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.
Unfortunately I've seen name brand professional broadcast equipment treated the same way during design back when I worked in broadcast engineering during the 80s & 90s. Bean counters control the process instead of design engineers. At my last job (QA) where railway related electronics were being serviced, repaired, and assembled,a number of the designs suffered the same issues and purchasing had to be watched like a hawk, especially on the hardware (non-electrical) components used on some electro-pneumatic equipment.
ReplyDeleteYep, I've seen it at places I worked for, too.
DeleteHope and pray you get over the bug, DrJim. Damsel is still shaking off a case of strep and congestion. I guess it's just that time of year for this stuff to happen although it has, thankfully, been a rare occurrence for us here in our corner of AZ.
ReplyDeleteStrep is not fun. I had a strep throat in High-School, and it knocked me down for most of a week.
DeleteWell, you sound like you have it under control. And a plan. Keep the frame, replace everything in it. Works for me.
ReplyDeleteSounds like electronic design is much like the way I build things. At least 50% overbuilt, if not 100% overbuilt.
And strange question, are the wires too small for the voltage being pushed through them? I've seen where cost-cutters have done that and had much the same issue. But since I'm definitely not a Sparky, I may just be talking out of my, well, you know, the thing.
Any chance to play with the snowblower yet?
It's not quite to the "Jack up the radiator cap and drive a new car under it" point, but I'm shaking my head over the components used in production.
DeleteWire size doesn't matter in this case. It's very important in power supplies and power circuits, but not here. Heath used solid wire instead of stranded for connecting things, and that can be a problem, but the size is adequate.
No snow yet, but I've been running it every couple of weeks just to make sure it works.
Wishing you a speedy recovery.
ReplyDeleteThanks, WSF!
DeleteHope you get well quickly, and maybe it's time to 'dump' those particular units rather than continue to let them frustrate you. Later Heathkits were definitely less reliable/broke more quickly than the good early units.
ReplyDeleteOne of the two analyzers I have was damaged in shipping to me, something I just noticed last night. I'll run a post on that later today.
DeleteGet well soon!
ReplyDeleteThanks, LL! Hope your travels are going well.
DeleteGet well soon, drjim!
ReplyDeleteYou all be safe and God bless.
Thanks, Linda!
DeleteHope you get better soon drjim. I have been having some bouts of various things. My daughter, a Nurse Practitioner, told me that as we interact more we are seeing stuff going around that wasn't because people weren't interacting.
ReplyDeleteI appreciate your pain in trouble shooting. I worked in a Metrology/Calibration/Repair Lab for about 10 years. When the technicians were stymied, I was called in to help troubleshoot. As long as you have the stage where it goes wrong, you can generally get it fixed which seems to be where you are.
That's why I'm going to shotgun all the under-rated capacitors. If there's one bad one, there's probably others.
DeleteI don't know how I became"Unknown". Oh, the vagaries of the Internet.
DeleteBillB
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