Dynaco SCA-80Q Amplifier Kit P2 • How Many Bad Capacitors & Resistors?

Описание: Welcome to part two of the Dynaco SCA-80Q series. In part one I unboxed the never-opened 1978 kit, and I speculated that many of the old components would now be bad.

I’ve now tested all of the kit’s 43 year-old capacitors, resistors, transistors and diodes and in this episode I’ll share my findings. At the end, I’ll also the test the transformer.

Before we start, I want you to take a guess about how many electrolytic capacitors you think are bad in our kit. Is it 10%? 50% 100?

And how about the other capacitors? The resistors? The transistors? The diodes? How many of those do you think are bad?

I guess we should start by defining what “bad” really means.

In the case of resistors, that’s easy. They’re color coded and tell you exactly how far out of range they can test for resistance. Resistance is tested with an ohm meter.

With diodes and transistors, it’s pretty easy, too. At this point, we just want to make sure they’re not open or shorted. We can do that with a multimeter or semiconductor analyzer.

Capacitors are first tested to see if they’re out of tolerance with a capacitance meter. With paper, mylar, ceramic and silver mica caps, that’s not a problem as they often have their tolerance stamped right on them. Either explicitly or with a code.

Electrolytics are bit trickier, though. Many circuits operate just fine with an electrolytic that’s 50% higher than rated capacitance. And most often they’re not marked for tolerance at all.

Most manufacturers claim their new electrolytics have a 20% tolerance, though, so for our purposes, any that test 20% or higher than rated capacitance will be considered bad.

In addition to capacitance, we also need to test an electrolytic’s loss and leakage.

To measure loss, we use an ESR meter which measures equivalent series resistance. Or, in other words, how much the capacitor resists passing an AC signal. There are generally-accepted standards for how much ESR is acceptable and a lookup table is often used as reference.

To measure leakage we use a leakage tester such as my tried-and-true antique Heathkit IT-28. It passes DC through a capacitor to see how much actually “leaks” or gets through. A good capacitor will pass no DC, which will be indicated by an open window on the magic eye display. A closed window indicates the capacitor is leaking DC and needs to be replaced.

Note that all types of capacitors can be tested for leakage, not just electrolytics.

So now that we have ground rules, one more chance to guess. How many capacitors, resistors, transistors and diodes do you think tested bad in our old kit?

Let’s look first at the capacitors.

As I expected, the typically reliable ceramic, silver mica and mylar capacitors tested well and 100% of the three types shown here tested perfectly. There was one ceramic disc capacitor that was out of tolerance, but 92% were fine. Overall, a solid showing.

Electrolytics are more likely to age poorly and predictably for me, 50% of the small electrolytics tested bad. Here’s a 47 microfarad cap measuring 45% high at 68 microfarad. And this one is completely open.

The three large electrolytics and multi-section can completely failed the leakage test. Here I’m testing one of the 80 volt, 5,000 microfarad caps and it didn’t even pass at 50 volts. Here’s the multi section electrolytic. The section I’m testing here is supposed to be good up to 75 volts, but was leaking DC even at 50 volts.

So, an overall bad showing for the electrolytics and that’s why I’ll be replacing all of them, even the ones that tested OK. That’s because even the ones that tested OK, still aren’t as good as modern equivalents. Compare this 470 microfarad cap from the kit that’s coming in at .2 ohms ESR. That’s good, but the replacement I’ll use has only .08 ohms of ESR. No contest.

OK. How about the resistors? Well 100% of the carbon film, ceramic and metal film resistors tested good. Impressive, considering these are gold-band resistors with a strict 5% tolerance. These are excellent components and no replacements will be necessary.

The carbon composition resistors, though, failed miserably. 50% tested bad. Here’s a 4.7K carbon comp, for example coming in at a 5.5K. That’s 17% high on a 5% tolerance component. So, I won’t be taking any chances with the carbon comp resistors and even the ones that tested well will be replaced.

I do have quite a few modern carbon comps in stock that I’ve had good luck with, so I may use those. Then again, carbon comps generally aren’t considered as stable as carbon films, so I’m not sure. In the end, the decision may come down to what I have in stock, so stay tuned.

Moving on to the semiconductors, all the transistors and diodes included with the kit tested fine. So no replacements necessary there.

Well, how did you do? Here’s the lineup of all the capacitors and resistors that tested poorly. Is this less, more or about what you expected? Let me know in the comments.

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