Author Topic: Damn, I'm good.  (Read 1767 times)

Gen. Dreedle

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Damn, I'm good.
« on: October 04, 2011, 05:54:52 PM »
No, not really. I just wanted to see how many folks I could hook with that intro.

A local brought over a Stax D-100M that was dead. Went into protect mode immediately.

This is an impressive, and not necessarily in the good sense, boat anchor of an amplifier. It has a bias switch on the front, that allows it to run Class AB (100W), or Class A for the first 25W or so.

Translation: lotsa heat sink area. At least they understood that they needed fin area, and thick fins are not much help in this scheme. How they get the heat from the outputs to the fins............I am not even going to try to describe it. Maybe I will find a way to shoot a pic of it.

Anyway............output goes to + rail, so, yeah, that is why is goes to protect. Disconnect the sense lead, and ground it, so I can see what is up. Hooked up a variac, and watched.

Got to around 80 V, and the output jumped to full +. Hmmm.............sounds like there are regulated supplies and the negative one is shot.

So, wait for the customer to get the schematic in a form that I can read. Oy vey.........I can't make much sense of it. Cross-coupled stuff everywhere. Ok, poke around where the - rail should be, and nada.

24 V zener on both supplies. Positve, ok. Negative, much lower. OK, enough clues.

Looked at the strange schematic some more, and............."Gotta be this transistor."

It was.

Yeah, I'm good. But if I was really good, i would have found it w/o the schematic.

See, told you that I wasn't that good. Maybe just good, and only marginally lucky.

But, unlike some guy I used to bowl with, Mike "I would rather be lucky than good any day" (who couldn't bowl for squat, but really was lucky..........I hated him.........), I would rather be good!

And today I was good enough. Luck only carries you so far for so long. Good sticks with you.

Moral of the story: no substitute for experience. Read all that you can, try everything at your disposal. It all counts in the long run.


(No, I don't know or care how it sounds. I'm only in it for the money. Just like Jimmy Carl Black, Indian of the group.)

(OK, he also wants beer and that waitress to...............)
"Major Danby, sir."
"Danby. D-A-N-B-Y."
"Take him out and shoot him."
"Sir?"
"I said take him out and shoot him. Can't you hear?"

Gen. Dreedle

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Re: Damn, I'm good.
« Reply #1 on: October 04, 2011, 05:55:21 PM »
No, because I don't have a cell phone!
"Major Danby, sir."
"Danby. D-A-N-B-Y."
"Take him out and shoot him."
"Sir?"
"I said take him out and shoot him. Can't you hear?"

Kryten

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I'm good.
« Reply #2 on: December 01, 2011, 07:39:57 PM »
Just get a Fluke 77 and use the audible diode test....and poke around until you find the duff semiconductor (usually about two minutes)......schematics pfffft !.

Davo.
Been fixing and listening to audio for waaay too long....

Gen. Dreedle

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Re: Damn, I'm good.
« Reply #3 on: December 05, 2011, 11:41:18 AM »
Uh..........I just fixed something that was not fixed by replacing the chip that no output, on pin X.

After replacing it, the second one also had not output, on pin X. But only after a few weeks! It initially worked.

The Fluke was of limited help: pin X was high Z, in both cases.

What other part I changed, and why it mattered...........well, I still have not figured that out, so i won't share that part of the story.
"Major Danby, sir."
"Danby. D-A-N-B-Y."
"Take him out and shoot him."
"Sir?"
"I said take him out and shoot him. Can't you hear?"