Yuck.....Under The Weather A Bit and Heathkit Distortion Analyzer Progress

 Picked up a "head cold" a couple of days ago, and today was the worst. Coughing, sneezing, runny nose, and feeling like a piece of ripe elephant dung has been the Order Of The Day. At least it feels like it's breaking up and on the way out.

And the Heathkit IM Distortion Analyzer has me stumped. I know exactly where in the circuit that proper operation goes South, but I can't seem to pin down why. The DC bias voltages are within specs, if you can call "+ / - 20%" a specification. Mostly they're within 5%, which should be OK, but when I get to the second transistor in the problem section of the Meter Amplifier, they disappear. It's either something glaringly obvious,  a very subtle error, or a degraded part that I'm overlooking. This is the first Heathkit I've seen where Heath's "late-stage cost-cutting death spiral" is so painfully obvious, and I've decided to correct these built-in / designed-in flaws so I have a proper foundation. The FM Stereo Multiplex Generator had a mild case of that, where they were using a 2 Watt resistor when a 5 Watt part was called for. The resistor was dissipating 1.8 Watts during normal operation, or 90% of it's rated value. That made the resistor run hot, which made it's value drift up over the years, and completely screwed up the power supply voltages. Back when I was a cub learning these things, I was taught to NEVER use a part at more than 75% of it's rated value, and running your parts at 50% of their ratings was a sure-fire way to get a better performing, longer lasting product. And in critical cases of circuit design, you run the parts at 25% of their rated value, or less. Yes, it costs money to use a better part, and it's usually a bit larger than a lower rated one, but you've just designed-in some extra reliability.

Whoever specified the parts used in this analyzer should have been fired. The capacitors in the power supply are OK. The rectifier has an output of 75 Volts, with a 100 Volt rated part. The next part downstream is a 75 Volt part with 65 Volts on it. Tight, but probably OK. The real sore spot is that ALL of the filtering/decoupling capacitors on the "15.5 Volt" power supply rail are rated at 15 Volts. That's a huge "Don't Do It". The parts have some tolerance as to what voltage they'll fail at, and a "15 Volt" part generally won't self-destruct with 16 Volts applied to it (at least not immediately), but you simply don't run electronic components at 100% of their ratings. 

So I'm now making up a list of capacitors to order, and I'm going to "shotgun" this particular issue by replacing them all. And since the next patient on the bench is another Heathkit of the same era, I'll be taking the covers off it, and going over the schematic  to see if they did the same thing. It's more convenient to place one order than two.