These Realistic receivers were made in Japan by Foster Electronics for Radio Shack/Tandy. A number of these such as STA-225 and STA-90 are similar in layout and design to the QTA-770. All the power amplifier stages are similar with only a few minor component changes. The tuner board - a nice 4 gang FM and 3 gang AM design is common to he STA-225 & QTA-770. This is a very sensitive module and works well even without an antenna attached. Overall these are well designed and built items (better than later Realistic stuff from Korea) - and offer above average performance.
This unit was picked up on eBay as a faulty item in need of repair. The seller had packed the item extremely well and survived its encounter with Aussie Post with no visible damage. Coming interstate these heavy receivers can take a right pounding during transit. One thing that was immediately obvious was the strong smell of this item. Not of electronic origin but of tobacco smoke and tar. Yuk! Anyway before even powering up this unit I removed the timber cover and steel bottom cover. This was fortunate as 2 miniature wood screws from the grill fitted to the timber cover has come out in transit. Turning to unit over the screws fell out and saved me from unnecessary repair work. Now the unit was given a complete inspection by eye. I was looking for cracked PCB's, burnt components or bulging capacitors and anything not looking well. From previous experience with these Realistic's my attention was drawn to the power supply board. Immediately apparent was 2 distended capacitors, a few burnt and discoloured resistors and brown glue on the PCB. Also noted was a discoloured resistor and surrounding PCB on the preamp/CD-4 board. The other boards appeared OK. The only other item of concern was the front panel dial lamps housing or reflector. This is made from heavy treated paper but after 35 years is burnt from the 4 panel lamps and starting to fall apart.
2. Action Plan
So an action plan was formed. Before powering up measure each of the 8 output transistors for shorts and good junctions. Then check and repair the power supply board. Followed by audio checks through the preamp and power amp stages repairing as I go. Then on to chassis items such as lamp replacement. Finally reassemble the unit and attend to cosmetic restoration of the front panel, knobs and timber cover.
3. Power Supply Repair
Below is a photo of the power supply PCB. This is mounted vertically between the two mains supply filter capacitors and the tuner board. The PCB is retained with two long self-tapping screws and two nylon spacers. Once the screws are removed the PCB can be careful lifted up to the extent of the attached wires. The PCB mounts bridge rectifiers for the main rails plus two voltage regulators using TO220 case transistors on small heatsinks and various other capacitors and zener diodes. The photo shows the repaired board with the darker blue capacitors being new replacements. Both the power transistors were removed for checking. One for the phono and tone control regulated rail was found to be completely open circuit on both junctions. I noted that both heatsinks had virtually no thermal grease - it may have evaporated over the years. I replaced the failed transistor and its mate with a slightly higher rated item. Also cleaned and filed the aluminium heatsink flat under the transistor and re-assembled with new thermal grease. The two distended capacitors and two others (the dark blue items in the photograph) were also replaced. When these items were made use of contact adhesive to mount capacitors was common. Over time and exposure to heat the glue darkens to chocolate colour. It also becomes hydroscopic and corrosive and attacks any metal in contact. This glue was removed before the new capacitors were installed. The large axial capacitors from previous experience are OK and not changed. I was now confident to apply power to the unit and test the power supply. All was well with the regulators now working. I plugged in my headphones for my first listen - unfortunately I only had sound on the right channels. So now onto the audio circuits.
4. Circuit Diagram
At this stage I needed a circuit diagram. An internet search revealed that no one seemed to have the diagram for this unit. Therefore I sat down and began the long process of drawing my own. Luckily I already had parts of the the diagram from other Realistic units in my collection. The tuner board for instance in identical to the STA-225 receiver. The power amplifier turned out to be similar to that used in many units including STA-90 and STA-225. These units are of similar age and are probably the work of the same design team at Foster Electronics at the time. Unfortunately being a Quadraphonic receiver the QTA-770 is fairly complicated with many boards, front panel switches and controls and heaps of interconnecting wiring. More than twice the content of a typical stereo version. These units must have been nightmares for the workers during assembly. For example one STA-225 that I restored I found that the tuner pointer lamps which switch from white to red with stereo reception didn't work. The lamp driver transistor had been installed in reverse from new! Anyway I started with the easier parts - tracing out the power supply and distrubution to the various boards. The power amplifiers next noting the minor variation to the STA-225 for which I have an original Radio Shack manual. The preamp was next and an overview (rather than an in depth) drawing of the complicated source and various quadraphonic mode switching. Life is too short... Then I tackled the CD-4 decoder. This is a separate board located on the underside of the unit. Rather than waisting too much time on this board I searched for CD-4 decoders and came across 2 other units. One from JVC (the inventors of CD-4) and a Marantz stand-alone decoder. Both had almost identical circuits diagrams. A look over the Realistic board soon revealed that it too was based on the same design. Easier than re-inventing the wheel. Once hand drawn across many pages of scrap paper I entered my findings into Visio. This makes for some nice diagrams and added together makes a valuable resource. Once the diagrams were complete I began audio testing and repair.