Troubleshooting Philosophies

OK - this all started from a series of threads where there seem to be two camps firmly ensconced and not quite as firmly divided. For the purposes of this question/post, I will describe them as, quite superficially:

a) Replace first, diagnose later.
b) Diagnose first, replace only as necessary.

For the purposes of full disclosure, I am in the b) camp.

When an amp starts to act out in my inventory, the *first* thing I will do is make sure that the bias is correct, all mechanical switches, sockets and so forth are behaving as they should, and that there are no visual clues or strange smells. Then, I will test the tubes. I will not replace tubes based on age or without some other indication. I will *substitute* tubes either individually or in pairs (if appropriate), but that is for purposes of elimination, all other things being equal.

I will state for the record that on few occasions has the replacement/substitution cured the problem, and on fewer occasions have the culprits been power-pentodes. Mostly small-signal tubes. Most of the time the problem has been one OEM cap or resistor or another that had either failed or drifted so far from spec. as to be useless.

And this is my concern. Would it not be better to be sure that the basic 'physical plant' is OK before replacing-by-rote? Now, I agree that the difference between my equipment and much that is here under discussion is that my most recent tube amp (ST70) is date-stamped 1964, and not one of Bob's fine options. So, perhaps my approach is colored by this condition. I am also blessed that over the years I have accumulated a fair amount of test equipment, meters and such, so I do not have to reach far for testing.

Any thoughts? Suggestions? Ideas?

Fault finding is a science in itself!
Luckily lot of fault's can be found with simple mechanical rulesets. This is what management
thinks that AI can do with support functions. And they do ( in some percentage of all cases).
But as more exclusive and unique the device is, the more an understanding of it's construction
and functioning is important. Add to that creeping addition of software and self-diagnose in
all kinds of devices, for bad or good. Luckily we see very little of that in out beloved tube amps.

Understanding is the key, and the ability to understand is depending on skill and information ( schematics, list of typical voltages etc). In addition diagnosing instruments is crucial, maybe not
in the amount some folks think, a voltmeter and a scope covers a lot of measurements.
The fact that most amps is stereo is a very good startingpoint where one channel is working
normally and the other is not, then some measurement might reveal good clues to the problem.

Time however is also a factor that impinges on the process. In a hurry we try to take shortcuts,
especially when we are experienced. It might be that "if x it's usually component y, lets replace y
and see what happens". And this works often enough. Given of course that the supply of good
quality parts is adequate ( a box of used components in unknown state is worse then bad)
But applying "replace and see" ( scenario (a) in previous post) by a uninformed and maybe
unskilled person ( or following the AI systems advice in absurdum) is a recipe for disaster. Here
is where thinking and fault-finding skills in general comes in in addition ( or substitution for
expertise of the technology )

Fault finding is a subject i happily will expand on in future articles!

As a hobbyist, short of failures associated with an obvious "event" (red tube, sparks, dark tube) I have have rarely seen a failed tube amp cured by replacing output tubes. When new tubes were indeed the "cure" the culprit usually turned out to be a compromised (miniature or octal) socket whose pin connections were improved by a tube, new or otherwise, with different diameter pins.

About the only "mindless" debug I suggest is checking voltages five times and in sequence--no tubes, no rectifier, one channel tubed, other channel tubed, all tubes in. And this only when I know the amp is in the hands of someone who realizes that tubes run hot and that there is high voltage present. Voltage measurements are easily communicated via telephone, e-mail, or text and often gives a clue as to what the next step should be.

I've had several professionals attempt to troubleshoot my philosophies over the years ... it's fun seeing them running out of the room, gibbering about this and that ...

That said, in many cases, a bit of quickie maintenance that should have been done regularly anyway can make things right. Not talking anything fancy, but just exercising the basic skill set any tube equipment owner needs to develop to keep the gear happy. First steps in ANY tube related problem would be cleaning and tensioning the sockets, especially with the octal types. Next, take a quick peek under the hood for obvious issues like burnt bias resistors and such. THEN pull out the test gear or take it to your amp guru for repairs.

Does poking around the chassis with a chopstick with the amp grooving along upside down at 500 volts looking for shorts and cold connections and bad components qualify? Seems we also used to spray some kind of super cold freeze stuff on components to find weak links and see which ones failed under stress. Ya just can't do that nowadays with class D amps and IC components.

wildiowa wrote:Does poking around the chassis with a chopstick with the amp grooving along upside down at 500 volts looking for shorts and cold connections and bad components qualify? Seems we also used to spray some kind of super cold freeze stuff on components to find weak links and see which ones failed under stress. Ya just can't do that nowadays with class D amps and IC components.

I thought about this considerably before this reply.

As any item I am testing or checking is coming through an isovariac, I do not typically worry about shocks even from a 500 V source. Not that I am careless, but I do have a layer of protection.
I do keep a number of plastic sticks for probing as you suggest. As well as a small rubber mallet with a wooden handle.
And non-metallic micro-spiral brushes small enough even for miniature-tube sockets - very handy.
So mechanical 'impact tests' are, initially, entirely valid for elimination purposes (Anyone ever 'flick' a light bulb for a few seconds of extra life?).
I also keep a number of dental tools that allow me to reach in/under things and pull on suspicious connections. Heat-shrink tubing for all but the last few mm eliminate possible bridging accidents.

I have always found Freeze Spray to be a rather crude means of testing.

My greater concern across this entire discussion is the concern about cascade effects. A fuse blows - the natural response is to replace it immediately. It holds, OK. Fuses age out. It blows again, right away - well... maybe a good tech or cautious owner stops before replacing it again. Worst, it holds for about an hour as it is one of those inventions of the devil, a wire-wound slow-blow fuse (heard this rant before, so I will spare all). Now, something else goes, or the part that caused the initial problem gives up entirely, taking other stuff with it. Now, I have a quite-vintage 1964 date-stamped ST70 that has been in my inventory for over 20 years, and in a record store with daily use 20 years before that - with the same set of tubes, all Dynaco-branded. The tubes test fine on a very good GM tester. The amp sounds just fine with neither hiss, nor hum nor other artifacts. One day, one channel went fuzzy. Some here would immediately leap to new tubes. I recapped the driver board and replaced two (2) resistors. The main filter passed both capacitance and ESR with flying colors - it is still there. The fuzz went away and has stayed away. But a little secret: I did also switch out the driver tubes - and the fuzz switched too. BUT! The actual culprits were the caps and drifted resistors. Meaning (to me) that one of the 7199s had aged to the point where marginal - but not yet fatally gone - other components were enough to cause the fuzz. A marginally better 7199 'fixed it'. 'Cept that it was not fixed. Had I stopped with the 7199, I would have set up the amp for a more spectacular failure. After the recapping and resistor changes (what happens to one channel happens to the other), the original 7199s went back in, and remain to this day in fine fettle.

Again, I admit to prejudice as most of my daily drivers are between 25 and 50+ years old. I do worry about

Thoughts?

Glad to see poking around is still an accepted practice and I do not own a can of freeze spray so I am OK. I can say that after a second fuse blowing I pretty much hang it up at that point. There is no sense going to a higher value or the dreaded slo-blo as Peter indicates you are only going to start taking more expensive stuff down so just stop there and find out what is wrong. Two strikes and you're out, in this case.

Temperature cycling has it's virtues. Using cool-spray with a small spraytube enables to
cool very small parts and thus identify a single component.
Usually the bulk of components are heated with a hairdryer or heatgun, then individual
components may be cooled one by one until something reacts.
It's especially useful on circuit boards with small surfacemounted components.
It's usage on tubeamps are usually resticted to signal caps and resistors, tubes don't like
to be sprayed.

A very good text about the Art Of Electronic Troubleshooting,
http://conradhoffman.com/troubleshoottut.htm