WhitakerAudio
 
New Project – AM/FM Stereo Tuner

This page documents the design and refinement of an AM/FM stereo tuner. The log below traces development of the tuner, shown on the right in finished form, from planning through construction and optimization.

AM/FM stereo tuner

Background

Finding the right components today for a tube-based AM or FM stereo tuner is a real challenge. A typical radio frequency (RF) product uses a number of high-frequency transformers and coils. Finding replacements for those devices today can be extraordinarily difficult. The coils could be custom-built; however, that raises both the complexity and cost of the project. I could also “roll my own,” but I’m not confident of success (or at least not yet).

Because of these considerations, the concept here will be to utilize the RF transformers and coils from old receivers and build the rest from new parts. This project will utilize printed wiring boards (PWBs) and ultimately wind up as a show-quality product. As currently planned, the unit will have the following general characteristics:
• AM tuner
• FM stereo tuner
• "Magic Eye" tuning indicator
• Styling and features similar to the other projects described in the book and on this site

The starting point for the circuits will be the following tube-based Heathkit products: BC-1A AM tuner, FM-3A FM tuner, and AC-11 FM multiplex adapter. These products are commonly available on eBay and from other sources. They seem as good a place to start as any. The coils, RF and IF transformers, and variable capacitors will be harvested from the old products and used in the new receiver. This project is challenging in that the mechanical layout is important to success and the front panel tuning dial display is not easy to implement. Having said that, there is no real reason to build this project other than it is a challenge.

Heathkit AM tuner

The old Heathkit turners are shown here; the AM radio on the left and the FM radio on the right. Notice the IF transformers and other coils, which will be repurposed in the new tuner. The tuning capacitors will also be reused. Everything else will be built up from new components. The old tuners are non-functional and no effort will be put into getting them to work, since I'm just looking for a supply of hard-to-find parts.

Heathkit FM tuner

A preliminary front panel design has been done that provides for individual AM and FM tuning controls, plus the other necessary functions. The tuning dials will be simple in nature. Each tuning knob will drive a 6:1 reduction device coupled to the variable capacitors. Current plans call for the same chassis size as the stereo preamplifier (17 inches wide by 10 inches deep), although it may be necessary to use a larger size chassis (14-inches deep) to accommodate all of the components.

The original concept was to pull power (heater and B+) from one of the stereo amplifiers, as is done with the stereo preamplifier. This will not be practical, however, for situations where the preamp is used—which is assumed to be a common scenario. Therefore, the better approach will be to include an integral power supply.

Update 1

The front panel has been completed, and I have arrived at a tuning scheme for the unit. The 6:1 drive reduction device works well and provides for a simple tuning mechanism that does not involve pulleys and string. The panel is shown below.

Mock-up of front panel

A new version of the panel has been ordered from Front Panel Express. In what should be the final version, the AM Tune and FM Tune controls are moved toward the edges of the panel by an inch or so, and a push-button power switch has been added.

Update 2

Considerable progress has been made on the tuner project. A complete parts lists has been developed and all parts have been ordered. Very few changes are being made in the basic Heathkit design, except with regard to the power supply, which will be similar to the supply used in the 20 W stereo amplifier. Because of the number of components, a larger chassis will be used (17-inch by 14-inch by 3-inch).

As mentioned elsewhere on this site (and in the book) I have used ExpressPCB for production of circuit boards. The company provides software to design the PWBs, which works quite well. They also offer circuit design software (ExpressSCH) that makes creating schematics relatively easy. The program is intuitive and links with the PWB software to assist in board layout. For previous projects I have used Adobe Illustrator, which produces excellent results but also can be time-consuming to work with. All of the schematic drawings of the tuner circuits are being done in ExpressSCH.

Update 3

All components have been collected for the AM/FM stereo tuner. The parts repurposed from the Heathkit AM and FM tuners have also been harvested from the old units. The coils and IF transformers are in good shape and should work well for this project. It is important to document the pinout of these devices as they are removed from the original equipment; the Heathkit assembly manuals are a considerable help in this regard.

As mentioned previously, the tuners were non-functional when purchased on eBay. Having looked closely at both units now, the failure modes are apparent. On the AM tuner, the main power supply filter capacitor had failed, which caused related damage to the supply. On the FM tuner, a low-voltage electrolytic capacitor in the ratio detector stage had failed. There may have been other problems with these units, but these are the most obvious.

The original Heathkit mechanical design for the BC-1A and FM-3A tuners was quite good, albeit camped. Given the space limitations, assembly would have been a challenge—a recognition of the skill of the builders back in the day. The main chassis for the tuners are beautifully done.

Update 4

The circuit board for the AM tuner section (and power supply) has been completed, as shown below. The board measures approximately 7-inches by 9-inches. Spacing of components is tight, but quite workable. Performance tests are pending.

AM circuit board

The AM tuner uses four tubes, plus a power supply rectifier and a "magic eye" tuning indicator. The coils and IF transformers harvested from the old Heathkit AM tuner can be seen in the photograph on the left.

As with the other projects, this one uses a two-sided printed wiring board (PWB). Components mounted external to the board include the power supply transformer and choke, and the tuning capacitor, a 3-gang device and associated trimmer capacitors.

The same tube mounting scheme used in other projects was used here as well, as shown in the photo on the right.

AM circuit board, foil side

Update 5

The circuit board for the FM tuner and stereo multiplex section was completed today, as shown below. The board is slightly larger than the AM PWB. It is somewhat crowded, but still manageable. Performance tests are pending.

FM circuit board

The FM tuner uses six tubes. The stereo multiplex section uses three tubes. As with the AM tuner, coils and RF transformers were taken from old inoperative units and re purposed for this project. Mounting of some of the devices presented a bit of a challenge, but the end results were good.

All adjustable coils and RF transformers can be accessed from the bottom of the unit. Test points have been identified on the board for alignment.

The FM tuner PWB is self-contained, except for the power supply, which is provided by the AM tuner PWB.

FM circuit board, foil side

Update 6

Construction has been completed on the AM/FM stereo tuner. Initial tests on the AM tuner section have gone very well. One of the features highlighted in the original Heathkit BC-1A tuner was wide audio frequency response. Based on first tests, the AM tuner excels in this category. The promotional literature for the BC-1A claims "FM-like sound" from AM. I was skeptical; however, this device delivers on audio performance.

The evolution of AM receivers from wide audio response to narrow audio response has been driven by a number of factors, first among them the increasing numbers of AM stations. As interference increased, receiver designers responded by limiting the audio frequency bandwidth of the output signal. This tuner was designed to take advantage of the high-fidelity broadcasts of the 1950s. The results are quite impressive.

The front-end and IF stages tune-up as specified in the original Heathkit manual, which provides a detailed procedure for broadbanding the IF circuits. A simple tune-for-peak approach will not result in the best frequency response from the tuner. So, while the procedure is time-consuming, it is well worth the effort.

Update 7

Initial tests on the FM tuner have been encouraging.

The front-end and IF stages were aligned today per the original Heathkit manual. Adjustment of the IF transformers resulted in a dramatic increase in gain over the initial settings. This increase in gain led to oscillation, which seems to be centered at the first IF stage. A check of the PWB implementation revealed no obvious issues. The next step in troubleshooting the oscillation will focus on power supply decoupling. There are some long traces on the board for the B+ supply, and so this seems like a good place to start. One option, of course, is to accept a lower amount of gain through the IF stages, but that's a poor choice.

Update 8

The FM tuner oscillation issue was solved with additional decoupling of the power supply traces and the AGC trace on the PWB. In addition, shielded probes were used for signal injection and voltage measurements. When tuned for peak response, the tuner is now quite stable. In addition to the RF and IF stages, the stereo multiplex circuits tuned up as expected.

As a way of increasing the accuracy of adjustments on the FM tuner stages, an aluminum shield was constructed that covers the bottom of the chassis (in addition to the bottom plate). Holes were punched in the shield to allow access to the RF, IF, and multiplex adjustments with the shield in place. This was particularly helpful when making adjustments on the FM RF circuits.

Update 9

The tuner was completed and placed into service today. Performance of the AM section is quite good, with high sensitivity for distant stations and wide frequency response for local stations. The FM tuner also performs well, although sensitivity leaves something to be desired. An attic-mounted antenna with a mast-mounted preamplifier is planned for the FM tuner to increase sensitivity for distant stations. All things considered, the project turned out very well.

AM/FM tuner front view

The AM/FM stereo tuner keeps the styling of the audio amplifiers already built, including the exposed tubes and acrylic cover. Separate AM and FM tuning controls are provided, which simplified construction and moreover provides for convenient operation.

Under the chassis, two circuit boards are used—one for the FM tuner and stereo demultiplexer, and the other for the AM tuner and power supply. Nearly all components are mounted on the PWBs.

Parts repurposed from old tuners are the tuning capacitors, IF transformers, and RF coils.

Tuner bottom view

Heat given off from the unit is relatively small, permitting it to easily be placed on a shelf. A minimum clearance above the power transformer of about 9 inches is recommended.

Update 10

Not content to leave well-enough alone, I decided to compare the performance of the tuner with the original Heathkit designs. One could reasonably assume that the Heathkit implementation represents the peak performance possible from the circuit, since many units were built over a period of many years. In order to make the comparison, I purchased from eBay another BC-1A AM tuner and FM-3A FM tuner. I checked all of the tubes in each product and ran a complete alignment, making repairs as needed. Once aligned and operating, these units served as the "reference" against which the performance of the new tuner would be judged.

The performance of the new AM tuner was nearly identical to the BC-1A reference. The performance of the new FM tuner, however, did not match the capabilities of the FM-3A. Gain through the IF stages was only about 20 percent of the reference. The input RF stage performed as expected, but was still subject to occasional oscillation when tuning.

Confronted with these results, I took the unusual (and expensive) step of designing and building a second version of the FM tuner board, as shown below. In the new layout, lead lengths were reduced by reorienting some of the tube sockets 180 degrees. In addition, the tuning capacitor was integrated onto the circuit board, which complicated the physical design but made for a more compact implementation.

Second generation FM tuner board

The redesigned PWB is shown on the left, with a close-up of the RF tuning section on the right. Note that the tuning capacitor mounts directly on the board, which provides for a more controlled operating environment.

The major layout constraint was that the new board had to physically fit in the chassis built for the old board. This meant that the tube sockets could not move (they could be turned 180 degrees, however) and the tuning capacitor shaft needed to be in the same position. These constraints proved to be easily accommodated.

Close up of FM front end

After the new board was completed, I dropped the old board out and inserted the new board. It was a straightforward and simple replacement. This project points out one of the benefits of using standardized board designs. The ability to drop out an entire circuit board and replace it with a new one is a very useful feature.

Performance tests on the new board are still underway, but initial results are very encouraging. The problem with low IF gain is solved in the new design. The gain at the input to the IF chain is identical between the new tuner and the FM-3A. The oscillator functions well and is quite stable. Additional work is needed on the first RF amplifier stage, but it is not a major issue. The specific problem is that the trimmer capacitor used to optimize the load of the RF amplifier will not peak as installed, even when run out to minimum capacitance. Adjustments to the tank are planned as a way of addressing this issue.

The RF stage work notwithstanding, the FM tuner performs well on strong signals, and reasonably well on weak signals.

Update 11

Touch-up tuning on the FM front-end and the addition of a shield between the tuning capacitor and the RF amplifier and mixer stages resulted in a considerable improvement in performance. With the changes today, the performance of the FM tuner is equivalent to the FM-3A reference design. The only problem remaining is oscillation at the high end of the FM band. A more extensive shield has been designed that better isolates the tuning capacitor, RF input, and oscillator/mixer stages.

Performance of the tuner at low- and mid-band is quite good on distant stations.

Update 12

As noted in Update #11, a more extensive shield was identified as necessary for the RF front-end of the FM tuner to control oscillations. The shield was constructed out of an un-etched copper-clad circuit board. The material was readily available and easy to work with.

FM front end shield

As show in the photo, the shield encloses the tuning capacitor and oscillator coils. It also completely encloses the RF input, oscillator, and mixer circuits. The shield mounts on standoffs connected to the tube sockets, which affords a good connection to ground and eliminated the need to drill additional holes in the FM tuner PWB.

Installation of the shield eliminated the oscillations that were experienced at the high end of the FM band. As expected, retuning of the RF input trimmer capacitor and first IF transformer were required after installation of the shield.

With this modification, performance of the FM tuner matches the reference Heathkit design.

Update 13

The tuner has been in service now for a number of months, with good results. I have noticed, however, some sensitivity to vibration. Exploring this more closely, two of the FM tuner IF transformers are somewhat microphonic. After some investigation, the solution was to refurbish the transformers themselves.

For any IF transformer in an FM tuner, there are primary and secondary windings and associated capacitors that form a tank circuit on the input and the output. Since 10.7 MHz IF transformers for vacuum tube circuits are no longer manufactured, and appear to be unavailable as NOS devices, the only practical sources for these components are old tuners. At the beginning of this project, I began with inoperative AM and FM tuners manufactured by Heathkit in the late 1950s or early 1960s. Prior to reusing the FM IF transformers, I cleaned the devices but otherwise did no further work on them. Faced with the microphonic issue, I investigated further.

The problem was traced to the capacitors used in the primary and secondary circuits. As is common in IF transformers, the capacitors are enclosed within the metal shield. In this particular device, each shunt capacitor was formed using extensions of the metal connection posts, placed one on top of another and separated by a thin dielectric (mica or something similar). This construction has the virtue of simplicity, but is also subject to several shortcomings, which include: 1) sensitivity to vibration, 2) sensitivity to temperature changes, and 3) detuning over time as the insulator becomes dirty or otherwise deteriorates.

The solution was to eliminate the "mounting clip" capacitors and replace them with high quality discrete mica capacitors. Using a bridge, the value of the "mounting clip" capacitors was measured at about 110 pF. The closest available standard value was 100 pF, which was used. Given the age of the transformers, considerable care was required to avoid damaging the coil leads inside the can.

Initial tests on the re-manufactured IF transformers were completed today on a testbed, with excellent results. Stable, broad peaks were found on both the primary and secondary, and the microphonics problem had disappeared.

Update 14

The new (rebuilt) IF transformers were installed on the FM tuner PWB today. As described above, the rebuilt transformers include replacement tank capacitors. In addition, a better method of grounding the shield can was implemented. Other changes included a simplified aluminum shield for the tuner front end and a slightly different mounting method for the tuning capacitor.

Improved FM front end shield

The revised board is shown in the photos (left and right). The improved front-end shield consists of two aluminum pieces. One fits over the circuitry of the input RF amplifier and mixer/oscillator stages; the other other wraps around the tuning capacitor and associated components.

The revised mounting of the tuning capacitor provides a more secure physical connection to the circuit board, and better grounding of the frame of the device to the component-side ground plane.

FM tuner board

Update 15

From time to time I get an idea for an incremental improvement in the tuner. As documented above, the individual changes are small, but they add up—particularly with regard to performance and stability. In addition, I have tried some new alignment techniques on the FM tuner that are aimed at optimizing performance. Rather than using a fixed frequency generator to align the IF stages, I acquired and put into service a sweep/marker generator, which greatly aids in broad-banding the FM IF circuits.

AM/FM tuner with tube shields in place

In earlier versions of the tuner I resisted using shields on the RF and IF tubes on the FM side. This decision had more to do with appearances than anything else. Recently, however, I discovered some interesting tube shields, available in various colors. (I picked red for the tuner.) For more information on the shields, see the Antique Electronic Supply Web site.

The shields fit well on the chassis and add an interesting look (see left). A bottom view of the completed tuner is shown at right.

Chassis view of second generation AM/FM tuner

Update 16

The cost of building any project is an important consideration for the audio enthusiast and hobbyist. The focus of this AM/FM stereo tuner project was to optimize the various elements that went into the product. Cost was a secondary consideration. Still, an effort was made to accurately determine the bill of material (BOM) costs and the time needed to build the tuner.

The total BOM (including shipping) for the AM/FM stereo tuner was $2,500. The major cost centers included:
• Electronic components = $750
• Printed wiring boards (PWB) = $800
• Tubes = $200
• Decals = 100
• Front panel = $150
• RF/IF components from old tuners = $200
• Plexiglas cover = $150

Builders could eliminate the PWB cost by using hand-wired terminal strip construction. Other potential areas of cost reductions include the back and top-side decals, and the Plexiglas cover. Note that the costs listed above do not include the first FM tuner PWB and components, which were replaced as described above. The costs shown here are what it would take to duplicate the final design.

The total time required to build the AM/FM stereo tuner—from ordering parts to completing performance measurements—was approximately 45 hours. Here again, this estimate is based on what it would take to duplicate the final product.

So, some final thoughts are in order here. Six months ago I set out to build a high-quality AM tuner and FM stereo tuner. That objective was achieved. It took considerably longer and was considerably more expensive than planned, but that is not anything really new. I have found with experience that whatever my initial cost and time estimates may be, they will likely end up being doubled before the project is done. You may have found a similar multiple in your own estimations.

As I mentioned in the first installment of this log, there is no real reason to build this tuner other than it is a challenge. It was that indeed. But the end product was worth the effort and the cost. And I learned a lot in the process.


Update 17 (January 2017)

Detailed step-by-step documentation is provided in a 166 page User and Assembly Manual that is available for download. If you are familiar with the Heathkit assembly manuals of the past, the approach taken with this new manual should seem comfortable. For those who want a printed version, it is available for purchase on Lulu—a web site for specialized, print-on-demand documents. (There is charge of $14.95 for the printed manual; see "AM/FM Stereo Tuner: User and Assembly Manual." on the Lulu web site.)

In addition, a ZIP file is available for download that will assist in building this tuner. The ZIP file contains the following individual files:

• Schematic diagrams of the AM and FM circuits as ".pdf" (Acrobat) files
• Bill of materials for the tuner as an Excel file
• Printed wiring board layout files for two boards as ".pcb" ( ExpressPCB) files
• Front panel layout as a ".fdp" ( Front Panel Express) file
• Chassis layout, bottom view, as an Acrobat file
• Chassis layout, top view, as an Acrobat file
• Drill pattern for the chassis as an Acrobat file
• Layout for the acrylic trim piece as an Acrobat file

Note that the files above are provided as-is. Every effort has been made to make sure they are complete and accurate, but no warranties are expressed or implied. Builders are encouraged to double-check the information contained in the above files prior to proceeding. For the front panel layout, users can customize the text as desired; e.g., "Built by John for Mary, January 2016."