Project National FB-7 - Conversion of a FB-7 to a FBXA

Click on photos to enlarge.

History of the FB-7 Receiver

This project began after I purchased an rough looking National FBXA receiver from a local radio collector. The FBXA is essentially a National FB-7 with the addition of a crystal filter unit. The "front end" of the FB-7 is rather wide so the crystal filter really helps during crowded band conditions and CW work.

The FB-7s came from National prepped for installation of this filter, the owner had only to remove the 1st IF can, drop the crystal filter unit in its place and solder two connections.

For reasons which escape me, one of the previous owners of this FBXA had drilled multiple holes in the cabinet, removed parts, even spray painted over the black crackle finish. The unit was a real mess but thankfully the National crystal filter unit was complete and undamaged. Whenever possible I try to restore and not part out old radios, but given its rough condition and already owning two FB-7s, I felt comfortable designating this FBXA a parts rig to keep my other two going.

I had a recently restored a nice National FB-7 receiver and entertained the conversion of this receiver into a FBXA, so I was very curious how well the crystal filter unit from the FBXA would work. My plan is to use this receiver for Antique Wiresless Association contests this winter, paired with a 1934 era homebrew transmitter which I picked up earlier this year.

After a month of consideration, I put aside any feelings of guilt and the rough FBXA radio became a donor to supply the crystal filter unit to the FB-7 I had recently restored. So on a rainy Saturday afternoon in September, the FBXA went under the knife - actually the soldering iron.

So It Begins

The freshly restored National FB-7 prior to starting the project.

I currently own three FB-7s, but the one which I restored did not have the required opening in the side cabinet for the crystal filter switch. You can see by the photos that the radio on the left has the slot for the thumbwheel used for selectivity adjustment, but it does not have the small opening in the middle of the cabinet for the three position switch.

Using the second radio, which does have such an opening, I created a template to modify the first radio. Normally the modification of a 70 year old radio is a sacrilege to me, but in this case the modification was an accepted factory upgrade to the FB-7 so I felt I was doing justice to the fine radio. I went about the process in a very methodical way as I wanted to make certain that I did not do a disservice to this old timer and desired to make my workmanship undetectable from a factory job.

Once cabinet side was center punched, I carefully drilled and then deburred the opening with a hand file. After the filing, with the hole now looking like it was machine punched, I used a flat black paint on the bare metal of the opening to match the cabinet. This took some time, but once done, you could not tell my handiwork from a panel stamped out by National.

The upgraded FBXA crystal filter unit mounts in place of the 1st IF can on the far right of the receiver.

This is the view before I removed the IF can. For clearance reasons, this can needs to be removed prior to drilling the cabinet. It looks rather packed in there, but it does come right out.

The underside view of the FB-7 before removal of the IF can.

To remove the IF can, turn the receiver upside down on a padded surface. Now take off the bottom cover for the receiver and locate the red, black and blue wires that enter the underside via a rubber grommet from the IF can. Unsolder these three wires and straighten them for removal. Now look for two small 1/4" nuts on either side of the IF can. Remove these nuts and small lock washers below and the IF can should fall out into your hands. 

The crystal filter was a factory option so the FB-7 chassis is factory drilled and prepped to accept the crystal filter unit.

This is the overall view of the FB-7 underside, well worth a closer look, pretty simple compared to today's solid state rigs. When was the last time you saw resistors, the tubular white units, hand lettered with a pen? Overall this unit has survived the last 70 years quite well, its is free of modifications and sloppy workmanship, which made my restoration that much easier.

With the IF can now removed, you can see the mounting holes used for the crystal filter unit. You can see my newly added opening in the photo to the right.

With the opening in place, filed and painted to match the cabinet, I next removed the vacuum tubes and shields around the opening to allow plenty of room to install the National crystal filter unit.

Here is the removed 1st IF can, saved for future use.

The National FBXA crystal filter unit as removed from the donor radio. I carefully cleaned and inspected this unit prior to installation. The crystal sits on the front of the unit, the selectivity control is on the rear and the three position switch on the side extends through the cabinet.

The bottom view of the National FBXA crystal filter unit as removed from the receiver. The switch on the of the unit has three positions. The counterclockwise position, as viewed from the side of the receiver, places the crystal in parallel. The middle switch position connects the crystal in series for "single signal" reception, ideal for CW work. The fully clockwise or right position removes the crystal from the circuit, and restores operation like the plain FB-7, ideal for AM reception.

This particular unit had a problem with the air variable capacitor used in the selectivity control. The shaft was binding and required a great deal of effort to rotate. I would imagine that this problem likely this has occurred for a lack of use as I understand the unit had been sitting unused for at least 2 decades!

To rectify the binding, I needed to remove and disassemble the air variable capacitor. Removal from the housing was a snap, there was only one large mounting nut on the top retaining it. With the nut removed I could now move the air variable out of the housing and desolder the two leads attached to the plates. Now the unit was free to be placed on the bench and inspected.

Surprisingly, the air variable capacitor proved somewhat easy to disassemble for cleaning and lubrication. The construction is really first rate, with individual adjustments for plate spacing on each side, they just don't build them like that anyone.

One needs to be careful not to bend or distort the plates during disassembly process. Once I had removed the two flat head screws that retained the rear stator plate, I was able to pull the rotor shaft from the unit. Before removal of the rotor unit I inspected shaft and had to deburr two small gouges at the end where the knob set screw had rested. With the shaft removed, I used a small round file to clean the shaft bushing on the front of the unit. It seems this bushing had shrunk over the years, it appears to be made of some sort of plastic material, and the shrinkage had created the binding issue. After a light clean up with the file, a touch of lubricant on the contact points was applied, and the unit reassembled.

Before reinstallation into the filter unit I checked my work with a ohmmeter while rotating the air variable as I wanted to be certain that none of the plates were touching. With a successful test, I soldered the two leads back onto the air variable and installed it back into the crystal filter unit housing.

The cleaned and lubricated air variable read for installation back into the filter unit, note the two fixed mica capacitors on the front and rear of the unit.

Here is the National FBXA crystal filter unit installed in the former FB-7 receiver.

Installation was quite straightforward, the unit must be tipped away from the panel to allow the mounting studs to enter the chassis. Once the studs have entered the chassis, you can now tip the crystal filter unit to the outside panel and install the retaining screw on the rear panel.

The crystal filter unit mounts with three bolts that are attached to its housing and stick down below the deck. A flat head screw secures the unit on the rear panel. Only two connections, a red and a blue/green wire need to be soldered to install the crystal filter unit. Use one lock washer and one 1/4" nut under each of the crystal filter unit mounting bolts.

With the transplanted crystal filter unit in place, I soldered the two wires from the unit into the receiver, installed the tubes, tube shields and grid connections.

One thing I noted on the crystal filter unit after installation. The switch does not sit in the absolute center of the opening on the side of the cabinet.

I thought perhaps I had made an error in my template and doubled checked my other two FB-7s. Both have the switch sitting to the lower right of the opening.

After some consideration I realized that National had perhaps done this intentionally as the crystal filter unit needs to be tilted for installation and the extra space at the top allows the switch shaft to fit through the side of the cabinet.

I carefully installed the receiver into the receiving position. The National FB series are simple receivers and need only power, antenna and speaker lead connections.
My FB-7 has the 2.5 volt filament tubes and fortunately I had a National "doghouse" power supply that would do the trick. As this was a Depression era receiver, many hams homebrewed their own power supplies as a cost saving measure. The power requirements are rather simple, 2.5 or 6.3 volts for the filament and 180 volts of B plus. 

Unlike the later HRO series, National made no provision on the rear panel to interrupt the receivers B plus supply, instead the operator must throw the left toggle switch on the front panel to break the B plus for transmission. The right toggle switch serves to turn on the BFO for CW reception.

It has taken nearly 3/4 of a century since it was first sold, but this FB-7 has now graduated into National FBXA.

The moment of truth at hand, I flipped on the power switch and the receiver came to life. After some tweaking of the dial, I was soon sipping my coffee and listening to New England AM group on 3885 KHZ.

All in all, not a bad project for a rainy Saturday afternoon.

The W1UJR station in Wiscasset

The National FBXA receiver paired up with a E.F. Johnson Valiant transmitter.