Heathkit AJ-10 AM/FM Tuner Restoration

​My Father's Stereo

In the 1960's, my father was an avid Heathkit builder. He put together many a kit including the AJ-10 Heathkit AM/FM Tuner that I am restoring in this blog. Back then you could build a high quality stereo system from a kit at a fraction of the cost of a fully assembled one. Heathkit builders came from all walks of life and varied electronic technical skills. The Heathkit manual was designed so that people from all skill levels could build a high quality electronic device. A novice could just follow the instructions step and step and complete the build. The manuals also contained circuit theory, schematics, calibration, and detailed troubleshooting steps for the more skilled builder. In additional, Heathkit offered phone support when required.

​The Save

The Heathkit AJ-10 perched atop of my Heathkit SA-2 Amplfier

After my dad had passed away, my mom re-organized her house for simplicity and donated or got rid of a lot of my fathers stuff only holding on to sentimental items. I happened to stop by my mom's house the night before trash collection and noticed that my dad's Heathkit stereo was at the top of the heap. I immediately liberated it from the trash and put if in the trunk of my car. I can see how my mom thought it was trash as it had been sitting in the corner of their basement since the early 1980s when I had used the system as my own personal stereo in my room. My Father had upgraded to a new transistorized Pioneer AM/FM Stereo amplifier that he purchased from Tokyo Shapiro and told me I could have his old stereo. Soon I had replaced the Heathkit stereo with a Lasonic Boom Box which was way cooler at the time. This led the Heathkit Stereo to be exiled into the basement never to see the light of day, until recently.

​About the Heathkit AJ-10

The Heathkit AJ-10 was offered in the 1960-1961 Heathkit catalog as an economy AM/FM Stereo Tuner kit at a price of $59.95. Actually, the AJ-10 enclosure houses a complete and separate AM and FM tuner with the only commonality is that both AM/FM sections share the same power supply. While being boasted as being a AM/FM Stereo Tuner kit it would be more appropriate to labeled as "FM Stereo Ready". An external multiplexer, such as a Heathkit AC-11, is required to separate A and B channels. Despite being marketed as an "economy" model, the AJ-10 did have advanced features such as magic eye tubes for both bands to aid in tuning. In addition, it offered AFC (Automatic Frequency Control) to aid in keeping the tuner locked into the desired FM channel and prevent frequency drift. In AM mode, the AJ-10 had an internal loop antenna and a bandwidth switch to aid in selectivity.

​Restoration

The rest of this blog is dedicated to the steps I took in the restoration of my father's Heathkit AJ-10 AM/FM Stereo Tuner.

​Caution Electrical Hazard

Like most vintage vacuum tube equipment, the AJ-10 Tuner employs high B+ voltages exceeding 100 volts DC which is used to power the vacuum tubes. Care must be taken when working around this equipment while it is powered on with the covers removed. I always wear rubber soled shoes, work on a bench that has a plastic insulated top, and only use one hand when taking measurements or making adjustments.

---

Step 1 Remove Top and Bottom Covers

Turn the AJ-10 Tuner upside down, remove the four screws circled in red then slide chassis out of the top cover.

The bottom cover is secured with two screws circled in red.

Top view of the Tuner chassis.

Bottom view of the Tuner chassis.

​Step 2 Vacuum Tube Testing/Replacement

Unlike solid state transistors and integrated circuits, vacuum tubes have a finite life span. As they age, fewer electronics are released from the cathode making them less efficient. In addition, the tube filaments burn out much like the filament in a light bulb.

All of the vacuum tubes in the AJ-10 Tuner were original. I can tell because they are Heathkit branded.

Test each vacuum tube in the AJ-10 Tuner. Replace any tubes that are defective or weak.

​Step 3 Electrolytic Capacitor Replacement

As electrolytic capacitors age, their electrolyte dries up causing their electrical capacity to drop and leakage current to increase. It is definitely a good idea to replace 50 year old electrolytic capacitors like the ones in the AJ-10 Tuner!

​Step 3a - Restoring the Multi-Section Capacitor

Multi-Section Capacitors, which were aluminum cans containing several discrete capacitors all connected to a common ground, were popular in the 1960s. There were used mostly in the power supply sections of vintage electronic devices. You can purchase replacement multi-section capacitors but they are expensive. I typically rebuild them by replacing their guts with inexpensive discrete capacitors of the same or slightly greater capacitance and working voltage.

View of the multi-section capacitor, circled in red, in the AJ-10 Tuner from the top of the chassis.

You must first un-solder all of the connections at the bottom of the multi-section capacitor before you can remove it from the chassis.

Two nuts, circled in red, hold the multi-section capacitor to the chassis.

The multi-section capacitor removed from the chassis. The side of the aluminum can indicates capacitance and working voltage of the internal capacitors.

The guts of a multi-section capacitor. I use needle nose pliers to un-crimp the bottom of the aluminum can exposing the innards. I always wear nitrile gloves when working with the guts of capacitors as the electrolyte may cause skin irritation.

The new replacement capacitors soldered to the multi-section capacitor base. I use a dab of hot glue to secure them in place.

This is a good time to check the two rectifiers connected to the base of the multi-section capacitor. Resistance should be high in one direction and low in the other when your meter is in Diode check mode.

You must re-solder all of the connections to the base of the rebuilt multi-section capacitor after it is once again mounted to the chassis. I also replaced two electrolytic capacitors, circled to the right, that connected to the multi-section cap.

I use a Dremel with cutoff tool to remove the area from the base of the multi-section capacitor's aluminum can that I bent with needle-nose pliers.

This is how the multi-section capacitor's aluminum can looks like after cutting and smoothing with fine grit sandpaper.

​I use hot glue to affix the aluminum can over the replacement capacitors connected to the multi-section capacitor base.

​Step 3b other Capacitor Replacement

All other electrolytic capacitors should be replaced with one of the same or slightly greater capacitance and working voltage rating.

Here is a picture of the other capacitors replaced in the AJ-10 Tuner.

Step 4 Cleaning and Lubrication

It is important to clean mechanical switch contacts and lubricate moving parts. Cleaning the chassis is not required but adds aesthetics to the AJ-10 Tuner.

I spray contact cleaner into all switches then work the switch back and forth to clean the internal contacts.

I also spray contact cleaner into the innards of potentiometers and rheostats then work them through their full motion of movement.

I use Labelle 107 model train oil to lubricate pulley shafts and other mechanisms. This oil will not harm plastics.

I use Labelle 106 model train grease to lubricate the bearings of the variable capacitors. This grease will not harm plastics.

Q-tips swabs moistened with Windex work great for cleaning hard to reach areas on the top of the chassis.

I also use Windex moistened Q-tips to clean the bottom of the chassis as they have the ability to clean between wire and electronic components.

I used Windex and paper towels to wipe away years of tobacco residue. This started out as a white paper towel.

Mr Clean Magic Eraser works great for cleaning up the original power cord!

I use Meguiar's Motorcycle Wax to bring back the luster of the AJ-10 Tuner's case.

A toothbrush works great for cleaning up the knobs. Plus the AJ-10 Tuner will not get cavities ;-)

The original sticker on the bottom of the tuner enclosure dried up and fell off the tuner years ago. I copied a picture of the connections from the manual and taped it to the bottom.

​Step 5 Testing and Troubleshooting

The Heathkit AJ-10 Tuner Manual has an extensive testing and troubleshooting section at the end of the assembly manual. The FM section of the AJ-10 Tuner comes pre-aligned from the factory and does not require any adjustment. Only a slight bit of alignment needs to be done on the AM section. An RF generator will be required to perform these steps.

Picture of my workbench area.

The first step is to check the power supply voltages against the schematic in the manual to make sure they are within spec. I also check the voltages on each pin of the vacuum tubes to make sure they are the same as on the schematic.

My tuner alignment setup. I use the short wave receiver in the background set to the AM broadcast band to check the frequency accuracy of my vintage Eico Model 325 RF Signal Generator.

You use the screws on the AM turner variable capacitor to adjust trimmer capacitors in order to calibrate the AM dial.

I connect an audio amplifier to the output ports an use test leads for an antenna to perform initial testing to see if I can pickup AM and FM signals.

​Final testing is accomplished by connected the Heathkit AJ-10 tuner to my Heathkit SA-2 amplifier and see how it sounds.

Step 6 Assembly

Follow "Step 1 Remove Top and Bottom Covers" section in reverse order to reinstall the top and bottom covers.

Picture of my newly restored AJ-10 Tuner.

​Conclusion

The restoration of my father's Heathkit AJ-10 Tuner was most gratifying. I think my dad would be happy to know that the tuner he built is still in use for background music in my basement workshop. This project took my mind off of the stresses of my daily job and reminded me of a simpler time when people had the time to sit around and listen to radio for enjoyment.