The Restoration Process – Gross CB-25 – Winter 2008/2009

Such a rig really deserved to be placed in, and kept in first class condition, so I began to consider my restoration strategy. Over the years the original black wrinkle finish had been redone, but the lower parts of the chassis, and undersides were showing rust. The finish was also soft, and the coil sets sitting on top had stuck to the finish, leaving circular impressions.

So first on the list was the paintwork, and last year, when I restored the Gross replica that John had built for Marshall W2ER, I made inquiries about the refinishing of vintage black wrinkle paint. Traditional black wrinkle paint, once so ubiquitous with gear from the 1920-30s, has all but disappeared. It seemed the only real alternative offering both durability, and a reasonably authentic appearance, would be powder coating.

Powder coating is a modern, and quite durable method to finish sheet metal. During the powder coating process, the sheet metal is electro-statically charged so the aerosol powdered paint will adhere to it, and is then baked at high temperatures. The result is a most resilient finish, and a close match to the old black wrinkle finishes. Numerous inquiries were made, until the trail pointed me to the door of Wayne Spring W6IRD in California. Wayne did a stellar job with the W1FPZ rig, so I naturally trusted the Gross CB-25 to him. Wayne is quite a gentleman, and most graciously said, “Just send it out”. With the refinishing tasked out, I began the long process of documentation and disassembly of the transmitter. 

Gross CB-25 Transmitter
Won for restoration of the W1FPZ tx

Since I planned a complete tear down and restoration, it was critical to properly document the layout of the rig for reassembly. Herein was the largest challenge I encountered during the entire project.

The Process
The CB-25 transmitter really consists of two separate decks, an RF and combined Modulator and Power Supply. So armed with my trusty camera, I carefully photographed the RF and Power Supply decks from every angle, to assure that I could properly disassemble, and then reassemble the units when the sheet metal came back from Wayne’s shop. This idea worked like a charm, and soon I had a better record of the transmitter than if I had only a schematic, for the photos showed not only where various wires were connected, but also the lead dress and layout.

One odd challenge I did encounter was with the insulation of the meter leads. Like much vintage equipment of the era, this unit used meters that were secured to the panel with their 3/8” mounting studs. Since plate voltage for the lower stages, as well as the output tubes, was present on these studs, it was imperative they be insulated from the metal panel. The original rubber insulator bushings were far too cracked and dry-rotted to be trusted with B+ voltage. My trusty local hardware store came to the rescue. I found the perfect solution, nylon bushings with a thin shoulder to insulate the meter studs from the metal of the panel. After a little attention with a file, the bushings were a perfect fit, and the meter panel looked quite authentic. 

Details, Details, Details
So it was with some trepidation that I made the big decision, and pulled apart the entire power supply, lettering and tagging each lead with paper labels. I used the digital photos I had taken earlier to label each component, assigning names such as “T1” and “R3” right on the photo. In this way, with the part names now noted on the paper tags, I could unsolder multiple component leads and still readily identify each connection point. 

The RF deck, unencumbered by transformers and chokes whose leads extended through the panel, was a much simpler process to disassemble. After several hours of work, I was left with a pile of parts, dozens and dozens of Ziploc baggies affixed with labels such as “RF Deck Knobs”, “Power Supply Hardware”, and “Gross Dial sets”. With the set of panels were heading west to Wayne to work his magic on, in the meantime, the pile of parts lay on the workbench, reminding me during each visit to the ham shack of the project which lay before me.

At the same time I sent off the panels, I also placed an order with the good folks at McMaster-Carr for an assortment of stainless steel fasteners. I was quite impressed with McMaster-Carr’s selection, and I was able to find some period looking hardware. Most early radio gear uses slotted rather than more modern Phillips heads, often with a countersunk design. I learned a trick some time back about using the countersunk washers under the front panel screws. You’ll often see these washers used on old Collins gear, with a small cork insert underneath to prevent marring of the front panel. Those cork gaskets are long gone, but you can also buy nylon countersunk washers as well, and they will fit right inside the stainless steel ones. By using this method you provide an authentic look, slotted head screws, metal countersunk washers, with the black nylon washer inside protecting the front panel. In a few weeks, the sheet metal panels came back from Wayne, looking every bit as nice as the first set he did. I was tempted to jump into the project right away, but I really wanted to make this the best that I could, so I waited until the weekend. 

While the rig was torn down, I took the time to inspect and clean each component, and repaint some of the transformers and capacitors. This was the ideal time to carry out cleaning and refinishing, and the results were most rewarding. Assembly of the unit took a little over 1 month, working 2-3 hours of time each session.

With the power supply and RF decks now assembled, I took time to check the power supply voltages before mounting the rig in the sheet metal cabinet. As Gross had fabricated the interconnect wiring harness for use in the rack; it was too short to run the transmitter with the decks side by side for bench testing. Sensing light at the end of the tunnel, I eagerly fabricated a longer cable for testing. All went off just fine, and soon I had my “dummy load” light bulb glowing brightly. Listening to the signal note in my monitor receiver, it was clear the unit was operating just fine. Confident of my work, I installed both in the chassis for final testing. 

Live To Air
On May 15, 2009, my 45hth birthday, the Gross CB-25 returned to the air. My initial testing consisted of using a 40-watt light bulb as a dummy load, as I peaked and dipped the various stages. Operation of the Gross rig is quite straightforward. To operate the transmitter, one turns on the small switch located on the left lower panel energizing the filament transformers. One then tunes each stage, working from right to left, starting with the oscillator and progressing to the buffer and final amplifiers. The two panel meters are plugged and unplugged, as each stage is set. When fully loaded, the CB-25 rig puts out a solid 20-25 watts of CW carrier, and is very stable.

With Thanks
In closing I’d like to thank my good friend Larry NE1S for his technical assistance, Wayne W6IRD for his masterful sheet metal work and powder coating, Tim W1GIG for sorting out John’s paperwork and forwarding over Gross materials, and finally my good friend John W1FPZ SK, who reminded me again of the true magic of radio. 

73 Bruce W1UJR
September 2009

Woolwich, Maine

1934 Gross CB-25 – About This Transmitter

Gross CB-25 Transmitter
Gross CB-25 Transmitter

The time was the 1930s, and America, like much of the world, was mired in the midst of the Great Depression. Jobs were gone, bread lines were common, and times were admittedly hard. Yet, to borrow a phrase from Charles Dickens, “It was the best of times, it was the worse of times.” for the darkest days for our country were the beginning of the Golden Days of radio. Americans needed distractions from the cares of the day, and radio had captured the American imagination like no other invention before. One could now “listen in” to the world, bringing news from around the globe into the living room each evening.

These were the halcyon days for the radio amateur as well, with radio stores popping up in small cities and towns, offering knowledge, parts and supplies to the budding home brewer. It was American ingenuity at its best, with whiskey shot glasses becoming insulators for the tuning capacitor, wooden breadboards and kitchen cake pans becoming the chassis for the latest receiver or transmitter creation, and tinfoil and plate glass becoming transmitting capacitors. 

With funds limited, the time honored tradition of home brewing or building your own set, was the word of the day. Those who bought “commercial jobs”, or assembled sets, were often looked upon with disdain, (perhaps even with a tad of envy) for breaking tradition by the more experienced of the amateur community. Although fone modulation was in use, CW was still the dominant mode, as it was simpler to build, therefore less expensive to the cash strapped amateur. Vacuum tubes were costly, and therefore most sets consisted of simple oscillators with one stage of gain, MOPAs, Hartleys, TPTG were the rule of the day.

Gross Radio Company
In late 1931 into this mix entered a new radio firm, the Gross Radio Company. Originally located at 25 Warren Street, and later at 51 Vessey Street in New York City, Gross Radio was founded by Jerry Gross W2AAE, after he left Leed’s Radio. Gross had worked at Leed’s designing radio kits, right down the street, in the area which later came to be known as “Radio Row”. During the 1930s, Gross Radio thrived, and ads could be found in a number of radio publications, including QST and Short Wave Craft, offering a variety of kits and assembled units. In fact the monthly QST ads were often ¾ or even full page, on the level of much larger companies like National and Leeds.

As these were the days of the Great Depression, many amateurs lacked the funds even to buy a radio kit, much less an assembled unit. So, aside from Gross transmitters and receivers, the company also sold radio parts and products manufactured by other radio firms. In one QST ad, the header began with “If We Don’t Carry It In Stock, We’ll Get It For You.” Gross also offered custom building for the discerning client, proudly advertising that “Our lab is equipped to build anything to your own specifications.” A Wednesday “Code Class” was even offered in the store from 6:30 to 7:30PM, with the advice to “bring your own phones.”.

Perhaps because of his parts sales business, Jerry was surprisingly tolerant of window shoppers and tire kickers, and “Jerry’s Place”, as the store came to be know, grew to become quite the gathering place for hams. Bill Orr W6SAI, writing in the February 1977 issue of CQ magazine, had this to say about his visits to Gross Radio as a young ham, “most amateurs spent their time in Jerry’s store examining the model transmitter carefully set atop a counter, and copying its circuit so they build the equipment out of scrap parts salvaged from a defunct broadcast receiver!”
The Gross CB-25 seems to have first marketed in the March 1934 ARRL publication QST, one of many products from the Gross Radio Company of New York City. The “CB-25” is best explained as the “C” stood for CW, the “B” for class B fone modulation, and “25” for the 25 watt RF output. 

This particular Gross Radio transmitter was originally owned by Bill Orr W6SAI, and given to my friend John Rollins W1FPZ SK to restore.
With John’s passing the unit came from his estate to my care. Overall, the transmitter is in remarkable condition thanks to John’s restoration efforts, but I’d like to find out a little more about it. Unfortunately W6SAI is now a silent key, so this may be a story left untold. 

Bill Orr W6SAI
Bill Orr W6SAI, the author of the famous “West Coast Handbook”, held high praise for the Gross, writing about it in at least two articles in CQ magazine. In these pieces Bill shared his joy of visiting “Jerry’s Place”, drooling over the various designs, and then heading home to build his own. At some point in later life, Bill became the proud owner of at least Gross transmitter, a story he shares so well in his February 1977 CQ article. Bill relates that in 1976, while browsing a flea market, he came across a mint CW-25, reigniting his childhood love of the sets. I’ll not spoil by attempting to relate Bill’s experiences here, suffice to say that Bill enjoyed the Gross so much that he rounded up yet another Gross set, this one making its way to Maine, and this is where our story really begins. 

John Rollins W1FPZ
A short time after I became a ham back in the mid 1990’s, my interest turned to the older 1930s sets. I joined the Antique Wireless Association, a group dedicated to collecting, documenting and restoring old equipment. Being involved with the AWA I became aware of a man named John Rollins W1FPZ who lived on the Maine coast. John was quite renowned for his building abilities, and had at that time just taken over one of the AWA’s on air contests from an ailing Bruce Kelly W2ICE.

In 2001 I moved to Maine, and later up the Maine coast, just a short drive from John’s home. John, aside from his wonderful building skills, was a most generous person with his time and materials. He was always willing to help out the radio newcomer, often building and giving away his creations. For me, John was a source of inspiration and knowledge. He was always ready and able to answer a question, assist with a part for my latest restoration. 

The Orr Rollins Connection
Sometime in the late 1990s, Bill Orr became aware of John also. Perhaps it was John’s article on restoring a Gross set in the 1998 AWA Old Timer’s Bulletin, or maybe it was just John’s reputation. In either case, Bill called John with a proposition, restore my old Gross transmitter, send me the photos, and you can keep the rig! Well for John this must have been irresistible, for it was his chance to get his hands on yet another Gross, John was likewise a fan of Jerry Gross’s creations, and a chance to bring a dead radio back to life. 

John was not about to let a deal like that go through his hand, and was prepared to fly out in his private plane to pick up the rig from Bill in San Francisco, but a friend stepped in, and offered to drive out to California and retrieve the rig for John. Soon the rig arrived back in John’s workshop in Arrowsic, Maine where it underwent a full restoration, Rollins style. Any correspondence that passed between John and Bill seems to be lost with John’s passing earlier this year, but John never ceased to proudly point out the rig when showing a visitor his ham shack. 

The CW/CB-25 Rig In Detail

The CW/CB-25 was quite robustly built as well, starting with the steel chassis, finished with the classic black crackle paint. The three air variable capacitors featured prominently on the front panel were General Radio products, as were the dials, printed with the Gross lightening bolt logo. Such quality was surprising for a bargain kit of its day, for General Radio was well known as a manufacturer of very high quality parts and equipment. Underneath the layout is a very traditional design, with straight-line heavy 12 gage tinned wire was used for the ground and RF bus, and 14 gage black insulated wire used elsewhere. A mixture of ceramic and fiber sockets are used for the coils, vacuum tubes and electrical connections. Two air variable capacitors, mounted on the rear of the top chassis, are used to neutralize the buffer and final stages. 

The RF circuit of the CW/CB-25 was based on the then popular type “46” tube, and the design was often referred to as the “46 job”. Starting from right to left on the RF chassis, a type “47” tube was used as a crystal oscillator as it would run on 160, 80, or 40 meter crystals. The 47 was designed as an audio pentode, but would provide decent RF output at 300 volts in a crystal oscillator. Output of the 47 went to a type 46 used as a buffer stage, which was neutralized for a vertically mounted capacitor located almost directly behind the tube. The buffer output was then directed onto a pair of 46s in a class C amplifier arrangement. A plug in coil was used for each stage, hence the need for three sets of coils per band. The crystal holder was mounted on the right front of the top panel, and Gross sold both the crystal holder and blank, you ground your own crystal in those days, for one dollar each.

During the tune up process, one used either an external meter, plugged into the jacks on the front panel of the RF deck, or the tried and true light bulb with an inductive loop of wire. Each stage was loaded and tuned to resonance, with the one caveat that the phone jacks were live with plate voltage! The CW/CB-25, like most sets of the day, was designed for a high impedance balanced antenna, so the link coupled output, again via the plug in coil, went directly to a set of porcelain insulators.

The modulator circuit on the CB-25 is a classic design that appears to have been taken from the early ARRL handbooks. A type “57” was used as the first stage in the speech amp, driving a single type 46 tube. The output of the single 46 was then sent into a pair of type 46 tubes in push pull, which used as modulators. Gross had designed the set for use with a carbon microphone, so the resulting audio was anything but “hi-fi”. Still the set did work, and offered a moderately priced entry into the fone band. A pair of type “83” mercury vapor rectifiers completed the rest of the tube compliment on the power deck.

Connection between the two chassis was via a cable running from the power supply to the jack on the rear of the RF deck. A set of “wander” leads were used on the front panel meters to switch in, measure and tune the various stages of the transmitter. A lower power meter on the right was used with the oscillator and buffer sections, with the higher power meter on the left measuring 46 amplifiers in push pull. Keying was made via the front panel jacks, and a set of insulated dummy plugs were used to block following sections when tuning. 

Contrasted with the Collins 32B rig, selling at $125, the Gross was a real bargain. Sadly it appears that few of the CB-25 variants survive today, its unknown if this was result of poor sales, or just the ravages of time.



Live To Air – National NC-101X

Live To Air – National NC-101X finally back together tonight newly installed coil rack, with the bronze bushings and fabricated band indicator. Slides like it’s on glass, super smooth and low effort.

Listening to the melodious tones of the old Gray Hair Net, using Amplitude Modulation, on 160 meters shortwave. Exact frequency is 1.945 MHz. Not bad for a 80 year old radio, I think the original owner W1KEK would be pleased to know that his treasured set, that he bought back in 1937, has returned to the air.

Very pleased with how this project turned out. The audio is very sweet, with some nice wide bandwidth for hifi AM audio. Just waiting for the electrolytic filter capacitors to finish the project.

Replacing Coil Housing Bushings – National NC-101X

Here’s the solution to the sliding coil deteriorated bushing problem on the National NC-101X shortwave receiver. Unlike that other National products with “plug in” coils, like the HRO, the early NC-100 series use a “catacomb” coil assembly that slides inside the chassis to switch bands. A very ingenious design, and it works very well..for 80 years or so, and then the plastic bushings deteriorate from age and use.

The chances of finding new old stock bushings is slim to none, and even if they were available, they’d be just as deteriorated as the ones inside my receiver.
The solution, order a 1/2” to 9/16” bronze bushing from McMaster Carr, machine it down so the outside diameter is .512 inch, press in place using Loctite to retain, ream the ID so the shaft slides through the bushings freely.

I’d like to thank Marc KA2QFX for his very gracious and kind skills in the machine shop, machining the new bushings to fit properly into the sliding coil form.
After two long months, I can finally see the end of the NC-101 project in sight.

Better, Stronger, Faster – Or Rebuilding the National Sliding Coil Housing

National NC-101X Project – Update

Fabricating a new band indicator became a necessity since spare parts for a 80 year old radio are just not something you can find at your local Radio Shack, if you can even find a Radio Shack.

So the old sheet metal, aluminum actually, band indicator, which provides a white dot in each of the band windows as the coil rack moves, was literally falling apart. Given the flimsy nature of the failed unit, I almost wondered if someone made this one.

So, one of the final steps on this project, now that the bushings are installed in the moving coil housing, was to fix or fabricate a new band indicator.

A trip to my local hardware store yielded me some nice 22ga sheet metal to start the project. Thin enough to work with on the bench with simple hand tools, yet rigid enough to last another 80 years. To get the correct form, I straightened the old one out, and used it as a template for the new replacement, scribing around it.

Carefully I trimmed it out, and then used a file for the final shape and deburring. Several coats of white Krylon did the trick.

Better than it was before, better, stronger and faster.

– Bruce

National NC-101X – Replacing the Catacomb Coil Bushings

While this is for the 101X, most it likely applies and hold true for any of the National moving coil products.
What had occurred to this unit, is that the plastic bushings had been brittle, loose and broken over the last 80 years, the coil became unusually binding when I attempted to move it. Concern that I might damage one of the unobtainable coil sets, or the contact fingers, let me to a solution. Since National is no more, I figured I could find a current product, and adapt that to the application. McMaster-Carr is an excellent source of such items, and small hardware, and a quick search of their online catalog found several possible candidates. Since the shipping was more than the $5 parts, I ordered two sets of the plastic and one set of the bronze bushings to try.

One thing I did have to satisfy myself on, was if the coil Assembly itself was a ground potential. Being insulted by the plastic bushings, I wasn’t sure if the coil housing was actually electrically insulated. Having another NC-101 on hand made it pretty straightforward work, I removed the bottom panel and using a ohm meter verified that there was continuity from ground to the coil assembly itself. I was glad to discover this, because it meant I could use bronze bushings without fear of grounding the sliding coil.

Step by Step Removal of the Sliding Coil Assembly

  1. The first step is to remove the bottom cover of the unit, this will expose the moving coil.
  2. Then the cabinet needs to be removed, which necessitates removal of the knobs, and disconnecting the s meter.

  3. The next step is the front rack and gear which moves the coil back-and-forth. You can actually start with this step if you want. There is a nut on the front panel it needs to be loosened and some hardware, including two insulator bushings, that attach a gear to the front panel. You’ll need to remove the top part of the sliding coil housing was necessitates removing six screws, so you can slide the gear up over the housing for removal. With that done, entire sliding coil assembly should lift free of the chassis.

  4. With a cabinet removed on the rear the chassis there are two flat head screws which retain the shaft on which the coil rack slides. Removal of the screws may take some effort, they likely been in there for over a years, and you don’t want to strip them out. The use of an impact screwdriver with the proper size bit, would be recommended.

  5. Once you have the screws loose on the sides, the rod will still be retained has there are two dimples in the chassis which fit into the countersunk ends of the rod. You’ll also note that there is a small braided ground cable, which is retained to the rod via a small machine screw and nut. Be sure to remove these, before attempting to remove the coil.

  6. Once you have the braided cable loose, using a large screwdriver carefully pry out the shaft from the counter sunk ends. It will take some effort, but the shaft will finally pop free.

  7. With the the shaft now for free of the chassis, you can carefully pick the sliding coil rack out from the receiver. I would recommend putting the cover back on the top of the catacomb, just so no stress is put on the fragile coil forms while working with it on the bench.
  8. In regards to the bushings, you’ll likely find that the bushings size is slightly larger any opening in a moving coil assembly. You’ll need to do one of two things, either machine be opening in a coil assembly to a slightly larger diameter to except the bushings, or machine the bushings to a slightly smaller outside diameter, for a tight press fit into the coil assembly. Either choice is acceptable, since the original bushings lasted 80 years, it is unlikely that replacements will be required again, especially if the bronze style bushing as fitted.

T-368 Transmitter Registry – Email Me To Add Yours

T-368 Singing For It’s Supper

Of all the T-368 transmitters known to have been manufactured, and yes there are ways to track these things, only a few are known to still exist.

Email me to add yours to the list!

Total manufactured: 2251
Total known units: 103

I remember visiting Fair Radio in Ohio in the lat 1990s, and seeing stacks and stacks of these sitting inside warehouses, outside rusting away, in various states of repair.