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First came the PAT-4 Preamp and the Stereo 120 Power Amp. Then came the Stereo 80, a slightly lower power version of the Stereo 120. The SCA-80Q was an integrated amp that combined a preamp based on the PAT-4 with a power amp based on the Stereo 80. The combination was dubbed the SCA-80 Integrated Amplifier. At some point, the Dynaco Quadaptor function was built in, and the result was the SCA-80Q.
My impression is that this was a very popular integrated amplifier. That impression is confirmed by the large number that have survived for 40 or more years.
Q. My SCA80 has developed a pretty bad hum. What's likely to be the problem?
A. C9 has probably gone bad. Of all the caps in an SCA80, it tends to be the first to go. Why does that cap fail so often? There are two identical caps in the amp, the two speaker coupling caps, and they don't fail nearly as often. Temperature is the answer. Temperature is the enemy of reliability. C9 sits right next to the PC-19 board that has 4 power resistors. Each one of the four resistors dissipates 36^2/400=3.24 Watts. Therefore, the four resistors dissipate 13 Watts. That's definitely an unlucky number for C9. If you put your hand over an operating SCA80, you'll definitely feel the warmth coming from those resistors. It's that warmth that cooks C9 to an early death.
Now, this is all relative. Can we really complain about a cap that lost its mojo after 30 or 40 years? Probably not, but we can replace those caps, and maybe get another 30 years. And we can make it better. That's the idea behind the SCA80C9 kit. The caps in the SCA80C9 kit aren't as tall as the original caps, so they probably are in a bit cooler spot. They're also brand new. Finally, there's three of them, and they have a total capacitance that's about twice what was in the original amp. That makes for lower hum and better bass. Still, perhaps the best answer, eventually, is to update the amplifier circuits to eliminate those power resistors. Stay tuned!
Q. Will the DRD4 (Distortion Reducer Kit for the PAT-4 Preamp) work in the SCA-80's preamp?
A. Yes, it will work, but it provides no real benefit. The high-level section of the PAT-4 preamp runs on 38 Volts and has a 600 Ohm collector load resistor. The SCA-80Q preamp's high level section runs on 24 Volts and has a 3300 Ohm collector load resistor. That results in very different operating points. The result is that the DRD-4 makes a dramatic difference to the PAT-4, but very little difference to the SCA-80Q.
SCA-80 with upgraded power supply and power amp kit installed
Q. Is there a kit to replace the power amps in the SCA-80Q?
A. Yes there is! This kit replaces the power supply and power amp boards. It includes:
This is a sweet kit. Like the stereo 120 upgrade kits, it provides much cleaner power than the original. It builds nicely, and drops into your SCA80(Q). You can take a look at the assembly manual here. It contains a detailed theory of operation section that will help you understand the design. You can buy the kit here.
Q. How are the knobs held on?
A. The knobs have 5-40 allen-head (hex) set screws. You'll need a 1/16" Allen wrench to undo them.
Does your SCA80 hum a lot more than it ever did? The biggest SCA-80 problem after all of these years is typically loss of capacitance. If your SCA-80 is humming, then the big filter caps, C9 and C11, are probably circling the drain. As you might imagine, I've developed two new kits for your SCA80Q:
Here's a movie showing the effect of a bad C9, and how you can replace it.
If the bass isn't what you remember, then the output (speaker) coupling capacitors (C7) may have lost some capacitance. Heck, they sometimes go almost open, and only the very highest sounds go through, making it sound like a tinny 1963 transistor radio. Even if the caps are up to spec, you can improve the bass by ordering and installing the C7X2 kit. It bumps the original 5000 uF for C7 up to 9900 uF.
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