Hi Chris,
The original AC cord is starting to dryrot, there are stress cracks in the insulation.
And while I do have good non-polarized replacement cords, I'd sooner feel better making this amp "modernized", particularly since the whole case is metal, and electrical codes are strict.
That dual "death cap" system is outdated, and to be removed, a single 0.047 cap from chassis to neutral/ground side of the AC cord is mandatory.
Well Chris, in as much as I'll admit that no one's perfect, I'll say that indeed a "I screwed up" thread would be somewhat amusing.
However, I pretty much doubt that I'd have anything much to enter there.
I just can't remember if and when I messed up anything, I'm prone to being "super careful" and detailed in my work.
Really, I could write a book. Maybe it's because I actually design stuff, and stuff that requires big hungry power supplies at that. Did you ever try to build an SMPS and wind the transformer yourself.....I know enough to place inverted coffee mugs over the mosfets before plugging it in. Nobody else ever got a diode or electrolytic in backwards.....or put the regular diode where the Zener goes, or worse the Zener where the 1N4007 goes?
I took one of Petes magic red boards well beyond its rated 18 WPC. It works great and sounds nice at 50 WPC, so I keep turning up the power supply watching the power meter climb....Somewhere north of 600 volts there is a very large bang, and a black stain on the board where a large electrolytic capacitor and resistor used to live. I had built out the board with 450 volt caps, just like Pete said......Still, it was a learning experience that allowed me to realize that two stacked power supplies were needed here, the board never sees more than 350 volts. The red wire to the OPT's gets 600 volts and 125 WPC flows out of that board. At least 10 were build by myself and several other builders. It might not have ever happened if I didn't blow some stuff up.
Maybe I design and simulate something, then build it and tweak on it for a week, eventually losing my patience and turn the power supply up full to smile as I watch it DIE!
Hi George,
Of course I've installed capacitors backwards, and other goofs. They are usually caught during the power up sequence, never reaching full power. But I did say that I correct the errors and make sure everything is working properly and reliably before releasing the equipment. If it doesn't meet original spec, I don't release it.
I was referring to all the gear I see come in that was released buggered up. Stuff that never worked properly and certainly did not meet spec after the previous repair. It blows me away how often things will work that should have failed very quickly, and they work for a few years often enough. It is amazing how much abuse some parts will take before they finally throw in the towel. Mind you, you do that for kicks.
I design things too. Sometimes they don't work. Most of the time they do, but I have never wound a transformer, RF transformer or switching supply transformer. In high power applications, you have it all over me. I generally only run tubes up to 475 volts maximum. Your work is well beyond mine in so many ways. Mostly, I repair known commercial designs that someone else owns. So I have to be careful, and I have to be correct in what I do. The end result anyway.
You know when I make mistakes? When I'm on the phone, or when someone is talking to me when I'm trying to work.
-Chris
Of course I've installed capacitors backwards, and other goofs. They are usually caught during the power up sequence, never reaching full power. But I did say that I correct the errors and make sure everything is working properly and reliably before releasing the equipment. If it doesn't meet original spec, I don't release it.
I was referring to all the gear I see come in that was released buggered up. Stuff that never worked properly and certainly did not meet spec after the previous repair. It blows me away how often things will work that should have failed very quickly, and they work for a few years often enough. It is amazing how much abuse some parts will take before they finally throw in the towel. Mind you, you do that for kicks.
I design things too. Sometimes they don't work. Most of the time they do, but I have never wound a transformer, RF transformer or switching supply transformer. In high power applications, you have it all over me. I generally only run tubes up to 475 volts maximum. Your work is well beyond mine in so many ways. Mostly, I repair known commercial designs that someone else owns. So I have to be careful, and I have to be correct in what I do. The end result anyway.
You know when I make mistakes? When I'm on the phone, or when someone is talking to me when I'm trying to work.
-Chris
Hell is the other(s), as Jean-Paul Sartre put it.
I must be one of the others, because things frequently go wrong with my projects.
I can report two things gone wrong just in the last week alone. First, I started on a much needed pickup swap in my South Korean ES-335 clone, only to find (a) that the neck tone control pot was damaged and spinning freely, and (b) after spending an hour fishing pots and wires through the guitar with strings tied to them, finding out that the replacement pot had too short a threaded bushing to get the nut on...
Second, a friend who plays acoustic guitar and sings hates the sound of all electric guitars, but was given one for free by another friend. Knowing she would use it if it didn't sound like an electric guitar, I said I'd take a stab at changing its personality.
Electric guitars sound much harsher than acoustic guitars, because the magnetic pickup itself suffers from a lot of harmonic distortion. This comes from the nonlinear way in which the magnetic field around the pole pieces varies with distance from the pole piece. After decades of hearing them, most of us have become so familiar with the sound of an electric guitar that we don't notice this any more - but a side-by-side comparison between an acoustic and an electric guitar, playing the same simple "cowboy chord" on both, makes it immediately obvious.
My friend only plays simple cowboy chords (her primary focus is singing), and doesn't like the way they sound on an electric guitar.
The other thing she didn't like much was the increased sustain of the electric guitar. Play a six-string, 2nd-fret A-major chord, then move immediately to a five-string D-major chord, and the "D" chord will sound horrible because the low "E" string continues to ring out, turning the D-major chord into a very muddy-sounding Dadd2 chord.
Those of us who play electric guitar invariably figure out ways to mute the unwanted strings, so that we don't get these unwanted and unwelcome additional notes. But my friend is not about to re-learn how to play guitar just so she can play an electric guitar that she doesn't really love in the first place; telling her she has to learn how to mute all those unwanted notes isn't going to fly.
Why bother with any of this at all, then? Well, my friend has some problems with arthritis in her left hand, and her days of brute-forcing big chords on acoustic guitars are probably numbered. Switching to an electric guitar would reduce the amount of force her left-hand fingers would have to apply, and likely, extend the number of years she can continue making music.
So, I need to make an electric guitar sound passably like a plugged-in acoustic guitar. My plan was simple: first, find a location on the guitar body where I could mount a small piezoelectric disc, then disconnect the stock magnetic pickups from the output jack, and wire the piezo to it instead. Second, add a strip of felt or something similar to the bridge, to permanently add a little mechanical damping to the strings, so that they didn't sustain any longer than a decent acoustic guitar. Simple, what could possibly go wrong?
Well, after I glued the piezo into the interior of the control cavity and wired the output jack to it, only faint tinny sounds came out of my guitar amp. If I place a fingertip on the piezo disc and push lightly, the sound suddenly becomes much louder, and much more balanced (bass shows up).
Cracked piezo, mebbe? But if I replace my fingertip with a pencil, a piece of wood, or one of my wive's plastic knitting needles, the sound goes back to thin and tinny.
Loose wire? Nope, solder joints are good, ohmmeter checks are good from output jack to solder-blob on piezo.
So: (a) the piezo doesn't do what it's supposed to, (b) my fingertip cures it, and (c) duplicating the pressure from my fingertip with some other insulating object does NOT cure it.
I haven't sorted this one out yet, but I wonder if my guitar amp is actually going unstable (oscillating), and my fingertip on the piezo is acting as a ground or something to stop the oscillation.
I'll get this figured out eventually, but in the meantime, Murphy himself is standing invisibly in the wings, snickering at me, as he so often does. Murphy's Law, the One Law to rule them all!
-Gnobuddy
My decades of experience stemmed from classroom training, aquiring certificates, and focused mainly on the service industry.
With all that, I had the "tools" to enable me to do some original work, like building amps, other projects.
So yeah, I am able to do my own designs, as well as upgrade and do modifications on commercial products.
It's not too hard, if you have the background, the fundamentals, taught to you by certified instructors.
But these days, a lot of people just want to "jump right in" and putz around and rely on the internet - making mistakes along the way.
Ignoring the training that I took, and am thankful for.
And god knows, relying on someone on the other end of the screen is not a guarantee of success - who the heck knows who's on the other end of that screen, there are self-professed goons all over the place.
Thanks for the suggestion.
I was, in fact, planning to try a parallel damping resistor across the piezo (it's 13nF, so anything well above 150k will be fine for guitar bandwidth). If that works, I'll probably replace it with a 250k volume pot, or re-purpose one of the existing 500k log pots in the guitar, as the owner will probably want a volume control on the guitar itself.
If the parallel resistor doesn't cure the problem, I'll certainly try a series one, as you suggest. A "grid stopper" at entirely the wrong place, in the guitar instead of inside the amp!
Here's the thing, though: as every engineering textbook on negative feedback amplifiers explains, a capacitive load on the output of an amplifier causes extra phase lag, and may cause oscillation if there is negative feedback involved. But a capacitive load on the input? That's not supposed to do anything to the stability margin!
But the textbooks always leave out Murphy's Law, don't they?
Incidentally, I got the piezo installed about five minutes before bed-time, so the one quick test I did was with my Boss Katana 50 amp. I have no idea if there will be any stability problems with any other amp.
This is not meant as any kind of criticism of the Katana 50, as it has been nothing but well-behaved in every other circumstance, and is an amp I very much enjoy using.
-Gnobuddy
Seriously, you've never had reality bite you in the behind unexpectedly?
One example: decades ago, I designed and built a solid-state audio power amp with the then-typical differential input stage, voltage amplification stage, and complimentary NPN/PNP output stage. I estimated the capacitor value needed for dominant-pole compensation from what transistor data I could get, and built the thing. This was long before LTSpice and personal computers, so the best I could do was ball-park approximate calculations.
When tested, the amplifier worked, but the output showed high-frequency oscillations on a 'scope - bu only on positive half-cycles! Something I'd never seen in any of the many electronics books I'd studied by then.
Once I saw the problem, I figured out the cause. You see, other things being equal, holes in P-type semiconductor have lower mobility than electrons in N-type semiconductors do. As a result, PNP transistors are usually slower than their NPN equivalents.
In this case, the PNP output transistor was slower than its NPN complimentary twin - so much slower that the additional phase lag it caused satisfied the Barkhausen criterion for oscillation, triggering the amplifier into oscillations when the PNP transistor was conducting, i.e., on positive half-cycles of the output.
I tweaked my dominant-pole compensation capacitor value accordingly, and the oscillations stopped. The amplifier went on to serve me well for several years. But this wasn't a problem I could have anticipated based on anything I'd learned until then.
In fact, though I've been building electronics since I was a young boy, my experience has been that only the simplest and most unimaginative circuits actually work exactly the way they're supposed to. The more creative the circuit, the more likely it is to exhibit some behaviour I didn't anticipate.
A very simple example from a different thread on diyAudio right now - mixing the output of two piezo sensors placed on a musical instrument. The plan was to mix the two outputs by floating a "mix" pot between them, then connecting the wiper of the "mix" pot conventionally to a volume pot. Pretty straightforward, just a passive resistive mixer, right?
The surprise: an LTSpice simulation shows that the bass response from each individual piezo develops a slight (roughly 0.5 dB) peak when connected in this way. This is caused by the interaction between the piezo capacitances, the three resistances in the circuit, and the piezo output voltages.
Betcha you didn't see that coming - even though it's simply the outcome of basic electronics principles we all know, i.e. Kirchoff's laws, and capacitive reactance.
If you're referring to Tubelab's comments earlier, you do know that he (Tubelab) has advanced electrical engineering degrees, as well as many decades of experience?
-Gnobuddy
All I know for sure is what I've been taught, and my level of expertise.
Which for all intents and purposes is more than competent for just about any "home entertainment products", which is the service profession that I focused on for decades.
In other words Gno.. I know what I know, for sure.... the reality that I live in.
For all I know, others typing across a screen could be some kind of boogieman, how can I know for sure who or what they are?.. what they post....
I don't really want to sound skeptical, but it's the internet, isn't it?
That's only a "virtual reality".
Hi John,
Good luck with the SCP-2. Absolutely, repair can be extremely difficult and vexing even if you know the circuit intimately as in your case. However, that may lead to assumptions and some times you just have to go "stupid" and check the obvious things.
Hi wiseoldtech,
I hear you. I also have a formal education and I know what I know. But. When I was finished with the organized education, I was ready to begin really learning. I was taught the framework to learn in, and I think that is the big difference between getting a post secondary education and not. Although more difficult (generally speaking), some folks out there attain extremely high levels of understanding without the formal education. I've learned you can dismiss anyone out of hand due to the lack of certificates on the wall.
-Chris
Good luck with the SCP-2. Absolutely, repair can be extremely difficult and vexing even if you know the circuit intimately as in your case. However, that may lead to assumptions and some times you just have to go "stupid" and check the obvious things.
Hi wiseoldtech,
I hear you. I also have a formal education and I know what I know. But. When I was finished with the organized education, I was ready to begin really learning. I was taught the framework to learn in, and I think that is the big difference between getting a post secondary education and not. Although more difficult (generally speaking), some folks out there attain extremely high levels of understanding without the formal education. I've learned you can dismiss anyone out of hand due to the lack of certificates on the wall.
-Chris
Yes Chris, the "formal education" we get gives us the edge on things, and it's without a doubt the most valuable facet of the business.
Like any tool, you gotta know how to use it first, what it's for, and what not to do with it.
The chances of success are enormously better, it goes without saying.
When I was a teen, and pre-tech school, I dabbled with things, yet, I read a lot, studied schematics, learned things like the famous resistor color codes, etc.
I always thought that I had a natural tendency to look at a schematic, and actually understand it.
Later on, in High School, that strict electronics shop teacher, Mr Connolly, always got on my case.
I hated that guy.
He took issue with me always "going past" his lessons and projects.
But I was stubborn. regardless.
While the rest of the class was making some dumb Graymark kit power supply, I was designing my own from scratch.
The Graymark kit was a simple 6 and 9 volt thing with 4 diodes and a filter cap and a switch. - in a cheap hinged plastic box.
The switch selected the transformer center tap, for 6 volts, and the bridge "back end" for 9 volts.
It was dumb.
The one I built was in an aluminum chassis box, pi filtered, and had a rotary switch with 4 positions - 3-6-9-12 volts, and a meter to show the output.
It included a pilot light and a toggle switch.
So what does ole Mr Connolly do?
He makes me go up in front of the class, and explain in detail what it's all about, and what the resistor/divider does, etc.
I got an A+ that year,.... LOL!
Some of the guys in class hated me...
Screw 'em.
I am what I am.
But you don't know what you don't know......That's where experience, learning from your own mistakes, and learning from others, so you don't have to make the same mistakes they made, really helps.
One of the many jobs I had at Motorola was training "freshouts" (a recent engineering school graduate) how to be an engineer. Many have never held a soldering iron, or operated any complex test equipment. None know how a complex digital two way radio or cell phone operates. These skills need to be mastered before someone can be expected to DESIGN one.
NO, you don't measure the harmonics on a 5 watt radio by connecting it directly to a spectrum analyzer and transmitting.....a $700 repair.
Yes, Motorola paid for me to get two engineering degrees. Neither was as important as 50+ years of building and fixing stuff, and blowing stuff up. All are part of the learning experience we call life.
Some people learn better by reading books and sitting in a classroom. Some people learn by jumping right in and putzing around. Much of my early learning was by simply wiring stuff up and plugging it in, without full knowledge of what I was doing. Often it didn't work....or worse.
Tubes and parts were plentiful in the trash dump, and I did eventually learn how to make a working guitar amp out of an old TV set, at age 10.
I was fortunate in that respect. I went through a 3 year 1080 hour vocational electronics program in high school. It taught the basic theory, and how to make money by fixing TV sets. That's where I finally learned about those impedance, biasing, and load line things that I had read about. By the beginning of the third year the teacher bet me that I could not make the outside metal envelope on a metal 6L6 glow red! I stunk up one whole wing of the school, but won that bet.
Couldn't agree more even though I know little about EE. I suppose that's the case with most practical level jobs. Formal education is just the foundation you build your skills on. This is especially true for my profession. You just simply don't learn this stuff by reading books.