In addition to tube-type communication gear, I occasionally enjoy working on old tube-type entertainment systems. This Stromberg-Carlson model SP-965-C "Integrity Series Component Ensemble" was a top of the line piece for 1960-61. The cabinet style is Italian Provincial according to the Stromberg-Carlson brochure.
Repairs of the amp
The ASR-444 amp is a bit difficult to remove from the cabinet and has to be done carefully to avoid damage. Carefully placing the whole cabinet on its face will let gravity help with the task.
All of the switches and controls were cleaned with deoxit. As usual, I did unpowered checks of the resistance in the B+ line, continuity checks of the output transformers, and checking for any line to chassis leakage. This amp had a factory installed fuse rather than the circuit breaker shown on the schematic. Schematics for both the amp and tuner are glued to their chassis bottom covers.
The amp power supply uses diodes and a voltage doubler. Two of the electrolytics had been replaced earlier in the amp's life. I used a variac to carefully reform the doubler caps to the voltages specified on the schematic while monitoring both sides of the doubler to assure the caps were evenly dividing the voltage.
The amp worked on both channels but with noticeable distortion. After some voltage checks, I replaced all 8 tubular coupling caps. Some distortion remained. The cause was insufficient voltage in the phase inverter circuits. Each of the 330K plate resistors for 6U8 pentode sections had climbed to over a megohm. Since the pentode plate is direct coupled to the grid of the triode section of the 6U8 used as phase inverter, the distortion cause was obvious. A couple of other resistors were also replaced. After repairs, the amp performed as expected.
Repairs of the tuner
The tuner needed a good cleaning of its slide switches. It then worked decently on both AM and FM, but I was not satisfied with the sensitivity on the AM side.
Repairs of the speaker systems
The 3 way speakers are quality pieces as well. Testing showed the mid-range driver on one of the speaker systems was open. Thinking the mid-range driver was probably permanently dead, I pulled it from the speaker cabinet and removed the bolted-on magnet structure and the dust cover. The aluminum voice coil had come unglued from the cone and had broken a voice coil wire at the glue point. I carefully scraped and cleaned about an eighth of an inch of the wire and soldered on a new piece of wire between the break and the point on the cone where the flex wire is attached. After resetting the voice coil to its proper position, I used a bit of cyanoacrylate glue to re-attach the aluminum to the cone. After the glue cured, I used clear fingernail polish to firmly fix the replacement wire to the cone and also added some of the polish to beef up the voice coil to cone joint. The dust cover was also re-attached using the fingernail polish. Luckily, there is enough play in the bolts that attach the magnet structure to the speaker body that I was able to re-attach the magnet so that the voice coil could move freely. I tightened the bolts at that point and put the driver back into the speaker cabinet for a successful repair.
I pulled the RC-88/4b Garrard changer. After a much-needed thorough cleaning, I tried the various speed settings. Although the record changing function slowed the turntable down, it did not stall but kept working. I lubed the partly-dried grease a bit. I noticed it was relatively quiet in operation. I suspect the changer has not been heavily used. Both channels were functional. The diamond needle did not appear to be worn. I decided it was probably OK to place it back into the console for more thorough tests later. It proved later to perform properly. The stylus pressure and tracking needed some adjustment.
Schematics and lots more Stromberg-Carlson info
The schematics, brochure, and lots more Stromberg-Carlson information can be found at Tom Bavis' website. Thanks, Tom.
This Stromberg-Carlson, with its 30 watt per channel amp and matched speakers is a powerful and clean music machine. It is an excellent sounding system. It has lots of head room. I doubt anyone would use even half of its audio power.
The Stromberg-Carlson has retired to Florida.
A Knightkit T-60 transmitter was the previous item on the bench.