors (including replacements) are marked EIA 911.

When this particular radio arrived it appeared at first glance to be a typical black "plain Jane" Royal 500, and one which had seen a few hard knocks to boot as shown in the photo below. On the other hand the knobs (removed for the photo) had bars through the middle indicating that this should be an early hand wired set. Further examination revealed another disappointment... the battery cover had been lost at some point. Even the white battery instruction plate which should surround the stamped serial number was missing. Oh well, I thought to myself, let's check out the number. My expectations were falling as I squinted at the digits which were staring back at me from the exposed battery compartment and my mind ticked them off...  0 - 0 - 5 - 0 - 6. What the? Can that be right? Yep, it really was 00506! Suddenly my curiosity kicked into high gear.

The next step was to remove the rear half of the case which was being held now by only a single screw. As it came loose and I moved it aside I could hardly believe the sight that greeted my eyes. The black or silver oval Sylvania transistors I had been expecting were absent.

Instead my gaze was drawn to a group of Raytheons on the left... four blue RF jobs with red and orange paint marks on top. But wait, what the heck is this? On the right were three black plastic Raytheon audio transistors with green and yellow paint marks. It couldn't get much better than this! After a few moments of reflection the reality finally sank in... this was a very special radio and only the 506th Royal 500 to come off the Zenith assembly line in Chicago fifty years ago this fall. Through the generous donation of a vistor to my web site I now have the honor and privilege of preserving what is surely one of the earliest surviving Royal 500s... and with a 7XT40Z chassis as well!

The Zenith service literature reveals that many minor modifications were incorporated into the Royal 500 "since transistor circuitry is new and improvements are continually being discovered." Although the majority of first generation Royal 500s seem to contain Sylvania transistors, two other hand wired chassis were produced... the 7XT40Z  which used Raytheon transistors (all PNP) and the 7XT40Z1 which used Texas Instruments transistors (both PNP and NPN). The 7XT40Z in particular seems to have been made in relatively low production quantities. More about that later.

The tuning capacitor date code is 538 which remains the earliest I have seen. Zenith must have purchased a truckload of these because they appear to have been used in all of the first 2500 or so Royal 500s. The audio driver and output transformer date codes are 544. The speaker was made by Jensen and is dated 545. The transistors in s/n 00506 are unique among the few other 7XT40Z Royal 500 chassis that I have been able to observe through pictures. This is because it contains both black plastic cased audio transistors and blue painted metal cased RF transistors.

The more common (if such a word can be applied to any 7XT40Z) chassis of the Raytheon based Royal 500s typically has a full set of blue painted metal cased transistors. (You can see such a set further down in this page - s/n 33930). This is due to the fact that Raytheon finished converting the majority of its transistor production from black plastic cases to blue painted metal cases shortly after Zenith introduced the Royal 500 in late November, 1955. To my knowledge there are no known examples of black plastic Raytheon CK725 transistors with a date code from 1956. This means there was a period of only 4 or 5 weeks during which a set such as s/n 00506 could have been assembled using new black plastic Raytheon CK725 transistors. Another indicator of the unique nature of this set is that none of the transistors bear the Zenith "121-XX" part numbers listed in the factory service literature which was printed sometime in 1956 after s/n 33240 was built.

Above: note the various paint marks to aid assembly workers with the proper placement of transistors. This is similar to the assembly process used in early transistor hearing aids. Below is a better look at the blue and black Raytheon family of transistors. The yellow swatch indicates the audio driver transistor and the green swatches are a matched pair of audio output transistors. The Raytheon CK725 is obviously a member of the more familiar CK718, CK721, CK722 series but is not as common today as its more famous cousins. The CK725 is listed in Raytheon sales literature for 1955, 1956, and 1957 but appears to have been made primarily for use by OEMs (Original Equipment Manufacturers) such as Zenith.

One observation that I find particularly interesting about the use of Raytheon transistors is related to the way they fit (or more accurately, don't fit) the sockets which Zenith used in the Royal 500s. The earliest Raytheon transistors (black plastic cases) simply had three equally spaced leads in a straight line and a red paint dot on the side to indicate the collector lead. By the mid-1950's the predominant transistor lead layout was a straight line with offset spacing on one end to physically key the collector orientation.

As shown above left, the Raytheon audio transistors required some lead adjustments to fit into the sockets properly. Such was also the case even with the blue Raytheon RF transistors which did have an offset lead design. It just didn't match the dimensions of the Zenith sockets! This was resolved when the industry eventually moved to the round case design with a triangular lead layout. Although this was more efficient I find it boring from a hobbyist prespective. I enjoy observing the accomodations that manufacturers like Zenith were sometimes forced to deal with in their early transistor based products.

As a side note the Zenith Royal-T hearing aids, which used Raytheon CK718 transistors (physically identical to the CK725), had equally spaced in line sockets which fit the transistors perfectly. Thus it appears that Zenith never intended to rely on Raytheon transistors extensively in its radios. Raytheon transistors were apparently used only as a last resort when Royal 500 production demand exceeded the supply of Sylvania devices. This conclusion is supported by the apparent scarcity of surviving Raytheon (and Texas Instruments) equipped radios.

The Raytheon RF transistors (2N111/CK759 and 2N112/CK760) are more common and were used in a few other first generation transistor radios as well as being sold individually to hobbyists. There were two additional members of this Raytheon RF series, the 2N113/CK761 and 2N114/CK762. The primary difference was in the upper cutoff frequency which increased with the type number. Consequently the 761s and 762s were rather expensive and are much less common today. Below is a view from the RF side of the chassis.

The detector diode in this radio (not shown) has the latest date code of any component, 548, or the fourth week of November, 1955. This corresponds very nicely to the official Zenith Royal 500 introduction date of November 22, 1955. Since the battery compartment cover was missing from s/n 00506 I still haven't been able to get a first hand observation of the correct manufacturers label for a 7XT40Z chassis. If anyone has a 7XT40Z with the red label, part # 102-1997, I would certainly appreciate receiving a scanned image of it.

SOME HISTORY...

It goes without saying that despite its somewhat rough external appearance, s/n 00506 is a very historic set and I am grateful to have been given the opportunity to preserve it. The donor kindly provided some very interesting information regarding the history of this Royal 500. Apparently his mother was a Zenith employee and she gave it to him as a graduation gift in June, 1956. It appeared to be new but he soon noticed that someone had engraved their initials on the bottom of the case. Although further questions were asked, his mother would only reveal that the radio had been a "test model". The little black Zenith spent the next half century following its owner and faithfully providing the service for which it was designed. It only required servicing one time to replace two dried out electrolytic capacitors (a common problem for all transistor sets of this era). Recently the donor was making plans to move and had decided it was finally time to part company with his old radio. He was going to throw it away but thankfully remembered seeing my web site and that's why you are able to read this story. I hate to think of how close this wonderful treasure came to being lost... buried in a landfill forever. What a loss that would have been for transistor radio collectors/historians. Now it is on virtual display for everyone to admire, enjoy, and learn from. Thanks Bill!

As I ponder the new information which s/n 00506 has provided I am beginning to think that Zenith may have produced an initial run of several hundred Royal 500s for use by employees and others as part of an in house testing program before they were released for sale to the general public. This makes some sense and a similar testing process was used for the first Zenith Royal-T hearing aids (described on another of my web pages).

This particular hand wired Royal 500 is interesting due to the fact that it also has a relatively low serial number.

The chassis number can be found printed on a label attached to the inside of the battery cover. Unfortunately it is not uncommon for the labels to have been damaged by leaking batteries over the years. This one was no exception so I created a digital reproduction of it as shown below.

Zenith service literature is a valuable source of documenting the variations among the first generation of Royal 500s. The first sets were hand wired and can be broken down into two major production runs (circuit #1 and circuit #2) although the basic chassis number, 7XT40,  is the same. Complicating matters is the fact that Zenith also produced two additional handwired chassis, the 7XT40Z (Raytheon transistors) and the 7XT40Z1 (Texas Instruments transistors). These two alternate chassis designs appear to have been sparsely produced during periods of time when Zenith could not obtain enough Sylvania transistors to meet production demands. More is said about about the 7XT40Z and 7XT40Z1 elsewhere in this page. Both versions of the "regular" 7XT40 used Sylvania NPN transistors. The first run of sets, circuit #1, used four 2N94 transistors in the RF stages and three 2N35 transistors in the audio stages and included serial numbers 00001 to 33240. The second run of hand wired sets, circuit #2, used a 2N194, a 2N193, two 2N216's and three 2N35's. This change was most likely due to the rapidly advancing state of the art regarding radio frequency junction transistors in the mid-1950's. Perhaps the most significant difference between the two 7XT40's is that circuit #1 had a 2000 ohm earphone jack tapped off the first audio driver transistor, while circuit #2 had a 15 ohm earphone jack tapped off the speaker voice coil circuit after the push pull output transistor pair.

Above is a rear view of the 7XT40 circuit #1 hand wired chassis. Observable date codes in the form of YWW (Y = last digit of year, WW = week of year) are: tuning capacitor 538, and audio output transformer 544. The transistors in this set were all originally black painted oval first generation Sylvania devices. Although I can't confirm it, my assumption for the numerals on the black transistors is that they also represent a date code and are in the form of YM, where Y = last digit of the year or 1955, and M = month of year, 3 for March or 4 for April. Zenith also had their own internal series of part numbers in the form of 121-XX even though they were not stamped on the transistors in the earliest Royal 500s. Interestingly, the Zenith 121-XX transistor part numbers for both chassis #1 and chassis #2 were identical (follow the link below to read the original Zenith service literature) even though some of the RETMA numbers differed (eg. 2N94 vs. 2N194, 2N193, or 2N216). The Zenith part numbers replaced the standard 2NXXX series numbers on later transistors although the manufacturer's RETMA code, such as 312 for Sylvania or 274 for RCA, remained to identify their true origin.

Two of the original black transistors (one 2N35 and one 2N94) in this radio had been exchanged at some point with General Electric "universal" devices. The larger size and unique round flanged shape of these GE replacements detracted from the radio's appearance. Fortunately I was eventually able to locate some early Sylvania units which are very close to the originals.

The photo above shows the front side of the hand wired chassis (the speaker was removed to take this picture). The diode detector is a beautiful red and white Raytheon 1N295 (seen on the left side about in the middle). Later Royal 500s, possibly beginning with the transition to the printed circuit version, used a black 1N87G with white lettering.

Here is another R500 which I acquired that adds some historic information to the survivor database. S/n 13906 is a good example of a typical Sylvania based R500 of the 7XT40 circuit #1 type. Fortunately it appears to have been left untouched over the years and therefore provides a good example of what factory production techniques were in place at the time it was made.

Below are the normal documentary photos of the external appearance and the chassis. The transistor complement is all Sylvania (four 2N94's and three 2N35's). Tuning capacitor and audio transformer dates are 602 and 604 respectively. There is also an apparent date stamped in black ink on the back of the tuning capacitor, 1 30 56. This would have been a Monday so that may be the actual assembly date for s/n 13906. There are also a few other interesting items which I have noted further below.

I believe this set is probably a member of the initial production run of maroon colored R500's. Zenith called this the 500R model as opposed to the basic black 500Y. Upon comparison with later sets this first maroon group has one striking and unique characteristic... the nylon plastic cabinet is semi-translucent. This fact is easily observable by holding each half of the cabinet up to a light after removing the chassis.

The views above are not reversed, they simply show how the nylon cabinet appears from the inside looking out. Of course there is no internal light source so normally the cabinet appears to be opaque, although upon close external examination it is slightly different than the later ones. At some point (yet to be determined) Zenith's cabinet supplier revised their formula so that later maroon cabinets are truely opaque. In the final analysis a translucent maroon cabinet appears to be an indicator of an early R500.

The photo above shows a minor but interesting development in the knobs... specifically the addition of a cardboard washer on the back to minimize scuffing the gold anodized face plate. I have not observed these washers in the earliest sets.

Perhaps the most interesting aspect of s/n 13906 is that it shows evidence of the evolution which was occurring in Sylvania transistors in early 1956. Although all of them are painted black, several differences are apparent. The three 2N35 audio transistors are ovals with round edged tops and are all stamped "54" on top. The photo above on the left provides a detail view. These 2N35s are identical to the first Sylvania transistors in R500 s/n 01653. The 2N94's tell a different story however. None of them have date stamps on top and only two are ovals with round edges. The remaining two 2N94's have oval cases but with "sharp" edges. The photo above on the right shows these "transistion" 2N94s in the lower left and upper right corners. This is very similar to the next Sylvania development which was simply to leave the transistor cases in their unpainted silver metal form. My conclusion is that these "sharp" edged 2N94s may be the "missing link" in the transition from black to silver Sylvania transistors.

I am indebted to a good friend for supplying the picture below of a later but by no means common 7XT40Z chassis. It was produced in early 1956.

The transistors are all painted blue and have metal cases. They are stamped with the normal Raytheon identification on one side and the Zenith part number on the other. Raytheon had discontinued the production of black plastic cased transistors by the end of 1955. More details on the transistor type numbers can be found in the "survivors" table further below.

SOME INSIGHT ON THE INFREQUENT APPEARANCE OF RAYTHEON BASED ROYAL 500s

The official Zenith Royal 500 factory service literature provides an interesting observation on the cost difference of producing these radios with Sylvania and Raytheon transistors. The earliest service notes, 7XT40 and 7ZT40 (supplement #1), included parts lists with prices for everything except the transistors themselves. Fortunately for collectors the 7ZT40 (supplement #2) which was issued in late 1956 or early 1957 included the price that Zenith was actually charging for replacement transistors. The lists are reproduced below.

7ZT40 Sylvania                        7ZT40Z Raytheon
---------------------    ---------------------
121-22 mixer    3.95     121-14 mixer    5.25
121-21 osc.     3.60     121- 9 osc.     5.25
121- 6 I.F.     3.30     121-10 I.F.     5.25
121- 6 I.F.     3.30     121-10 I.F.     5.25
121- 7 driver   2.50     121-11 driver   5.25
121- 8 output*  5.65     121-12 output* 10.50
               -----                    -----
         total 22.30              total 36.75

Notice the significant price difference between the Sylvania and Raytheon parts! The Raytheon transistors commanded a "premium" of $14.45 or 64.8%!!! Of course these are replacement prices, not what Zenith paid as an OEM, but I assume their cost was somewhat proportional. Based on these figures it appears that Zenith took a significant hit on the profit margin for each Royal 500 that was produced with Raytheon transistors. This is because the Royal 500 sold for $75.00 regardless of its internal components. This seems to be hard evidence to explain why Raytheon based Royal 500s seem to be so scarce. My conclusion is that there was really no incentive for Zenith to produce them except to keep product on the shelves when other transistor sources temporarily dried up.

To check this further I obtained pices for roughly equivalent Sylvania and Raytheon transistors from the 1957 Allied Radio catalog. For the Sylvania: 2N35 = $1.65 and 2N94 = $2.85, for a total "Allied" cost of $16.35. For the Raytheon: CK725 = $2.80, 2N111 & 2N112 both = $2.80, for a total "Allied" cost of $19.60. It seems clear that Zenith was making quite a profit on replacement parts but the "premium" price of the Raytheon product is still apparent although not nearly as much.

I am also fortunate to have an example of a hand wired 7XT40 built with the second circuit variation. Below is a picture of the front of its chassis.

The serial number is 68430, the tuning capacitor date code is 636 and the audio output transformer date code is 626. The transistor complement is identical to the description I provided above. They all have the unpainted silver flat oval cases that Sylvania migrated to for its second generation of transistors. Only the standard "2N" RETMA identification is present. There are no Zenith 121-XX series part numbers but all the transistors are marked with a stylized "Z" logo above the number 312 on one side. 312 is the manufacturers code for Sylvania and the "Z" obviously refers to the fact that they were made for Zenith.

Based on my research with other collectors it seems that the 7XT40 circuit #2 hand wired chassis used at least two varieties of battery cover labels. Unfortunately this radio has no label. Even more unusual is that a close examination of the battery cover reveals not even the slightest trace of old paper or glue, just virgin plastic. Zenith quality control was either napping when this set slipped by or the battery door is an unlabeled replacement part. I have created a restored version of the label variety which I believe is appropriate for this set and is shown below on the left. Another and possibly earlier variety of this label is shown below on the right. The major difference is the use of  Zenith "121-XX" part numbers and a date of 6-55. Both restorations are based on photographs of somewhat deteriorated originals.

This is a late production hand wired set (black case) and is interesting due to the fact that it contains the fourth (and also less common) chassis variation using Texas Instruments transistors. It also has a couple of "crossover" production items. According to the data I have collected the earliest printed circuit R500 of which I am aware is s/n 92354, only about 5000 units away.

Above left is the chassis front. Note that the detector diode is mostly obscured by the pair of large and small disc capacitors on the left side. I was expecting to find a typical red Raytheon 1N295, but instead found a black 1N87G which is generally associated with printed circuit models. Above right is a look at the Texas Instruments transistors. They are similar in appearance to the second generation Sylvania's but there are visual differences. The tops of the TI's are free from the date code which Sylvania used. Also, notice the circular dimple in the tops of the three audio stage transistors. Sylvania devices do not have this characteristic. The transistor markings are summarized below along with detailed front and rear views of two of them.
                                Function     Front   Back  RETMA

I am not absolutely sure what the "R" numbers represent. They appear on one side of all the transistors along with the RETMA number and are probably for internal TI inventory control.

Above are the left and right side views of the chassis. Everything is pretty typical for a hand wired R500. Visible on the left is the 121-18 audio driver transistor. On the right you can see green and yellow paint dot marks on the oscillator and mixer transistors.

Above are views of both sides of the battery cover. On the left is a crossover item, a sticker boasting about independent test lab results. This was a refernce to the positive review that the R500 received in Consumer Reports magazine in May, 1956. An image of a much better example is available at the bottom of this article. I had previously only seen this on printed circuit models but this set proves that it was placed on both types. On the right is the manufacturers label which is in a sad (but typical) condition due to batteries that leaked sometime in the past. Notice that it is printed in green ink which corresponds to a green paint dot on the battery instruction plate.

Later Zenith advertising in the 1960's bragged about the benefits of "handcrafted quality". Despite this admonition, the Royal 500 chassis evolved from a hand wired design to a printed circuit design although the transistors remained mounted in sockets. Based on actual set information from other collectors it appears that the hand wired sets were phased out by late 1956 as Zenith ramped up production on the printed circuit models. The most likely reason for this change was to reduce the manufacturing cost (and increase the profit!) on what had become a very successful product with a high sales volume.

The following general conclusion can be drawn from Zenith service literature and collecting experience. The 7XT40 hand wired circuit #1 was used on sets from s/n 00001 through about 33240. The 7XT40 hand wired circuit #2 was used from about s/n 33240 through almost 100000.
When Zenith began to migrate to the printed circuit design in late 1956 they changed the basic chassis number from 7XT40 to 7ZT40. The earliest printed circuit chassis I have seen is a 7ZT40 with s/n 92354. The latest hand wired chassis I have seen is a 7XT40Z1 with s/n 121707. This evidence clearly establishes the fact that there was a definite transition period during which both hand wired and printed circuit chassis were in production.

I also have a relatively early printed circuit model in my collection, s/n 157570, made in late 1956. The 7ZT40 seems to simply be a printed circuit continuation of the 7XT40 hand wired circuit #2. This is suggested by several observations. First, Zenith documentation states that the battery label part number for the hand wired 7XT40 chassis #2 was 102-2263. My own printed circuit 7ZT40 has a battery label part number of 102-2265 which is a nice sequential fit.

Second, the battery label color on my printed circuit 7ZT40 is maroon, identical to the color Zenith stated was used for the hand wired 7XT40 circuit #2. Third, the transistor complement in my printed circuit 7ZT40 is identical to the one in my hand wired 7XT40 circuit #2 including standard RETMA transistor numbers (a 2N194, a 2N193, two 2N216's and three 2N35's). Details are:

Fourth, the progression of serial numbers and tuning capacitor date codes in the database below seems to show a relatively smooth transition from hand wired to printed circuits. Hopefully additional Zenith documentation will turn up to help answer these questions with more certainty.

On the unusual side, my 7ZT40 battery label is dated 12-55 which is several months prior to when the switch to printed circuit chassis seems to have been made. The battery label date is very tiny and appears directly above the Zenith logo. (It should not be confused with the sticker on the left side from North Akron Radio dated Dec 12 1956) Again, I assume this discrepancy reflects the typical time lag between engineering and production.

In their November, 1956 Christmas buying guide, Consumer Reports gave the Royal 500 a further recommendation. Such articles undoubtedly helped to boost the sales of this radio which accounts for their relative abundance in today's collector market.

Zenith was quick to promote the positive test reports. The label (above right) is on the back of my printed circuit 7ZT40 Royal 500 which also has a sticker inside from the store that originally sold it. The date was December 12, 1956. I like to imagine that some lucky person received this set as a Christmas present that year. If so it was a very nice gift indeed because $75.00 in 1956 would have been equivalent to about $460.00 today.

The Zenith Royal 500 family of transistor radios was noteworthy for its high level of quality and performance. The long product life cycle saw the incorporation of many changes such as vernier (geared) tuning and an eventual increase in the transistor count from 7 to 8. Such revisions indicate that Zenith was constantly striving to improve the Royal 500 and undoubtedly viewed it as a flagship product which they intended to be around for quite some time. Everyone with a broad interest in collecting transistor radios should own one of these fine sets and the good news is that common examples are not too difficult to find. Best of all, most of them still work and continue to provide listening pleasure even though they are a half century old.

Reference: "Zenith Transistor Radios - Evolution of a Classic", by Norman Smith, published by Schiffer Publications Ltd. This is a great book for the Zenith fan and includes descriptions of all their radios from the mid 1950's through the mid 1960's.