I was lucky enough to get to work on a very nice Scott 233 stereo integrated amplifier. This example was wonderfully well preserved, coming out of a storage unit looking like it had just left the showroom floor. Unfortunately, though, the electronics weren’t in such great shape after sitting for so many years and this amp’s owner reported it would start to smoke when powered on. A bit concerning of a start!
Underneath it was all original, with Ceracap sealed paper capacitors, and a handful of electrolytic and ceramic caps.
Checking all the iron, everything seemed in order, so it was on to a component replacement.
There’s not enough room under the chassis for a full set of terminal strips for the electrolytic capacitors, so I fitted this amplifier with CE Manufacturing new manufacture twist-lock capacitors.
The second can cap had a bit more wiring.
There was enough room for the 100 uF 75V caps used for filtering the bias voltage; the original early bias rectifier diodes were also replace with 1N4007s.
All the tubes tested good:
Time for the first power-up. Success! Sound and everything. There were a couple of problems, though. Immediately at low volumes, there was a bit of a channel imbalance. Shown here at the input to the driver/phase inverter tube for each channel with the good channel in yellow and the weaker channel in blue, with a 1 KHz test signal.
Working backwards, though, the signal was fine – with the only item in between being the compensation network on the volume control.
The failure traced to one of the 0.02 uF ceramic coupling capacitors was fading, going low in value. For good measure, I replaced both channel’s 0.02 uF capacitors, both ceramic 47 pF capacitors in the compensation network, and tested the resistors which were in-spec. One couldn’t be saved, though, during the operation and was also replaced. The larger were replaced with CDE film caps, and the smaller with CDE mica capacitors which are nearly perfectly stable over a range of temperatures and bias voltages.
Much better, now both channels get an equal signal at the inputs to their driver stage:
Total power output is very, very uneven though. While one channel plays well, the other caps out at about 1W of power and won’t go any higher. With the oscilloscope, it showed that the driver stage of V3/V103 wasn’t properly amplifying; the yellow trace demonstrated the good channel while the blue the channel lacking output power.
I spot-checked and replaced a couple of components which seemed questionable, but overall the DC voltages were correct for both channels, and those turned out not to be the root cause.
It’s tough to track down the specific issue with the feedback loop involved, so I disconnected the feedback. With no feedback connected, both channels rose in power output equally, so the trouble had to be in the feedback circuit.
Ultimately, I opted to replace all of the components in the feedback loop, including a 68 Ohm resistor, 8.2K resistor, 270 pF capacitor, 270K resistor, and the 4/250 capacitors for a second time. Once again, I used mica capacitors in the feedback loop for their precision and stability, and replaced the electrolytic screen filters with film capacitors for good measure.
Succcess! One of the small ceramic caps had failed, making the feedback circuit fail to operate correctly. With all those components replaced, it worked great on both channels.
Time for biasing. This unit has both DC balance, and DC bias controls. The idle current adjustment is set with the DC bias control. Scott specified 220 mV with no signal originally, but for an additional safety factor with today’s higher line voltages, I set this to 200 mV. One channel was low, at 125 mV and the other high at 320 mV to start.
The DC Balance adjustment relates to the relative bias of the driver tubes, and should be adjusted for lowest distortion, but it also interacts with the bias control so I needed to measure both distortion and voltage output at the same time to a high degree of precision. For the first time, the Audio Precision DCX-127’s voltmeter came in handy here. It’s calibrated and accurate, just like the Keithley, which I used to measure the bias voltage while adjusting for lowest distortion, then re-adjusting bias.
Shown here measuring 0.69% THD to start, which adjusted down to < 0.5% THD.
Looking good so far – but there was still some trouble with the phono input. While the Mag Input worked well when set to Tape Head, the RIAA curve below the 1 KHz turnover point for the pre-amp equalizer dropped off sharply. Both signals passed through the small, red PEC 222ER couplets near the input selector. While in the Phono position, the input signal was coupled partially through that 150 pF capacitor as the only difference between both modes; that pointed to the internal cap as the cause of failure. Fortunately, it’s connected just across pins 2 and 4 internally, so could be easily replaced with an external mica capacitor by simply clipping Pin 4 at the base, and wiring the external cap directly between the switch terminal and the couplet’s Pin 2.
Both channels had failed in the same manner, so were both replaced with the yellow 150 pF mica capacitors shown.
With that last problem sorted it was time to pack everything up and run performance tests to determine the sensitivity of the various inputs. Quite a bit came out of this amp:
Testing shows very closely matched channels (seen here before re-scaling):
The output tubes have a great glow.
And back in the case, this amp looks fantastic.
Ultimately this example of the Scott Stereomaster 233 produced a full scale output of 28W RMS, all channels driven into an 8 Ohm load, with frequency response from 26 Hz – 20 KHz +0 / -3 dB, THD < 0.5%, relative to 1 KHz. The Mag Low input had 4 mV sensitivity for full output; Mag High 8 mV sensitivity, and Ceramic 500 mV RMS sensitivity. Both the Tuner and Extra inputs also demonstrated 500 mV sensitivity for maximum output at full volume.